Synergistic Pre-Clinical Activity of Combined Epigenetic Therapy with the Novel Histone Methyltransferase EZH2 Inhibitor 3-Deazaneplanocin and Histone Deacetylase Inhibitor Panobinostat against Human AML Cells.

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 (