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Research teams from around the world have made remarkable progress in the past decade in elucidating the genomic landscape of most types of childhood cancer. A decade ago it was possible to hope that targetable oncogenes, such as activated tyrosine kinases, might be identified in a high percentage of childhood cancers. However, it is now clear that the genomic landscape of childhood cancer is highly varied, and in many cases is quite distinctive from that of the common adult cancers.
There are examples of genomic lesions that have provided immediate therapeutic direction, including the following:
For some cancers, the genomic findings have been highly illuminating in the identification of genomically defined subsets of patients within histologies that have distinctive biological features and distinctive clinical characteristics (particularly in terms of prognosis). In some instances, identification of these subtypes has resulted in early clinical translation as exemplified by the WNT subgroup of medulloblastoma. Because of its excellent outcome, the WNT subgroup will be studied separately in future medulloblastoma clinical trials so that reductions in therapy can be evaluated with the goal of maintaining favorable outcome while reducing long-term morbidity. However, the prognostic significance of the recurring genomic lesions for some other cancers remains to be defined.
A key finding from genomic studies is the extent to which the molecular characteristics of childhood cancers correlate with their tissue (cell) of origin. As with most adult cancers, mutations in childhood cancers do not arise at random, but rather are linked in specific constellations to disease categories. A few examples include the following:
Another theme across multiple childhood cancers is the contribution of mutations of genes involved in normal development of the tissue of origin of the cancer and the contribution of genes involved in epigenomic regulation.
Structural variations play an important role for many childhood cancers. Translocations resulting in oncogenic fusion genes or overexpression of oncogenes play a central role, particularly for the leukemias and sarcomas. However, for other childhood cancers that are primarily characterized by structural variations, functional fusion genes are not produced. Mechanisms by which these recurring structural variations have oncogenic effects have been identified for osteosarcoma (translocations confined to the first intron of TP53) and medulloblastoma (structural variants juxtapose GFI1 or GFI1B coding sequences proximal to active enhancer elements leading to transcriptional activation [enhancer hijacking]).[1,2] However, the oncogenic mechanisms of action for recurring structural variations of other childhood cancers (e.g., the segmental chromosomal alterations in neuroblastoma) need to be elucidated.
Understanding of the contribution of germline mutations to childhood cancer etiology is being advanced by the application of whole-genome and exome sequencing to cohorts of children with cancer. Estimates for rates of pathogenic germline mutations approaching 10% have emerged from studies applying these sequencing methods to childhood cancer cohorts.[3,4,5] In some cases, the pathogenic germline mutations are clearly contributory to the patient's cancer (e.g., TP53 mutations arising in the context of Li-Fraumeni syndrome), whereas in other cases the contribution of the germline mutation to the patient's cancer is less clear (e.g., mutations in adult cancer predisposition genes such as BRCA1 and BRCA2 that have an undefined role in childhood cancer predisposition).[4,5] The frequency of germline mutations varies by tumor type (e.g., lower for neuroblastoma and higher for osteosarcoma), and many of the identified germline mutations fit into known predisposition syndromes (e.g., DICER1 for pleuropulmonary blastoma, SMARCB1 and SMARCA4 for rhabdoid tumor and small cell ovarian cancer, TP53 for adrenocortical carcinoma and Li-Fraumeni syndrome cancers, RB1 for retinoblastoma, etc.). The germline contribution to the development of specific cancers is discussed in the disease-specific sections that follow.
Each section of this document is meant to provide readers with a brief summary of current knowledge about the genomic landscape of specific childhood cancers, an understanding that is critical in considering how to apply precision medicine concepts to childhood cancers.
Acute Lymphoblastic Leukemia (ALL)
The genomics of childhood ALL has been extensively investigated and multiple distinctive subtypes based on cytogenetic and molecular characterizations have been defined, each with its own pattern of clinical and prognostic characteristics. Figure 1 illustrates the distribution of ALL cases by cytogenetic/molecular subtype.Figure 1. Subclassification of childhood ALL. Blue wedges refer to B-progenitor ALL, yellow to recently identified subtypes of B-ALL, and red wedges to T-lineage ALL. Reprinted from Seminars in Hematology, Volume 50, Charles G. Mullighan, Genomic Characterization of Childhood Acute Lymphoblastic Leukemia, Pages 314-324, Copyright (2013), with permission from Elsevier.
The genomic landscape of B-precursor ALL is typified by a range of genomic alterations that disrupt normal B-cell development and in some cases by mutations in genes that provide a proliferation signal (e.g., activating mutations in RAS family genes or mutations/translocations leading to kinase pathway signaling). Genomic alterations leading to blockage of B-cell development include translocations (e.g., TCF3-PBX1 and ETV6-RUNX1), point mutations (e.g., IKZF1 and PAX5), and intragenic/intergenic deletions (e.g., IKZF1, PAX5, EBF, and ERG).
The genomic alterations in B-precursor ALL tend not to occur at random, but rather to cluster within subtypes that can be delineated by biological characteristics such as their gene expression profiles. Cases with recurring chromosomal translocations (e.g., TCF3-PBX1 and ETV6-RUNX1, and MLL (KMT2A)-rearranged ALL) have distinctive biological features and illustrate this point, as do the examples below of specific genomic alterations within distinctive biological subtypes:
Activating point mutations in kinase genes are uncommon in high-risk B-precursor ALL, and JAK genes are the primary kinases that are found to be mutated. These mutations are generally observed in patients with Ph-like ALL that have CRLF2 abnormalities, although JAK2 mutations are also observed in approximately 15% of children with Down syndrome ALL.[4,9,10] Several kinase genes and cytokine receptor genes are activated by translocation as described below in the discussion of Ph-positive ALL and Ph-like ALL. FLT3 mutations occur in a minority of cases (approximately 10%) of hyperdiploid ALL and MLL (KMT2A)-rearranged ALL, and are rare in other subtypes.
Understanding of the genomics of B-precursor ALL at relapse is less advanced than understanding of ALL genomics at diagnosis. Childhood ALL is often polyclonal at diagnosis and under the selective influence of therapy, some clones may be extinguished and new clones with distinctive genomic profiles may arise. Of particular importance are new mutations that arise at relapse that may be selected by specific components of therapy. As an example, mutations in NT5C2 are not found at diagnosis whereas specific mutations in NT5C2 were observed in 7 of 44 (16%) and 9 of 20 (45%) cases of B-precursor ALL with early relapse that were evaluated for this mutation.[12,13]NT5C2 mutations are uncommon in patients with late relapse, and they appear to induce resistance to 6-mercaptopurine and thioguanine. Another gene that is found mutated only at relapse is PRSP1, a gene involved purine biosynthesis. Mutations were observed in 13.0% of a Chinese cohort and 2.7% of a German cohort, and were observed in patients with on-treatment relapses. The PRSP1 mutations observed in relapsed cases induce resistance to thiopurines in leukemia cell lines. CREBBP mutations are also enriched at relapse and appear to be associated with increased resistance to glucocorticoids.[12,15] With increased understanding of the genomics of relapse, it may be possible to tailor upfront therapy to avoid relapse or detect resistance-inducing mutations early and intervene before a frank relapse.
Specific genomic and chromosomal alterations are provided below, with a focus on their prognostic significance.
A number of recurrent chromosomal abnormalities have been shown to have prognostic significance, especially in precursor B-cell ALL. Some chromosomal alterations are associated with more favorable outcomes, such as high hyperdiploidy (51-65 chromosomes) and the ETV6-RUNX1 fusion. Others historically have been associated with a poorer prognosis, including the Philadelphia chromosome (t(9;22)(q34;q11.2)), rearrangements of the MLL (KMT2A) gene, hypodiploidy, and intrachromosomal amplification of the AML1 gene (iAMP21).
In recognition of the clinical significance of many of these genomic alterations, the 2016 revision of the World Health Organization classification of tumors of the hematopoietic and lymphoid tissues lists the following entities for precursor B-cell ALL:
These and other chromosomal and genomic abnormalities for childhood ALL are described below.
High hyperdiploidy, defined as 51 to 65 chromosomes per cell or a DNA index greater than 1.16, occurs in 20% to 25% of cases of precursor B-cell ALL, but very rarely in cases of T-cell ALL. Hyperdiploidy can be evaluated by measuring the DNA content of cells (DNA index) or by karyotyping. In cases with a normal karyotype or in which standard cytogenetic analysis was unsuccessful, interphase fluorescence in situ hybridization (FISH) may detect hidden hyperdiploidy. High hyperdiploidy generally occurs in cases with clinically favorable prognostic factors (patients aged 1 to <10 years with a low white blood cell [WBC] count) and is itself an independent favorable prognostic factor.[18,19,20] Within the hyperdiploid range of 51 to 65 chromosomes, patients with higher modal numbers (58-66) appeared to have a better prognosis in one study. Hyperdiploid leukemia cells are particularly susceptible to undergoing apoptosis and accumulate higher levels of methotrexate and its active polyglutamate metabolites, which may explain the favorable outcome commonly observed in these cases.
While the overall outcome of patients with high hyperdiploidy is considered to be favorable, factors such as age, WBC count, specific trisomies, and early response to treatment have been shown to modify its prognostic significance.[22,23]
Patients with trisomies of chromosomes 4, 10, and 17 (triple trisomies) have been shown to have a particularly favorable outcome as demonstrated by both Pediatric Oncology Group (POG) and Children's Cancer Group analyses of NCI standard-risk ALL. POG data suggest that NCI standard-risk patients with trisomies of 4 and 10, without regard to chromosome 17 status, have an excellent prognosis.
Chromosomal translocations may be seen with high hyperdiploidy, and in those cases, patients are more appropriately risk-classified based on the prognostic significance of the translocation. For instance, in one study, 8% of patients with the Philadelphia chromosome (t(9;22)(q34;q11.2)) also had high hyperdiploidy, and the outcome of these patients (treated without tyrosine kinase inhibitors) was inferior to that observed in non-Philadelphia chromosome-positive (Ph+) high hyperdiploid patients.
Certain patients with hyperdiploid ALL may have a hypodiploid clone that has doubled (masked hypodiploidy). These cases may be interpretable based on the pattern of gains and losses of specific chromosomes. These patients have an unfavorable outcome, similar to those with hypodiploidy.
Near triploidy (68-80 chromosomes) and near tetraploidy (>80 chromosomes) are much less common and appear to be biologically distinct from high hyperdiploidy. Unlike high hyperdiploidy, a high proportion of near tetraploid cases harbor a cryptic ETV6-RUNX1 fusion.[28,29,30] Near triploidy and tetraploidy were previously thought to be associated with an unfavorable prognosis, but later studies suggest that this may not be the case.[28,30]
The genomic landscape of hyperdiploid ALL is represented by mutations in genes of the receptor tyrosine kinase (RTK)/RAS pathway in approximately one-half of cases. Genes encoding histone modifiers are also present in a recurring manner in a minority of cases. Analysis of mutation profiles demonstrates that chromosomal gains are early events in the pathogenesis of hyperdiploid ALL.
Precursor B-cell ALL cases with fewer than the normal number of chromosomes have been subdivided in various ways, with one report stratifying based on modal chromosome number into the following four groups:
Most patients with hypodiploidy are in the near-haploid and low-hypodiploid groups, and both of these groups have an elevated risk of treatment failure compared with nonhypodiploid cases.[27,32] Patients with fewer than 44 chromosomes have a worse outcome than do patients with 44 or 45 chromosomes in their leukemic cells. One study of 20 patients with near-haploid or low-hypodiploid ALL indicated that MRD may have prognostic significance in the hypodiploid population.
The recurring genomic alterations of near-haploid and low-hypodiploid ALL appear to be distinctive from each other and from other types of ALL. In near-haploid ALL, alterations targeting RTK signaling, RAS signaling, and IKZF3 are common. In low-hypodiploid ALL, genetic alterations involving TP53, RB1, and IKZF2 are common. Importantly, the TP53 alterations observed in low-hypodiploid ALL are also present in nontumor cells in approximately 40% of cases, suggesting that these mutations are germline and that low-hypodiploid ALL represents, in some cases, a manifestation of Li-Fraumeni syndrome.
Fusion of the ETV6 gene on chromosome 12 to the RUNX1 gene on chromosome 21 is present in 20% to 25% of cases of precursor B-cell ALL but is rarely observed in T-cell ALL. The t(12;21)(p12;q22) produces a cryptic translocation that is detected by methods such as FISH, rather than conventional cytogenetics, and it occurs most commonly in children aged 2 to 9 years.[35,36] Hispanic children with ALL have a lower incidence of t(12;21)(p13;q22) than do white children.
Reports generally indicate favorable event-free survival (EFS) and overall survival (OS) in children with the ETV6-RUNX1 fusion; however, the prognostic impact of this genetic feature is modified by the following factors:[38,39,40,41,42]
In one study of the treatment of newly diagnosed children with ALL, multivariate analysis of prognostic factors found age and leukocyte count, but not ETV6-RUNX1, to be independent prognostic factors. It does not appear that the presence of secondary cytogenetic abnormalities, such as deletion of ETV6 (12p) or CDKN2A/B (9p), impacts the outcome of patients with the ETV6-RUNX1 fusion.[42,43] There is a higher frequency of late relapses in patients with ETV6-RUNX1 fusion compared with other precursor B-cell ALL.[38,44] Patients with the ETV6-RUNX1 fusion who relapse seem to have a better outcome than other relapse patients, with an especially favorable prognosis for patients who relapse more than 36 months from diagnosis. Some relapses in patients with t(12;21)(p13;q22) may represent a new independent second hit in a persistent preleukemic clone (with the first hit being the ETV6-RUNX1 translocation).[47,48]
The Philadelphia chromosome t(9;22)(q34;q11.2) is present in approximately 3% of children with ALL and leads to production of a BCR-ABL1 fusion protein with tyrosine kinase activity (refer to Figure 2). Figure 2. The Philadelphia chromosome is a translocation between the ABL-1 oncogene (on the long arm of chromosome 9) and the breakpoint cluster region (BCR) (on the long arm of chromosome 22), resulting in the fusion gene BCR-ABL1. BCR-ABL1 encodes an oncogenic protein with tyrosine kinase activity.
This subtype of ALL is more common in older children with precursor B-cell ALL and high WBC count, with the incidence of the t(9;22)(q34;q11.2) increasing to about 25% in young adults with ALL.
Historically, the Philadelphia chromosome t(9;22)(q34;q11.2) was associated with an extremely poor prognosis (especially in those who presented with a high WBC count or had a slow early response to initial therapy), and its presence had been considered an indication for allogeneic hematopoietic stem cell transplantation (HSCT) in patients in first remission.[26,49,50,51] Inhibitors of the BCR-ABL1 tyrosine kinase, such as imatinib mesylate, are effective in patients with Ph+ ALL. A study by the Children's Oncology Group (COG), which used intensive chemotherapy and concurrent imatinib mesylate given daily, demonstrated a 5-year EFS rate of 70% ± 12%, which was superior to the EFS rate of historical controls in the pre-tyrosine kinase inhibitor (imatinib mesylate) era.[53,54]
Rearrangements involving the MLL (KMT2A) gene occur in approximately 5% of childhood ALL cases overall, but in up to 80% of infants with ALL. These rearrangements are generally associated with an increased risk of treatment failure.[55,56,57,58] The t(4;11)(q21;q23) is the most common rearrangement involving the MLL gene in children with ALL and occurs in approximately 1% to 2% of childhood ALL.[56,59]
Patients with the t(4;11)(q21;q23) are usually infants with high WBC counts; they are more likely than other children with ALL to have central nervous system (CNS) disease and to have a poor response to initial therapy. While both infants and adults with the t(4;11)(q21;q23) are at high risk of treatment failure, children with the t(4;11)(q21;q23) appear to have a better outcome than either infants or adults.[55,56] Irrespective of the type of MLL (KMT2A) gene rearrangement, infants with leukemia cells that have MLL gene rearrangements have a worse treatment outcome than older patients whose leukemia cells have an MLL gene rearrangement.[55,56] Whole-genome sequencing has determined that cases of infant ALL with MLL gene rearrangements have few additional genomic alterations, none of which have clear clinical significance. Deletion of the MLL gene has not been associated with an adverse prognosis.
