Abstract 5056: The bromodomain inhibitor JQ1 sensitizes homologous recombination proficient ovarian cancer cells to the PARP inhibitor olaparib
Authors: Wilson Andrew J. , Thompson Janese , Osman Abdirahman , Saskowski Jeanette , Khabele Dineo 
Published: July 2017

Introduction: Lack of effective treatment options for high-grade serous ovarian cancers (HGSOC) retaining functional homologous recombination (HR) DNA repair pathways is a significant clinical problem. HR-proficient HGSOC tumors, for example those harboring cyclin E amplifications, have poorer clinical outcomes and show relative resistance to DNA-damaging platinum agents and newer poly ADP ribose polymerase inhibitors (PARPi). We and others have shown that using epigenetic drugs to reduce HR efficiency in HR-proficient HGSOC sensitizes these cancer cells to DNA damaging agents. One mechanism by which these drugs reduce HR efficiency is by transcriptional down-regulation of HR pathway components. An emerging class of epigenetic mediators of pro-tumorigenic transcription is the bromodomain (BRD) family of proteins, and BRD inhibitors (BRDi) have shown promising preclinical anti-tumor efficacy. However, it is unknown whether BRDi sensitize HR-proficient HGSOC to DNA damaging agents. Aims: To test the hypothesis that BRDi decrease efficiency of HR DNA repair in HR-proficient ovarian cancer cells, thereby sensitizing them to PARPi.

Methods: The HR-proficient ovarian cancer cell lines, OVCAR-3 (cyclin E-amplified) and SKOV3, were treated with 0.01% DMSO vehicle, the PARPi olaparib (Astra Zeneca), the BRDi JQ1 or with the olaparib/JQ1 combination. Sulforhodamine B (SRB) assays assessed cell growth and viability (72 hours treatment). Immunofluorescence (IF) assays assessed markers of DNA damage (pH2AX), apoptosis (cleaved caspase-3), and HR efficiency (RAD51 foci, and GFP expression in cells co-transfected with I-Sce1 endonuclease and DRGFP HR reporter plasmids) (24 hours treatment). Steady state levels of the HR protein BRCA1, pH2AX and cleaved caspase-3 were assessed by western blot (24 hours treatment).

Results: The combination of JQ1 and olaparib synergistically reduced cancer cell viability following isobologram analyses of SRB experiments. Compared to either drug alone, the JQ1/olaparib combination also significantly reduced BRCA1 expression and increased protein levels of cleaved caspase-3 and pH2AX in western blots, and also increased the number of cells displaying DNA damage and apoptosis in IF assays. Finally, JQ1 and olaparib combined to significantly reduce HR efficiency in our RAD51 foci formation and DRGFP assays compared to olaparib alone.

Conclusions: Our results suggest that BRDi sensitize HR-proficient cells to DNA damaging drugs, in part by reducing efficiency of HR DNA repair. These findings have important implications for expanding the use of PARPi in HR-proficient HGSOC through rational combinations with epigenetic drugs such as BRDi that target the HR pathway.

Citation Format: Andrew J. Wilson, Janese Thompson, Abdirahman Osman, Jeanette Saskowski, Dineo Khabele. The bromodomain inhibitor JQ1 sensitizes homologous recombination proficient ovarian cancer cells to the PARP inhibitor olaparib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5056. doi:10.1158/1538-7445.AM2017-5056