Characterization of the hOGG1 promoter and its expression during the cell cycle.

The human OGG1 gene codes for a 38kD protein with an antimutator activity related to its capacity to excise the mutagenic base 8-OH-Guanine from DNA. Mutant forms of this gene have been found in lung and kidney tumors. The determination of the start of transcription allowed the definition of the promoter sequences for the gene. By transient transfection and a luciferase reporter assay a 135 base pair region immediately upstream of the transcription start is shown to have full promoter activity. Two CpG islands and an Alu repeat were identified within the promoter and the 5' sequences of the transcribed region. The lack of TATA or CAAT boxes suggests that OGG1 is a housekeeping gene. Consistently, its expression, measured as the transcription from the promoter or as the enzymatic activity in cultured fibroblast cell lines, does not vary during the cell cycle.

Detection of 8-oxoG DNA glycosylase activity and OGG1 transcripts in the rat CNS.

The oxoguanine DNA glycosylase (Ogg1) is a DNA repair enzyme that excises 7,8-dihydro-8-oxoguanine present in DNA damaged by oxidative stress. We have investigated the expression of the OGG1 gene in different regions of the rat CNS. Biochemical studies on brain homogenates of adult rats have shown that Ogg1 nicking activity is present at relatively similar levels in the cerebral cortex, the hypothalamus, the pons and the cerebellum. Following in situ hybridization with radiolabeled OGG1 cDNA or specific antisense oligonucleotides, OGG1 transcripts showed a widespread but heterogeneous distribution pattern among distinct brain regions of adult rats: high levels of this transcript were detected in the CA1-CA3 layers and the gyrus dentate of the hippocampal formation, the piriform cortex, the supraoptic nuclei, the olivary complex as well as in the pyramidal cells of layer V of the cortex and the Purkinje cells of the cerebellum. In peripheral organs such as the lungs, the stomach and the spleen, OGG1 transcript is however expressed in specific subpopulations of cells. Using a semi-quantitative reverse transcription - polymerase chain reaction assay on total mRNA from the frontal cortex, OGG1 mRNA was determined to be expressed with relatively the same levels in 1-day-old and 7-day-old rats as well as in adult rats. These results provide evidence for the widespread expression of the OGG1 gene in developing and adult brains.

Overexpression of Ogg1 in mammalian cells: effects on induced and spontaneous oxidative DNA damage and mutagenesis.

Chinese hamster ovary cell lines (AA8 and AS52) were stably transfected to overexpress hOgg1 protein, the human DNA repair glycosylase for 7,8-dihydro-8-oxoguanine (8-oxoG). In the transfectants, the repair rate of 8-oxoG residues induced by either potassium bromate or the photosensitizer [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl)-carbo nyl ]-2-pyrrolidinemethanolplus light was up to 3-fold more rapid than in the parental cells. However, the improved repair had little effect on the mutagenicity of potassium bromate in the guanine phosphoribosyl transferase (gpt) locus of the OGG1-transfected AS52 cells. The steady-state (background) levels of DNA base modifications sensitive to Fpg protein, which include 8-oxoG, in cells not exposed to a damaging agent were not reduced by the overexpression of Ogg1 protein. Moreover, the spontaneous mutation rates in the gpt locus were similar in OGG1-transformed and vector-only-transformed cells. The results demonstrate the potential of Ogg1 protein to remove its substrate modifications from most of the chromosomal DNA. They indicate, on the other hand, that the Ogg1 protein alone may not be rate limiting for the repair of the residual substrate modifications observed in cells under normal growth conditions.