Decitabine-mediated DNA methylation dynamics at pericentromeric satellite 2 repeats

The hypomethylating agents (HMAs) 5-azacytidine (vidaza-AZA) and 5-aza-2'-deoxycytidine (decitabine-DAC) are part of the standard of care for the treatment of myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). However, the molecular events mediated by HMAs in MDS and AML are poorly understood, and the efficacy of MDS and AML treatments is still improvable. The majority of CpG dinucleotides are located at satellite repeats, and their methylation levels are known to play a fundamental role in ensuring the genomic stability of cells. HMAs are believed to act through a plethora of effects, including DNA demethylation and the consequent re-expression of aberrantly silenced genes, DNA damage due to the covalent trapping of DNA methyltransferases (DNMTs) on DNA, and endogenous retroelements (EREs) reactivation associated with the induction of a cell-intrinsic antiviral response. DNA demethylation of satellite repeats and the consequent genomic destabilization and mitotic impairment of leukemic cells are also believed to play important roles. Although the demethylating activity of HMAs on gene promoters has been extensively investigated, little is known about their effects on satellite DNA methylation during treatment, especially when the selective pressure of the treatment ends. Here, we characterized the dynamics of satellite 2 DNA methylation mediated by decitabine in a human AML cell line model (U937 cells). We demonstrate that the initial demethylation of satellite 2 repeats is followed by complete recovery after 48 h of culture. The observed regain of methylation is associated with increased expression of DNMT3B, the de novo DNMT known to target satellite 2 repeats. In the intent of deciphering the regulation of DNMT3B expression, we found that DAC significantly increased the level of H3 acetylation at the DNMT3B promoter. These preliminary data shed light on DAC-mediated methylation dynamics at satellite 2 repeats, suggesting that satellite 2 remethylation could limit the genomic-destabilizing effects mediated by HMAs in tumor cells and, thus, the future evaluation of strategies to impair this methylation regain and to improve HMAs activity against tumor cells.