Involvement of mammalian OGG1(MMH) in excision of the 8-hydroxyguanine residue in DNA.

8-Hydroxyguanine (7,8-dihydro-8-oxoguanine, abbreviated as 8-OH-G or 8-oxoG) is the site of a frequent mutagenic DNA lesion produced by oxidative damage. MutM of E. coli and OGG1 of Saccharomyces cervisiae are known to possess 8-OH-G glycosylase and apurinic (AP) site lyase activity. cDNA clones of four isoforms (types 1a, 1b, 1c, and 2) of human OGG1 homologs (hMMH) were isolated. In order to examine whether expression of hMMH (hOGG1) protein actually occurs in human cells, we prepared type 1a specific antibody, and by using this antibody, we showed that type 1a protein isolated from HeLaS3 has 8-OH-G glycosylase/lyase activity. Furthermore, we showed that type 1a protein is a major enzyme for repair of the 8-OH-G lesion in human cells. In our second study, we generated a mouse line carrying an inactivated mutant Mmh allele by targeted gene disruption. Liver extracts of Mmh homozygous mutant mice were found to have loss of the nicking activity for the 8-OH-G site. In addition, the amount of endogenous 8-OH-G in liver DNA of the homozygous mice increased linearly with age, reaching 7-fold increase in 14 week old mice, over that of wild-type or heterozygous mice. Furthermore, when homozygous mice were fed the oxygen radical-forming agent KBrO3, to provide oxidative stress, the level of 8-OH-G in kidney DNA was tremendously increased: more than 200-fold as that of control mice without oxidative stress after 12 weeks of age. These results indicate that Ogg1/Mmh plays an essential role in the repair of the 8-OH-G residue in DNA produced by oxidative stress.