Clinical characterization and therapeutic outcomes of patients (pts) with colorectal cancer (CRC) harboring somatic BRCA1/2 mutations.

Abstract

134 Background: Somatic BRCA1/2 mutations in BRCA-associated cancers increase therapeutic sensitivity to platinum-based chemotherapies and PARP inhibitors, but the implications of BRCA mutations in non- BRCA-associated cancers such as CRC is unknown and hypothesized to be shaped by tumor lineage. We thus conducted a retrospective analysis of outcomes with platinum-based chemotherapy and PARP inhibition in pts with CRC, where the estimated prevalence of BRCA1/2 alterations is 3-4%. Methods: Pts with blood or tissue next generation sequencing (NGS) positive for a somatic BRCA1/2 variant and a diagnosis of CRC were identified from a single institution database. The pathogenicity of each mutation was annotated using public genomic databases (e.g. ClinVar). Pt characteristics were compared using Fisher’s exact test. Log-rank test was used for comparison of survival distribution among groups. Results: 8,258 NGS reports with a BRCA1/2 alteration from 1760 cancer pts were identified of which 189 (11%) had a diagnosis of CRC. 54/189 CRC pts (29%) had ≥1 pathogenic BRCA1/2 variant, 19 pts (10%) had ≥1 benign variant, and 134 pts (69%) had 1 ≥ variant of unknown significance (VUS); 26 pts (14%) had more than one BRCA1/2 variant. Pts with ≥1 pathogenic mutation (n=54) were more likely to have an inactivating DNA polymerase epsilon (POLE) mutation versus pts with only benign/VUS BRCA1/2 variants (n=135) (20% vs 6%, p=0.004) while prevalence of microsatellite instability (MSI-H) (19% vs 13%, p=0.37) and Ras/Raf (53% vs 53%, p=1.00) were similar between groups. Median progression free survival (PFS) among 115 CRC pts (unselected for BRCA1/2 pathogenicity) who received an oxaliplatin-based regimen for advanced disease in the 1st line (n=52) was similar to the median PFS among pts who received an irinotecan-based regimen (n=63) (7.0 mths vs 7.0 mths, p=0.48). Median PFS with an oxaliplatin-based regimen in the 1st line among 52 pts with pathogenic BRCA1/2 variants (n=13) was 5.1 mths vs 7.4 mths among pts with benign/VUS BRCA1/2 variants (n=39) (p=0.398). There were no significant differences in median PFS among pts with a pathogenic vs benign/VUS BRCA1/2 variants when the analysis was restricted to pts with BRCA1/2 variants identified in tissue (p=0.76) and to pts without co-occurring driver mutations (Ras/Raf, MSI-H, and/or POLE) (p=0.1). Overall survival in patients with advanced disease was similar between pts with a pathogenic (n=52) vs benign/VUS (n=122) BRCA variants (37.1 mths vs 45.1 mths, p=0.14). 6 pts (4 with pathogenic BRCA1/2 and 3 with VUS) received a PARP inhibitor after a median of 3 lines of treatment and the best response was disease progression in 6 pts (100%). Conclusions: In the context of CRC, somatic BRCA1/2 mutations frequently co-occur with pathogenic POLE mutations, but do not appear to confer therapeutic benefit to platinum-based chemotherapy and PARP inhibitors.