Clustered damages and total lesions induced in DNA by ionizing radiation: Oxidized bases and strand breaks

Ionizing radiation induces both isolated DNA lesions and clustered damages- multiple closely spaced lesions (strand breaks, oxidized purines, oxidized pyrimidines, or abasic sites within a few helical turns). Such clusters are postulated to be difficult to repair and thus potentially lethal or mutage nic lesions. Using highly purified enzymes that cleave DNA at specific clas ses of damage and electrophoretic assays developed for quantifying isolated and clustered damages in high molecular length genomic DNAs, we determined the relative frequencies of total lesions and of clustered damages involvi ng both strands, and the composition and origin of such clusters. The relat ive frequency of isolated vs clustered damages depends on the identity of t he lesion, with similar to 15-18% of oxidized purines, pyrimidines, or abas ic sites in clusters recognized by Fpg, Nth, or Nfo proteins, respectively, but only about half that level of frank single strand breaks in double str and breaks. Oxidized base clusters and abasic site clusters constitute abou t 80% of complex damages, while double strand breaks comprise only similar to 20% of the total. The data also show that each cluster results from a si ngle radiation (track) event, and thus clusters will be formed at low as we ll as high radiation doses.