Correlation of the colony-forming abilities of xeroderma pigmentosum fibroblasts with repair-specific DNA incision reactions catalyzed by cell-free extracts.

Several normal and XP group A fiblast cell lines were exposed to the weakly carcinogenic and toxic agent methyl methanesulfonate, and the differences in their abilities to form colonies were determined. The XP group A cell lines investigated exhibited higher sensitivity towards methyl methanesulfonate than normal cell lines. Correspondingly, cell-free extracts of the same XP cell lines differed from normal ones in cleaving methyl methane-sulfonate-treated double-stranded DNA less rapidly. Since depurinated DNA was cleaved by XP and normal cell lines at equal rates, it was concluded that the differences observed with methylated DNA were due to a reaction preceding cleavage at apurinic sites. In control experiments using extracts from Chinese hamster ovary cells liberation of m3Ade was observed indicating the presence of 3-methyl-adenine DNA glycosylase activity. Furthermore, extracts from a normal fibroblast line liberated small amounts of m3Ade, whereas the one of a XP group A cell line was found to be less effective. The possible role of 3-methyl-adenine DNA glycosylase activity as a rate-limiting factor in the incision step has been discussed.