Of interest, the t(11;19)(q23;p13.3) involving MLL (KMT2A) and MLLT1/ENL occurs in approximately 1% of ALL cases and occurs in both early B-lineage and T-cell ALL. Outcome for infants with the t(11;19) is poor, but outcome appears relatively favorable in older children with T-cell ALL and the t(11;19).
The t(1;19) occurs in approximately 5% of childhood ALL cases and involves fusion of the TCF3 gene on chromosome 19 to the PBX1 gene on chromosome 1.[63,64] The t(1;19) may occur as either a balanced translocation or as an unbalanced translocation and is the primary recurring genomic alteration of the pre-B ALL immunophenotype (cytoplasmic Ig positive). Black children are relatively more likely than white children to have pre-B ALL with the t(1;19).
The t(1;19) had been associated with inferior outcome in the context of antimetabolite-based therapy, but the adverse prognostic significance was largely negated by more aggressive multiagent therapies.[64,68] However, in a trial conducted by St. Jude Children's Research Hospital (SJCRH) on which all patients were treated without cranial radiation, patients with the t(1;19) had an overall outcome comparable to children lacking this translocation, with a higher risk of CNS relapse and a lower rate of bone marrow relapse, suggesting that more intensive CNS therapy may be needed for these patients.[69,70]
The t(17;19) resulting in the TCF3-HLF fusion occurs in less than 1% of pediatric ALL cases. ALL with the TCF3-HLF fusion is associated at diagnosis with disseminated intravascular coagulation and with hypercalcemia. Outcome is very poor for children with the t(17;19), with a literature review noting mortality for 20 of 21 cases reported. In addition to the TCF3-HLF fusion, the genomic landscape of this ALL subtype was characterized by deletions in genes involved in B-cell development (PAX5, BTG1, and VPREB1) and by mutations in RAS pathway genes (NRAS, KRAS, and PTPN11).
Approximately 5% of standard-risk and 10% of high-risk pediatric precursor B-cell ALL patients have a rearrangement involving DUX4 that leads to its overexpression.[72,73] The frequency in older adolescents (aged >15 years) is approximately 10%. The most common rearrangement produces IGH-DUX4 fusions, with ERG-DUX4 fusions also observed. Approximately 50% of DUX4-rearranged cases have focal intragenic deletions involving ERG that are not observed in other ALL subtypes,[72,73] and DUX4-rearranged cases show a distinctive gene expression pattern that was initially identified as being associated with these focal deletions in ERG.[5,6,7]IKZF1 alterations are observed in 35% to 40% of DUX4-rearranged ALL.[72,73]ERG deletion connotes an excellent prognosis, with OS exceeding 90%; even when the IZKF1 deletion is present, prognosis remains highly favorable.[5,6,7] Patients with DUX4 rearrangements who lack ERG deletion also appear to have favorable prognosis.
This entity is included in the 2016 revision of the WHO classification of tumors of the hematopoietic and lymphoid tissues. The finding of t(5;14)(q31.1;q32.3) in patients with ALL and hypereosinophilia in the 1980s was followed by the identification of the IL3-IGH fusion as the underlying genetic basis for the condition.[74,75] The joining of the IGH locus to the promoter region of the interleukin-3 gene (IL3) leads to dysregulation of IL3 expression. Cytogenetic abnormalities in children with ALL and eosinophilia are variable, with only a subset resulting from the IL3-IGH fusion.
The number of cases of IL3-IGH ALL described in the published literature is too small to assess the prognostic significance of the IL3-IGH fusion.
iAMP21 with multiple extra copies of the RUNX1 (AML1) gene at 21q22 occurs in approximately 2% of precursor B-cell ALL cases and is associated with older age (median, approximately 10 years), presenting WBC of less than 50 × 109 /L, a slight female preponderance, and high end-induction minimal residual disease (MRD).[78,79,80]
The United Kingdom (UK)-ALL clinical trials group initially reported that the presence of iAMP21 conferred a poor prognosis in patients treated in the MRC ALL 97/99 trial (5-year EFS, 29%). In their subsequent trial (UKALL2003 [NCT00222612]), patients with iAMP21 were assigned to a more intensive chemotherapy regimen and had a markedly better outcome (5-year EFS, 78%). Similarly, the COG has reported that iAMP21 was associated with a significantly inferior outcome in NCI standard-risk patients (4-year EFS, 73% for iAMP21 vs. 92% in others), but not in NCI high-risk patients (4-year EFS, 73% vs. 80%). On multivariate analysis, iAMP21 was an independent predictor of inferior outcome only in NCI standard-risk patients. The results of the UKALL2003 and COG studies suggest that treatment of iAMP21 patients with high-risk chemotherapy regimens abrogates its adverse prognostic significance and obviates the need for SCT in first remission.
IKZF1 deletions, including deletions of the entire gene and deletions of specific exons, are present in approximately 15% of precursor B-cell ALL cases. Less commonly, IKZF1 can be inactivated by deleterious point mutations. Cases with IKZF1 deletions tend to occur in older children, have a higher WBC count at diagnosis, and are therefore, more common in NCI high-risk patients than in NCI standard-risk patients.[2,81,82,83] A high proportion of BCR-ABL1 cases have a deletion of IKZF1,[3,82] and ALL arising in children with Down syndrome appears to have elevated rates of IKZF1 deletions.IKZF1 deletions are also common in cases with CRLF2 genomic alterations and in Philadelphia chromosome (Ph)-like (BCR-ABL1-like) ALL (see below).[5,82,85]
Multiple reports have documented the adverse prognostic significance of an IKZF1 deletion, and most studies have reported that this deletion is an independent predictor of poor outcome on multivariate analyses.[5,81,82,85,86,87,88,89,90,91]; [Level of evidence: 2Di] That said, the prognostic significance of IKZF1 may not apply equally across ALL biological subtypes, as illustrated by the apparent lack of prognostic significance in patients with ERG deletion.
BCR-ABL1-negative patients with a gene expression profile similar to BCR-ABL1-positive patients have been referred to as BCR-ABL1-like.[81,85] This occurs in 10% to 15% of pediatric ALL patients, increasing in frequency with age, and has been associated with a poor prognosis and with IKZF1 deletion or mutation.[9,81,85,89,93] The 5-year EFS rate of 90% observed in one study of 40 patients with BCR-ABL1-like ALL suggested that the adverse prognostic significance of this subtype may be abrogated when patients are treated with risk-directed therapy based on MRD levels. Six of these 40 patients were classified as high risk and all proceeded to allogeneic SCT.[Level of evidence: 2A]
The hallmark of BCR-ABL1-like ALL is activated kinase signaling, with 50% containing CRLF2 genomic alterations  and half of those cases containing concomitant JAK mutations. Additional information about BCR-ABL1-like ALL cases with CRLF2 genomic alterations is provided below.
Many of the remaining cases of BCR-ABL1-like ALL have been noted to have a series of translocations with a common theme of involvement of kinases, including ABL1, ABL2, CSF1R, JAK2, and PDGFRB.[4,93] Fusion proteins from these gene combinations have been noted in some cases to be transformative and have responded to tyrosine kinase inhibitors both in vitro and in vivo, suggesting potential therapeutic strategies for these patients. Point mutations in kinase genes, aside from those in JAK1 and JAK2, are uncommon in Ph-like ALL cases.
Genomic alterations in CRLF2, a cytokine receptor gene located on the pseudoautosomal regions of the sex chromosomes, have been identified in 5% to 10% of cases of precursor B-cell ALL; they represent approximately 50% of cases of BCR-ABL1-like ALL.[97,98] The chromosomal abnormalities that commonly lead to CRLF2 overexpression include translocations of the IgH locus (chromosome 14) to CRLF2 and interstitial deletions in pseudoautosomal regions of the sex chromosomes, resulting in a P2RY8-CRLF2 fusion.[9,95,97,98]CRLF2 abnormalities are strongly associated with the presence of IKZF1 deletions and JAK mutations;[9,82,95,96,98] they are also more common in children with Down syndrome. Point mutations in tyrosine kinase genes other than JAK1 and JAK2 are uncommon in CRLF2-overexpressing cases.
Although the results of several retrospective studies suggest that CRLF2 abnormalities may have adverse prognostic significance on univariate analyses, most do not find this abnormality to be an independent predictor of outcome.[95,97,98,99,100] For example, in a large European study, increased expression of CRLF2 was not associated with unfavorable outcome in multivariate analysis, while IKZF1 deletion and BCR-ABL1-like expression signatures were associated with unfavorable outcome. Controversy exists about whether the prognostic significance of CRLF2 abnormalities should be analyzed based on CRLF2 overexpression or on the presence of CRLF2 genomic alterations.[99,100]
Approximately 9% of BCR-ABL1-like ALL cases result from rearrangements that lead to overexpression of a truncated erythropoietin receptor (EPOR). The C-terminal region of the receptor that is lost is the region that is mutated in primary familial congenital polycythemia and that controls stability of the EPOR. The portion of the EPOR remaining is sufficient for JAK-STAT activation and for driving leukemia development.
A number of polymorphisms of genes involved in the metabolism of chemotherapeutic agents have been reported to have prognostic significance in childhood ALL.[102,103,104] For example, patients with mutant phenotypes of thiopurine methyltransferase (TPMT, a gene involved in the metabolism of thiopurines, such as mercaptopurine [6-MP]), appear to have more favorable outcomes, although such patients may also be at higher risk of developing significant treatment-related toxicities, including myelosuppression and infection.[106,107] Patients with homozygosity for TPMT variants associated with low enzymatic activity tolerate only very low doses of mercaptopurine (approximately 10% of the standard dose) and are treated with reduced doses of mercaptopurine to avoid excessive toxicity. Patients who are heterozygous for this mutant enzyme gene generally tolerate mercaptopurine without serious toxicity, but they do require more frequent dose reductions for hematologic toxicity than do patients who are homozygous for the normal allele.[108,109]
Germline variants in nucleoside diphosphate-linked moiety X-type motif 15 (NUDT15) that reduce or abolish activity of this enzyme also lead to diminished tolerance to thiopurines.[108,110] The variants are most common in East Asians and Hispanics, and they are rare in Europeans and Africans. Patients homozygous for the risk variants tolerate only very low doses of mercaptopurine, while patients heterozygous for the risk alleles tolerate lower doses than do patients homozygous for the wild-type allele (approximately 25% dose reduction on average), but there is broad overlap in tolerated doses between the two groups.[108,111]
Gene polymorphisms may also affect the expression of proteins that play central roles in the cellular effects of anticancer drugs. As an example, patients who are homozygous for a polymorphism in the promoter region of CEP72 (a centrosomal protein involved in microtubule formation) are at increased risk of vincristine neurotoxicity.
Genome-wide polymorphism analysis has identified specific single nucleotide polymorphisms associated with high end-induction MRD and risk of relapse. Polymorphisms of IL-15, as well as genes associated with the metabolism of etoposide and methotrexate, were significantly associated with treatment response in two large cohorts of ALL patients treated on SJCRH and COG protocols. Polymorphic variants involving the reduced folate carrier and methotrexate metabolism have been linked to toxicity and outcome.[114,115] While these associations suggest that individual variations in drug metabolism can affect outcome, few studies have attempted to adjust for these variations; it is unknown whether individualized dose modification based on these findings will improve outcome.
(Refer to the PDQ summary on Childhood Acute Lymphoblastic Leukemia Treatment for information about the treatment of childhood ALL.)
Acute Myeloid Leukemia (AML)
Pediatric AML is typically a disease of recurring chromosomal alterations, with conventional cytogenetics detecting structural and numerical cytogenetic abnormalities in 70% to 80% of children with AML, while the recently recognized cryptic translocations (e.g., NUP98/NSD1, CBFA2T3/GLIS2, and NUP98/KDM5A) and mutations (e.g., CEBPA and NPM1) account for many of the remaining cases.[116,117]
Comprehensive molecular profiling of AML in pediatric and adult cases has characterized AML as a disease showing both commonalities and distinct differences between the age groups.[117,118,119]Figure 3 (A) illustrates the frequencies of recurring gene mutations in adult and pediatric AML, showing that some mutations are differentially present between pediatric and adults cases (e.g., IDH1 and DNMT3A mutations being much more common in adults than children).Figure 3 (B) shows that common genomic alterations in adult AML (FLT3-ITD, NPM1, and CEBPA mutations) are uncommon in children younger than 5 years but increase in frequency with age.Figure 3. (A) Prevalence of AML-associated mutations in pediatric versus adult AML, demonstrating lower incidence of mutations in pediatric AML. Bordered panel shows 2 newly discovered mutations in adults that are absent in pediatric AML. (B) Age-based prevalence of common AML-associated mutations. Reprinted from Pediatric Clinics of North America, Volume 62, Katherine Tarlock, Soheil Meshinchi, Pediatric Acute Myeloid Leukemia: Biology and Therapeutic Implications of Genomic Variants, Pages 75-93, Copyright (2015), with permission from Elsevier.
Figure 4 (A) shows the marked variation in MLL (KMT2A)-rearranged AML by age, with much higher frequencies for infants compared with older children and adults. Normal karyotype AML and core-binding factor AML show an opposing pattern, with very low rates in infancy and with increasing rates in the first two decades of life. Figure 4 (B) shows specific cryptic translocations that occur primarily in children (NUP98/NSD1, CBFA2T3/GLIS2, and NUP98/KDM5A) and vary by age.Figure 4. Age-based prevalence of specific karyotypic (A) or cryptic (B) translocations in AML. Reprinted from Pediatric Clinics of North America, Volume 62, Katherine Tarlock, Soheil Meshinchi, Pediatric Acute Myeloid Leukemia: Biology and Therapeutic Implications of Genomic Variants, Pages 75-93, Copyright (2015), with permission from Elsevier.
The genomic landscape of pediatric AML cases can change from diagnosis to relapse, with mutations detectable at diagnosis dropping out at relapse and conversely with new mutations appearing at relapse. A key finding in a study of 20 cases for which sequencing data were available at diagnosis and relapse was that the variant allele frequency at diagnosis strongly correlated with persistence of mutations at relapse. Approximately 90% of the diagnostic variants with variant allele frequency greater than 0.4 persisted to relapse compared with only 28% with variant allele frequency less than 0.2 (P < .001). This observation is consistent with previous results showing that presence of the FLT3-ITD mutation predicted for poor prognosis only when there was a high FLT3-ITD allelic ratio.
Chromosomal analyses of leukemia (using either conventional cytogenetic methods and/or molecular methods) should be performed on children with AML because chromosomal abnormalities are important diagnostic and prognostic markers.[121,122,123,124,125,126] Clonal chromosomal abnormalities have been identified in the blasts of about 75% of children with AML and are useful in defining subtypes with particular characteristics (e.g., t(8;21), t(15;17), inv(16), 11q23 abnormalities, t(1;22)). Leukemias with the chromosomal abnormalities t(8;21) and inv(16) are called core-binding factor leukemias; core-binding factor (a transcription factor involved in hematopoietic stem cell differentiation) is disrupted by each of these abnormalities.
A unifying concept for the role of specific mutations in AML is that mutations that promote proliferation (Type I) and mutations that block normal myeloid development (Type II) are required for full conversion of hematopoietic stem/precursor cells to malignancy.[127,128] Support for this conceptual construct comes from the observation that there is generally mutual exclusivity within each type of mutation, such that a single Type I and a single Type II mutation are present within each case. Further support comes from genetically engineered models of AML for which cooperative events rather than single mutations are required for leukemia development. Type I mutations are commonly in genes involved in growth factor signal transduction and include mutations in FLT3, KIT, NRAS, KRAS, and PTNP11. Type II genomic alterations include the common translocations and mutations associated with favorable prognosis (t(8;21), inv(16), t(16;16), t(15;17), CEBPA, and NPM1). MLL (KMT2A) rearrangements (translocations and partial tandem duplication) are also classified as Type II mutations.
