Abstract A33: Loss of ZC3H18 disrupts homologous recombination repair and sensitizes ovarian cancer cells to PARP inhibitors and DNA cross-linking agents
Authors: Kanakkanthara Arun , Huntoon Catherine J. , Hou Xiaonan , Zhang Minzhi , Heinzen Ethan P , Oberg Ann L. , Weroha John S. , Kaufmann Scott H. , Karnitz Larry M. 
Published: September 2020

Abstract
About 50% of high-grade serous ovarian cancers (HGSOC) have defects in homologous recombination (HR) DNA repair. Intriguingly, HR-defective tumors are highly susceptible to poly(ADP-ribose) polymerase (PARP) inhibitors and platinum-based chemotherapy. Mutations in BRCA1/2 or epigenetic silencing of BRCA1 via promoter hypermethylation contributes to ~30% of the HR defects, but other genetic alterations that cause HR deficiency in HGSOC remain unclear. To identify novel genes that may regulate HR, we studied Zinc Finger CCCH-Type Containing 18 (ZC3H18), encoded by ZC3H18. Notably, ZC3H18 lies within 16q24.2, a region with frequent copy number loss in HGSOC. Here we demonstrate that BRCA1 is transcriptionally activated by a previously unknown function of ZC3H18. We show that ZC3H18 interacts with an E2F site in the BRCA1 promoter where it facilitates recruitment of E2F4 to an adjacent E2F site to promote BRCA1 transcription. Consistent with the role of ZC3H18 in promoting BRCA1 expression, ZC3H18 depletion induces BRCA1 promoter methylation, reduces BRCA1 expression, disrupts HR, and sensitizes ovarian cancer cells to PARP inhibitors and platinum agents. Moreover, in patient-derived xenografts and primary HGSOC tumors, ZC3H18 mRNA levels were positively correlated with BRCA1 mRNA levels, further supporting the role of ZC3H18 in regulating BRCA1. Together, these findings suggest that loss and/or reduced ZC3H18 expression may help identify HGSOC patients most likely to benefit from PARP inhibitor and platinum-based therapies.
Summary of the Unpublished Data/Significance/Innovation: These studies are innovative on several fronts. First, although BRCA1 methylation is a frequent event that mediates tumorigenesis and therapeutic responses in HGSOC, how BRCA1 methylation is regulated remains unknown. These studies will be the first to provide a mechanistic basis for how BRCA1 is methylated in HGSOC. Second, our studies are the first to show a role for ZC3H18 in any cancer. Third, we show that ZC3H18 regulates HR and that its depletion sensitizes ovarian cancer cells to PARP inhibitors and platinum therapy. Thus, these studies will provide important basis for developing ZC3H18 loss/decreased abundance as a novel biomarker for ovarian cancers that may respond to PARP inhibitors and platinum agents.
Citation Format: Arun Kanakkanthara, Catherine J. Huntoon, Xiaonan Hou, Minzhi Zhang, Ethan P Heinzen, Ann L. Oberg, John S. Weroha, Scott H. Kaufmann, Larry M. Karnitz. Loss of ZC3H18 disrupts homologous recombination repair and sensitizes ovarian cancer cells to PARP inhibitors and DNA cross-linking agents [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr A33.