Clustered Damages and Total Lesions Induced in DNA by Ionizing Radiation:  Oxidized Bases and Strand Breaks

Abstract

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 mutagenic lesions. Using highly purified enzymes that cleave DNA at specific classes 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 involving both strands, and the composition and origin of such clusters. The relative frequency of isolated vs clustered damages depends on the identity of the lesion, with approximately 15-18% of oxidized purines, pyrimidines, or abasic sites in clusters recognized by Fpg, Nth, or Nfo proteins, respectively, but only about half that level of frank single strand breaks in double strand breaks. Oxidized base clusters and abasic site clusters constitute about 80% of complex damages, while double strand breaks comprise only approximately 20% of the total. The data also show that each cluster results from a single radiation (track) event, and thus clusters will be formed at low as well as high radiation doses.

Cellular Responses to the DNA Strand-scission Enediyne C-1027 Can Be Independent of ATM, ATR, and DNA-PK Kinases

“…The main mechanism of DNA scission by IR or radiomimetic drugs is based on oxidation of deoxyribose in DNA, initiated by hydrogen abstraction at C-1Ј, C-4Ј, and/or C-5Ј carbon atoms (33,34). In addition to strand breaks, IR produces a multitude of different base and nucleotide damage in DNA, including heterogeneous clustered damage (35). In contrast, radiomimetic drugs usually cause only a subset of the IR-induced DNA damage.…”

Low-dose radiation: Thresholds, bystander effects, and adaptive responses

“…This heterogeneity results from random collateral DNA damage generated near the DSB sites by other free radicals usually from the same cluster of reactive oxygen species. The existence of these sites, termed locally multiply damaged sites (16), has been substantiated by using enzymes specific for different types of DNA damage (17,18). Of the DNA lesions in locally multiply damaged sites, Ϸ20% were found to be DSBs, and the rest were composed of single-strand breaks, altered bases, and damaged deoxyribose backbones.…”

Abortive base-excision repair of radiation-induced clustered DNA lesions in Escherichia coli

“…When two opposing single-strand breaks are formed within a cluster, a potentially lethal double-strand break results. In addition, a number of studies using irradiated DNA have shown that DNA double-strand breaks can be formed by postirradiation treatment with enzymes that recognize single DNA lesions (11)(12)(13)(14)(15), suggesting that these lesions may be closely opposed to each other or to single-strand breaks.…”