A single BRCA2 BRC repeat supports viability while multiple repeats ensure resilience under stress.

BRC repeats are integral to BRCA2 function and mediate RAD51 loading during homologous recombination (HR). Their number varies across species, but mammals, including mice and humans, harbor eight repeats. Prior studies have suggested functional redundancy among these repeats, but the biological significance of maintaining multiple repeats remains unresolved. Here, we demonstrate that the presence of a single BRC repeat, either BRC2 or BRC4, is sufficient for mouse embryonic stem cell (mESC) viability, RAD51 loading, PARP inhibitor resistance and protection of stalled replication forks. Consistent with these findings, we show that knock-in mice with a single BRC repeat 2 or 4 are viable and exhibit normal growth and fertility. In contrast, embryonic fibroblasts from these mice display genomic instability and impaired RAD51 recruitment. Notably, this defect is rescued under low oxygen culture conditions, which mimics physoxic levels, whereas exposure to oxidative stress impairs RAD51 recruitment in mESCs harboring a single BRC repeat. Together, our findings indicate that while a single BRC repeat is sufficient under physiological conditions, the evolutionary retention of multiple BRC repeats likely ensures robust genome stability under extreme oxidative stress.