Defining germline genetics of germ cell tumor: Implications for genetic testing and clinical management.

5007 Background: Epidemiologic studies on germ cell tumors (GCT) in men have shown GCT has a high heritability. Yet, the genetic mechanisms underlying development of GCT in men remain unknown. We sought to determine the prevalence of pathogenic or likely pathogenic (P/LP) germline variants in cancer predisposition genes in men with GCT and identify clinical and pathologic factors associated with the presence of P/LP variants. Methods: We retrospectively identified men with testicular (T) or mediastinal (M) GCT who underwent matched tumor--germline sequencing of ≥ 310 genes as part of an institutional genomic profiling initiative from 04/2015 to 12/2020. Presence of germline variants in clinically actionable genes associated with hereditary cancer syndromes were analyzed. Clinicopathologic characteristics, including primary site, histology and family history were assessed by the presence of P/LP variants and compared using Fisher’s exact test. Results: The study included 480 men, 70 (15%) with M-GCT and 410 (85%) with T-GCT. The median age of diagnosis was 30 years (interquartile range 24 – 39). A total of 81% had non-seminoma; 5% had localized disease; 46%, 14%, and 34% had good, intermediate, and poor risk advanced disease, respectively; 6% had family history of GCT. Among men with T-GCT, 4% had bilateral T-GCT. P/LP variants in clinically actionable genes were identified in 58 (12%) men in the entire cohort. Men with M-GCT had higher frequency of P/LP variants compared to those with T-GCT (15/70 [21%] vs 43/410 [10%], p = 0.02). A total of 7 (10%) men with M-GCT and 20 (5%) with T-GCT had P/LP variants in moderate or high penetrance cancer predisposition genes (p = 0.09). Most P/LP variants in both M-GCT (11/15, 73%) and T-GCT (33/43, 77%) were in genes involved in DNA damage repair pathways. The spectrum of moderate or high penetrance DNA damage repair genes identified is shown in the Table 1. Family history and histology (seminoma vs. non-seminoma) were not associated with the presence of a P/LP variants in the overall cohort (p = 0.8 and 0.7, respectively). Bilateral disease was not associated with the presence of P/LP variants in men with T-GCT (p = 0.4). Conclusions: Approximately 1 in 5 men with M-GCT carried a P/LP variant in a clinically actionable cancer susceptibility gene and 10% had a P/LP variant in a high-risk gene such as TP53 and PMS2. As men with GCT tend to develop disease in early adulthood, identification of germline P/LP variant has significant implications for enhanced cancer screening and surveillance, cascade testing for at-risk family members, and potential preconception genetic counseling. [Table: see text]