Pathogenic Germline Variants in Non-Brca1/2 Homologous Recombination Genes in Ovarian Cancer: Analysis of Tumor Phenotype and Survival.

5565 Background: BRCA1/2-associated ovarian cancer (OC) exhibits homologous recombination (HR)-deficiency (HRD) and a better prognosis than BRCA1/2-wildtype (WT) OC. However, it is unclear if patients with germline pathogenic variants (gPV) in other HR-related genes have a similar tumor phenotype. We sought to define OC molecular features from patients with gPV in other HR genes and analyze survival compared to patients with BRCA1/2 and WT OC. Methods: We identified patients with OC treated at our institution who underwent tumor-normal sequencing using MSK-IMPACT targeting 341-505 cancer-related genes with germline analysis of ≥76 genes from 7/2015-12/2020. Biallelic inactivation was inferred via assessments of loss of heterozygosity (LOH) at gPV in non- BRCA1/2 HR-related genes ( ATM, BARD1, BRIP1, FANCA, FANCC, NBN, PALB2, RAD50, RAD51B, RAD51C, and RAD51D). Clinical characteristics were compared to patients with BRCA1/2-associated and WT OC using non-parametric tests. Progression-free (PFS) and overall-survival (OS) were calculated from date of pathologic diagnosis using the Kaplan-Meier method. Left truncation at date of MSK-IMPACT consent was applied. Whole-exome sequencing (WES) was performed in a subset of OCs. Results: Of 882 patients with OC, 56 (6.3%) had gPV in non- BRCA1/2 HR-related genes compared to 95 (10.8%) patients with BRCA1-associated OC (58 germline, 37 somatic) and 59 (6.7%) patients with BRCA2-associated OC (40 germline, 19 somatic). Patients with a deleterious genetic alteration were diagnosed with OC at younger age and more likely to have high-grade serous histology compared to the WT group (p < 0.01). High rates of biallelic alterations were observed amongst gPV in BRIP1 (11/13), PALB2 (3/4), RAD51B (3/4), RAD51C (3/4), and RAD51D (8/10), and WES was performed in a subset (n = 27) of tumors from these patients with adequate tumor purity (>30%). We observed a higher tumor mutational burden (TMB), median 2.5 (1.1-6.0) vs. 1.2 (0.6-2.6) mut/Mb, and enrichment of HRD mutational signatures in tumors associated with PALB2 and RAD51B/C/D compared with BRIP1 (p < 0.01), although markers of telomeric-allelic imbalance (TAI), large-scale state transitions (LST) and fraction of genome altered (FGA) were similar. PFS and OS varied by gene group with best survival in BRCA1/2-associated OC, even after adjustment for clinical covariates in multivariable models (p < 0.01). Although we observed heterogeneity in PFS and OS for those with gPV in other HR-related genes by biallelic status and HRD phenotype, none had significantly better survival than those with WT OC. Conclusions: OCs associated with gPV in non- BRCA1/2 HR-related genes represent a heterogenous group. OCs in those with gPV in PALB2 and RAD51B/C/D preferentially harbored biallelic alterations and displayed an HRD phenotype.