Functional Cooperation of Oggl and Mutyh in Preventing G: C ~ T: A Transversions in Mice

Abstract Oxygen radicals generated through normal cellular metabolism induce a variety of types of oxidative damage into DNA and its precursors. Among such types of oxidative damage, 7, 8-dihydro-8-oxoguanine (8-oxoG), an oxidized form of guanine, is known to be abundant and highly mutagenic. 8-0xoG can pair with adenine as well as cytosine, thus causing G: C to T: A transversions after DNA replication, if not repaired. Organisms are equipped with elaborate systems to avoid such mutations caused by 8-oxoG. In Escherichia coli, two DNA glycosylases have been identified to suppress these mutations. One is MutM, an 8-oxoguanine DNA glycosylase that removes 8-oxoG from 8- oxoG: C base pairs. The other is MutY, an adenine DNA glycosylase that excises adenine from 8-oxoG: A mismatches. Mammals also have such DNA glycosylases; OGGl (Oggl) is the functional counterpart of MutM, and MUTYH (Mutyh) is the MutY homologue. In order to investigate the roles of these two enzymes in the avoidance of 8-oxoG-related mutagenesis in mammals, we analyzed spontaneous mutagenesis in the small intestine of Oggl-deficient ( Ogg1-1-) and Oggl-, Mutyh- double deficient ( Ogg1-1-; Mutyh-1-) mice at the age of 4 ~ 5 weeks using the prokaryotic rpsL transgene as a reporter. The observed mutation frequency was 1. 00 x 10- 5 in both wild type and Ogg1-1-mice, and 1. 91 x 10-5 in Ogg1-1-; Mutyh- 1- mice, indicating that the overall spontaneous mutation frequency was increased in Ogg1-1-; Mutyh-1- mice, but not in Ogg1-1- mice. Analysis of the mutation spectrum revealed that the frequency of G: C to T: A transver-sions were significantly increased in both Ogg1-1- and Ogg1-1-; Mutyh- 1- mice; a 5-fold increase in Ogg1-1- mice, and a 41-fold increase in Ogg1-1-; Mutyh-1- mice when compar-ed with wild type mice. A previous study in our laboratory indicated that a defect in Mutyh caused a 4- fold increase in the frequency of G: C to T: A transversions in mice. Combined, these observations suggest that a cooperative function between Oggl and Mutyh exists to prevent 8- oxoG-related mutagenesis in mammals