(P 224) COMPREHENSIVE EVALUATION OF DNA DAMAGE RESPONSE PATHWAY ALTERATIONS AND ASSOCIATED IMMUNOTHERAPY-RESPONSE BIOMARKERS IN SARCOMAS

Objective: Alterations in DNA damage response (DDR) pathways contribute to genomic instability and malignant progression and have been shown to be of clinical significance in several carcinomas and solid tumors. Genomic instability has recently gained interest because it can result in the creation of neoantigens that may be responsive to immunotherapies. Although some studies have investigated this relationship in certain types of tumors, information regarding the role of DDR pathway alterations and related immunotherapy-response biomarkers in sarcoma is limited.

Methods: Sarcoma patient samples (N = 5309), representing 38 pediatric and adult histologic subtypes, underwent NGS of DNA (592 gene panel or whole exome) and RNA (whole transcriptome sequencing, N = 3612) at a CLIA-certified laboratory (Caris Life Sciences, Phoenix, AZ). DDR pathway alterations included pathogenic/likely pathogenic variants detected in any of 51 DDR pathway-associated genes (Table 2). High tumor mutational burden (TMB-H) was defined as 10 mut/Mb. Immunohistochemistry (IHC) was performed for PD-L1 (SP142; 2+|5% = positive). The gene expression profiles of patient samples that underwent whole transcriptome sequencing were further analyzed for immunotherapy-related biomarker associations with DDR pathway alteration, including analysis of a gene expression signature that predicts response to PD1/PDL-1 blockade (T cell-inflamed score) and quantification of the relative abundance of immune and stromal cell populations (MCP-counter).

Results: Overall, DDR pathway alterations were detected in 842 (15.9%) sarcoma samples, with varying prevalences (range 0-39%) across sarcoma subtypes (Table 1). T cell-inflamed scores were significantly increased in DDR pathway-altered synovial sarcoma (SYNS; +0.15 arbitrary units, AU) and angiosarcoma (ANGS; +0.10 AU), with numerically increased median scores observed for several other subtypes, which correlated with increased abundance of pro-inflammatory cell populations in subtypes with higher T cell-inflamed scores (Figure 1). DDR pathway alterations were associated with increased frequency of PD-L1+ IHC overall (+7.3%) and among rhabdomyosarcoma (RMS; +39.1%), malignant phyllodes tumor (PT; +37%), undifferentiated uterine sarcoma (UUS; +27.9%), and ‘Sarcoma, NOS’ (SARCNOS; +9.3%) subtypes (all P< 0.05). The frequency of dMMR/MSI-High and TMB-High was also increased in DDR pathway-altered sarcoma compared to DDR pathway-WT overall (+4.4% and +3.7%, respectively), as well as in high-grade endometrial stromal sarcoma (HGESS; +13.0% and +27.2%), UUS (+11.1% and +44.4%), RMS (+19.1% and +17.3%), uterine carcinosarcoma (UCS; +26.9% and +41.2%), leiomyosarcoma (LMS; +3.8% and +4.6%), and SARCNOS (+4.9% and +11.9%) subtypes (all P< 0.05). Additional subtypes with an increased frequency of dMMR/MSI-High among DDR pathway-altered tumors included ‘Round cell sarcoma, NOS’ (RCSNOS; +33.3%), spindle cell sarcoma (SCSARC; +9.5%), osteosarcoma (OS; +8.3%), pleomorphic sarcoma (PLSARC; +8.3%), and gastrointestinal stromal tumor (GIST; +1.5%), while TMB-high frequencies were increased in DDR pathway-altered tumors among clear cell sarcoma (CCS; +95.8%) and ‘Uterine sarcoma, Other’ (OUSARC; +25.8% TMB-High) subtypes (all P< 0.05).

Conclusion: DDR pathway alterations are present in numerous histologic subtypes of sarcoma. In many subtypes, DDR pathway-altered tumors were found to have increased rates of PD-L1+, dMMR/MSI-high, and TMB-high biomarkers commonly used to identify patients who may benefit from immunotherapy. Further studies to validate these associations are needed associations and could lead to a novel clinical trial exploring the use of immune checkpoint inhibitors in DDR-mutated sarcoma.