BRCA1 (Breast Cancer 1, early onset) is linked to breast and ovarian cancer predisposition. Still, the risks conferred by a significant portion of BRCA1 variants identified in the population remains unknown. Most of these variants of uncertain significance are missense alterations. However, the functional implications of small in-frame deletions and/or insertions (indels) are also difficult to predict. Our group has previously evaluated the functional impact of 347 missense variants using an extensively validated transcriptional activity assay. Here we show a systematic assessment of 30 naturally occurring in-frame indels located at the C-terminal region of BRCA1. We identified positions sensitive and tolerant to alterations, expanding the knowledge of structural determinants of BRCA1 function. We further designed and assessed the impact of four single codon deletions in the tBRCT linker region and six nonsense variants at the C-terminus end of BRCA1. Amino acid substitutions, deletions or insertions in the disordered region do not significantly impact activity and are not likely to constitute pathogenic alleles. On the other hand, a sizeable fraction of in-frame indels at the BRCT domain significantly impact function. We then use a Bayesian integrative statistical model to derive the probability of pathogenicity for each variant. Our data highlights the importance of assessing the impact of small in-frame indels in BRCA1 to improve risk assessment and clinical decisions for carriers.
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