Combination Of Inhibitors Against DNA Methyltransferases and Histone Deacetylases Synergistically Suppresses Cell Viability In Acute Myeloid Cells In a p53-Dependent Manner

Acute myeloid leukemia (AML) accounts for 18% of childhood leukemia diagnoses. Although, the 5-year survival rate for children with AML is estimated at 64%, novel therapeutic options are required to improve the outcome for certain subsets of AML that are refractory. The contribution of epigenetic modifiers to the pathogenesis of AML in children is becoming evident following the discovery of mutations in patients, which affect DNA and/or histone methylation. Although DNA methyltransferase (DNMT) inhibitors have been approved in AML, their efficacy as single agents has been limited. This could be due to the crosstalk between DNA methylation and histone modifications that together regulate gene expression. This raises the possibility that optimal re-expression of silenced tumor suppressor genes in AML requires treatment with both DNMT and histone deacetylase (HDAC) inhibitors.

We evaluated the effect of a combination of DNA methyltransferase inhibitor azacytidine with HDAC inhibitors panobinostat or romidepsin on the viability of three pediatric AML cell lines- MV4;11 (expressing wild-type p53), THP-1 (with no detectable p53 mRNA) and AML-193 (with mutated p53). We observed that azacytidine as a single-agent only moderately reduced cell viability of MV4;11 cells (IC50 – 7 uM), and it failed to induce cell death in THP-1 and AML-193 cells. However, azacytidine in combination with panobinostat and romidepsin was more effective at reducing cell viability in MV4;11 and THP-1 cells (with a combination index of 0.5 to 0.6). AML-193 cells were almost insensitive to cell death induced by the combination treatment. This suggests that p53 is required for induction of cell death by azacytidine and panobinostat/romidepsin combination. Although THP-1 cells have no detectable p53 expression, other studies have reported the upregulation of p53 following stimulation with nitric oxide or 7-oxysterols. The effect on cell viability in THP-1 cells is likely due to the reexpression of silenced p53 in these cells. Taken together, our data suggests that combining azacytidine with panobinostat/romidepsin induces synergistic cell death in a p53-dependent manner.