Abstract IA04: Targeting DNA repair in ovarian cancer therapy.

Large-scale genomic studies have shown that half of epithelial ovarian cancers (EOCs) have alterations in genes regulating homologous recombination (HR) repair. Loss of HR accounts for the genomic instability and replicative stress of EOCs and for their cellular hyperdependence on alternative poly-ADP ribose polymerase (PARP)-mediated DNA repair mechanisms. On the basis of this hyperdependence, in 2014, the FA approved the PARP inhibitor, olaparib, for patients with advanced ovarian cancer associated with BRCA1 and BRCA2 germline mutations, whose tumors are HR deficient. Previous studies have implicated the DNA polymerase POLQ in an error-prone pathway required for the repair of DNA double-strand breaks, referred to as alternative endjoining (alt-EJ). In Drosophila melanogaster and Caenorhabditis elegans, POLQ functions in alt-EJ and in the maintenance of genome stability at sites of G-quadruplex structures. Whether POLQ interacts with canonical DNA repair pathways to prevent genomic instability at stalled replication forks has remained unknown. We have recently demonstrated an inverse correlation between HR activity and POLQ expression in EOCs (Ceccaldi R. et al, Nature, 518: 258-262, 2015). While knockdown of POLQ in HR-proficient cells upregulates HR activity and RAD51 nucleofilament assembly, knockdown of POLQ in HR-deficient EOCs results in enhanced cell death. Consistent with these results, genetic inactivation of an HR gene (Fancd2) and Polq in mice results in embryonic lethality. Moreover, POLQ contains a repeat of RAD51 binding motifs, and it blocks D-loop formation and recombination. Importantly, PARP1 activity is required for the recruitment of POLQ to sites of DNA repair, accounting, in part, for the PARP1 hyperdependence of HR-deficient ovarian tumor cells. Taken together, these results reveal a synthetic lethal relationship between the HR pathway and PARP1/POLQ-mediated alternative end-joining in EOCs, and identify POLQ as a novel druggable target for ovarian cancer therapy. Such inhibitors may be useful in the treatment of PARP inhibitor-resistant ovarian cancers. Citation Format: Alan D. D9Andrea. Targeting DNA repair in ovarian cancer therapy. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr IA04.