Eradication of leukemia stem cells by inhibitors of DNA methyltransferase, EZH2 and G9a histone methyltransferases

Abstract

Complete eradication of leukemic stem cells (LSCs) in patients with acute myeloid leukemia (AML) is required for curative therapy. Epigenetic alterations that involve gene-silencing by DNA methylation by DNMT1, methylation of H3K27 by EZH2 histone methyltransferase (HMT) and methylation of H3K9 by G9a HMT may play a major role in the development of AML. The major action of these epigenetic alterations is the silencing of the genes that program differentiation of AML cells. Inhibitors of DNA and histone methylation have the potential to reverse this block in differentiation. If tumor suppressor genes (TSGs) contain two gene-silencing markers, such as DNA methylation and H3K27me3, they may not be fully reactivated with only an inhibitor of DNA methylation, such as 5-aza-2’-deoxycytidine (5-AZA-CdR), but may also require an inhibitor of EZH2. In support of this model is the synergistic antileukemic action as shown by a colony assay on AML cells using 5-AZA-CdR in combination with 3-deazaneplanocin A (DZNep), a potent inhibitor of EZH2. A similar type of interaction can occur when TSGs are silenced by DNA methylation and the G9a methylation of H3K9me2, a second gene-silencing marker. Treatment of these AML cells with 5-AZA-CdR and BIX01294, an inhibitor of G9a, also results in a synergistic antileukemic action. Leukemic cells that contain 3 different gene-silencing markers: DNA methylation, H3K27me3 and H3K9me2 may require 3 different inhibitors for maximal antineoplastic activity. This result was observed when the AML cells were treated in with 5-AZA-CdR, DZNep and BIX01294. The aim of this study was to demonstrate that epigenetic agents that target DNA and histone methylation have remarkable antineoplastic activity against myeloid leukemia cells. The second aim was to propose a dose-schedule for these epigenetic agents that can be evaluated in a clinical trial in patients with advanced AML for its potential to eradicate LSCs. One of the most sensitive targets for chemotherapeutic intervention in LSCs is the block in differentiation due to gene-silencing by DNA and histone methylation. Epigenetic agents that have the potential to reverse this block merit clinical investigation with high priority.