Increased mutability and decreased repairability of a three-lesion clustered DNA-damaged site comprised of an AP site and bi-stranded 8-oxoG lesions

PURPOSE Ionizing radiation induces DNA damage, some of which are present in clusters, defined as two or more lesions within one to two helical turns of DNA by passage of a single radiation track. These clusters are thought to contribute to the detrimental effects of radiation, in part due to the compromised repair of clustered DNA damaged sites. MATERIALS AND METHODS The repair of three-lesion cluster present in oligonucleotides were determined in vitro using the hamster cell line CHO-K1 nuclear extract or purified proteins involved in base excision repair. The mutagenic potential of these clusters present in a plasmid was determined using an Escherichia coli reporter assay. RESULTS We have shown that the repair of an abasic (AP) site within a three-lesion cluster, comprised of an AP site and bi-stranded 8-oxo-7,8-dihydroguanine (8-oxoG) lesions, is retarded compared to that of an isolated AP site in an in vitro base excision repair (BER) assay. Further, the mutation frequency of the clustered damaged site is up to three times greater than that of an isolated 8-oxoG lesion. CONCLUSIONS As a consequence of enhanced mutagenic potential of clusters, non-double-strand break (DSB) DNA damage may contribute to the detrimental effects of radiation, in addition to the effects of DSB.