酸化ストレスによるクラスターDNA損傷機構とその変異における役割(日本環境変異原学会第32回大会シンポジウム2「DNA損傷と変異のメカニズム」)

Reactive oxygen species generated by environmental factors, such as radiation, UV and chemicals can cause sequence-specific DNA damage and play important roles in mutagenesis and carcinogenesis. We have investigated sequence specificity of oxidative stress-mediated DNA damage by using ^<32>P-labeled DNA fragments obtained from the human c-Ha-ras-1, p53 and p16 genes. Free hydroxyl radicals cause DNA damage with no marked site specificity. Copper-hydroperoxo complex caused DNA damage at thymine, cytosine and guanine residues. ^1O_2 preferentially induces lesions at guanine residues. Benzoyloxyl radical specifically causes damage to the 5'-G in GG sequence ; this sequence is easily oxidized because a large part of the highest occupied molecular orbital of this radical is distributed on this site. Recently, we demonstrated that BP-7, 8-dione, a metabolite of carcinogenic benzo [a] pyrene (BP), strongly damaged the G and C of the 5'-ACG-3' sequence complementary to codon 273 of the p53 gene in the presence of NADH and Cu (II). BP-7, 8-dione also caused preferential double base lesion at 5'-TG-3' sequences. Since clustered DNA damage is poorly repaired, it is speculated that induction of the double base lesions in DNA might lead to activation of proto-oncogene or inactivation of the tumor suppressor gene. Therefore, oxidative DNA damage induced by BP-7,8-dione, especially double base lesions, may participate in the expression of carcinogenicity of BP in addition to DNA adduct formation. Here, we discuss the mechanisms of sequence-specific DNA damage including clustered DNA damage in relation to mutagenesis and carcinogenesis.