Title: Epigenetic shielding
Description: Maintaining the balance between dynamic DNA methylation and demethylation is crucial to mammalian development and pathogenesis. In vitro methylation at the C-5 position of cytosine enhances cyclobutane pyrimidine dimer (CPD) formation and promotes transition mutations. While the loss of 5-hydroxymethylcytosine (5hmC) and inactivation of the ten-eleven translocation (TET) family have been implicated in cancers, and repeated exposure to UV radiation is a known risk factor for developing skin cancers, the link between DNA demethylation and UV damage has not yet been illustrated. We report that hydroxylation and carboxylation of 5-methylcytosine mitigate methylation-induced CPD enhancement upon UV irradiation. However, 5hmC also increases UV induction of (6-4) photoproducts. In a melanoma cell model, this duality by 5hmC in modulating the UV response is accentuated through TET2 overexpression. These findings implicate the DNA demethylation intermediates 5-hydroxymethylcytosine (5hmC) and 5-carboxylcytosine (5caC) as selective epigenetic shields against UV induction of DNA photoproducts.