Teresa Roldán-Arjona and Rafael R. Ariza
Eukaryotic DNA methylation is performed by DNA‑methyltransferases that catalyze transfer of a methyl group from S‑adenosyl‑L‑methionine to carbon 5 of cytosine bases in DNA, giving rise to 5‑methylcytosine (5‑meC). Cytosine methylation is used as an epigenetic mark for maintenance of gene silencing across cellular divisions. However, this chemically stable modification may be removed from DNA through demethylation. DNA demethylation may take place as a passive process due to lack of maintenance methylation during several cycles of DNA replication, or as an active mechanism in the absence of replication. Extensive demethylation of the mammalian genome occurs in preimplantation embryos, first in the male pronucleus through an active mechanism independent of DNA replication and subsequently in both paternal and maternal chromosomes through a passive process. Localized demethylation at specific genes takes place later throughout development and tissue differentiation and rapid cycles of DNA methylation and demethylation of CG dinucleotides at gene promoters have been recently reported. Despite many attempts to identify the mechanism responsible for active DNA demethylation in animal cells, its enzymatic basis remains controversial, although there is evidence for a role of thymine‑DNA glycosylase after deamination of 5‑meC to thymine. In plants, genetic and biochemical studies have revealed that the Arabidopsis DNA glycosylase domain‑containing proteins DME and ROS1 initiate DNA demethylation. Both DME and ROS1 catalyze the release of 5‑meC from DNA by a glycosylase/lyase mechanism, cleaving the phosphodiester backbone at the 5‑meC removal site by successive β,δ‑elimination and leaving a gap that has to be further processed to generate a 3\'‑OH terminus suitable for polymerization and ligation. This repair‑like pathway provides a mechanism to exchange methylated cytosines with cytosines.