Specific recurring cytogenetic and molecular abnormalities are briefly described below. The abnormalities are listed by those in clinical use that identify patients with favorable or unfavorable prognosis, followed by other abnormalities.
Molecular abnormalities associated with favorable prognosis include the following:
Although both RUNX1-RUNX1T1 and CBFB-MYH11 fusion genes disrupt the activity of core-binding factor, which contains RUNX1 and CBFB, cases with these genomic alterations have distinctive secondary mutations.[136,137] Both subtypes commonly show mutations in genes that activate receptor tyrosine kinase signaling (e.g., NRAS, FLT3 and KIT); NRAS and KIT are the most commonly mutated genes for both subtypes. RUNX1-RUNX1T1 cases additionally have frequent mutations in genes regulating chromatin conformation (e.g., ASXL1 and ASXL2) and genes encoding members of the cohesin complex (approximately 40% and 20% of cases, respectively). Mutations in ASXL1 and ASXL2 and mutations in members of the cohesin complex are rare in CBFB-MYH11 leukemias.[136,137] Data exist (primarily from adults) that these secondary mutations may have prognostic significance,[136,137,138] but further study is required to understand their prognostic significance in children. Additionally, KIT mutations may indicate increased risk of treatment failure for patients with core-binding factor AML, although the prognostic significance of KIT mutations may be dependent on the mutant-allele ratio (high ratio unfavorable) and/or the specific type of mutation (exon 17 mutations unfavorable).[136,137]KIT exon 17 mutations are enriched in patients with RUNX1-RUNX1T1 fusions when compared with patients with CBFB-MYH11 fusions.
Studies of children with AML suggest a lower rate of occurrence of NPM1 mutations in children compared with adults with normal cytogenetics. NPM1 mutations occur in approximately 8% of pediatric patients with AML and are uncommon in children younger than 2 years.[128,150,151,152]NPM1 mutations are associated with a favorable prognosis in patients with AML characterized by a normal karyotype.[128,151,152] For the pediatric population, conflicting reports have been published regarding the prognostic significance of an NPM1 mutation when a FLT3-ITD mutation is also present, with one study reporting that an NPM1 mutation did not completely abrogate the poor prognosis associated with having a FLT3-ITD mutation,[151,153] but with other studies showing no impact of a FLT3-ITD mutation on the favorable prognosis associated with an NPM1 mutation.[128,152]
CEBPA mutations occur in 5% to 8% of children with AML and have been preferentially found in the cytogenetically normal subtype of AML with FAB M1 or M2; 70% to 80% of pediatric patients have double-mutant alleles, which is predictive of a significantly improved survival and similar to the effect observed in adult studies.[160,161] Although both double- and single-mutant alleles of CEBPA were associated with a favorable prognosis in children with AML in one large study, a second study observed inferior outcome for patients with single CEBPA mutations. However, very low numbers of children with single-allele mutants were included in these two studies (only 13 in toto), making a conclusion regarding the prognostic significance of single-allele CEBPA mutations in children premature. In newly diagnosed patients with double-mutant CEBPA AML, in addition to usual family history queries, germline screening should be considered because 5% to 10% of these patients are reported to have a germline CEBPA mutation.
Molecular abnormalities associated with an unfavorable prognosis include the following:
In the past, patients with del(7q) were also considered to be at high risk of treatment failure and data from adults with AML support a poor prognosis for both del(7q) and monosomy 7. However, outcome for children with del(7q), but not monosomy 7, appears to be comparable to that of other children with AML.[125,165] The presence of del(7q) does not abrogate the prognostic significance of favorable cytogenetic characteristics (e.g., inv(16) and t(8;21)).[121,165,166]
Chromosome 5 and 7 abnormalities appear to lack prognostic significance in AML patients with Down syndrome who are 4 years of age and younger.
For APL, FLT3-ITD and point mutations occur in 30% to 40% of children and adults.[171,174,175,181,182,183,184] Presence of the FLT3-ITD mutation is strongly associated with the microgranular variant (M3v) of APL and with hyperleukocytosis.[174,183,185,186] It remains unclear whether FLT3 mutations are associated with poorer prognosis in patients with APL who are treated with modern therapy that includes all-trans retinoic acid and arsenic trioxide.[181,182,185,187,188]
Activating point mutations of FLT3 have also been identified in both adults and children with AML, though the clinical significance of these mutations is not clearly defined.
Other molecular abnormalities observed in pediatric AML include the following:
Outcome for patients with de novo AML and MLL (KMT2A) gene rearrangement is generally reported as being similar to that for other patients with AML.[121,124,191,192] However, as the MLL gene can participate in translocations with many different fusion partners, the specific fusion partner appears to influence prognosis, as demonstrated by a large international retrospective study evaluating outcome for 756 children with 11q23- or MLL-rearranged AML. For example, cases with t(1;11)(q21;q23), representing 3% of all 11q23/MLL-rearranged AML, showed a highly favorable outcome with 5-year event-free survival (EFS) of 92%. While reports from single clinical trial groups have variably described more favorable prognosis for cases with t(9;11), in which the MLL gene is fused with the AF9 gene, the international retrospective study did not confirm the favorable prognosis of the t(9;11)(p22;q23) subgroup.[121,124,191,193,194,195] An international collaboration evaluating pediatric AMKL observed that the presence of t(9;11), which was seen in approximately 5% of AMKL cases, was associated with an inferior outcome compared with other AMKL cases.
Several 11q23/MLL (KMT2A)-rearranged AML subgroups appear to be associated with poor outcome. For example, cases with the t(10;11) translocation are a group at high risk of relapse in bone marrow and the CNS.[121,125,196] Some cases with the t(10;11) translocation have fusion of the MLL gene with the AF10-MLLT10 at 10p12, while others have fusion of MLL with ABI1 at 10p11.2.[197,198] The international retrospective study found that these cases, which present at a median age of approximately 1 year, have a 5-year EFS in the 20% to 30% range. Patients with t(6;11)(q27;q23) and with t(4;11)(q21;q23) also have a poor outcome, with a 5-year EFS of 11% and 29%, respectively, in the international retrospective study. A follow-up study by the international collaborative group demonstrated that additional cytogenetic abnormalities further influenced outcome of children with MLL translocations, with complex karyotypes and trisomy 19 predicting poor outcome and trisomy 8 predicting a more favorable outcome.
Controversy exists regarding the prognostic significance of the t(1;22) in pediatric AMKL. In a report from the Berlin-Frankfurt-Münster (BFM) study group of 97 non-Down syndrome AMKL patients, presence of t(1;22) (n = 8) was associated with a significantly inferior outcome (5-year EFS, 38% vs. 53% in other AMKL patients), although all of the observed events in patients with t(1;22) were related to treatment-related mortality. An international collaborative retrospective study with a larger number of t(1;22) cases reported that patients with this abnormality had a 5-year EFS of 54.5% and an OS of 58.2%, similar to the rates for other children with AMKL. In another international retrospective analysis of 153 cases with non-Down syndrome AMKL with samples available for molecular analysis, the 4-year EFS for patients with t(1;22) was 59% and OS was 70%, significantly better than AMKL patients with other specific genetic abnormalities (CBFA2T3/GUS2, NUP98/KDM5A, KMT2A rearrangements, monosomy7).
Activating mutations in CSF3R are also observed in patients with severe congenital neutropenia. These mutations are not the cause of severe congenital neutropenia, but rather arise as somatic mutations and can represent an early step in the pathway to AML. In one study of patients with severe congenital neutropenia, 34% of patients who had not developed a myeloid malignancy had CSF3R mutations detectable in peripheral blood neutrophils and mononuclear cells, while 78% of patients who had developed a myeloid malignancy showed CSF3R mutations. A study of 31 patients with severe congenital neutropenia who developed AML or MDS observed CSF3R mutations in approximately 80%, and also observed a high frequency of RUNX1 mutations (approximately 60%), suggesting cooperation between CSF3R and RUNX1 mutations for leukemia development within the context of severe congenital neutropenia.
(Refer to the PDQ summary on Childhood Acute Myeloid Leukemia/Other Myeloid Malignancies Treatment for information about the treatment of childhood AML.)
Juvenile Myelomonocytic Leukemia (JMML)
The genomic landscape of JMML is characterized by mutations in one of five genes of the Ras pathway:[282,283]NF1, NRAS, KRAS, PTPN11, and CBL. In a series of 118 consecutively diagnosed JMML cases with Ras pathway-activating mutations, PTPN11 was the most commonly mutated gene, accounting for 51% of cases (19% germline and 32% somatic) (refer to Figure 5). Patients with mutated NRAS accounted for 19% of cases, and patients with mutated KRAS accounted for 15% of cases. NF1 and CBL mutations accounted for 8% and 11% of cases, respectively. Although mutations among these five genes are generally mutually exclusive, 10% to 17% of cases have mutations in two of these Ras pathway genes,[282,283] a finding that is associated with poorer prognosis.
The mutation rate in JMML leukemia cells is very low, but additional mutations beyond those of the five Ras pathway genes described above are observed.[282,283] Secondary genomic alterations are observed for genes of the transcriptional repressor complex PRC2 (e.g., ASXL1 was mutated in 7%-8% of cases). Some genes associated with myeloproliferative neoplasms in adults are also mutated at low rates in JMML (e.g., SETBP1 was mutated in 7%-9% of cases).[282,283,284]JAK3 mutations are also observed in a small percentage (4%-12%) of JMML cases.[282,283,284] Cases with germline PTPN11 and germline CBL mutations showed low rates of additional mutations (refer to Figure 5).
Figure 5. Alteration profiles in individual JMML cases. Germline and somatically acquired alterations with recurring hits in the RAS pathway and PRC2 network are shown for 118 patients with JMML who underwent detailed genetic analysis. Blast excess was defined as a blast count ≥10% but <20% of nucleated cells in the bone marrow at diagnosis. Blast crisis was defined as a blast count ≥20% of nucleated cells in the bone marrow. NS, Noonan syndrome. Reprinted by permission from Macmillan Publishers Ltd: Nature Genetics (Caye A, Strullu M, Guidez F, et al.: Juvenile myelomonocytic leukemia displays mutations in components of the RAS pathway and the PRC2 network. Nat Genet 47 : 1334-40, 2015), copyright (2015).
General characteristics of leukemia cells provide both prognostic information and guidance regarding therapeutic opportunities for JMML:
Mature B-cell Lymphoma
The mature B-cell lymphomas include Burkitt and Burkitt-like lymphoma, diffuse large B-cell lymphoma, and primary mediastinal B-cell lymphoma.
Burkitt and Burkitt-like lymphoma
The malignant cells show a mature B-cell phenotype and are negative for the enzyme terminal deoxynucleotidyl transferase. These malignant cells usually express surface immunoglobulin, most bearing a clonal surface immunoglobulin M with either kappa or lambda light chains. A variety of additional B-cell markers (e.g., CD19, CD20, CD22) are usually present, and most childhood Burkitt and Burkitt-like lymphomas/leukemias express CALLA (CD10).
Burkitt lymphoma/leukemia expresses a characteristic chromosomal translocation, usually t(8;14) and more rarely t(8;22) or t(2;8). Each of these translocations juxtaposes the c-myc oncogene and immunoglobulin locus regulatory elements, resulting in the inappropriate expression of c-myc, a gene involved in cellular proliferation.[2,3,4] The presence of one of the variant translocations t(2;8) or t(8;22) does not appear to affect response or outcome.
The distinction between Burkitt and Burkitt-like lymphoma/leukemia is controversial. Burkitt lymphoma/leukemia consists of uniform, small, noncleaved cells, whereas the diagnosis of Burkitt-like lymphoma/leukemia is highly disputed among pathologists because of features that are consistent with diffuse large B-cell lymphoma.
Cytogenetic evidence of c-myc rearrangement is the gold standard for diagnosis of Burkitt lymphoma/leukemia. For cases in which cytogenetic analysis is not available, the World Health Organization (WHO) has recommended that the Burkitt-like diagnosis be reserved for lymphoma resembling Burkitt lymphoma/leukemia or with more pleomorphism, large cells, and a proliferation fraction (i.e., MIB-1 or Ki-67 immunostaining) of 99% or greater. BCL2 staining by immunohistochemistry is variable. The absence of a translocation involving the BCL2 gene does not preclude the diagnosis of Burkitt lymphoma/leukemia and has no clinical implications.
Studies have demonstrated that the vast majority of Burkitt-like or atypical Burkitt lymphoma/leukemia has a gene expression signature similar to Burkitt lymphoma/leukemia.[8,9] Additionally, as many as 30% of pediatric diffuse large B-cell lymphoma cases will have a gene signature similar to Burkitt lymphoma/leukemia.[8,10]
(Refer to the PDQ summary on Childhood Non-Hodgkin Lymphoma Treatment for information about the treatment of childhood non-Hodgkin lymphoma.)
Diffuse large B-cell lymphoma
The World Health Organization (WHO) classification system does not recommend subclassification of diffuse large B-cell lymphoma on the basis of morphologic variants (e.g., immunoblastic, centroblastic).
Diffuse large B-cell lymphoma in children and adolescents differs biologically from diffuse large B-cell lymphoma in adults in the following ways:
Primary mediastinal B-cell lymphoma
Primary mediastinal B-cell lymphoma was previously considered a subtype of diffuse large B-cell lymphoma, but is now a separate entity in the most recent World Health Organization (WHO) classification. These tumors arise in the mediastinum from thymic B-cells and show a diffuse large cell proliferation with sclerosis that compartmentalizes neoplastic cells.
Primary mediastinal B-cell lymphoma can be very difficult to distinguish morphologically from the following types of lymphoma:
Primary mediastinal B-cell lymphoma is associated with distinctive chromosomal aberrations (gains in chromosomes 9p and 2p in regions that involve JAK2 and c-rel, respectively) [18,19] and commonly shows inactivation of SOCS1 by either mutation or gene deletion.[20,21] Primary mediastinal B-cell lymphoma has a distinctly different gene expression profile from diffuse large B-cell lymphoma, but its gene expression profile has features similar to those seen in Hodgkin lymphoma.[22,23]
Lymphoblastic lymphomas are usually positive for terminal deoxynucleotidyl transferase, with more than 75% having a T-cell immunophenotype and the remainder having a precursor B-cell phenotype.[2,24]
As opposed to pediatric acute lymphoblastic leukemia, chromosomal abnormalities and the molecular biology of pediatric lymphoblastic lymphoma are not well characterized. The Berlin-Frankfurt-Münster group reported that loss of heterozygosity at chromosome 6q was observed in 12% of patients and NOTCH1 mutations were seen in 60% of patients, but NOTCH1 mutations are rarely seen in patients with loss of heterozygosity in 6q16.[25,26]
Anaplastic Large Cell Lymphoma
While the predominant immunophenotype of anaplastic large cell lymphoma is mature T-cell, null-cell disease (i.e., no T-cell, B-cell, or natural killer-cell surface antigen expression) does occur. The World Health Organization (WHO) classifies anaplastic large cell lymphoma as a subtype of peripheral T-cell lymphoma.
All anaplastic large cell lymphoma cases are CD30-positive. More than 90% of pediatric anaplastic large cell lymphoma cases have a chromosomal rearrangement involving the ALK gene. About 85% of these chromosomal rearrangements will be t(2;5)(p23;q35), leading to the expression of the fusion protein NPM-ALK; the other 15% of cases are composed of variant ALK translocations. Anti-ALK immunohistochemical staining pattern is quite specific for the type of ALK translocation. Cytoplasm and nuclear ALK staining is associated with NPM-ALK fusion protein, whereas cytoplasmic staining only of ALK is associated with the variant ALK translocations.
In adults, ALK-positive anaplastic large cell lymphoma is viewed differently from other peripheral T-cell lymphomas because prognosis tends to be superior. Also, adult ALK-negative anaplastic large cell lymphoma patients have an inferior outcome compared with patients who have ALK-positive disease. In children, however, this difference in outcome between ALK-positive and ALK-negative disease has not been demonstrated. In addition, no correlation has been found between outcome and the specific ALK-translocation type.[31,32,33]
In a European series of 375 children and adolescents with systemic ALK-positive anaplastic large cell lymphoma, the presence of a small cell or lymphohistiocytic component was observed in 32% of patients and was significantly associated with a high risk of failure in the multivariate analysis, controlling for clinical characteristics (hazard ratio, 2.0; P = .002). The prognostic implication of the small cell variant of anaplastic large cell lymphoma was also shown in the COG-ANHL0131 (NCT00059839) study, despite a different chemotherapy backbone.
