RAD51C breast cancer-identified variants with attenuated homologous recombination function exhibit sensitivity to combined PARP1 and ATR inhibition

missense mutations identified in patients are variants of unknown significance. In addition to promoting HR, RAD51C also protects damaged replication forks. Here, we characterized 33 clinically identified breast cancer variants and assayed their ability to perform HR. We identified one HR-deficient variant and five variants with intermediate HR proficiency. While these variants have modestly attenuated HR, they misregulate replication fork progression and accumulate single-strand DNA gaps. We then assessed their sensitivity to PARP1 and ATR inhibition. While RAD51C variants with attenuated HR are largely resistant to monotherapy, cells expressing a subset of these RAD51C variants exhibit increased sensitivity to combination PARP1i and ATRi treatment, Saruparib and Ceralasertib, respectively. Collectively, we pinpoint the key functional regions of RAD51C and uncover a new C-terminal region proximal to the ATP-binding site in the folded RAD51C structure. Together, our findings suggest that variants with partial HR function can cause profound defects in the repair of replicative damage, and that this, in turn, may lead to distinct therapeutic responses.