Ovarian Cancers Harbour Defects in Non-Homologous End Joining Resulting in Resistance to Rucaparib.

DNA damage defects are common in ovarian cancer and can be used to stratify treatment. Although most work has focussed on Homologous Recombination (HR), DNA double strand breaks are repaired primarily by non-homologous end joining (NHEJ). Defects in NHEJ have been shown to contribute to genomic instability and have been associated with the development of chemoresistance.,NHEJ was assessed in a panel of ovarian cancer cell lines and 47 primary ascitic derived ovarian cancer cultures, by measuring the ability of cell extracts to end-join linearized plasmid monomers into multimers. mRNA and protein expression of components of NHEJ was determined using RT-qPCR and western blotting. Cytotoxicities of cisplatin and the Poly(ADP-ribose) polymerase-1 (PARP) inhibitor rucaparib were assessed using sulforhodamine B (SRB) assays. HR function was assessed using γH2AX/RAD51 foci assay.,NHEJ was defective (D) in 4 of 6 cell lines and 20 of 47 primary cultures. NHEJ function was independent of homologous recombination (HR) competence (C). NHEJD cultures were resistant to rucaparib (p=0.0022). When HR and NHEJ functions were taken into account, only NHEJC/HRD cultures were sensitive to rucaparib (compared to NHEJC/HRC p=0.034, NHEJD/HRC p=0.0002, and NHEJD/HRD p=0.0045). The DNA-PK inhibitor, NU7441 induced resistance to rucaparib (p=0.014) and HR function recovery in a BRCA1 defective cell line.,This study has shown that NHEJ is defective in 40% of ovarian cancers, which is independent of HR function and associated with resistance to PARP inhibitors in ex vivo primary cultures.