抄録
Ionizing radiation generates free radicals along its track and produces locally multiply damaged sites in DNA molecules. Among these sites, those containing base lesions and/or strand breaks (SSBs) within a few helical turns of DNA across both strands are referred to as clustered damage. Clustered damage is important to elucidate the biological consequences of radiation since it is believed to be repair-resistant or to lead to error prone repair. Clustered damage containing base lesions (clustered base damage) is repaired by the base excision repair (BER) pathway. During the initial step of BER, SSB is formed by the AP lyase activity of DNA glycosylase or AP endnuclease. Therefore, processing of clustered base damage by BER could result in the formation of fatal DSB lesions.
In the present study, we have examined the activity of hOGG1 for clustered base damage containing two 8-oxoguanine (8-oxoG) residues and analyzed the formation of DSBs. In substrates, the 8-oxoG residue in the top strand is 1 to 5 bp away from the second one in the bottom strand in the 3' or 5' direction. Substrates were incubated with hOGG1, and the amounts of SSBs and DSBs induced by hOGG1 were analyzed by PAGE. The excision of the first 8-oxoG was not influenced significantly by closely opposed 8-oxoG. The subsequent excision of 8-oxoG was retarded moderately and resulted in DSBs. Interestingly, some 8-oxoG clusters were converted to DSBs very inefficiently. We are also examining the activity of hOGG1 in the presence of hAPE1.
