p53-Independent, Normal Stem Cell Sparing Epigenetic Differentiation Therapy for Myeloid and Other Malignancies

Abstract

Summary Cytotoxic chemotherapy for acute myeloid leukemia (AML) usually produces only temporary remissions, at the cost of significant toxicity and risk for death. One fundamental reason for treatment failure is that it is designed to activate apoptosis genes (eg., TP53) that may be unavailable because of mutation or deletion. Unlike deletion of apoptosis genes, genes that mediate cell cycle exit by differentiation are present in myelodysplastic syndrome (MDS) and AML cells but are epigenetically repressed: MDS/AML cells express high levels of key lineage-specifying transcription factors (TF). Mutation in these TF (eg., CEBPA) or their cofactors (eg., RUNX1) affect transactivation function and produce epigenetic repression of late-differentiation genes that antagonize MYC. Importantly, this aberrant epigenetic repression can be redressed clinically by depleting DNA methyltransferase 1 (DNMT1, a central component of the epigenetic network that mediates transcription repression) using the deoxycytidine analogue decitabine (DAC) at non-cytotoxic concentrations. The DNMT1 depletion is sufficient to trigger upregulation of late-differentiation genes and irreversible cell cycle exit by p53-independent differentiation mechanisms. Fortuitously, the same treatment maintains or increases self-renewal of normal hematopoietic stem cells (HSC), which do not express high levels of lineage-specifying TF. The biological rationale for this approach to therapy appears to apply to cancers other than MDS/AML also. DAC or 5-azacytidine dose and schedule can be rationalized to emphasize this mechanism of action, as an alternative or complement to conventional apoptosis-based oncotherapy.

Evaluation of noncytotoxic DNMT1-depleting therapy in patients with myelodysplastic syndromes

“…Several groups have observed that terminal differentiation is induced in vitro when treating myeloid and other cancer cells with drugs or conditions that inhibit gene-silencing, chromatin-modifying enzymes (chromatin relaxation) (reviewed in ref. 13). These differentiation-mediated cell-cycle exits, like those that occur during normal tissue differentiation, do not require p53 and are readily induced in p53/p16-null cancer cells (10,(14)(15)(16).…”

“…The reasons for this cell context-dependent response have been evaluated: differentiation is driven by relatively few master transcription factors (25). Myeloid cancer cells express master myeloid differentiation-driven transcription factors (e.g., CEBPA and PU.1) at high levels, yet the target genes of these transcription factors are epigenetically silenced (10,13,14,20,(26)(27)(28)(29) because of aberrant recruitment of silencing -instead of activating -chromatin-modifying enzymes to the transcription factors (20,27,30). Inhibition of silencing enzymes with drugs such as decitabine restores expression of numerous target genes of the transcription factors, including MYC antagonists (e.g., CEBPE and p27/ CDKN1B), that terminate proliferation (10, 14, 17, 20-22, 27, 30-32).…”

“…Several in vitro studies have detailed that low-range decitabine concentrations can deplete DNMT1 without cytotoxicity or γH2AX increases (reviewed in ref. 13). One caveat is that DNMT1 depletion by shRNA has been shown to decrease mismatch repair (MMR) protein levels and destabilize unmethylated microsatellite repeats in vitro (56), and such a mechanism for genetic instability would be expected to operate even with noncytotoxic DNMT1 depletion.…”

Homocysteine induces blood vessel global hypomethylation mediated by LOX-1

“…Maintenance of the methylation pattern is achieved by DNMT1 function during DNA replication, whereas de novo methylation is primarily catalyzed by DNMT3a and DNMT3b (Pazienza et al, 2012). DNMT1 is a central component of the epigenetic network that mediates transcription repression (Saunthararajah et al, 2012). Overexpression of DNMT1 promotes global DNA hypermethylation (Biniszkiewicz et al, 2002).…”

Artesunate-enhanced apoptosis of human high-risk myelodysplastic cells induced by the DNA methyltransferase inhibitor decitabine

“…The hypermethylation of CpG islands within promoter regions, and the resultant silencing of tumor-associated genes, are significant events involved in the pathogenesis of MDS (1). Previous studies have revealed that the DNA methyltransferase (DNMT) 1 and 3A enzymes contribute to aberrant methylation in MDS (2,3). Therefore, DNMTs have become potential targets for the treatment of MDS (4)(5)(6)(7)(8).…”