Abstract
Ionizing radiation produces a unique form of DNA damage called clustered damage, which contains two or more lesions such as single-strand breaks and oxidative base damage induced within the one or two helical turns of DNA. Clustered damage would be less readily repaired than isolated lesions. Therefore clustered damage might be biologically significant. In this report we show that E. coli mutM nth nei triple mutant was less sensitive to the lethal effect of sparsely ionizing radiation (gamma-rays and X-rays) than the wild-type strain. Overproduction of MutM, Nth and Nei increased the sensitivity to gamma-rays, whereas it did not affect the sensitivity to alpha-particles. Increased sensitivity to gamma-rays also occurred in E. coli overproducing human hOgg1. Treatment of gamma-irradiated plasmid DNA with purified MutM converted the covalently closed circular to the linear form of the DNA. Furthermore, the present study revealed that HeLaS3 cells transfected by Ogg1 type1a or type2a plasmid were more sensitive to gamma-rays than HeLaS3 cells without plasmid. Ogg1 protein localized in nuclei and Ogg1 type2a in mitochondria. Overexpression of these Ogg1 proteins in HeLa cells was confirmed by RT-PCR procedures. Excessive excision by DNA glycosylases converts nearly opposite base damage in clustered DNA damage to double-strand breaks, which are potentially lethal. We are currently investigating the biological effects and repair mechanisms of clustered DNA damages in human cells.
