Abstract 4486: Epigenetic vulnerabilities in ovarian cancer

Abstract

Background: Histone acetylation regulates gene expression in cancer and can be exploited for therapeutic targeting. Histone deacetylase inhibitors (HDACi) increase histone acetylation, promoting transcription of genes involved in growth arrest and apoptosis and have been used as anti-cancer agents. BRCA mutations are the best characterized genetic alteration associated with ovarian cancer and can be targeted therapeutically by recently approved PARP inhibitors. We hypothesized that the epigenetic landscape of ovarian cancer is altered by the presence of BRCA mutations and can be manipulated by HDACi for therapeutic benefit. Materials and Methods: We utilized isogenic cell lines carrying mutated/functional BRCA 1 (UWB1.289; UWB1.289-BRCA), and respectively BRCA 2 genes (PE01/PEO4). We measured HDAC expression and activity by western blotting and an enzymatic assay and characterized globally chromatin marked by H3K9ac (active chromatin) and H3K27ac (poised chromatin) by using ChIP sequencing. To determine whether HDAC inhibitors increase susceptibility to PARP inhibitors, cellular proliferation was measured in ovarian cancer cells expressing mutated and wild type (WT) BRCA. Results: HDAC1 expression and activity were decreased in cell lines carrying defective BRCA proteins. ChIP sequencing identified chromatin regions differentially marked by H3K9ac (178 promoters in BRCA1-null compared to BRCA WT cells) and H3K27ac (131 promoters in BRCA WT compared to BRCA null cells) suggesting differences in gene transcription dependent on histone acetylation in BRCA mutated vs BRCA functional cancer cells. A subset of genes differentially marked by H3K9ac were validated at mRNA level by RT PCR confirming BRCA dependent regulation (TGM2, Wnt7a, DKK1, MET, FLI1, AXL, TG2, PMEPA1, TWIST2 upregulated) and (E2F2, NID2 downregulated) in BRCA1-null ovarian cancer cells. HDAC1 inhibition with trichostatin A (TSA) and entinostat (MS-275) increased H3K9ac and was associated with an increase in TGM2, FLI1, DKK1 and E2F2, and a decrease in TWIST2 gene expression in both BRCA1-wt and -null cell lines confirming epigenetic regulation of this subset of genes. Functionally, HDACi synergized with PARP inhibitors in BRCA null ovarian cancer cells (combination index 0.63), consistent with the altered gene expression changes induced by differences in chromatin marks. Conclusions: In conclusion, the chromatin landscape in BRCA null ovarian cancer cells is altered and can be targeted by HDAC inhibitors. Identifying pathways and specific genes altered by histone acetylation may enable future rational targeted combination therapies. Citation Format: Jessica A. Thomes Pepin, Horacio Cardenas, Cody Moore, Salvatore Condello, Jiang Guanglong, Liu Yunlong, Daniela Matei. Epigenetic vulnerabilities in ovarian cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4486.