Central nervous system (CNS) tumors include pilocytic astrocytomas and other astrocytic tumors, diffuse astrocytic tumors, brain stem gliomas, CNS atypical teratoid/rhabdoid tumors, medulloblastomas, nonmedulloblastoma embryonal tumors, and ependymomas.
The terminology of the 2016 World Health Organization (WHO) Classification of Tumors of the Central Nervous System is used below. The 2016 WHO CNS classification incorporates genomic features in addition to histology, and it includes multiple changes from the previous 2007 WHO classification. Of particular relevance for childhood brain cancers is the new entity diffuse midline glioma, H3 K27M-mutant, which includes diffuse intrinsic pontine glioma (DIPG) with the H3 K27M mutation and other high-grade gliomas of the midline with the H3 K27M mutation. Other examples of molecularly defined entities discussed below are RELA-fusion-positive ependymoma, WNT-activated and SHH-activated medulloblastoma, and embryonal tumor with multilayered rosettes, C19MC-altered.
Pilocytic Astrocytomas and Other Astrocytic Tumors
Genomic alterations involving activation of BRAF and the ERK/MAPK pathway are very common in sporadic cases of pilocytic astrocytoma, a type of low-grade glioma.
BRAF activation in pilocytic astrocytoma occurs most commonly through a BRAF-KIAA1549 gene fusion, producing a fusion protein that lacks the BRAF regulatory domain.[2,3,4,5,6] This fusion is seen in most infratentorial and midline pilocytic astrocytomas, but is present at lower frequency in supratentorial (hemispheric) tumors.[2,3,7,8,9,10,11,12]
Presence of the BRAF-KIAA1549 fusion predicted a better clinical outcome (progression-free survival [PFS] and overall survival [OS]) in one report that described children with incompletely resected low-grade gliomas. However, other factors such as CDKN2A deletion, whole chromosome 7 gain, and tumor location may modify the impact of the BRAF mutation on outcome.; [Level of evidence: 3iiiDiii] Progression to high-grade glioma is rare for pediatric low-grade glioma with the BRAF-KIAA1549 fusion.
BRAF activation through the BRAF-KIAA1549 fusion has also been described in other pediatric low-grade gliomas (e.g., pilomyxoid astrocytoma).[10,11]
Other genomic alterations in pilocytic astrocytomas that can activate the ERK/MAPK pathway (e.g., alternative BRAF gene fusions, RAF1 rearrangements, RAS mutations, and BRAF V600E point mutations) are less commonly observed.[3,5,6,16]BRAF V600E point mutations are also observed in nonpilocytic pediatric low-grade gliomas, including ganglioglioma, desmoplastic infantile ganglioglioma, and approximately two-thirds of pleomorphic xanthoastrocytomas.[17,18,19] One retrospective study of 53 children with gangliogliomas demonstrated BRAF V600E staining in approximately 40% of tumors. Five-year recurrence-free survival was worse in the V600E-mutated tumors (about 60%) than in tumors that did not stain for V600E (about 80%). Similarly, children with diencephalic low-grade astrocytomas with a BRAF V600E mutation had a 5-year PFS of 22%, compared with a 52% PFS in children who were BRAF wildtype.[Level of evidence: 3iiiDiii] The frequency of the BRAF V600E mutation was significantly higher in pediatric low-grade glioma that transformed to high-grade glioma (8 of 18 cases) than was the frequency of mutation in cases that did not transform (10 of 167 cases).
Angiocentric gliomas have been noted to largely harbor MYB-QKI fusions, a putative driver mutation for this relatively rare class of gliomas.
As with neurofibromatosis type 1 (NF1) deficiency in activating the ERK/MAPK pathway, activating BRAF genomic alterations are uncommon in pilocytic astrocytoma associated with NF1.
Activating mutations in FGFR1, PTPN11, and in NTRK2 fusion genes have also been identified in noncerebellar pilocytic astrocytomas. In pediatric grade II diffuse astrocytomas, the most common alterations reported (up to 53% of tumors) are rearrangements in the MYB family of transcription factors.[24,25]
Most children with tuberous sclerosis have a mutation in one of two tuberous sclerosis genes (TSC1/hamartin or TSC2/tuberin). Either of these mutations results in activation of the mammalian target of rapamycin (mTOR) complex 1. These children are at risk of developing subependymal giant cell astrocytomas, cortical tubers, and subependymal nodules. Because subependymal giant cell astrocytomas are driven by mTOR activation, mTOR inhibitors are active agents that can induce tumor regression in children with these tumors.
(Refer to the PDQ summary on Childhood Astrocytomas Treatment for information about the treatment of low-grade childhood astrocytomas.)
Diffuse Astrocytic Tumors
This category includes, among other diagnoses, diffuse astrocytomas (grade II) and pediatric high-grade gliomas (anaplastic astrocytoma [grade III], glioblastoma [grade IV], and diffuse midline glioma, H3 K27M-mutant (grade IV]).
For pediatric diffuse astrocytomas (grade II), rearrangements in the MYB family of transcription factors (MYB and MYBL1) are the most commonly reported genomic alteration.[24,25,27] Other alterations observed include FGFR1 alterations (primarily duplications involving the tyrosine kinase domain),[25,27]BRAF alterations, NF1 mutations, and RAS family mutations.[24,25]IDH1 mutations, which are the most common genomic alteration in adult diffuse astrocytomas, are uncommon in children with diffuse astrocytomas and, when present, are observed almost exclusively in older adolescents.[24,28]
Anaplastic astrocytomas and glioblastomas
Pediatric high-grade gliomas, especially glioblastoma multiforme, are biologically distinct from those arising in adults.[28,29,30,31]
Pediatric glioblastoma multiforme tumors are separated into relatively distinct subgroups on the basis of epigenetic patterns (DNA methylation), with distinctive chromosome copy number gains/losses and gene mutations.[34,35] Two subgroups have identifiable recurrent mutations in H3F3A (the gene encoding histone 3.3), suggesting disrupted epigenetic regulatory mechanisms, with the most recognized subgroup having mutations at K27 (lysine 27) and the other group having mutations at G34 (glycine 34). The subgroups are the following:
The H3F3A K27 and G34 mutations appear to be unique to high-grade gliomas and have not been observed in other pediatric brain tumors. Both mutations induce distinctive DNA methylation patterns compared with the patterns observed in IDH-mutated tumors, which occur in young adults.[32,33,34,37,38]
Pediatric glioblastoma multiforme patients whose tumors have IDH1 mutations are almost exclusively older adolescents (median age in a pediatric population, 16 years) with hemispheric tumors. IDH1-mutated cases often show TP53 mutations, MGMT promoter methylation, and a glioma-CpG island methylator phenotype (G-CIMP).[34,35] Pediatric patients with IDH1 mutations show a more favorable prognosis than do other pediatric glioblastoma multiforme patients.
A fourth group of pediatric glioblastoma multiforme patients identified by DNA methylation analysis are those lacking both histone mutations and IDH1 mutations. This is a heterogeneous group with higher rates of gene amplifications than other pediatric glioblastoma multiforme subtypes. The most commonly amplified genes are PDGFRA, EGFR, CCND/CDK, and MYC/MYCN.[34,35]
DNA methylation analysis of tumor tissue may identify pediatric tumors with a histologic diagnosis of glioblastoma multiforme, but with the molecular characteristics of other pediatric gliomas. For example, one study found that approximately 14% of patients with a diagnosis of glioblastoma multiforme had molecular characteristics that are associated with pleomorphic xanthoastrocytomas (e.g., high rates of BRAF V600E mutations).
Infants and young children with a glioblastoma multiforme diagnosis appear to have tumors with distinctive molecular characteristics when compared with tumors of older children. One report that applied DNA methylation analysis to glioblastoma multiforme tumors observed a group of patients (representing approximately 7% of pediatric patients with a histologic diagnosis of glioblastoma multiforme) whose tumors had molecular characteristics consistent with low-grade gliomas. The median age for this group of patients was 1 year, and they showed a favorable prognosis (3-year overall survival, approximately 90%). A second report investigated gene copy number gains and losses and mutation status of selected genes for glioblastoma multiforme tumors from children younger than 36 months. Molecular alterations observed at appreciable rates in older children (e.g., K27M, CDKN2A loss, PDGFRA amplification, and TERT promoter mutations) were rare in the tumors of these young children, and novel abnormalities (e.g., loss of SNORD on chromosome 14q32) were observed in some cases.
Childhood secondary high-grade glioma (high-grade glioma that is preceded by a low-grade glioma) is uncommon (2.9% in a study of 886 patients). No pediatric low-grade gliomas with the BRAF-KIAA1549 fusion transformed to a high-grade glioma, whereas low-grade gliomas with the BRAF V600E mutations were associated with increased risk of transformation. Seven of 18 patients (approximately 40%) with secondary high-grade glioma had BRAF V600E mutations, with CDKN2A alterations present in 8 of 14 cases (57%).
(Refer to the PDQ summary on Childhood Astrocytomas Treatment for information about the treatment of high-grade childhood astrocytomas.)
Diffuse Midline Glioma,H3 K27M-Mutant (Including Diffuse Intrinsic Pontine Gliomas [DIPGs])
The diffuse midline glioma, H3 K27M-mutant, category includes tumors previously classified as DIPG; most of the data is derived from experience with DIPG. This category also includes gliomas with the H3 K27M mutation arising in midline structures such as the thalamus.
The genomic characteristics of DIPGs appear to differ from those of most other pediatric high-grade gliomas and from those of adult high-grade gliomas. The molecular and clinical characteristics of DIPGs align with those of other midline high-grade gliomas with a specific H3 K27M mutation in histone H3.1 (H3F3A) or H3.3 (HIST1H3B and HIST1H3C), which led the World Health Organization to group these tumors together into a single entity. In one report of 64 children with thalamic tumors, 50% of high-grade gliomas (11 of 22) had an H3 K27M mutation, and approximately 10% of tumors with low-grade morphological characteristics (5 of 42) had an H3 K27M mutation. Five-year overall survival (OS) was only 6% (1 of 16). In another study that included 202 children with glioblastoma, 68 of the tumors were midline (primarily thalamic) and had an H3 K27M mutation. Five-year OS for this group was only 5%, which was significantly inferior to the survival rates of the remaining patients in the study.
A number of chromosomal and genomic abnormalities have been reported for DIPG, including the following:
The gene expression profile of DIPG differs from that of non-brain stem pediatric high-grade gliomas, further supporting a distinctive biology for this subset of pediatric gliomas. Pediatric H3 K27M-mutant tumors rarely show MGMT promoter methylation, which explains the lack of efficacy of temozolomide when it was tested in patients with DIPG.
(Refer to the PDQ summary on Childhood Brain Stem Glioma Treatment for information about the treatment of childhood brain stem gliomas.)
Central Nervous System (CNS) Atypical Teratoid/Rhabdoid Tumors (AT/RT)
AT/RT was the first primary pediatric brain tumor for which a candidate tumor suppressor gene, SMARCB1 (also known as INI1 and hSNF5), was identified.SMARCB1 is genomically altered in the majority of rhabdoid tumors, including CNS, renal, and extrarenal rhabdoid malignancies. Additional genomic alterations (mutations and gains/losses) in other genes are very uncommon in patients with SMARCB1-associated AT/RT, and there are no other genes that are recurrently mutated.[51,52,53]
SMARCB1 is a component of a switch (SWI) and sucrose non-fermenting (SNF) adenosine triphosphate-dependent chromatin-remodeling complex. Rare familial cases of rhabdoid tumors expressing SMARCB1 and lacking SMARCB1 mutations have also been associated with germline mutations of SMARCA4/BRG1, another member of the SWI/SNF chromatin-remodeling complex.[55,56]
The 2016 WHO classification defines AT/RT by the presence of either SMARCB1 or SMARCA4 alterations. Tumors with histological features of AT/RT that lack these genomic alterations are termed CNS embryonal tumor with rhabdoid features.
Despite the absence of recurring genomic alterations beyond SMARCB1 (and, more rarely, other SWI/SNF complex members), biologically distinctive subsets of AT/RT have been identified.[57,58] The following three distinctive subsets of AT/RT were identified through the use of DNA methylation arrays for 150 AT/RT tumors and gene expression arrays for 67 AT/RT tumors:
In addition to somatic mutations, germline mutations in SMARCB1 have been reported in a substantial subset of AT/RT patients.[50,60] A study of 65 children with rhabdoid tumors found that 23 (35%) had germline mutations and/or deletions of SMARCB1. Children with germline alterations in SMARCB1 presented at an earlier age than did sporadic cases (median age, approximately 5 months vs. 18 months) and were more likely to present with multiple tumors. One parent was found to be a carrier of the SMARCB1 germline abnormality in 7 of 22 evaluated cases showing germline alterations, with four of the carrier parents being unaffected by SMARCB1-associated cancers. This indicates that AT/RT shows an autosomal dominant inheritance pattern with incomplete penetrance.
Gonadal mosaicism has also been observed, as evidenced by families in which multiple siblings are affected by AT/RT and have identical SMARCB1 alterations, but both parents lack a SMARCB1 mutation/deletion.[61,62] Screening children diagnosed with AT/RT for germline SMARCB1 mutations may provide useful information for counseling families on the genetic implications of their child's AT/RT diagnosis.
(Refer to the PDQ summary on Childhood Central Nervous System Atypical Teratoid/Rhabdoid Tumors Treatment for information about the treatment of childhood CNS atypical teratoid/rhabdoid tumors.)
Multiple medulloblastoma subtypes have been identified by integrative molecular analysis.[63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78] Since 2012, the general consensus is that medulloblastoma can be molecularly separated into at least four core subtypes. These subtypes remain stable across primary and metastatic components. It is likely that further subclassification will occur.[76,77,80] The 2016 World Health Organization (WHO) classification has endorsed this consensus by adding the following categories for genetically defined medulloblastoma:
The following four core molecularly defined subtypes of medulloblastoma have been identified:[76,77,81,82]
The WNT subset is primarily observed in older children, adolescents, and adults and does not show a male predominance. The subset is believed to have brain stem origin, from the embryonal rhombic lip region. WNT medulloblastomas are associated with a very good outcome, especially in individuals whose tumors have beta-catenin nuclear staining and proven 6q loss and/or CTNNB1 mutations.[78,84]
SHH medulloblastomas show a bimodal age distribution and are observed primarily in children younger than 3 years and in older adolescence/adulthood. The tumors are believed to emanate from the external granular layer of the cerebellum.
Prognosis for patients with SHH medulloblastoma appears to be negatively affected by other molecular genetic changes, such as chromosome 17p loss, chromosome 3q gain, chromothripsis, p53 amplification, TP53 mutation, and the finding of large cell/anaplastic histology.[77,85] The outcome for patients with SHH medulloblastoma is relatively favorable, primarily in children younger than 3 years and in adults. This is likely because of the type of mutation present in the SHH pathway, given that patients with mutations upstream of the SHH signaling pathway, such as PTCH1, PTCH2, and SUFU, have a more favorable prognosis than do patients with downstream genomic aberrations, such as GLI2 and MYCN amplification.[86,87] Overall outcome in adolescents and young adults with SHH medulloblastoma is not different from that seen in patients with non-WNT pathway-activated tumors, except for patients with TP53 mutations and downstream SHH pathway mutations. Patients with unfavorable molecular findings have an unfavorable prognosis, with less than 50% of patients surviving after conventional treatment.[81,85,86,87,88]
The 2016 WHO classification identifies SHH medulloblastoma with a TP53 mutation as a distinctive entity (medulloblastoma, SHH-activated and TP53-mutant). Approximately 25% of SHH-activated medulloblastoma cases have TP53 mutations, with a high percentage of these cases also showing a TP53 germline mutation (9 of 20 in one study). These patients are commonly between the ages of 5 years and 18 years and have an inferior outcome (overall survival at 5 years, <50%). The tumors often show large cell anaplastic histology.
