DNA ADDUCT 8-HYDROXYL-2'-DEOXYGUANOSINE (8-HYDROXYGUANINE) AFFECTS FUNCTION OF HUMAN DNA METHYLTRANSFERASE

8-Hydroxyl-2'-deoxyguanosine (also referred to as 8-hydroxyguanine [8-OH-dG] or 7,8-dihydro-8-oxoguanine), a common DNA adduct resulting from injury to DNA via reactive oxygen species, affects the in vitro methylation of nearby cytosine moieties by the human DNA methyltransferase. The exact position of 8-OH-deoxyguanosine relative to a CpG dinucleotide appears important to this effect. Our data indicate that 8-OH-deoxyguanosine diminishes the ability of the methyltransferase to methylate a target cytosine when the 8-OH-deoxyguanosine is one or two nucleotides 3' from the cytosine, on the same strand. On the other hand 8-OH-deoxyguanosine does not diminish the ability of the enzyme to respond to a methyl director (5-methylcytosine) when the 8-OH-deoxyguanosine is on the same strand but one or two nucleotides 3' from the methyl director. Differences in methylation rates as great as 13-fold have been detected using various 8-OH-deoxyguanosine-containing oligonucleotides as substrates in methylation assays. Our findings suggest that oxidative damage of parental strand guanines would permit normal copying of methylation patterns through maintenance methylation, while oxidative damage of guanines in the nascent strand DNA would inhibit such methylation.