Replication protein A protects lagging strand gaps, restricting PARP inhibitor-induced synthetic lethality in BRCA1-deficient tumors.

Replication Protein A (RPA) is a key single-stranded DNA (ssDNA)-binding protein essential for maintaining genome integrity during DNA replication, repair, and recombination. In this study, we elucidate the mechanisms by which a small-molecule RPA inhibitor induces functional RPA exhaustion. Using non-small cell lung cancer and BRCA1-deficient breast and ovarian cancer models, we demonstrate that RPA is critical for sustaining replication fork speed under normal conditions and for facilitating replication restart following fork stalling. Disruption of replication fork-associated processes, including Okazaki fragment processing and ssDNA gap suppression, increases cellular dependence on RPA for ssDNA protection. Chemical inhibition of RPA exacerbates genome instability in BRCA1-deficient cancer models treated with PARP inhibitors, leading to loss of ssDNA gap protection, chromosome shattering, and ultimately, cell death. Combining genetic and pharmacologic approaches to induce ssDNA accumulation alongside RPA exhaustion in vivo shows therapeutic efficacy in BRCA1-deficient breast cancer. These findings provide a mechanistic framework for targeting RPA-mediated ssDNA protection as a therapeutic strategy in cancers experiencing endogenous or therapy-induced replication stress.