Group 3 medulloblastomas occur throughout childhood and may occur in infants. Males outnumber females in a 2:1 ratio in this medulloblastoma subtype. Patients with group 3 medulloblastomas have a variable prognosis. Patients with MYC amplification or MYC overexpression have a poor prognosis, with less than 50% of these patients surviving 5 years after diagnosis. This poor prognosis is especially true in children younger than 4 years at diagnosis. However, patients with group 3 medulloblastoma without MYC amplification or MYC overexpression who are older than 3 years have a prognosis similar to that of most patients with medulloblastoma, with a 5-year progression-free survival (PFS) rate higher than 70%.
Group 4 medulloblastomas occur throughout infancy and childhood and into adulthood. They also predominate in males. The prognosis is better than group 3 medulloblastoma but not as good as WNT medulloblastoma. Prognosis for group 4 medulloblastoma patients is affected by additional factors such as the presence of metastatic disease and chromosome 17p loss.[76,77]
Optimal ways of identifying the four core medulloblastoma subtypes for clinical use is under active study, and both immunohistochemical methods and methods based on gene expression analysis are under development.[84,89] The classification of medulloblastoma into the four major subtypes will be altered in the future.[90,91] Further subdivision within subgroups based on molecular characteristics is likely as each of the subgroups is further molecularly dissected, although there is no consensus regarding an alternative classification.[76,80,87]
Whether the classification for adults with medulloblastoma has similar predictive ability in children is unknown.[77,81] In one study of adult medulloblastoma, MYC oncogene amplifications were rarely observed, and tumors with 6q deletion and WNT activation (as identified by nuclear beta-catenin staining) did not share the excellent prognosis seen in pediatric medulloblastomas, although another study did confirm an excellent prognosis for WNT-activated tumors in adults.[77,81]
(Refer to the PDQ summary on Childhood Central Nervous System Embryonal Tumors Treatment for information about the treatment of childhood medulloblastoma.)
Nonmedulloblastoma Embryonal Tumors
This section describes the genomic characteristics of embryonal tumors other than medulloblastoma and atypical teratoid/rhabdoid tumor. The 2016 WHO classification removed the term primitive neuroectodermal tumors (PNET) from the diagnostic lexicon. This change resulted from the recognition that many tumors previously classified as CNS PNETs have the common finding of amplification of the C19MC region on chromosome 19. These entities included ependymoblastoma, embryonal tumors with abundant neuropil and true rosettes (ETANTR), and some cases of medulloepithelioma. The 2016 WHO classification now categorizes tumors with C19MC amplification as embryonal tumor with multilayered rosettes (ETMR), C19MC-altered. Tumors previously classified as CNS PNETs are now termed CNS embryonal tumor, NOS, with the recognition that tumors in this category will likely be classified by their defining genomic lesions in future editions of the WHO classification.
A study applying unsupervised clustering of DNA methylation patterns for 323 nonmedulloblastoma embryonal tumors found that approximately one-half of these tumors diagnosed as nonmedulloblastoma embryonal tumors showed molecular profiles characteristic of other known pediatric brain tumors (e.g., high-grade glioma, atypical teratoid/rhabdoid tumor). This observation highlights the utility of molecular characterization to assign this class of tumors to their appropriate biology-based diagnosis.
Among the same collection of 323 tumors diagnosed as nonmedulloblastoma embryonal tumors, molecular characterization identified genomically and biologically distinctive subtypes, including the following:
ETMRs are defined at the molecular level by high-level amplification of the microRNA cluster C19MC and by a gene fusion between TTYH1 and C19MC.[93,94,95] This gene fusion puts expression of C19MC under control of the TTYH1 promoter, leading to high-level aberrant expression of the microRNAs within the cluster. The World Health Organization (WHO) allows histologically similar tumors without C19MC alteration to be classified as ETMR.
Medulloepithelioma is identified as a histologically discrete tumor within the WHO classification system.[98,99] Medulloepithelioma tumors are rare and tend to arise most commonly in infants and young children. Medulloepitheliomas, which histologically recapitulate the embryonal neural tube, tend to arise supratentorially, primarily intraventricularly, but may arise infratentorially, in the cauda, and even extraneurally, along nerve roots.[98,99] Medulloepithelioma with the classic molecular change is considered an ETMR.
Pineoblastoma, which was previously conventionally grouped with embryonal tumors, is now categorized by the WHO as a pineal parenchymal tumor. Given that therapies for pineoblastoma are quite similar to those utilized for embryonal tumors, the previous convention of including pineoblastoma with the CNS embryonal tumors is followed here. Pineoblastoma is associated with germline mutations in both the retinoblastoma (RB1) gene and in DICER1, as described below:
(Refer to the PDQ summary on Childhood Central Nervous System Embryonal Tumors Treatment for information about the treatment of childhood PNETs.)
Molecular characterization studies have identified several biological subtypes of ependymoma based on their distinctive DNA methylation and gene expression profiles and on their distinctive spectrum of genomic alterations.[105,106,107]
Figure 6. Graphical summary of key molecular and clinical characteristics of ependymal tumor subgroups.Schematic representation of key genetic and epigenetic findings in the nine molecular subgroups of ependymal tumors as identified by methylation profiling. CIN, Chromosomal instability. Reprinted from Cancer Cell, Volume 27, Kristian W. Pajtler, Hendrik Witt, Martin Sill, David T.W. Jones, Volker Hovestadt, Fabian Kratochwil, Khalida Wani, Ruth Tatevossian, Chandanamali Punchihewa, Pascal Johann, Juri Reimand, Hans-Jorg Warnatz, Marina Ryzhova, Steve Mack, Vijay Ramaswamy, David Capper, Leonille Schweizer, Laura Sieber, Andrea Wittmann, Zhiqin Huang, Peter van Sluis, Richard Volckmann, Jan Koster, Rogier Versteeg, Daniel Fults, Helen Toledano, Smadar Avigad, Lindsey M. Hoffman, Andrew M. Donson, Nicholas Foreman, Ekkehard Hewer, Karel Zitterbart, Mark Gilbert, Terri S. Armstrong, Nalin Gupta, Jeffrey C. Allen, Matthias A. Karajannis, David Zagzag, Martin Hasselblatt, Andreas E. Kulozik, Olaf Witt, V. Peter Collins, Katja von Hoff, Stefan Rutkowski, Torsten Pietsch, Gary Bader, Marie-Laure Yaspo, Andreas von Deimling, Peter Lichter, Michael D. Taylor, Richard Gilbertson, David W. Ellison, Kenneth Aldape, Andrey Korshunov, Marcel Kool, and Stefan M. Pfister, Molecular Classification of Ependymal Tumors across All CNS Compartments, Histopathological Grades, and Age Groups, Pages 728-743, Copyright (2015), with permission from Elsevier.
Approximately two-thirds of childhood ependymomas arise in the posterior fossa, and two major genomically defined subtypes of posterior fossa tumors are recognized. Similarly, most pediatric supratentorial tumors can be categorized into one of two genomic subtypes. These subtypes and their associated clinical characteristics are described below. Among these subtypes, the 2016 World Health Organization (WHO) classification has accepted ependymoma, RELA fusion-positive as a distinct diagnostic entity.
The most common posterior fossa ependymoma subtype is EPN-PFA and is characterized by the following:
Figure 7. Identification of Subgroup-Specific Copy Number Alterations in the Posterior Fossa Ependymoma Genome. (A) Copy number profiling of 75 PF ependymomas using 10K array-CGH identifies disparate genetic landscapes between Group A and Group B tumors. Toronto and Heidelberg copy number datasets have been combined and summarized in a heatmap. The heatmap also displays the association of tumors to cytogenetic risk groups 1, 2, and 3 (Korshunov et al., 2010). Statistically significant chromosomal aberrations (black boxes) are also displayed between both subgroups, calculated by Fisher's exact test. Witt H, Mack SC, Ryzhova M, et al.: Delineation of two clinically and molecularly distinct subgroups of posterior fossa ependymoma. Cancer Cell 20 (2): 143-57, 2011, doi:10.1016/j.ccr.2011.07.007. Copyright © 2011 Elsevier Inc. All rights reserved.
The EPN-PFB subtype is less common than the EPN-PFA subtype in children and is characterized by the following:
The largest subset of pediatric supratentorial (ST) ependymomas are characterized by gene fusions involving RELA,[109,110] a transcriptional factor important in NF-κB pathway activity. This subtype is termed ST-EPN-RELA and is characterized by the following:
A second, less common subset of supratentorial ependymomas, termed ST-EPN-YAP1, has fusions involving YAP1 and are characterized by the following:
Clinical implications of genomic alterations
The absence of recurring mutations in the EPN-PFA and EPN-PFB subtypes at diagnosis precludes using their genomic profiles to guide therapy. The RELA and YAP1 fusion genes present in supratentorial ependymomas are not directly targetable with agents in the clinic, but can provide leads for future research.
(Refer to the PDQ summary on Childhood Ependymoma Treatment for information about the treatment of childhood ependymoma.)
Genomic abnormalities related to hepatoblastoma include the following:
Genomic abnormalities related to hepatocellular carcinoma include the following:
(Refer to the PDQ summary on Childhood Liver Cancer Treatment for information about the treatment of liver cancer.)
The genomic landscape of osteosarcoma is distinctive from that of other childhood cancers. It is characterized by an exceptionally high number of structural variants with relatively small numbers of single nucleotide variants in comparison to many adult cancers.[1,2]
Key observations regarding the genomic landscape of osteosarcoma are summarized below:
Figure 9. Circos plots of osteosarcoma cases from the National Cancer Institute's Therapeutically Applicable Research to Generate Effective Treatments (TARGET) project. The red lines in the interior circle connect chromosome regions involved in either intra- or inter-chromosomal translocations. Osteosarcoma is distinctive from other childhood cancers because it has a large number of translocations. Credit: National Cancer Institute.
A number of germline mutations are associated with susceptibility to osteosarcoma; Table 1 summarizes the syndromes and associated genes for these conditions. Mutations in TP53 are the most common germline alterations associated with osteosarcoma. Mutations in this gene are found in approximately 70% of patients with Li-Fraumeni syndrome (LFS), which is associated with increased risk of osteosarcoma, breast cancer, various brain cancers, soft tissue sarcomas, and other cancers. While rhabdomyosarcoma is the most common sarcoma arising in patients aged 5 years and younger with TP53-associated LFS, osteosarcoma is the most common sarcoma in children and adolescents aged 6 to 19 years. One study observed a high frequency of young osteosarcoma cases (age <30 years) carrying a known LFS- or likely LFS-associated TP53 mutation (3.8%) or rare exonic TP53 variant (5.7%), with an overall TP53 mutation frequency of 9.5%. Another study observed germline mutations in TP53 in 7 of 59 (12%) osteosarcoma cases subjected to whole-exome sequencing. Other groups have reported lower rates (3%-7%) of TP53 germline mutations in patients with osteosarcoma.[5,6]
Refer to the following summaries for more information about these genetic syndromes:
(Refer to the PDQ summary on Osteosarcoma and Malignant Fibrous Histiocytoma Treatment for information about the treatment of osteosarcoma.)
The detection of a translocation involving the EWSR1 gene on chromosome 22 band q12 and any one of a number of partner chromosomes is the key feature in the diagnosis of Ewing sarcoma (refer to Table 2). The EWSR1 gene is a member of the TET family [TLS/EWS/TAF15] of RNA-binding proteins. The FLI1 gene is a member of the ETS family of DNA-binding genes. Characteristically, the amino terminus of the EWSR1 gene is juxtaposed with the carboxy terminus of the STS family gene. In most cases (90%), the carboxy terminus is provided by FLI1, a member of the family of transcription factor genes located on chromosome 11 band q24. Other family members that may combine with the EWSR1 gene are ERG, ETV1, ETV4 (also termed E1AF), and FEV. Rarely, TLS, another TET family member, can substitute for EWSR1. Finally, there are a few rare cases in which EWSR1 has translocated with partners that are not members of the ETS family of oncogenes. The significance of these alternate partners is not known.
Besides these consistent aberrations involving the EWSR1 gene at 22q12, additional numerical and structural aberrations have been observed in Ewing sarcoma, including gains of chromosomes 2, 5, 8, 9, 12, and 15; the nonreciprocal translocation t(1;16)(q12;q11.2); and deletions on the short arm of chromosome 6. Trisomy 20 may be associated with a more aggressive subset of Ewing sarcoma.
Three papers have described the genomic landscape of Ewing sarcoma and all show that these tumors have a relatively silent genome, with a paucity of mutations in pathways that might be amenable to treatment with novel targeted therapies.[22,23,24] These papers also identified mutations in STAG2, a member of the cohesin complex, in about 15% to 20% of the cases, and the presence of these mutations was associated with advanced-stage disease. CDKN2A deletions were noted in 12% to 22% of cases. Finally, TP53 mutations were identified in about 6% to 7% of cases and the coexistence of STAG2 and TP53 mutations is associated with a poor clinical outcome.[22,23,24]
Figure 10 below from a discovery cohort (n = 99) highlights the frequency of chromosome 8 gain, the co-occurrence of chromosome 1q gain and chromosome 16q loss, the mutual exclusivity of CDKN2A deletion and STAG2 mutation, and the relative paucity of recurrent single nucleotide variants for Ewing sarcoma.
Figure 10. A comprehensive profile of the genetic abnormalities in Ewing sarcoma and associated clinical information. Key clinical characteristics are indicated, including primary site, type of tissue, and metastatic status at diagnosis, follow-up, and last news. Below is the consistency of detection of gene fusions by RT-PCR and whole-genome sequencing (WGS). The numbers of structural variants (SV) and single-nucleotide variants (SNV) as well as indels are reported in grayscale. The presence of the main copy-number changes, chr 1q gain, chr 16 loss, chr 8 gain, chr 12 gain, and interstitial CDKN2A deletion is indicated. Listed last are the most significant mutations and their types. For gene mutations, "others" refers to: duplication of exon 22 leading to frameshift (STAG2), deletion of exon 2 to 11 (BCOR), and deletion of exons 1 to 6 (ZMYM3). Reprinted from Cancer Discovery, Copyright 2014, 4 (11), 1342-53, Tirode F, Surdez D, Ma X, et al., Genomic Landscape of Ewing Sarcoma Defines an Aggressive Subtype with Co-Association of STAG2 and TP53 mutations, with permission from AACR.
Ewing sarcoma translocations can all be found with standard cytogenetic analysis. A more rapid analysis looking for a break apart of the EWS gene is now frequently done to confirm the diagnosis of Ewing sarcoma molecularly. This test result must be considered with caution, however. Ewing sarcomas that utilize the TLS translocations will have negative tests because the EWSR1 gene is not translocated in those cases. In addition, other small round tumors also contain translocations of different ETS family members with EWSR1, such as desmoplastic small round cell tumor, clear cell sarcoma, extraskeletal myxoid chondrosarcoma, and myxoid liposarcoma, all of which may be positive with a EWS fluorescence in situ hybridization (FISH) break-apart probe. A detailed analysis of 85 patients with small round blue cell tumors that were negative for EWSR1 rearrangement by FISH with an EWSR1 break-apart probe identified eight patients with FUS rearrangements. Four patients who had EWSR1-ERG fusions were not detected by FISH with an EWSR1 break-apart probe. The authors do not recommend relying solely on EWSR1 break-apart probes for analyzing small round blue cell tumors with strong immunohistochemical positivity for CD99.
