Combining Homologous Recombination and Phosphopeptide-binding Data to Predict the Impact of <i>BRCA1</i> BRCT Variants on Cancer Risk

Abstract

<i>BRCA1</i> mutations have been identified that increase the risk of developing hereditary breast and ovarian cancers. Genetic screening is now offered to patients with a family history of cancer, to adapt their treatment and the management of their relatives. However, a large number of <i>BRCA1</i> variants of uncertain significance (VUS) are detected. To better understand the significance of these variants, a high-throughput structural and functional analysis was performed on a large set of <i>BRCA1</i> VUS. Information on both cellular localization and homology-directed DNA repair (HR) capacity was obtained for 78 BRCT missense variants in the UMD-BRCA1 database and measurement of the structural stability and phosphopeptide-binding capacities was performed for 42 mutated BRCT domains. This extensive and systematic analysis revealed that most characterized causal variants affect BRCT-domain solubility in bacteria and all impair BRCA1 HR activity in cells. Furthermore, binding to a set of 5 different phosphopeptides was tested: all causal variants showed phosphopeptide-binding defects and no neutral variant showed such defects. A classification is presented on the basis of mutated BRCT domain solubility, phosphopeptide-binding properties, and VUS HR capacity. These data suggest that HR-defective variants, which present, in addition, BRCT domains either insoluble in bacteria or defective for phosphopeptide binding, lead to an increased cancer risk. Furthermore, the data suggest that variants with a WT HR activity and whose BRCT domains bind with a WT affinity to the 5 phosphopeptides are neutral. The case of variants with WT HR activity and defective phosphopeptide binding should be further characterized, as this last functional defect might be sufficient <i>per se</i> to lead to tumorigenesis. IMPLICATIONS: The analysis of the current study on BRCA1 structural and functional defects on cancer risk and classification presented may improve clinical interpretation and therapeutic selection.