Synergistic effects of PARP inhibitors by Schlafen 11 and BRCA2-deficiency through accumulation of single-strand DNA gaps behind a fork

Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPis) induce synthetic lethality in breast cancer gene (BRCA)-deficient tumors. Besides the original model proposed by accumulation of double-strand DNA breaks due to the impaired homologous recombination, accumulation of single-strand DNA (ssDNA) gaps due to impaired BRCA-mediated Okazaki fragment processing has emerged as an alternative mechanism of synthetic lethality. Accordingly, PARPis induce ssDNA gaps behind a replication fork in BRCA-deficient cells. Schlafen 11 (SLFN11), a member of the SLFN family, binds replication protein A (RPA)-coated ssDNA gaps and sensitizes cancer cells to DNA-damaging anticancer agents. These facts motivated us to examine the combinational effects of SLFN11 and BRCA-deficiency on PARPis sensitivity. Here, we show that SLFN11 and BRCA2-deficiency synergistically increased sensitivity to PARPis (talazoparib, niraparib, olaparib, and veliparib) at specific concentrations, where SLFN11 alone showed marginal effects. Using chromatin-bound proteins and alkaline BrdU comet assays in human cancer cells, we revealed that ssDNA gaps induced by PARPis were increased by SLFN11 or BRCA2-deficiency and that the combination of the two had the greatest effect. SLFN11 was recruited to and colocalized with chromatin-bound RPA2 under PARPis. SLFN11 recruited around a fork under DNA damage blocked replication, whereas SLFN11 recruited behind a fork under PARPis did not. Chromatin recruitment of SLFN11 and RPA2 was attenuated by the MRE11 inhibitor mirin. Hence, our studies showed that BRCA2-deficiency increased ssDNA gaps behind a fork under PARPis treatment, where SLFN11 bound and further increased the gaps. Our findings provide a mechanistic understanding of favorable responses to PARPis in SLFN11-proficient and BRCA-deficient tumors. ### Competing Interest Statement The authors have declared no competing interest.