A Step Forward in Realizing the Promise of Genomic Medicine for Childhood Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a rare mesenchymal malignancy that is primarily a disease of children and adolescents with approximately 50% of patients being diagnosed in the first decade of life.¹ Clinicians recognized early the value of histologic classification dividing RMS broadly into embryonal rhabdomyosarcoma (ERMS) and alveolar rhabdomyosarcoma (ARMS) subtypes with the former having a more favorable prognosis.² The earliest known molecular finding in RMS was the loss of heterozygosity at chromosome 11p15.³ Soon after, almost 30 years ago, the discovery of the recurring translocations between chromosome 2 or 1 and chromosome 13 resulting in the fusion of the PAX3 or PAX7 gene with the FOXO1 (FKHR) gene became pathognomonic of ARMS as it was seen in the majority of cases.^4-6 In addition to loss of heterozygosity at 11p15 that was more frequently noted in ERMS, loss of imprinting at the same locus was noted in ARMS.⁷ Subsequently, a smaller subset of alveolar histology that did not harbor the FOXO1 fusion was found to have a gene expression profile and a more favorable prognosis similar to ERMS.^8,9 However, there was significant hesitancy for many years to classify RMS molecularly into two broad categories of FOXO1 fusion–positive (FP) or FOXO1 fusion–negative (FN) subgroups.¹⁰ It is relatively recent that sufficient data have been generated to broadly use this molecular classification together with Clinical Stage and Group.^11,12 Other molecular findings described in RMS include copy number variations including gene amplifications, and point mutations culminating in a landmark report of the genomic landscape of childhood RMS^13-15 and have been recently summarized.¹⁶ As such, ERMS may be considered a malignancy featured by point mutations and aneuploidy, whereas ARMS is a malignancy of gene fusions and amplifications. Pleomorphic rhabdomyosarcoma represents a third genomic form of RMS, with greater kinship to undifferentiated pleomorphic sarcoma than ERMS or ARMS.^17,18 Pleomorphic rhabdomyosarcoma is found nearly exclusively in adults and is not studied in pediatric clinical trials.

Over the past five decades, cooperative group trials in RMS using multiagent chemotherapy, surgery, and/or radiation therapy initially resulted in significant improvements in survival outcomes for patients with low-risk and intermediate-risk disease, whereas those with distant metastases continued to fare poorly; however, these outcomes have since plateaued.¹⁹ Given the young age of most patients with RMS and the use of radiation therapy for local tumor control, the burden of late effects of treatment in patients with RMS is immense.^20,21 Molecular targeted agents have only recently been used in clinical trials in combination with classic cytotoxic chemotherapy for poor prognosis patients,^22-24 and patient-specific strategies are yet to be incorporated. Traditional clinical and pathologic risk assignment results in a large majority of patients being lumped together in the Children's Oncology Group (COG) intermediate-risk group with some patient having a prognosis closer to low-risk patients and others closer to those with disseminated disease.²⁵ Recent analysis has helped further delineate risk by incorporation of fusion status together with clinicopathologic features.^26,27

In this issue, Shern et al²⁸ report on genomic findings from 641 clinically annotated RMS patient tumor samples collected on COG, MMT, and RMS2005 trials spanning a 22-year period and analyze survival outcomes to identify additional genetic risk factors beyond FOXO1 fusion status that can potentially further refine risk stratification. Approximately 20% of samples had no known driver mutation. TP53 mutations were found at a higher incidence than previously reported and portended a poorer prognosis in both FP and FN tumors. MYOD1 mutations were restricted to FN tumors, were not limited to spindle or sclerosing histology, and were associated with a dismal prognosis. The magnitude of effect on prognosis appears to be greater for the presence of a MYOD1 rather than a TP53 mutation in FN RMS. As expected, gene amplification events involving CDK4 and MYC were observed in FP tumors, whereas RAS pathway mutations were noted in more than 50% of FN tumors. 37% of FN tumors had more than one mutation and had a poorer prognosis compared with those with one or no candidate gene mutation in the COG cohort. The authors also propose a risk stratification incorporating MYOD1 and TP53 mutation status where any MYOD1-mutant tumor and TP53-mutant tumor irrespective of clinical features are classified as very or ultra-high risk (expected long-term failure-free survival [FFS] < 20%); stage 4, clinical group IV FN tumor with more than one metastatic site, any stage 4, clinical group IV FP TP53 wild-type tumor, or any TP53-mutant tumor that is not low risk or very high risk as high risk (expected FFS 20 to < 40%); any stage, clinical group III nonorbit primary, FN stage 3, clinical group I or II, FN stage 4, clinical group IV with one metastatic site, FN low-risk TP53-mutant FN tumors and FP stage 1-3, clinical group I-III tumors that are TP53 wild-type as intermediate risk (FFS 60%-75%); and stage 1-2, clinical group I-II and stage 1, clinical group III orbit primary FN tumors as low risk (FFS > 85%). This collaborative effort between the COG and researchers in the United Kingdom is to be applauded as it allows for further refinement of risk stratification for RMS, hopefully leading to more precision in determining risk groups allowing for potential opportunities to affect outcomes by improving survival, decreasing morbidity, and considering targeted therapies that are more patient specific.

