Accurate classification of BRCA1 variants with saturation genome editing

Variants of uncertain significance (VUS) fundamentally limit the clinical utility of genetic information. The challenge they pose is epitomized by BRCA1, a tumor suppressor in which germline loss-of-function variants predispose women to breast and ovarian cancer. Although BRCA1 has been sequenced in millions of women, the risk associated with most newly observed variants cannot be definitively assigned. Here, we employ saturation genome editing to assay 96.5% of all possible single nucleotide variants (SNVs) in 13 exons encoding functionally critical domains of BRCA1. Functional effects for nearly 4,000 SNVs are bimodally distributed and almost perfectly concordant with established assessments of pathogenicity. Over 400 non-functional missense SNVs are identified, as well as ~300 SNVs that disrupt expression. We predict that these results will be immediately useful for clinical interpretation of BRCA1 variants, and that this paradigm can be extended to overcome the challenge of VUS in additional clinically actionable genes.