Abstract
Clustered DNA damage is defined as two or more lesions generated within 1-2 helical turns (10-20bp) of DNA. Ionizing radiation has a characteristic to deposit energy to local area on DNA and to induce clustered DNA damage. Theoretical studies and experimental data suggest an increased complexity and severity of clustered DNA damage with increasing LET of ionizing radiation. It is proposed that ionizing radiation produces clustered DNA damage including single strand break (SSB) and double strand break (DSB) with several oxidative base lesions. It is postulated that clustered DNA damage is incompletely or incorrectly repaired, and is strongly related to biological effects, such as cell death and mutation.
We measured mutation frequencies induced by two types of clustered damage carrying a SSB and an 8-oxo-7,8-dihydroguanine (8-oxoG) each on the complementary strands or both on the same DNA strand in E. coli. We found that the clustered damage with 8-oxoG and SSB each placed on complementary strands had a higher mutation frequency than single 8-oxoG, while the clustered damage with 8-oxoG and SSB positioned on the same strand had a slightly lower mutation frequency than single 8-oxoG either in wild type or in glycosylase (Fpg and/or MutY) deficient mutants. These results suggest that SSB opposed to 8-oxoG retards the excision of 8-oxoG by formamidopyrimidine DNA glycosylase (Fpg). However, processing of the tandem lesion of SSB and 8-oxoG remains to be elucidated. In this study, using in vitro enzyme assay, we investigated the processing of clustered damage containing 8-oxoG and SSB on the same DNA strand.