Small round blue cell tumors of bone and soft tissue that are histologically similar to Ewing sarcoma but do not have rearrangements of the EWSR1 gene have been analyzed and translocations have been identified. These include BCOR-CCNB3, CIC-DUX4, and CIC-FOX4.[27,28,29,30] The molecular profile of these tumors is different from the profile of EWS-FLI1 translocated Ewing sarcoma, and limited evidence suggests that they have a different clinical behavior. In almost all cases, the patients were treated with therapy designed for Ewing sarcoma on the basis of the histologic and immunohistologic similarity to Ewing sarcoma. There are too few cases associated with each translocation to determine whether the prognosis for these small round blue cell tumors is distinct from the prognosis of Ewing sarcoma of similar stage and site.[27,28,29,30]
A genome-wide association study identified a region on chromosome 10q21.3 associated with an increased risk of Ewing sarcoma. Deep sequencing through this region identified a polymorphism in the EGR2 gene, which appears to cooperate with the gene product of the EWSR1-FLI1 fusion that is seen in most patients with Ewing sarcoma. The polymorphism associated with the increased risk is found at a much higher frequency in whites than in blacks or Asians, possibly contributing to the epidemiology of the relative infrequency of Ewing sarcoma in the latter populations.
(Refer to the PDQ summary on Ewing Sarcoma Treatment for information about the treatment of Ewing sarcoma.)
The embryonal and alveolar histologies have distinctive molecular characteristics that have been used for diagnostic confirmation, and may be useful for assigning risk group, determining therapy, and monitoring residual disease during treatment.[33,34,35,36,37]
Embryonal histology with anaplasia: Anaplasia has been reported in a minority of children with rhabdomyosarcoma, primarily arising in children with the embryonal subtype who are younger than 10 years.[43,44] Rhabdomyosarcoma with nonalveolar, anaplastic morphology may be a presenting feature for children with Li-Fraumeni syndrome and germline TP53 mutations. Among eight consecutively presenting children with rhabdomyosarcoma and TP53 germline mutations, all showed anaplastic morphology. Among an additional seven children with anaplastic rhabdomyosarcoma and unknown TP53 germline mutation status, three of the seven children had functionally relevant TP53 germline mutations. The median age at diagnosis of the 11 children with TP53 germline mutation status was 40 months (range, 19-67 months).
The alveolar histology that is associated with the PAX7 gene in patients with or without metastatic disease appears to occur at a younger age and may be associated with longer event-free survival rates than those associated with PAX3 gene rearrangements.[49,50,51,52,53,54] Patients with alveolar histology and the PAX3 gene are older and have a higher incidence of invasive tumor (T2). Around 22% of cases showing alveolar histology have no detectable PAX gene translocation.[37,47] In addition to FOXO1 rearrangements, alveolar tumors are characterized by a lower mutational burden than are fusion-negative tumors, with fewer genes having recurring mutations.[41,42]BCOR and PIK3CA mutations and amplification of MYCN, MIR17HG, and CDK4 have also been described.
In older children and adults with spindle cell/sclerosing rhabdomyosarcoma, a specific MYOD1 mutation (p.L122R) has been observed in a large proportion of patients.[56,57,58,59] Activating PIK3CA mutations were common in MYOD1-mutated cases (4 of 10); when they were present, they were associated with sclerosing histology. The presence of the MYOD1 mutation is associated with an increased risk of treatment failure.[56,57,58] In one study that included nine children aged 1 year or older with spindle cell/sclerosing histology and MYOD1 mutations, seven had a fatal outcome despite aggressive multimodality treatment.
These findings highlight the important differences between embryonal and alveolar tumors. Data demonstrate that PAX-FOX01 fusion-positive alveolar tumors are biologically and clinically different from fusion-negative alveolar tumors and embryonal tumors.[37,60,61,62,63] In a study of Intergroup Rhabdomyosarcoma Study Group cases, which captured an entire cohort from a single prospective clinical trial, the outcome for patients with translocation-negative alveolar rhabdomyosarcoma was better than that observed for translocation-positive cases. The outcome was similar to that seen in patients with embryonal rhabdomyosarcoma and demonstrated that fusion status is a critical factor for risk stratification in pediatric rhabdomyosarcoma.
(Refer to the PDQ summary on Childhood Rhabdomyosarcoma Treatment for information about the treatment of childhood rhabdomyosarcoma.)
Studies published in 1994 showed clonality in Langerhans cell histiocytosis (LCH) using polymorphisms of methylation-specific restriction enzyme sites on the X-chromosome regions coding for the human androgen receptor, DXS255, PGK, and HPRT.[1,2] Biopsies of lesions with single-system or multisystem disease were found to have a proliferation of LCH cells from a single clone. The discovery of recurring genomic alterations (primarily BRAF V600E) in LCH (see below) confirmed the clonality of LCH in children. Pulmonary LCH in adults is usually nonclonal and it is possible that this group represents a reactive process to smoking. However, a subset appeared to be clonal, as an analysis of BRAF mutations showed that a significant proportion of patients (25%-30%) have evidence for mutant BRAF V600E.
Figure 11. Courtesy of Rikhia Chakraborty, Ph.D. Permission to reuse the figure in any form must be obtained directly from Dr. Chakraborty.
The genomic basis of LCH was advanced by a report in 2010 of an activating mutation of the BRAF oncogene (V600E) that was detected in 35 (57%) of 61 cases. Multiple subsequent reports have confirmed the presence of BRAF V600E mutations in 50% or more of LCH cases in children.[6,7,8] Another BRAF mutation (BRAF 600DLAT) was identified, which resulted in the insertion of four amino acids and also appeared to activate signaling.ARAF mutations are infrequent in LCH, but when present, can also lead to RAS-MAPK pathway activation. No clinical characteristics associated with the BRAF V600E mutation have been identified.[5,6,7]
The RAS-MAPK signaling pathway (Figure 11) transmits signals from a cell surface receptor (e.g., a growth factor) through the RAS pathway (via one of the RAF proteins [A, B, or C]) to phosphorylate MEK and then the extracellular signal-regulated kinase (ERK), which leads to nuclear signals affecting cell cycle and transcription regulation. The V600E mutation of BRAF leads to continuous phosphorylation, and thus activation, of MEK and ERK without the need for an external signal. Activation of ERK occurs by phosphorylation, and phosphorylated ERK can be detected in virtually all LCH lesions.[5,10]
Because RAS-MAPK pathway activation can be detected in all LCH cases, but not all cases have BRAF mutations, the presence of genomic alterations in other components of the pathway was suspected. Whole-exome sequencing of BRAF-mutated versus BRAF-wild-type LCH biopsies revealed that 7 of 21 BRAF-wild-type specimens had MAP2K1 mutations, while no BRAF-mutated specimens had MAP2K1 mutations. The mutations in MAP2K1 (which codes for MEK) were activating, as indicated by their induction of ERK phosphorylation. Another study showed MAP2K1 mutations exclusively in 11 of 22 BRAF-wild-type cases. Finally, in-frame BRAF deletions and in-frame FAM73A-BRAF fusions have occurred in the group of BRAF V600E and MAP2K1 mutation-negative cases. Studies to date support the universal activation of ERK in LCH, with activation in most cases being explained by BRAF and MAP2K1 alterations.[5,10,12]
The presence of BRAF V600E mutation in blood and bone marrow was studied in a series of 100 patients, of which 65% tested positive for the BRAF V600E mutation by a sensitive quantitative polymerase chain reaction technique. Circulating cells with the BRAF V600E mutation could be detected in all high-risk patients and in a subset of low-risk multisystem patients. The presence of circulating cells with the mutation conferred a twofold increased risk of relapse. The myeloid dendritic cell origin of LCH was confirmed by finding CD34+ stem cells with the mutation in the bone marrow of high-risk patients. Those with low-risk disease had more mature myeloid dendritic cells with the mutation, suggesting the stage of cell development is critical in defining the clinical characteristics of LCH, which can now be considered a myeloid neoplasia in most cases.
A study of 173 patients with the BRAF V600E mutation, and 142 patients without the mutation, revealed that the mutation occurred in 88% of patients with high-risk disease, 69% of patients with multisystem low-risk LCH, and 44% of patients with single-system low-risk LCH. The mutation was also found in 75% of patients with neurodegenerative syndrome and 73% of patients with pituitary involvement. Resistance to initial treatment and relapse were higher in patients with the mutation.
Clinical implications of the described genomic findings include the following:
(Refer to the PDQ summary on Langerhans Cell Histiocytosis Treatment for information about the treatment of childhood LCH.)
Neuroblastoma can be subdivided into a biologically defined subset that has a very favorable prognosis (i.e., low-risk neuroblastoma) and another group that has a guarded prognosis (i.e., high-risk neuroblastoma). While neuroblastoma in infants with tumors that have favorable biology is highly curable, only 50% of children with high-risk neuroblastoma are alive at 5 years from diagnosis, at best.
Low-risk neuroblastoma is usually found in children younger than 18 months with limited extent of disease; the tumor has changes, usually increases, in the number of whole chromosomes in the neuroblastoma cell. Low-risk tumors are hyperdiploid when examined by flow cytometry.[1,2] In contrast, high-risk neuroblastoma generally occurs in children older than 18 months, is often metastatic to bone, and usually has segmental chromosome abnormalities. They are near diploid or near tetraploid by flow cytometric measurement.[1,2,3,4,5,6,7] High-risk tumors also show exonic mutations (refer to the Exonic mutations in neuroblastoma section of this summary for more information), but most high-risk tumors lack mutations in genes that are recurrently mutated. Compared with adult cancers, neuroblastomas show a low number of mutations per genome that affect protein sequence (10-20 per genome).
Key genomic characteristics of high-risk neuroblastoma that are discussed below include the following:
Segmental chromosomal aberrations (includingMYCNgene amplification)
Segmental chromosomal aberrations, found most frequently in 1p, 1q, 3p, 11q, 14q, and 17p (and MYCN amplification), are best detected by comparative genomic hybridization and are seen in almost all high-risk and/or stage 4 neuroblastomas.[3,4,5,6,7] Among all patients with neuroblastoma, a higher number of chromosome breakpoints correlated with the following, whether or not MYCN amplification was considered:[3,4,5,6,7][Level of evidence: 3iiD]
In a study of unresectable primary neuroblastomas without metastases in children older than 12 months, segmental chromosomal aberrations were found in most, and older children were more likely to have them and to have more of them per tumor cell. In children aged 12 to 18 months, the presence of segmental chromosomal aberrations had a significant effect on event-free survival (EFS) but not on overall survival (OS). However, in children older than 18 months, there was a significant difference in OS in children with segmental chromosomal aberrations versus children without segmental chromosomal aberrations (67% vs. 100%), regardless of the histologic prognosis.
Segmental chromosomal aberrations are also predictive of recurrence in infants with localized unresectable or metastatic neuroblastoma without MYCN gene amplification.[1,2]
MYCN amplification (defined as more than 10 copies per diploid genome) is one of the most common segmental chromosomal aberrations, detected in 16% to 25% of tumors. For high-risk neuroblastoma, 40% to 50% of cases show MYCN amplification. In all stages of disease, amplification of the MYCN gene strongly predicts a poorer prognosis in both time to tumor progression and OS in almost all multivariate regression analyses of prognostic factors.[1,2] Within the localized MYCN-amplified cohort, ploidy status may further predict outcome. However, patients with hyperdiploid tumors with any segmental chromosomal aberrations do relatively poorly.
Most unfavorable clinical and pathobiological features are associated, to some degree, with MYCN amplification; in a multivariable logistic regression analysis of 7,102 International Neuroblastoma Risk Group patients, pooled segmental chromosomal aberrations and gain of 17q were the only poor prognostic features not associated with MYCN amplification. However, segmental chromosomal aberrations at 11q are almost mutually exclusive of MYCN amplification.
Exonic mutations in neuroblastoma
Multiple reports have documented that a minority of high-risk neuroblastomas have a small number of low-incidence, recurrently mutated genes. The most commonly mutated gene is ALK, which is mutated in approximately 10% of patients (see below). Other genes with even lower frequencies of mutation include ATRX, PTPN11, ARID1A, and ARID1B.[12,13,14,15,16,17,18] As shown in Figure 12, most neuroblastoma cases lack mutations in genes that are altered in a recurrent manner.
Figure 12. Data tracks (rows) facilitate the comparison of clinical and genomic data across cases with neuroblastoma (columns). The data sources and sequencing technology used were whole-exome sequencing (WES) from whole-genome amplification (WGA) (light purple), WES from native DNA (dark purple), Illumina WGS (green), and Complete Genomics WGS (yellow). Striped blocks indicate cases analyzed using two approaches. The clinical variables included were gender (male, blue; female, pink) and age (brown spectrum). Copy number alterations indicates ploidy measured by flow cytometry (with hyperdiploid meaning DNA index >1) and clinically relevant copy number alterations derived from sequence data. Significantly mutated genes are those with statistically significant mutation counts given the background mutation rate, gene size, and expression in neuroblastoma. Germline indicates genes with significant numbers of germline ClinVar variants or loss-of-function cancer gene variants in our cohort. DNA repair indicates genes that may be associated with an increased mutation frequency in two apparently hypermutated tumors. Predicted effects of somatic mutations are color coded according to the legend. Reprinted by permission from Macmillan Publishers Ltd: Nature Genetics (Pugh TJ, Morozova O, Attiyeh EF, et al.: The genetic landscape of high-risk neuroblastoma. Nat Genet 45 (3): 279-84, 2013), copyright (2013).
ALK, the exonic mutation found most commonly in neuroblastoma, is a cell surface receptor tyrosine kinase, expressed at significant levels only in developing embryonic and neonatal brains. Germline mutations in ALK have been identified as the major cause of hereditary neuroblastoma. Somatically acquired ALK-activating mutations are also found as oncogenic drivers in neuroblastoma.
The presence of an ALK mutation correlates with significantly poorer survival in high-risk and intermediate-risk neuroblastoma patients. ALK mutation was examined in 1,596 diagnostic neuroblastoma samples.ALK tyrosine kinase domain mutations occurred in 8% of samples-at three hot spots and 13 minor sites-and correlated significantly with poorer survival in patients with high-risk and intermediate-risk neuroblastoma. ALK mutations were found in 10.9% of MYCN-amplified tumors versus 7.2% of those without MYCN amplification. ALK mutations occurred at the highest frequency (11%) in patients older than 10 years. The frequency of ALK aberrations was 14% in the high-risk neuroblastoma group, 6% in the intermediate-risk neuroblastoma group, and 8% in the low-risk neuroblastoma group.
Small-molecule ALK kinase inhibitors such as crizotinib are being developed and tested in patients with recurrent and refractory neuroblastoma. (Refer to the Treatment Options Under Clinical Evaluation for Recurrent or Refractory Neuroblastoma section in the PDQ summary on Neuroblastoma Treatment for more information about crizotinib clinical trials.)
Genomic evolution of exonic mutations
There are limited data regarding the genomic evolution of exonic mutations from diagnosis to relapse for neuroblastoma. Whole-genome sequencing was applied to 23 paired diagnostic and relapsed neuroblastomas to define somatic genetic alterations associated with relapse, while a second study evaluated 16 paired diagnostic and relapsed specimens. Both studies identified an increased number of mutations in the relapsed samples compared with the samples at diagnosis.
In addition, three relapse samples showed structural alterations involving MAPK pathway genes consistent with pathway activation, so aberrations in this pathway were detected in 18 of 23 relapse samples (78%). Aberrations were found in ALK (n = 10), NF1 (n = 2), and one each in NRAS, KRAS, HRAS, BRAF, PTPN11, and FGFR1. Even with deep sequencing, 7 of the 18 alterations were not detectable in the primary tumor, highlighting the evolution of mutation presumably leading to relapse and the importance of genomic evaluations of tissues obtained at relapse.
In a study of 276 neuroblastoma samples of all stages and from patients of all ages, very deep (33,000X) sequencing of two amplified ALK mutational hot spots revealed 4.8% clonal mutations and an additional 5% subclonal mutations, suggesting that subclonal mutations are common. Deep sequencing can reveal the presence of mutations in tiny subsets of tumor cells that may be able to survive during treatment and grow to constitute a relapse.
