In vivo repair of methylation damage in Aag 3-methyladenine DNA glycosylase null mouse cells

3-Methyladenine (3MeA) DNA glycosylases initiate base excision repair by re moving 3MeA, These glycosylases also remove a broad spectrum of spontaneous and environmentally induced base lesions in vitro. Mouse cells lacking the Aag 3MeA DNA glycosylase (also known as the Mpg, APNG or ANPG DNA glycosyl ase) are susceptible to 3MeA-induced S phase arrest, chromosome aberrations and apoptosis, but it is not known if Aag is solely responsible for repair of 3MeA in vivo. Here we show that in Aag(-/-) cells, 3MeA lesions disappe ar from the genome slightly faster than would be expected by spontaneous de purination alone, suggesting that there may be residual repair of 3MeA, How ever, repair of 3MeA is at least 10 times slower in Aag(-/-) cells than in Aag(+/+) cells. Consequently, 24 h after exposure to [H-3]MNU, 30% of the o riginal 3MeA burden is intact in Aag(-/-) cells, while 3MeA is undetectable in Aag(+/+) cells. Thus, Aag is the major DNA glycosylase for 3MeA repair. We also investigated the in vivo repair kinetics of another Aag substrate, 7-methylguanine. Surprisingly, 7-methylguanine is removed equally efficien tly in Aag(+/+) and Aag(-/-) cells, suggesting that another DNA glycosylase acts on lesions previously thought to be repaired by Aag.