An in vivo analysis of MMC-induced DNA damage and its repair Young-Ju Lee1, Su-Jung Park1,3,

Mitomycin C (MMC) induces various types of DNA dam-ages that cause significant cytotoxicity to cells. Accord-ingly, repair of MMC-induced damages involves multiple repair pathways such as nucleotide excision repair, homo-logous recombination repair and translesion bypass repair pathways. Nonetheless, repair of the MMC-induced DNA damages in mammals have not been fully delineated. In this study, we investigated potential roles for Xeroderma pigmentosum (XP) proteins in the repair of MMC-induced DNA damages using an assay that detects the ssDNA patches generated following treatment with MMC or 80-methoxy-psoralen (8-MOP) þ UVA (ultraviolet light A). Human wild-type cells formed distinctive ssDNA foci fol-lowing treatment with MMC or 8-MOP þ UVA, but not with those inducing alkylation damage, oxidative damage or strand-break damage, suggesting that the foci represent ssDNA patches formed during the crosslink repair. In con-trast to wild-type cells, mutant defective in XPE orXPG did not form the ssDNA foci following MMC treatment, while XPF mutant cells showed a significantly delayed response in forming the foci. A positive role for XPG in the repair of MMC-induced DNA damages was further supported by observations that cells treated with MMC induced a tight association of XPG with chromatin, and a targeted inhibi-tion of XPG abolished MMC-induced ssDNA foci forma-tion, rendering cells hypersensitive to MMC. Together, our results suggest that XPG along with XPE and XPF play unique role(s) in the repair of MMC-induced DNA dam-ages