Co-occurring rare germline DNA repair gene variants in BRCA1/BRCA2 implicated hereditary breast cancer families.

As gene-panels expand to include candidate breast cancer predisposing genes (CPGs) involved in diverse DNA repair pathways, we investigated an index breast cancer (BC) case from 56 BRCA1/BRCA2 implicated high-risk hereditary BC families using gene-based approach for co-occurring, rare germline variants predicted to be damaging and clinically relevant in 276 DNA repair genes (DRGs). Using whole exome sequencing analyses, we identified a total of 287 variants in 55% of DRGs, of which 24 loss-of-function and 36 other predicted damaging variants were identified in 72% and 76% of BRCA1 and BRCA2 implicated cases, respectively. We also identified 60 variants of clinical interest in various known CPGs, including those involved in risk to other cancers, in 72% of BRCA1 and 60% of BRCA2 cases. The number of variants predicted to be deleterious in diverse DRGs raises questions about the interpretability of findings as panel testing expands beyond clinically established breast CPGs.