A high-throughput approach to identify BRCA1-downregulating compounds to enhance PARP inhibitor sensitivity

Highlights•PARP inhibitors (PARPi) are highly effective in cancers with a loss of BRCA1 protein•Reducing BRCA1 expression with drugs could enhance the effectiveness of PARPi•BRCA1-HiBiT reporter cells enabled the screening of over 6,000 compounds•Three drugs were identified to decrease BRCA1 expression and synergyze with PARPiSummaryPARP inhibitors (PARPi) are efficacious in BRCA1-null tumors; however, their utility is limited in tumors with functional BRCA1. We hypothesized that pharmacologically reducing BRCA1 protein levels could enhance PARPi effectiveness in BRCA1 wild-type tumors. To identify BRCA1 downregulating agents, we generated reporter cell lines using CRISPR-mediated editing to tag endogenous BRCA1 protein with HiBiT. These reporter lines enable the sensitive measurement of BRCA1 protein levels by luminescence. Validated reporter cells were used in a pilot screen of epigenetic-modifying probes and a larger screen of more than 6,000 compounds. We identified 7 compounds that could downregulate BRCA1-HiBiT expression and synergize with olaparib. Three compounds, N-acetyl-N-acetoxy chlorobenzenesulfonamide (NANAC), A-443654, and CHIR-124, were validated to reduce BRCA1 protein levels and sensitize breast cancer cells to the toxic effects of olaparib. These results suggest that BRCA1-HiBiT reporter cells hold promise in developing agents to improve the clinical utility of PARPi.