Reduced penetrance BRCA1 and BRCA2 pathogenic variants in clinical germline genetic testing.

Prior studies have suggested the existence of reduced penetrance pathogenic variants (RPPVs) in BRCA1 and BRCA2 (BRCA) which pose challenges for patient counseling and care. Here, we sought to establish RPPVs as a new category of variants. Candidate BRCA RPPVs provided by two large clinical diagnostic laboratories were compiled to identify those with the highest likelihood of being a RPPV, based on concordant interpretations. Sixteen concordant candidate BRCA RPPVs across both laboratories were systematically assessed. RPPVs included missense, splice site, and frameshift variants. Our study establishes RPPVs as a new class of variants imparting a moderately increased risk of breast cancer, which impacts risk-informed cancer prevention strategies, and provides a framework to standardize interpretation and reporting of BRCA RPPVs. Further work to define clinically meaningful risk thresholds and categories for reporting BRCA RPPVs is needed to personalize cancer risks in conjunction with other risk factors.