Cytosine methylation is believed to play a pivotal role in eucaryotic cellular development as well as in viral latency. We have been investigating chemical mechanisms for the perturbation of methylation patterns, including the effects of ultraviolet radiation. We observed that, upon exposure to UV light, 5-methylcytosine (5mC) was converted to thymine, cytosine, and a series of 5-substituted cytosine derivatives as analyzed by gas chromatography/mass spectrometry. Deamination of 5mC to thymine proceeds via formation of the intermediate photohydrate. Formation of 5-substituted cytosine derivatives results from oxidation of the 5-methyl group with initial formation of 5-(hydroxymethyl)cytosine (hmC). Upon exposure to UV light, hmC is converted to cytosine. The conversion of hmC to cytosine likely results from photohydration and elimination of formaldehyde. It is proposed that endogenous oxidation and hydrolysis could result in demethylation of 5mC residues in DNA. Whereas hydrolytic deamination of 5mC to thymine has been widely discussed, demethylation of 5mC has not as yet been described.
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