DNA methylation and mechanism of action of 5-azacytidine

Mabaera et al 1 addressed the role of DNA hypomethylation in the induction of -globin expression by 5-azacytidine. On the basis of the failure of siRNA-mediated down-regulation of DNMT1 to induce expression of -globin in cultured human erythroid progenitors, the authors suggest "that the decrease in -globin promoter methylation seen with 5-azacytidine is not the primary cause of gene induction and fetal hemoglobin (HbF) production, but is a secondary effect related to gene activation by some other mechanism." As an alternative, the authors propose changes in posttranscriptional RNA stability and translational efficiency possibly related to 5-azacytidine incorporation into RNA transcripts (5-azacytidine is a ribose). In proposing this mechanism, the authors fail to consider that decitabine (5-aza-2-deoxycytidine), a deoxyribose analog of 5-azacytidine that is primarily incorporated into DNA, 2-4 produces equivalent elevations of HbF in baboons and in patients with sickle cell disease at molar doses 10% to 20% that of 5-azacytidine Furthermore, the ribonucleotide reductase inhibitor hydroxyurea, which would be expected to block conversion of 5-azacytidine into the deoxyribose form and its incorporation into DNA but not into RNA, blocks the ability of 5-azacytidine to induce HbF in baboons. These results are most consistent with a requirement for 5-azacytidine incorporation into DNA, necessary for DNA methyltransferase inhibition, in its mechanism of action.