Cold Spring Harbor Symposia on Quantitative Biology, Volume LXV.© 2000 Cold Spring Harbor Laboratory Press 0-87969-605-2/00. 127 bile glycosyl bond of 7-methyldeoxyguanosine residues. The generation and subsequent loss of this common base lesion contribute significantly to the total DNA depurination rate with about 3000 abasic sites formed per day. A steady state of about 4000 7-methylguanine residues per genome apparently occurs (Rydberg and Lindahl 1982). In normal repair-proficient mammalian cells, 7-methylguanine is most likely the main aberrant base residue regularly present in DNA; it seems unlikely that any potentially cytotoxic or promutagenic base would be allowed to remain at such a level. The existence of minor endogenous methylating agents other than SAM has been indicated by spontaneous mutator phenotypes of repair-deficient bacteria (Rebeck and Samson 1991). These agents may be more closely related to methylnitrosourea, induce formation of both O6-methylguanine and 3-methyladenine in DNA, and can arise by nitrosation of endogenous metabolites (Taverna and Sedgwick 1996). Bacteria and eukaryotic cells have specific repair enzymes to remove the various major types of oxidative damage from DNA. A defect in DNA repair of the most common mutagenic base lesion, 8-hydroxyguanine (8-oxoG), leads to a strong mutator phenotype in bacteria and yeast (Michaels and Miller 1992; Thomas et al. 1996). These data show that repair of endogenous DNA damage generated by reactive oxygen species is relevant in vivo and that significant endogenous oxidative damage to DNA occurs continuously, perhaps at a similar level as damage due to hydrolysis and endogenous alkylation. However, the relative importance of this form of DNA decay remains unclear in comparison with other types of intrinsic DNA damage. This is because the intracellular, and especially intranuclear, concentrations of the relevant reactive oxygen species have not been determined, whereas the intracellular concentration of, for example, water that accounts for hydrolytic degradation is known to be 55 M. Reactive oxygen species that damage DNA are generated by iron-mediated Fenton reactions (Henle and Linn 1997), and the frequency of such reactions in the cell nucleus is not known. Furthermore, in eukaryotic cells, most oxygen metabolism has been delegated to mitochondria, so the cell nucleus is practically anoxic (Joenje 1989). Processes such as lipid peroxidation in the nucleus and cytoplasm are clearly relevant, however, and cellular stress may increase the amount of 8-oxoG formed in DNA (Conlon et al. 2000). Exocyclic DNA base adducts generated from lipid peroxidation by-products have been characterized. The most abundant of these appears to be the pyrimidopurinone M1G, which is generated by reaction between a G residue in DNA and the lipid peroxidation product, malondialdehyde (Fink et al. 1997). In addition, lipid peroxidation may yield acrolein and crotonaldehyde, which are readily metabolized to epoxides that can generate exocyclic etheno modifications of DNA bases. Two such bases, etheno-A and etheno-C, are excised efficiently by DNA glycosylases (Hang et al. 1998; Saparbaev and Laval 1998), which strongly suggests that generation of such adducts occurs at sufficiently high rates in vivo to endanger genomic stability.

Oxygen free radicals generate small amounts of bulky DNA damage that require repair by the nucleotide excision repair pathway, in addition to the many base alterations corrected by BER. A particularly interesting lesion of this class is the 5, 8-cyclopurine deoxynucleoside (Brooks et al …