Genomic alterations promoting telomere lengthening
Lengthening of telomeres, the tips of chromosomes, promotes cell survival. Telomeres otherwise shorten with each cell replication, resulting eventually in the lack of a cell's ability to replicate. Low-risk neuroblastomas have little telomere lengthening activity. Aberrant genetic mechanisms for telomere lengthening have been identified for high-risk neuroblastoma.[12,13,22] Thus far, the following three mechanisms, which appear to be mutually exclusive, have been described:
Additional biological factors associated with prognosis
MYC and MYCN expression
Immunostaining for MYC and MYCN proteins on 357 undifferentiated/poorly differentiated neuroblastomas has demonstrated that elevated MYC/MYCN protein expression is prognostically significant. Sixty-eight tumors highly expressed MYCN protein, and 81 were MYCN amplified. Thirty-nine tumors expressed MYC highly and were mutually exclusive of high MYCN expression. Segmental chromosomal aberrations were not examined in this study, except for MYCN amplification.
Most neuroblastomas with MYCN amplification in the International Neuroblastoma Pathology Classification system have unfavorable histology, but about 7% have FH. Of those with MYCN amplification and FH, most do not express MYCN, despite the gene being amplified, and have a more favorable prognosis than those that express MYCN. Segmental chromosomal aberration at 11q is almost mutually exclusive of MYCN amplification.
Neurotrophin receptor kinases
Expression of neurotrophin receptor kinases and their ligands vary between high-risk and low-risk tumors. TrkA is found on low-risk tumors, and absence of its ligand NGF is postulated to lead to spontaneous tumor regression. In contrast, TrkB is found in high-risk tumors that also express its ligand, BDNF, which promotes neuroblastoma cell growth and survival.
Immune system inhibition
Anti-GD2 antibodies, along with modulation of the immune system to enhance antineuroblastoma activity, are often used to help treat neuroblastoma. The anti-GD2 antibody (3F8), used for treating neuroblastoma exclusively at one institution, utilizes natural killer cells to kill the neuroblastoma cells. However, the natural killer cells can be inhibited by the interaction of HLA antigens and killer immunoglobulin receptor subtypes. Thus, the patient's immune system genes can help determine response to immunotherapy for neuroblastoma.[27,28] A report on the effects of immune system genes on response to dinutuximab, a commercially available anti-GD2 antibody, awaits publication.
(Refer to the PDQ summary on Neuroblastoma Treatment for information about the treatment of neuroblastoma.)
Retinoblastoma is a tumor that occurs in heritable (25%-30%) and nonheritable (70%-75%) forms. Heritable disease is defined by the presence of a germline mutation of the RB1 gene. This germline mutation may have been inherited from an affected progenitor (25% of cases) or may have occurred in a germ cell before conception or in utero during early embryogenesis in patients with sporadic disease (75% of cases). The presence of positive family history or bilateral or multifocal disease is suggestive of heritable disease.
Heritable retinoblastoma may manifest as unilateral or bilateral disease. The penetrance of the RB1 mutation (laterality, age at diagnosis, and number of tumors) is probably dependent on concurrent genetic modifiers such as MDM2 and MDM4 polymorphisms.[1,2] All children with bilateral disease and approximately 15% of patients with unilateral disease are presumed to have the heritable form, even though only 25% have an affected parent.
In heritable retinoblastoma, tumors tend to be diagnosed at a younger age than in the nonheritable form of the disease. Unilateral retinoblastoma in children younger than 1 year raises concern for heritable disease, whereas older children with a unilateral tumor are more likely to have the nonheritable form of the disease.
The genomic landscape of retinoblastoma is driven by alterations in RB1 that lead to biallelic inactivation.[4,5] A rare cause of RB1 inactivation is chromothripsis, which may be difficult to detect by conventional methods. Other recurring genomic changes that occur in a small minority of tumors include BCOR mutation/deletion, MYCN amplification, and OTX2 amplification.[4,5,6] A study of 1,068 unilateral nonfamilial retinoblastoma tumors reported that a small percentage of cases (approximately 3%) lacked evidence of RB1 loss. Approximately one-half of these cases with no evidence of RB1 loss (representing approximately 1.5% of all unilateral nonfamilial retinoblastoma) showed MYCN amplification.
(Refer to the PDQ summary on Retinoblastoma Treatment for information about the treatment of retinoblastoma.)
Wilms tumor is thought to arise from clonal expansion of a nephrogenic rest. Mutations in many genes can perturb renal development and lead to cancer. This is in contrast to retinoblastoma, for example, in which a mutation in a single gene (RB1) is the oncogenic driver. Approximately one-third of Wilms tumor cases involve mutations in WT1, CTNNB1, or WTX.[1,2] Another subset of Wilms tumor cases result from mutations in miRNA processing genes (miRNAPG), including DROSHA, DGCR8, DICER1, and XPO5.[3,4,5,6] Other genes that are recurrently mutated in Wilms tumor include SIX1 and SIX2 (transcription factors that play key roles in early renal development) [3,4] and MLLT1 (a gene involved in transcriptional elongation in early development). Anaplastic Wilms tumor is characterized by the presence of TP53 mutations.
Elevated rates of Wilms tumor are observed in a number of genetic disorders, including WAGR (Wilms tumor, aniridia, genitourinary anomalies, and mental retardation) syndrome, Beckwith-Wiedemann syndrome, hemihypertrophy, Denys-Drash syndrome, and Perlman syndrome. Other genetic causes that have been observed in familial Wilms tumor cases include germline mutations in REST and CTR9.[9,10]
The genomic and genetic characteristics of Wilms tumor are summarized below.
Wilms tumor 1gene (WT1)
The WT1 gene is located on the short arm of chromosome 11 (11p13). WT1 is a transcription factor that is required for normal genitourinary development and is important for differentiation of the renal blastema.WT1 mutations are observed in 10% to 20% of cases of sporadic Wilms tumor.[1,11,12]
Wilms tumor with a WT1 mutation is characterized by the following:
Germline WT1 mutations are more common in children with Wilms tumor and one of the following:
Syndromic conditions with germline WT1 mutations include WAGR syndrome, Denys-Drash syndrome, and Frasier syndrome.
Inactivating mutations or deletions in the PAX6 gene lead to aniridia, while deletion of WT1 confers the increased risk of Wilms tumor. Sporadic aniridia in which WT1 is not deleted is not associated with increased risk of Wilms tumor. Accordingly, children with familial aniridia, generally occurring for many generations, and without renal abnormalities, have a normal WT1 gene and are not at an increased risk of Wilms tumor.[23,24]
Wilms tumor in children with WAGR syndrome is characterized by an excess of bilateral disease, intralobar nephrogenic rests-associated favorable-histology (FH) tumors of mixed cell type, and early age at diagnosis. The mental retardation in WAGR syndrome may be secondary to deletion of other genes, including SLC1A2 or BDNF.
Germline WT1 point mutations produce genetic syndromes that are characterized by nephropathy, 46XY disorder of sex development, and varying risks of Wilms tumor.[27,28]
WT1 mutations in Denys-Drash syndrome are most often missense mutations in exons 8 and 9, which code for the DNA binding region of WT1. By contrast, WT1 mutations in Frasier syndrome typically occur in intron 9 at the KTS site, and they affect an alternative splicing, thereby preventing production of the usually more abundant WT1 +KTS isoform.
Studies evaluating genotype/phenotype correlations of WT1 mutations have shown that the risk of Wilms tumor is highest for truncating mutations (14 of 17 cases, 82%) and lower for missense mutations (27 of 67 cases, 42%). The risk is lowest for KTS splice site mutations (1 of 27 cases, 4%).[27,28] Bilateral Wilms tumor was more common in cases with WT1-truncating mutations (9 of 14 cases) than in cases with WT1 missense mutations (3 of 27 cases).[27,28] These genomic studies confirm previous estimates of elevated risk of Wilms tumor for children with Denys-Drash syndrome and low risk of Wilms tumor for children with Frasier syndrome.
Late effects associated with WAGR syndrome and Wilms tumor include the following:
(Refer to the Late effects after Wilms tumor therapy section of the PDQ summary on Wilms Tumor and Other Childhood Kidney Tumors Treatment for more information about the late effects associated with Wilms tumor.)
Somatic activating mutations of the CTNNB1 gene have been reported to occur in 15% of patients with Wilms tumor.[2,12,14,32] These CTNNB1 mutations result in activation of the WNT pathway, which plays a prominent role in the developing kidney.CTNNB1 mutations commonly occur with WT1 mutations, and most cases of Wilms tumor with WT1 mutations have a concurrent CTNNB1 mutation.[12,14,32] Activation of beta-catenin in the presence of intact WT1 protein appears to be inadequate to promote tumor development because CTNNB1 mutations are rarely found in the absence of a WT1 or WTX mutation, except when associated with a MLLT1 mutation.[2,34]CTNNB1 mutations appear to be late events in Wilms tumor development because they are found in tumors but not in nephrogenic rests.
Wilms tumor gene on the X chromosome (WTX)
WTX, which is also called FAM123B, is located on the X chromosome at Xq11.1. It is altered in 15% to 20% of Wilms tumor cases.[35,1,2,12,36] Germline mutations in WTX cause an X-linked sclerosing bone dysplasia, osteopathia striata congenita with cranial sclerosis (MIM300373). Despite having germline WTX mutations, individuals with osteopathia striata congenita are not predisposed to tumor development. The WTX protein appears to be involved in both the degradation of beta-catenin and in the intracellular distribution of APC protein.[34,38]WTX is most commonly altered by deletions involving part or all of the WTX gene, with deleterious point mutations occurring less commonly.[1,12,35] Most Wilms tumor cases with WTX alterations have epigenetic 11p15 abnormalities.
WTX alterations are equally distributed between males and females, and WTX inactivation has no apparent effect on clinical presentation or prognosis.
Imprinting Cluster Regions (ICRs) on chromosome 11p15 (WT2) and Beckwith-Wiedemann syndrome
A second Wilms tumor locus, WT2, maps to an imprinted region of chromosome 11p15.5; when it is a germline mutation, it causes Beckwith-Wiedemann syndrome. About 3% of children with Wilms tumor have germline epigenetic or genetic changes at the 11p15.5 growth regulatory locus without any clinical manifestations of overgrowth. Like children with Beckwith-Wiedemann syndrome, these children have an increased incidence of bilateral Wilms tumor or familial Wilms tumor.
Approximately 80% of patients with Beckwith-Wiedemann syndrome have a molecular defect of the 11p15 domain. Various molecular mechanisms underlying Beckwith-Wiedemann syndrome have been identified. Some of these abnormalities are genetic (germline mutations of the maternal allele of CDKN1C, paternal uniparental isodisomy of 11p15, or duplication of part of the 11p15 domain) but are more frequently epigenetic (loss of methylation of the maternal ICR2/KvDMR1 or gain of methylation of the maternal ICR1).[26,40]
Several candidate genes at the WT2 locus comprise the two independent imprinted domains IGF2/H19 and KIP2/LIT1. Loss of heterozygosity, which exclusively affects the maternal chromosome, has the effect of upregulating paternally active genes and silencing maternally active ones. A loss or switch of the imprint for genes (change in methylation status) in this region has also been frequently observed and results in the same functional aberrations.[26,39,40]
A relationship between epigenotype and phenotype has been shown in Beckwith-Wiedemann syndrome, with a different rate of cancer in Beckwith-Wiedemann syndrome according to the type of alteration of the 11p15 region. The overall tumor risk in patients with Beckwith-Wiedemann syndrome has been estimated to be between 5% and 10%, with a risk between 1% (loss of imprinting at ICR2) and 30% (gain of methylation at ICR1 and paternal 11p15 isodisomy). Development of Wilms tumor has been reported in patients with only ICR1 gain of methylation, whereas other tumors such as neuroblastoma or hepatoblastoma were reported in patients with paternal 11p15 isodisomy.[42,43,44]
Loss of imprinting or gene methylation is rarely found at other loci, supporting the specificity of loss of imprinting at 11p15.5. Interestingly, Wilms tumor in Asian children is not associated with either nephrogenic rests or IGF2 loss of imprinting.
Approximately one-fifth of patients with Beckwith-Wiedemann syndrome who develop Wilms tumor present with bilateral disease, and metachronous bilateral disease is also observed.[23,47,48] The prevalence of Beckwith-Wiedemann syndrome is about 1% among children with Wilms tumor reported to the National Wilms Tumor Study (NWTS).[48,49]
Other genes and chromosomal alterations
Additional genes and chromosomal alterations that have been implicated in the pathogenesis and biology of Wilms tumorinclude the following:
In an analysis of FH Wilms tumor from 1,114 patients from NWTS-5 (COG-Q9401/NCT00002611), 28% of the tumors displayed 1q gain.
These conflicting results may arise from the greater prognostic significance of 1q gain described above. Loss of heterozygosity of 16q and 1p loses significance as independent prognostic markers in the presence of 1q gain. However, in the absence of 1q gain, loss of heterozygosity of 16q and 1p retains their adverse prognostic impact. The loss of heterozygosity of 16q and 1p appears to arise from complex chromosomal events that result in 1q loss of heterozygosity or 1q gain. The change in 1q appears to be the critical tumorigenic genetic event.
Germline mutations in miRNAPG are observed for DICER1 and DIS3L2, with mutations in the former causing DICER1 syndrome and mutations in the latter causing Perlman syndrome.
Figure 13. The miRNA processing pathway is commonly mutated in Wilms tumor. Expression of mature miRNA is initiated by RNA polymerase-mediated transcription of DNA-encoded sequences into pri-miRNA, which form a long double-stranded hairpin. This structure is then cleaved by a complex of Drosha and DGCR8 into a smaller pre-miRNA hairpin, which is exported from the nucleus and then cleaved by Dicer (an RNase) and TRBP (with specificity for dsRNA) to remove the hairpin loop and leave two single-stranded miRNAs. The functional strand binds to Argonaute (Ago2) proteins into the RNA-induced silencing complex (RISC), where it guides the complex to its target mRNA, while the nonfunctional strand is degraded. Targeting of mRNAs by this method results in mRNA silencing by mRNA cleavage, translational repression, or deadenylation. Let-7 miRNAs are a family of miRNAs highly expressed in ESCs with tumor suppressor properties. In cases in which LIN28 is overexpressed, LIN28 binds to pre-Let-7 miRNA, preventing DICER from binding and resulting in LIN28-activated polyuridylation by TUT4 or TUT7, causing reciprocal DIS3L2-mediated degradation of Let-7 pre-miRNAs. Genes involved in miRNA processing that have been associated with Wilms' tumor are highlighted in blue (inactivating) and green (activating) and include DROSHA, DGCR8, XPO5 (encoding exportin-5), DICER1, TARBP2, DIS3L2, and LIN28. Copyright © 2015 Hohenstein et al.; Published by Cold Spring Harbor Laboratory Press. Genes Dev. 2015 Mar 1; 29(5): 467-482. doi: 10.1101/gad.256396.114. This article is distributed exclusively by Cold Spring Harbor Laboratory Press under a Creative Commons License (Attribution-NonCommercial 4.0 International), as described at http://creativecommons.org/licenses/by-nc/4.0/.
Figure 14 summarizes the genomic landscape of a selected cohort of Wilms tumor patients selected because they experienced relapse despite showing FH. The 75 FH Wilms tumor cases were clustered by unsupervised analysis of gene expression data, resulting in six clusters. Five of six MLLT1-mutant tumors with available gene expression data were in cluster 3, and two were accompanied by CTNNB1 mutations. This cluster also contained four tumors with a mutation or small segment deletion of WT1, all of which also had either a mutation of CTNNB1 or small segment deletion or mutation of WTX. It also contained a substantial number of tumors with retention of imprinting of 11p15 (including all MLLT1-mutant tumors). The miRNAPG-mutated cases clustered together and were mutually exclusive with both MLLT1 and with WT1/WTX/CTNNB1-mutated cases.
