Abstract AP32: THE METHYLTRANSFERASE EZH2 IS A VULNERABLE TARGET IN SMALL CELL CARCINOMA OF THE OVARY, HYPERCALCEMIC TYPE (SCCOHT)

Abstract

Abstract PURPOSE: SCCOHT is a rare, poorly differentiated and very deadly ovarian tumor that mainly affects young women with the median age in twenties. Recently, we and others have discovered that the genome of SCCOHT was minimally disturbed compared to common malignancies with the inactivation mutations of SMARCA4 being the only recurrent genetic alternation in SCCOHT genome. SMARCA4 is the ATPase of the SWI/SNF chromatin-remodeling complex, which is the master regulator of gene transcription through controlling the accessibility of chromatin and therefore plays critical roles in many biological processes such as cell cycle control, apoptosis, and differentiation. Recently, a genetic antagonism has been observed between some SWI/SNF complex components and the histone methyltransferase EZH2 of the polycomb repressive complex 2 (PRC2), suggesting that EZH2 may be a vulnerable target in certain SWI/SNF-deficient tumor. Therefore, we aimed to determine the promise of defeating SCCOHT through targeting EZH2 in the present study. METHODS: The EZH2 protein expression was determined by immunohistochemistry on the SCCOHT tissue microarray. Cell viability, cell cycle and apoptosis were measured by crystal violet staining, FACS analysis and live cell imaging with activated caspase-3/7 staining, respectively. The proteomic profiles of SCCOHT cells were determined by Mass Spectrometry using the TMT-labeled peptide fractions. Two SCCOHT xenograft mouse models were established from BIN67 and SCCOHT1 cells, respectively, in immune-deficient mice were used for the evaluation of EPZ-6438 efficacy in vivo. RESULT: All (24/24) SCCOHT expressed abundant EZH2 protein. In comparison to other ovarian cell lines, SCCOHT cells displayed hypersensitivity to two EZH2 inhibitors (GSK126 and EPZ-6438) and EZH2 shRNA. Furthermore, EPZ-6438 induced cell cycle arrest, apoptosis and cell differentiation in SCCOHT cells, accompanying by the induction of genes involved in cell proliferation, apoptosis and differentiation. Treatment of EPZ-6438 in SCCOHT xenograft-bearing mice delayed tumor progression, improved the survival of mice. Further, re-expression of SMARCA4 suppressed the expression of EZH2 and the global level of histone H3K27 trimethylation, induced the expression of PRC2 targets that are induced by EZH2 inhibitors and triggered differentiation of SCCOHT cells. In addition, we also discovered that SCCOHT cells are hypersensitive to multiple inhibitors of histone deacetylase (HDAC), which is known to cooperate with EZH2 to silence certain target genes. Combined treatment of EPZ-6438 and Quisinostat (an HDAC inhibitor) synergistically suppressed the proliferation of SCCOHT cells. CONCLUSION: Our data suggest that EZH2 is a promising therapeutic target for fighting SCCOHT. Combined pharmacological targeting of both EZH2 and HDAC may provide a novel treatment option for SCCOHT patients. Citation Format: Yemin Wang, Shary Chen, Anthony Karnezis, Nancy Dos Santos, Pilar Ramos, Sarah Maines-Bandiera, Christine Chow, Bernard Weissman, Jeffrey Trent, David Huntsman. THE METHYLTRANSFERASE EZH2 IS A VULNERABLE TARGET IN SMALL CELL CARCINOMA OF THE OVARY, HYPERCALCEMIC TYPE (SCCOHT) [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr AP32.