Evaluation of ctDNA in patients with CRPC with pathogenic germline mutations in BRCA2.

253 Background: Approximately 3-5% of advanced prostate cancer patients have pathogenic BRCA2 mutations in germline tests. In this study, we examine the relationship between pathogenic germline BRCA2 mutation and somatic changes in ctDNA. Methods: Germline screenings were performed by Invitae multi-cancer gene panel which includes 50-84 genes. ctDNA alterations were detected by Guardant 360 assays which report somatic changes in 70-83 genes. All ctDNA samples were collected in post-abiraterone and/or enzalutamide (in CRPC patients). Any pathogenic/likely pathogenic somatic alterations in the ctDNAs with more than 0.1% of allelic fraction were included in this cohort. The type of mutation detected in ctDNA was also assessed (truncating, point, etc.). Statistical significance for comparison is calculated with Fischer Exact Probablity Test and Chi-Square Test. Results: A total of 11 patients had germline BRCA2 pathogenic mutations and ctDNA assays; 267 patients had no germline DNA pathogenic alterations and ctDNA assays. Compared to germline normal patients, the germline BRCA2 mutations were less likely to have AR alterations on ctDNA (OR=0.2133, 95% C.I. [0.087, 0.525], p-value = 0.0003). BRCA2 germline positive patients were also more likely to have a mutated BRCA1, BRCA2, and TP53 ctDNA (OR=7.899, 95% C.I. [1.2745, 48.9548], p=0.055), (OR=7.899, 95% C.I. [1.7529, 16.059], p=0.008), and (OR=6.442, 95% C.I. [2.449, 16.946], p=0.00001), respectively. All other ctDNA assessed genes were mutated at a similar frequency between germline BRCA2 mutated and “normal” germline patients. BRCA2 germline mutations patients are less likely to have copy number alterations (CNVs) (OR=0.3992, 95% C.I. [0.2168, 0.7352], p=0.0031) and more likely to have frameshift mutations (OR=2.3182, 95% C.I. [1.169, 4.5972], p=0.0183). Conclusions: The ctDNA testing in this CRPC population (after abiraterone and/or enzalutamide) was less likely to find AR alterations and more likely to find pathogenic mutations for BRCA1, BRCA2, and TP53 in BRCA2 germline positive patients. In addition, BRCA2 germline positive CRPC patients were more likely to have frameshift mutations and less likely to have CNVs than those with an intact germline. Characterizing the mutational landscape in BRCA2 germline mutated patients may help to better define underlying disease biology. Those with BRCA mutated germline may be less likely to have AR driven tumors. More study is needed to better understand patients with underlying DNA repair defects.