DNA demethylation is the process of removal of a methyl group from nucleotides in DNA. DNA demethylation could be passive or active. The passive process takes place in the absence of methylation of newly synthesised DNA strands by DNMT1 during several replication rounds (for example, upon 5-Azacytidine treatment). Active DNA demethylation occurs via direct removal of a methyl group independently of DNA replication.
Examples DNA Demethylation
All the cases of DNA demethylation can be divided as global (genome wide) or locus-specific (when just specific sequences are demethylated). The genome-wide DNA demethylation occurs:
- In mammals:
- Possibly in amphibia - during midblastula transition.
Examples of specific DNA demethylation:
- Genomic imprinting during plant reproduction;
- Electroconvulsive stimulation-induced demethylation of neurotrophic factor genes in dentate gyrus neurons in the mouse brain.
Possible mechanisms of active DNA demethylation
There are several proposed hypothetical mechanisms of active DNA demethylation:
A Direct removal of 5-methylcytosine
- Direct removal of methyl group. This process has quite low thermodynamic probability.
- Removal of methylated bases (either by direct removal of methylcytosine, or through cytosine deamination followed by removal of thymine from thymine/guanosine mismatch), followed by insertion of unmethylated one using base excision repair machinery (BER).
- Removal of entire DNA patch and following filling it with new nucleotides by nucleotide excision repair (NER).
B Removal of 5-methylcytosine via further modified cytosine bases
Oxidation of the methyl group generates 5-Hydroxymethylcytosine. Several mechanisms have been proposed to mediate demethylation of 5-hydroxymethylcytosines. This base can be either deaminated by AID/Apobec enzymes to give 5-Hydroxymethyluracil. Alternatively, TET enzymes can further oxidize 5-hydroxymethylcytosine to 5-Formylcytosine and 5-Carboxylcytosine.
- Both the deamination and the oxidation products have been shown to be repaired by TDG, a glycosylase which is involved in base excision repair. A base excision mediated demethylation mechanism would yield double strand breaks if it occurs on large scale in CpG islands.
- The carboxyl and formyl groups of 5-Formylcytosine and 5-Carboxylcytosine could be enzymatically removed without excision of the base. Precedent for similar reactions is found in biosynthetic pathways.
DNA hydroxymethylation has been proposed to act as a specific epigenetic mark opposing DNA methylation, rather than a passive intermediate in the de-methylation pathway. DNA hydroxymethylation in vivo is sometimes associated with labile nucleosomes, which are more easy to disassemble and to out-compete by transcription factors during cell development 
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