Figure 14. Unsupervised analysis of gene expression data. Non-negative Matrix Factorization (NMF) analysis of 75 FH Wilms tumor resulted in six clusters. Five of six MLLT1 mutant tumors with available gene expression data occurred in NMF cluster 3, and two were accompanied by CTNNB1 mutations. This cluster also contained a substantial number of tumors with retention of imprinting of 11p15 (including all MLLT1-mutant tumors), in contrast to other clusters, where most cases showed 11p15 loss of heterozygosity or retention of imprinting. Almost all miRNAPG-mutated cases were in NMF cluster 2, and most WT1, WTX, and CTNNB1 mutations were in NMF clusters 3 and 4. Copyright © 2015 Perlman, E. J. et al. MLLT1 YEATS domain mutations in clinically distinctive Favourable Histology wilms tumours. Nat. Commun. 6:10013 doi: 10.1038/ncomms10013 (2015). This article is distributed by Nature Publishing Group, a division of Macmillan Publishers Limited under a Creative Commons Attribution 4.0 International License, as described at http://creativecommons.org/licenses/by/4.0/.
Renal Cell Carcinoma
Translocation-positive carcinomas of the kidney are recognized as a distinct form of renal cell carcinoma (RCC) and may be the most common form of RCC in children, accounting for 40% to 50% of pediatric RCC. In a Children's Oncology Group (COG) prospective clinical trial of 120 childhood and adolescent patients with RCC, nearly one-half of patients had translocation-positive RCC. These carcinomas are characterized by translocations involving the transcription factor E3 gene (TFE3) located on Xp11.2. The TFE3 gene may partner with one of the following genes:
Another less-common translocation subtype, t(6;11)(p21;q12), involving an Alpha-transcription factor EB (TFEB) gene fusion, induces overexpression of TFEB. The translocations involving TFE3 and TFEB induce overexpression of these proteins, which can be identified by immunohistochemistry.
Previous exposure to chemotherapy is the only known risk factor for the development of Xp11 translocation RCCs. In one study, the postchemotherapy interval ranged from 4 to 13 years. All reported patients received either a DNA topoisomerase II inhibitor and/or an alkylating agent.[77,78]
Controversy exists as to the biological behavior of translocation RCC in children and young adults. Whereas some series have suggested a good prognosis when RCC is treated with surgery alone despite presenting at a more advanced stage (III/IV) than TFE-RCC, a meta-analysis reported that these patients have poorer outcomes.[79,80,81] The outcomes for these patients are being studied in the ongoing COG AREN03B2 (NCT00898365) biology and classification study. Vascular endothelial growth factor receptor-targeted therapies and mammalian target of rapamycin (mTOR) inhibitors seem to be active in Xp11 translocation metastatic RCC. Recurrences have been reported 20 to 30 years after initial resection of the translocation-associated RCC.
Diagnosis of Xp11 translocation RCC needs to be confirmed by a molecular genetic approach, rather than using TFE3 immunohistochemistry alone, because reported cases have lacked the translocation. There is a rare subset of RCC cases that is positive for TFE3 and lack a TFE3 translocation, showing an ALK translocation instead. This subset of cases represents a newly recognized subgroup within RCC that is estimated to involve 15% to 20% of unclassified pediatric RCC. In the eight reported cases in children aged 6 to 16 years, the following was observed:[84,85,86,87]
(Refer to the PDQ summary on Wilms Tumor and Other Childhood Kidney Tumors Treatment for information about the treatment of renal cell carcinoma.)
Rhabdoid Tumors of the Kidney
Rhabdoid tumors in all anatomical locations have a common genetic abnormality-loss of function of the SMARCB1/INI1/SNF5/BAF47 gene located at chromosome 22q11.2. The following text refers to rhabdoid tumors without regard to their primary site. SMARCB1 encodes a component of the SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling complex that has an important role in controlling gene transcription.[88,89] Loss of function occurs by deletions that lead to loss of part or all of the SMARCB1 gene and by mutations that are commonly frameshift or nonsense mutations that lead to premature truncation of the SMARCB1 protein.[89,90] A small percentage of rhabdoid tumors are caused by alterations in SMARCA4, which is the primary ATPase in the SWI/SNF complex.[91,92] Exome sequencing of 35 cases of rhabdoid tumor identified a very low mutation rate, with no genes having recurring mutations other than SMARCB1, which appeared to contribute to tumorigenesis.
Germline mutations of SMARCB1 have been documented in patients with one or more primary tumors of the brain and/or kidney, consistent with a genetic predisposition to the development of rhabdoid tumors.[94,95] Approximately one-third of patients with rhabdoid tumors have germline SMARCB1 alterations.[89,96] In most cases, the mutations are de novo and not inherited. The median age at diagnosis of children with rhabdoid tumors and a germline mutation or deletion is younger (6 months) than that of children with apparently sporadic disease (18 months). Germline mosaicism has been suggested for several families with multiple affected siblings. It appears that patients with germline mutations may have the worst prognosis.[98,99] Germline mutations in SMARCA4 have also been reported in patients with rhabdoid tumors.[91,100]
(Refer to the PDQ summary on Wilms Tumor and Other Childhood Kidney Tumors Treatment for information about the treatment of rhabdoid tumor of the kidney.)
Clear Cell Sarcoma of the Kidney
Clear cell sarcoma of the kidney is an uncommon renal tumor that comprises approximately 5% of all primary renal malignancies in children, and is observed most often before age 3 years. The molecular background of clear cell sarcoma of the kidney is poorly understood due to its rarity and lack of experimental models.
Several biological features of clear cell sarcoma of the kidney have been described, including the following:
(Refer to the PDQ summary on Wilms Tumor and Other Childhood Kidney Tumors Treatment for information about the treatment of clear cell tumor of the kidney.)
Melanoma-related conditions with malignant potential that arise in the pediatric population can be classified into three general groups:
The genomic characteristics of each tumor are summarized in Table 3.
The genomic landscape of conventional melanoma in children is represented by many of the genomic alterations found in adults with melanoma. A report from the Pediatric Cancer Genome Project observed that 15 cases of conventional melanoma had a high burden of somatic single-nucleotide variations, TERT promoter mutations (12 of 13), and activating BRAF V600 mutations (13 of 15), as well as a mutational spectrum signature consistent with ultraviolet light damage. In addition, two-thirds of the cases had MC1R variants associated with an increased susceptibility to melanoma.
The genomic landscape of spitzoid melanomas is characterized by kinase gene fusions involving various genes including RET, ROS1, NTRK1, ALK, MET, and BRAF.[2,3,4] These fusion genes have been reported in approximately 50% of cases and occur in a mutually exclusive manner.[1,3]TERT promoter mutations are uncommon in spitzoid melanocytic lesions and were observed in only 4 of 56 patients evaluated in one series. However, each of the four cases with TERT promoter mutations experienced hematogenous metastases and died of their disease. This finding supports the potential of TERT promoter mutations in predicting aggressive clinical behavior in children with spitzoid melanocytic neoplasms, but further study is needed to define the role of wild-type TERT promoter status in predicting clinical behavior in patients with primary site spitzoid tumors.
Large congenital melanocytic nevi are reported to have activating NRAS Q61 mutations with no other recurring mutations noted. Somatic mosaicism for NRAS Q61 mutations has also been reported in patients with multiple congenital melanocytic nevi and neuromelanosis.
(Refer to the PDQ summary on Unusual Cancers of Childhood Treatment for information about the treatment of childhood melanoma.)
Studies have shown subtle differences between the genetic profiling of childhood differentiated thyroid carcinomas and that of adult tumors. In one study, a higher prevalence of RET/PTC rearrangements was reported in pediatric papillary carcinoma (45%-65% in children vs. 3%-34% in adults).BRAF V600E mutations are seen in more than 50% of adults with papillary thyroid carcinoma; although it likely occurs in a similar frequency in pediatric patients, studies have revealed a wide variation in frequency of this mutation.[1,2,3,4] In children, the correlation between the genomic alteration and stage or prognosis has not been well defined. While two studies failed to show a correlation,[3,4] one study that included 55 pediatric thyroid carcinoma cases demonstrated a significant correlation between the presence of a BRAF V600E mutation and an increased risk of recurrence. Differentiated thyroid carcinoma has been associated with germline DICER1 mutations and it is considered part of the DICER1 syndrome.
(Refer to the PDQ summary on Unusual Cancers of Childhood Treatment for information about the treatment of childhood thyroid cancer.)
The most salient clinical and genetic alterations of the multiple endocrine neoplasia (MEN) syndromes are shown in Table 5.
A study documented the initial symptoms of MEN1 syndrome occurring before age 21 years in 160 patients. Of note, most patients had familial MEN1 syndrome and were followed up using an international screening protocol.
Germline mutations of the MEN1 gene located on chromosome 11q13 are found in 70% to 90% of patients; however, this gene has also been shown to be frequently inactivated in sporadic tumors. Mutation testing is combined with clinical screening for patients and family members with proven at-risk MEN1 syndrome.
It is recommended that screening for patients with MEN1 syndrome begin by the age of 5 years and continue for life. The number of tests or biochemical screening is age specific and may include yearly serum calcium, parathyroid hormone, gastrin, glucagon, secretin, proinsulin, chromogranin A, prolactin, and IGF-1. Radiologic screening should include a magnetic resonance imaging of the brain and computed tomography of the abdomen every 1 to 3 years.
A germline activating mutation in the RET oncogene (a receptor tyrosine kinase) on chromosome 10q11.2 is responsible for the uncontrolled growth of cells in medullary thyroid carcinoma associated with MEN2A and MEN2B syndromes.[7,8,9]
A pentagastrin stimulation test can be used to detect the presence of medullary thyroid carcinoma in these patients, although management of patients is driven primarily by the results of genetic analysis for RET mutations.[13,14]
Guidelines for genetic testing of suspected patients with MEN2 syndrome and the correlations between the type of mutation and the risk levels of aggressiveness of medullary thyroid cancer have been published.[14,15]
The most-recent literature suggests that this entity should not be identified as a form of hereditary medullary thyroid carcinoma that is separate from MEN2A and MEN2B. Familial medullary thyroid carcinoma should be recognized as a variant of MEN2A, to include families with only medullary thyroid cancer who meet the original criteria for familial disease. The original criteria includes families of at least two generations with at least two, but less than ten, patients with RET germline mutations; small families in which two or fewer members in a single generation have germline RET mutations; and single individuals with a RET germline mutation.[14,16]
(Refer to the PDQ summary on Unusual Cancers of Childhood Treatment for information about the treatment of childhood MEN syndromes.)
The PDQ cancer information summaries are reviewed regularly and updated as new information becomes available. This section describes the latest changes made to this summary as of the date above.
Revised text in the Acute Myeloid Leukemia (AML) section to state that both RUNX1-RUNX1T1 and CBFB-MYH11 subtypes commonly show mutations in genes that activate receptor tyrosine kinase signaling; NRAS and KIT are the most commonly mutated genes for both subtypes. Also added text to state that KIT mutations may indicate increased risk of treatment failure for patients with core-binding factor AML, although the prognostic significance of KIT mutations may be dependent on the mutant-allele ratio and/or the specific type of mutation. KIT exon 17 mutations are enriched in patients with RUNX1-RUNX1T1 fusions when compared with patients with CBFB-MYH11 fusions (cited Faber et al. and Eisfeld et al. as references 137 and 138, respectively).
Revised text in the Acute Myeloid Leukemia (AML) section to state that cases with CBFB-MYH11 and cases with RUNX1-RUNX1T1 have distinctive secondary mutations; CBFB-MYH11 secondary mutations are primarily restricted to genes that activate receptor tyrosine kinase signaling.
Central Nervous System Tumors
The Diffuse Midline Glioma, H3 K27M-Mutant (Including Diffuse Intrinsic Pontine Gliomas [DIPGs]) section was extensively revised.
Added text to the Central Nervous System (CNS) Atypical Teratoid/Rhabdoid Tumors (AT/RT) section to state that the 2016 World Health Organization (WHO) classification defines AT/RT by the presence of either SMARCB1 or SMARCA4 alterations. Tumors with histological features of AT/RT that lack these genomic alterations are termed CNS embryonal tumor with rhabdoid features (cited Louis et al. as reference 1).
Added text to the Central Nervous System (CNS) Atypical Teratoid/Rhabdoid Tumors (AT/RT) section to state that for patients with AT/RT TYR, the mean overall survival (OS) is 37 months. Cribriform neuroepithelial tumor is a brain cancer that also presents in young children and has genomic and epigenomic characteristics that are very similar to AT/RT TYR (cited Johann et al. as reference 59).
Added text to the Central Nervous System (CNS) Atypical Teratoid/Rhabdoid Tumors (AT/RT) section to state that for patients with AT/RT SHH, the mean OS is 16 months.
Added text to the Central Nervous System (CNS) Atypical Teratoid/Rhabdoid Tumors (AT/RT) section to state that for patients with AT/RT MYC, the mean OS is 13 months.
Added text to the Ependymomas section to state that the 2016 WHO classification has accepted ependymoma, RELA fusion-positive, as a distinct diagnostic entity.
Added text to the Ewing Sarcoma section to state that a detailed analysis of 85 patients with small round blue cell tumors that were negative for EWSR1 rearrangement by fluorescence in situ hybridization (FISH) with an EWSR1 break-apart probe identified eight patients with FUS rearrangements. Four patients who had EWSR1-ERG fusions were not detected by FISH with an EWSR1 break-apart probe. The authors do not recommend relying solely on EWSR1 break-apart probes for analyzing small round blue cell tumors with strong immunohistochemical positivity for CD99 (cited Chen et al. as reference 26).
Langerhans Cell Histiocytosis
Added text to the Langerhans Cell Histiocytosis section to state that in-frame BRAF deletions and in-frame FAM73A-BRAF fusions have occurred in the group of BRAF V600E and MAP2K1 mutation-negative cases (cited Chakraborty et al. as reference 12).
Added text to the Neuroblastoma section about subclonal ALK mutations in neuroblastoma (cited Bellini et al. as reference 21).
This summary is written and maintained by the PDQ Pediatric Treatment Editorial Board, which is editorially independent of NCI. The summary reflects an independent review of the literature and does not represent a policy statement of NCI or NIH. More information about summary policies and the role of the PDQ Editorial Boards in maintaining the PDQ summaries can be found on the About This PDQ Summary and PDQ® - NCI's Comprehensive Cancer Database pages.
Purpose of This Summary
This PDQ cancer information summary for health professionals provides comprehensive, peer-reviewed, evidence-based information about the genomics of childhood cancer. It is intended as a resource to inform and assist clinicians who care for cancer patients. It does not provide formal guidelines or recommendations for making health care decisions.
Reviewers and Updates
This summary is reviewed regularly and updated as necessary by the PDQ Pediatric Treatment Editorial Board, which is editorially independent of the National Cancer Institute (NCI). The summary reflects an independent review of the literature and does not represent a policy statement of NCI or the National Institutes of Health (NIH).
Board members review recently published articles each month to determine whether an article should:
Changes to the summaries are made through a consensus process in which Board members evaluate the strength of the evidence in the published articles and determine how the article should be included in the summary.
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Levels of Evidence
Some of the reference citations in this summary are accompanied by a level-of-evidence designation. These designations are intended to help readers assess the strength of the evidence supporting the use of specific interventions or approaches. The PDQ Pediatric Treatment Editorial Board uses a formal evidence ranking system in developing its level-of-evidence designations.
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The preferred citation for this PDQ summary is:
PDQ® Pediatric Treatment Editorial Board. PDQ Childhood Cancer Genomics. Bethesda, MD: National Cancer Institute. Updated <MM/DD/YYYY>. Available at: https://www.cancer.gov/types/childhood-cancers/pediatric-genomics-hp-pdq. Accessed <MM/DD/YYYY>. [PMID: 27466641]
Images in this summary are used with permission of the author(s), artist, and/or publisher for use within the PDQ summaries only. Permission to use images outside the context of PDQ information must be obtained from the owner(s) and cannot be granted by the National Cancer Institute. Information about using the illustrations in this summary, along with many other cancer-related images, is available in Visuals Online, a collection of over 2,000 scientific images.
Based on the strength of the available evidence, treatment options may be described as either "standard" or "under clinical evaluation." These classifications should not be used as a basis for insurance reimbursement determinations. More information on insurance coverage is available on Cancer.gov on the Managing Cancer Care page.
More information about contacting us or receiving help with the Cancer.gov website can be found on our Contact Us for Help page. Questions can also be submitted to Cancer.gov through the website's Email Us.
Last Revised: 2017-04-25
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