Effect of DNA Methylation Modulators on UV Damage Formation and Repair

Background/Objectives: DNA methylation is a key epigenetic modification involved in regulating many cellular processes, including gene expression and the maintenance of genome stability. Ultraviolet (UV) radiation induces DNA damage in the form of pyrimidine-pyrimidone (6-4) photoproducts [(6-4)PPs] and cyclobutane pyrimidine dimers (CPDs), which can lead to mutations if not efficiently repaired. While cytosine methylation has been implicated in influencing UV-induced DNA damage formation, the effect of DNA methylation modulators such as S-adenosyl-L-methionine (SAM) and RG108 on UV damage formation and repair remains unclear. Methods: Here, using immunoslot blot assays, we investigated the effects of SAM and RG108 on UV-induced DNA damage formation and repair in human lymphoblastoid cells. Results: We found that SAM, but not RG108, rapidly suppresses the formation of both (6-4)PP and CPD, with detectable effects within minutes of exposure. Although SAM pretreatment was associated with modestly accelerated early (6-4)PP repair, this effect was accompanied by substantially lower initial damage levels. When cells were treated with SAM or RG108 immediately after UV irradiation to ensure equivalent initial damage burden, no significant differences in repair were observed for either lesion type, demonstrating that the accelerated early (6-4)PP repair reflects reduced lesion burden rather than increased intrinsic nucleotide excision repair (NER). Global 5-methylcytosine (5mC) levels remained stable following SAM or RG108 treatment and during UV damage repair, suggesting that these effects occur independently of global alterations in DNA methylation. Conclusions: Together, our findings reveal that SAM modulates UV damage susceptibility at the level of lesion formation without altering repair, highlighting a previously unrecognized role for DNA methylation modulators in regulating genome stability.