Efficiency of excision of 8-oxo-guanine within DNA clustered damage by XRS5 nuclear extracts and purified human OGG1 protein

A major DNA lesion is the strongly mutagenic 8-oxo-7,8-dihydroguanine (8-ox oG) base, formed by oxidative attack at guanine and which leads to a high l evel of G-C-T-A transversions. Clustered DNA damages are formed in DNA foll owing exposure to ionizing radiation or radiomimetic anticancer agents and are thought to be biologically severe. The presence of 8-oxoG within cluste red DNA damage may present a challenge to the repair machinery of the cell, if the OGG1 DNA glycosylase/AP lyase protein, present in eukaryotic cells, does not efficiently excise its substrate, 8-oxoG. In this study, specific oligonucleotide constructs containing an 8-oxoG located in several positio ns opposite to another damage (5,6-dihydrothymine (DHT), uracil, 8-oxoG, AP site, or various types of single strand breaks) were used to determine the relative efficiency of purified human OGG1 and mammalian XRS5 nuclear extr acts to excise 8-oxoG from clustered damages. A base damage (DHT, uracil, a nd 8-oxoG) on the opposite strand has little or no influence on the rate of excision of 8-oxoG whereas the presence of either an AP site or various ty pes of single strand breaks has a strong inhibitory effect on the formation of a SSB due to the excision of 8-oxoG by both hOGG1 and the nuclear extra ct. The binding of hOGG1 to 8-oxoG is not significantly affected by the pre sence of a neighboring lesion.