hOGG1-Cys326 variant cells are hypersensitive to DNA repair inhibition by nitric oxide.

The repair of 8-oxo-7,8-dihydroguanine in the DNA of mammalian cells is initiated by 8-oxoguanine DNA glycosylase (OGG1). A frequent polymorphism in the human OGG1 gene, rs1052133, causes the substitution of serine by cysteine at amino acid 326 of the protein and has been associated with an altered risk for various types of cancer in some populations. The OGG1-Cys326 protein appears to have normal enzymatic activity, but greater sensitivity to oxidation than the serine variant. Here, we describe a comparison of the cellular repair by the two OGG1 variants under stress conditions characteristic of inflammation, namely in cells pretreated with nitric oxide (NO) or pre-exposed to hyperthermia. The results show that NO at concentrations causing negligible DNA damage and little cytotoxicity strongly reduces the repair rates of oxidized purines in the DNA of HeLa cells overexpressing the OGG1-Cys326 variant. The reduction in repair was much less pronounced in isogenic cells overexpressing the OGG1-Ser326 variant. Similar results were observed in EBV-transformed lymphocytes from donors homozygous for the two OGG1 variant alleles. In contrast, hyperthermia-induced stress caused a repair retardation that was independent of the OGG1 polymorphism. The repair inhibition by NO in the variant cells gave rise to increased genetic instability, measured as increased micronuclei formation after oxidant exposure. The results could explain a higher risk of malignant transformation in inflamed tissues of carriers of this variant allele.

Influences of histone deacetylase inhibitors and resveratrol on DNA repair and chromatin compaction.

Accessibility of DNA is a prerequisite for both DNA damage and repair. Therefore, the chromatin structure is expected to have major impact on both processes, with opposite consequences for the stability of the genome. To analyse the influence of chromatin compaction on the generation and repair of various types of DNA modifications, we modulated the global chromatin structure of AS52 Chinese hamster ovary cells and HeLa cells by treatment with either histone deacetylase inhibitors or resveratrol and measured the repair kinetics of (i) pyrimidine dimers induced by ultraviolet B, (ii) oxidised purines generated by photosensitisation and (iii) single-strand breaks induced by H2O2, using an alkaline elution technique. The decrease of chromatin compaction (detected as reduced DNA accessibility to DNase I) after treatment with trichostatin A or butyrate slightly increased the damage generation but had no significant effect on the global repair rates. In contrast, incubation of AS52 cells with resveratrol at concentrations that caused significant chromatin compaction and that had only moderate influence on cell proliferation gave rise to a strong decrease of the repair rates of all three types of DNA modifications. Similar, but less pronounced effects were observed in HeLa cells. The effects of resveratrol on the repair rates were not antagonised by the sirtuin inhibitor EX-527 or by an increase of the intracellular thiol levels.