Schlafen 11 further sensitizes BRCA-deficient cells to PARP inhibitors through single-strand DNA gap accumulation behind replication forks

The preferential response to PARP inhibitor (PARPi) olaparib in BRCA-deficient and Schlafen 11 (SLFN11)-expressing ovarian cancers has been documented, yet the underlying molecular mechanisms remain unclear. As the accumulation of single-strand DNA (ssDNA) gaps behind replication forks is key for the lethality induced by PARPis, we investigated the combined effects of SLFN11 expression and BRCA deficiency on PARPis sensitivity and ssDNA gap formation in human cancer cells. PARPis increased chromatin-bound RPA2 and ssDNA gaps in SLFN11-expressing cells and even more in cells with BRCA1 or BRCA2 deficiency. SLFN11 was co-localized with chromatin-bound RPA2 under PARPis treatment, with enhanced recruitment in BRCA2-deficient cells. Notably, the chromatin-bound SLFN11 under PARPis did not block replication, contrary to its function under replication stress. SLFN11 recruitment was attenuated by the MRE11 inhibitor, mirin. Hence, under PARPis treatment, MRE11 expression and BRCA deficiency lead to ssDNA gaps behind replication forks, where SLFN11 binds and increases their formation. Ovarian cancer patients who super-responded (progression-free survival > 2 years) to olaparib maintenance therapy had a significantly higher SLFN11-positivity than short-responders (< 6 months). Our findings provide a mechanistic understanding of the favorable responses to PARPis in SLFN11-expressing and BRCA-deficient tumors and highlight the clinical implications of SLFN11.