Patient-derived separation-of-function mutations reveal specific role for BRCA1 coiled coil domain in genome stability.

Specific inherited mutations of BRCA1 compromise homologous recombination repair and increase the risk of cancer. Here, we employ a panel of isogenic chicken DT40 cell lines for the comparative study of the main BRCA1 protein domains and of common BRCA1 founder mutations in genome stability and DNA repair. Whole genome sequencing of cell clones demonstrates that the L1407P-equivalent coiled coil domain variant is a separation-of-function mutant with an increased rate of base substitution mutagenesis but not of deletions or large-scale genomic instability, reflecting its proficiency in repairing DNA double strand breaks and promoting sister chromatid exchanges. The BRCA1 coiled coil domain mediates interaction with PALB2, and PALB2 coiled coil domain mutations also primarily affect base substitution mutagenesis. In contrast, truncating and missense mutations in the RING and BRCT domains affect all functions of BRCA1, inducing larger deletions and rearrangements typical of BRCA1 loss. PARP inhibitor sensitivity is marginal in the coiled coil domain mutants, instead it correlates with deletion mutagenesis, degradation of stalled replication forks, and impaired RAD51 loading following topoisomerase I inhibition. Our results suggest a specific function for the BRCA1-PALB2 interaction in promoting error-free DNA damage bypass as an alternative to mutagenic translesion DNA synthesis.