(P 05) GENETIC ANALYSIS OF ADULT RMS: ARE THERE MOLECULAR ALTERATIONS THAT DRIVE INFERIOR OUTCOMES?

Objective: Rhabdomyosarcoma (RMS) is the most common type of soft tissue sarcoma in the pediatric population, and much rarer in adults. Alveolar RMS (ARMS) and embryonal RMS (ERMS) defined by copy number, structural, and single nucleotide variants are the most common entities in pediatrics with pleomorphic RMS (PRMS) occurring nearly exclusively in adults, while spindle/sclerosing RMS (SRMS) is seen across all ages. Despite multimodal therapy, adults with RMS tend to have a poor outcome. Furthermore, adults with ERMS/ARMS appear to have worse outcomes than children with the ‘same’ disease. Thus, the aim of our study was 1) to define genetic alterations of an adult RMS cohort through whole exome sequencing and 2) to compare the genetic landscape of adult RMS to pediatric RMS.

Methods: Sample collection was approved by the institutional review boards of University Health Network (UHN) and Sinai Health (SH). 136 patients with RMS (Toronto Adult RMS cohort) from Jan 1980 to May 2022 were identified and data was extracted from the CanSaRCC (Canadian Sarcoma Research and Clinical Collaboration) database. An expert pathologist in soft tissue sarcoma reviewed slides and diagnostic reports from all available cases; unavailable cases were classified as RMS, not otherwise specified (NOS). Patients were excluded if reclassified as an alternative diagnosis, or the patient was < 17 years of age at diagnosis. Unstained slides from 41 patients were available for DNA extraction and molecular profiling that included whole exome sequencing (WES) using the Agilent SureSelect v8 capture and Illumina NovoSeq 6000 platform. Variants of interest in a curated set of genes were identified using the DRAGEN analysis pipeline and a variety of in house algorithms. Description of the frequency of alterations between the adult cohort and pediatric population were made by comparison with the recently published international pediatric cohort. Patients were stratified by RMS subtype and baseline characteristics compared using Kruskal-Wallis tests for continuous variables and chi-squared tests for proportions. Time to progression (TTP), defined as time from diagnosis to disease relapse or progression was estimated using Kaplan-Meier survival methods. Patients were censored at death or time of last follow-up.

Results: Of 136 patients, 98 patients had complete clinical data with known mutational status and were stratified into genomic groups: ARMS (n=30, 30.6%), ERMS (n=21, 21.4%), PRMS (n=22, 22.5%), SRMS (n=4, 4.1%), and RMS, NOS (n=21, 21.4%) (Table 1). Groups differed in age, primary site and extent of disease (p< 0.007) (Table 1). 2-year TTP for patients, indicating free from progression or relapse, with ARMS, ERMS, PRMS, SRMS, and RMS, NOS was 25.2% (95%CI: 12.9-49.4%), 59.4% (95%CI: 41.0%-85.9%), 38.1% (95%CI: 21.1%-68.7%), 25.0% (95%CI: 4.6%-100%), and 43.9% (95%CI: 26.4%-72.8%), respectively (log-rank p=0.17, Figure 1). Genomic alterations varied by subtype (Figure 2). As expected, FOXO1 fusion genes were present in all ARMS cases and focal amplifications of CDK4 or MYCN were observed. Of 34 FOXO1- patients (15 ERMS, 13 PRMS, 6 SRMS), RAS isoforms were common in the ERMS, TP53/CDKN2A were frequent in PRMS and MYOD1 L122R point mutations were nearly universal in SRMS. Detailed copy number, clonal analysis and comparison to the pediatric cohort is underway.

Conclusion: Genomic analysis of RMS subtypes in adults confirm clinical disparities in presentation and outcome. Deeper analyses will inform exceptional cases within our cohort. Further characterization and comparison are very important to understand the RMS adult population and if poor outcomes are a result of tumor genomic factors or patient factors.

CTOS 2022 Adult RMS Figures.docx