These data have several limitations. They were generated from 470 (14.7%) of 3,184 unique patients treated on seven therapeutic trials and enhanced by 171 samples from patients enrolled on D9902, a biology study where limited therapeutic information was available. This data set should therefore be considered a convenience cohort, the deficiencies of which have been eloquently discussed previously.²⁹ This is likely why mutations reported previously in RMS such as MTOR, ALK, SOS2, PDGFRA, BRAF, AKT, and others were not detected in this study. It may also be the reason why more than one mutation was not associated with inferior outcomes in the United Kingdom cohort. Therapy delivered is a critical variable in determining outcome. During the time covered by the clinical trials that provided the clinically annotated tumor samples, similar risk groups had significantly different FFS particularly in ERMS/FN RMS with the caveat that the comparisons were with historical controls.^30-32 ERMS and FN RMS appear to benefit from alkylator intensification by dose or duration,^33-35 whereas therapy seems to be a less important variable in FP RMS.²⁷ Paired germline testing was not performed, and therefore, the contribution of germline mutations in TP53 and other known cancer predisposition genes to outcome could not be assessed. As such, these data need to be validated prospectively and is planned for the next COG low-risk RMS trial.

Nevertheless, this report opens many opportunities in addition to more precise risk stratification to be considered in improving the outcomes of patients with RMS. Given that only approximately 350 patients are diagnosed with RMS each year in the United States (US) and a similar number of patients in Western Europe, it is going to be almost impossible to conduct traditional randomized phase III clinical trials with cytotoxic chemotherapy given the number of patients needed and the duration to accrue them. Furthermore, there have been only four positive randomized clinical trials to date conducted in RMS.^24,36-38 The recently formed International Soft-Tissue Sarcoma Database Consortium (INSTRuCT), a collaboration of the COG Soft-Tissue Sarcoma Committee, the European Pediatric Soft-Tissue Sarcoma Study Group, and the Cooperative Weichteilsarkom Studiengruppe ,will help immensely in the harmonization of data collection and its prospective implementation across cooperative groups for RMS clinical trials. This will result in more robust data sets to analyze for outcomes across clinical trials including providing opportunities to investigate RMS gene signatures,^39,40 allowing for meaningful tools to risk stratify or assign a specific treatment.

Compelling questions to be addressed in RMS include the possibility of minimizing late effects of treatment, targeting the FOXO1 fusion, targeting other genetic and epigenetic aberrations, and addressing germline mutations and tumor heterogeneity, all this while at least maintaining outcomes achieved by surgery and/or radiation and multiagent chemotherapy. There are several examples in medical oncology where molecular targeted therapy has been shown to be superior to chemotherapy including targeting epidermal growth factor receptor with gefitinib in pulmonary adenocarcinoma⁴¹ and ALK with crizotinib in anaplastic lymphoma kinase–positive lung cancer,⁴² encorafenib, binimetinib, and cetuximab in BRAF V600E–mutated colorectal cancer,⁴³ and alpelisib in PIK3CA-mutated, hormone receptor–positive advanced breast cancer.⁴⁴ This was achievable in part given the low progression-free survival on chemotherapy alone in these cancers. It is unclear whether simple intensification of chemotherapy may improve outcomes for patients with MYOD1- or TP53-mutated RMS. It is highly unlikely that a single or dual molecular targeted agent will prove superior to standard chemotherapy and radiation in RMS. Significant response rates and improved progression-free survival have been reported in cancers that are driven by fusions, notably tropomyosin receptor kinase fusions in both adults and children,^45-47 and rearranged-during-transfection alterations in adults.^48,49 However, targeting the FOXO1 fusion in RMS has been elusive thus far. A recent publication describing germline cancer predisposition variants in 615 patients with pediatric RMS found 7.3% to harbor a molecular finding consistent with a cancer predisposition including TP53, NF-1, and HRAS mutations, which were also reported as somatic mutations in RMS. The use of poly ADP-ribose polymerase inhibition has been shown to be beneficial in metastatic human epidermal growth factor receptor 2–negative breast cancer patients with BRCA mutations,⁵⁰ raising the possibility of harnessing germline mutations to inform treatment of RMS patients with cancer predisposition. Targeting the BCR-ABL gene fusion in childhood Philadelphia chromosome–positive acute lymphoblastic anemia with imatinib together with standard cytotoxic chemotherapy significantly improved the survival outcomes of these poor prognosis patients.⁵¹ A similar approach targeting a genetic alteration in RMS together with standard treatment may lead to improved outcomes. Another avenue to pursue could be to target more than one mutation when indicated. For example, combination therapy with trametinib and alpelisib may be considered in a patient whose RMS harbors an RAS and PIK3CA mutation. Finally, limiting the morbidity of late effects in patients with RMS is still worthwhile. Observation following resection of stage 1 testicular germ cell tumors is standard of care in pediatrics with a 100% salvage rate with standard chemotherapy, thus eliminating the risk of ototoxicity, pulmonary toxicity, and second malignancy in most of the stage 1 patients. Although chemotherapy is universally recommended for all patients with RMS at this time, a future consideration within the context of a clinical trial may be observation for a selected group of patients with RMS age < 10 years with stage 1, clinical group I small primary FN RMS tumors that do not harbor a deleterious genetic mutation.

Given the limited number of patients diagnosed each year with RMS, the report by Shern et al²⁸ paves the way for a possible international collaborative umbrella clinical trial in newly diagnosed patients with RMS, seeking to improve outcomes by tailoring therapy according to a genomic classification and targeting molecular alterations as feasible to improve the overall survival—a definite step forward in realizing the promise of genomic medicine for childhood RMS.

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A Step Forward in Realizing the Promise of Genomic Medicine for Childhood Rhabdomyosarcoma

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