Project description:Hepatocellular carcinoma (HCC) is the most common primary liver malignancy with cases increasing dramatically in the USA. Developing more effective targeted therapies for HCC is an urgent need. The FDA-approved DNA methyltransferase inhibitors (DNMTis) 5-azacytidine (5-aza-CR, Vidaza) and 5-aza-2’-deoxycytidine (5-aza-CdR, Decitabine) represented the first clinical breakthrough to target aberrant cancer epigenomes. However, the clinical efficacies of DNMTis are still limited, in part due to an “epigenetic switch” mechanism, in which a large group of genes demethylated by DNMTi treatment remain silenced by Polycomb Repressive Complex 2 (PRC2) occupancy. Overexpression of PRC2 subunits, especially EZH2, is correlated with poor patient survivor in HCC. In this study, we tested the combination of 5-Aza-CdR and the EZH2 inhibitor (EZH2i) GSK-126 in human HCC cell lines (SNU398, HepG2, and SNU475) on DNA methylation, nucleosome accessibility, and gene expression profiles. Compared with single agent treatments, all cell lines showed increased sensitivity after receiving both drugs concomitant with prolonged anti-proliferative changes and sustained reactivation of nascently silenced genes. The increased number of up-regulated genes, including tumor suppressors, were correlated with prolonged anti-proliferation effects and nucleosome accessibility but negatively correlated with gene promoter DNA methylation. Detailed epigenome analyses reveal a mechanistic rationale in which the combination treatment of 5-aza-CdR with an EZH2 inhibitor (EZH2i) activates demethylated promoters that are repressed by PRC2 occupancy. Furthermore, about 13.6% to 31% of genes down-regulated by DNA methylation in primary HCC tumors can be reactivated through this combination treatment scheme in vitro. Finally, combination treatment also exacerbates viral mimicry activation as well as leukocyte, lymphocyte and T cell mediated immunity, while most those genes or pathways were downregulated in over 50% of primary HCC tumors in TCGA dataset. Conclusion: we have linked the anti-tumor effects of DNMTi and EZH2i combination treatments to detailed epigenetic alterations in HCC cells, identified potential therapeutic targets and provided a rationale for combination treatment efficacy in HCC patients.

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