Lysine specific histone methylation and deacetylation and DNA hypermethylation are involved in the epigenetic silencing of tumor suppressor genes (TSG), e.g., p15 and p16. The multi-protein complex PRC (polycomb repressive complex) 2 that contains the three core proteins EZH2, SUZ12 and EED, has intrinsic histone methyltransferase (HMTase) activity. This is mediated by the SET domain of EZH2, which induces tri-methylation (3Me) of lysine (K)-27 on histone H3, regulates the expression of HOX genes and promotes cell proliferation and aggressiveness of neoplastic cells. In the present studies we demonstrate that treatment with the S-adenosylhomocysteine hydrolase inhibitor 3-Deazaneplanocin A (DZNep) dose-dependently (200 nM to 2.0 uM) depletes EZH2, SUZ12 and EED levels, inhibits 3Me K27 on H3 while inducing K27 H3 acetylation in the cultured human AML HL60 and OCI-AML3 cells and in primary, patient-derived AML blasts. DZNep treatment also induced the levels of p16, p21, p27 and FBXO32 while depleting cyclin E and HOXA9 levels. Treatment with DZNep induced PARP cleavage activity of the caspases and apoptosis in the cultured and primary AML cells. DZNep promoted proteasomal degradation of EZH2 and SUZ12, since co-treatment with bortezomib significantly restored EZH2 and SUZ12 levels in the AML cells. We had previously reported that treatment with the pan-histone deacetylase (HDAC) inhibitor panobinostat (PS) (LBH589, Novartis Pharmaceutical Corp) also depletes the levels of EZH2, SUZ12 and EED in cultured and primary AML cells (

Mol Cancer Ther.
). Within the PRC2 complex, EZH2 bound and recruited the DNA methyltransferases DNMT1, and treatment with PS also disrupted the interaction of EZH2 with DNMT1, attenuated DNMT1 levels and its binding to the EZH2-targeted gene promoters, e.g., p16 and JunB. Here, we also demonstrate that, as compared to treatment with either agent alone, co-treatment with DZNep and PS caused more depletion of EZH2, SUZ12 and EED, more induction of p16, p21 and p27, as well as synergistically induced apoptosis of AML cells (combination indices < 1.0). Additionally, DZNep induced apoptosis of HL-60/LR cells that are resistant to HDACs including PS, as well as sensitized HL-60/LR cells to PS. Taken together, these findings indicate that targeting EZH2 and the PRC2 complex is an effective epigenetic therapy of AML that also overcomes resistance to HDAC inhibitors. Additionally, combined epigenetic therapy with DZNep and PS exerts synergistic in vitro activity against human AML cells, suggesting that this combination may be a promising novel treatment for AML.

Disclosures: Ataja:Novartis: Employment. Bhalla:Novartis: Honoraria, Research Funding; Merck: Research Funding.

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