PARG inhibition induces replication catastrophe in ovarian cancer cells with down-regulated DNA replication genes

The discovery that PARP1/2 inhibitors selectively kill BRCA mutant cells has led to a paradigm shift in the treatment of women with homologous recombination (HR)-deficient high-grade serous ovarian cancer (HGSOC), driving unprecedented improvements in progression-free and, more recently, overall survival. However, because most HGSOC cases are not HR-defective, and are therefore unlikely to benefit from PARPi-based therapies, additional strategies will be required to improve outcomes for women with HR-proficient disease. To develop novel therapeutic strategies, considerable attention is now being focused on inhibitors targeting PARG, the poly(ADP ribose) glycohydrolase that counterbalances PARP1/2 activity. Here we characterise ten ovarian cancer cell lines in response to the PARG inhibitor PDD00017273, hereafter PARGi. We demonstrate that six lines are resistant while four are sensitive, and that sensitivity correlates with several markers of persistent DNA replication stress, DNA damage and replication catastrophe, namely the accumulation of asymmetric DNA replication fibres, {gamma}H2AX and RPA foci, KAP1 and Chk1 phosphorylation, a pre-mitotic cell cycle block and, following prolonged exposure, a pan-nuclear {gamma}H2AX phenotype that indicates RPA exhaustion. We demonstrate that PARGi-sensitive cell lines have down-regulated DNA replication genes, including components of the fork protection complex, namely TIMELESS, TIPIN and CLASPIN. These observations suggest that a subset of HGSOC may respond to PARG inhibitors and that "replication stress" gene expression signature could serve as a predictive biomarker to guide the design of clinical trials.