Regulation of GC box activity by 8-oxoguanine.

The oxidation-induced DNA modification 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) was recently implicated in the activation and repression of gene transcription. We aimed at a systematic characterisation of the impacts of 8-oxodG on the activity of a GC box placed upstream from the RNA polymerase II core promoter. With the help of reporters carrying single synthetic 8-oxodG residues at four conserved G:C base pairs (underlined) within the 5'-TGGGCGGAGC-3' GC box sequence, we identified two modes of interference of 8-oxodG with the promoter activity. Firstly, 8-oxodG in the purine-rich (but not in the pyrimidine-rich) strand caused direct impairment of transcriptional activation. In addition, and independently of the first mechanism, 8-oxodG initiated a decline of the gene expression, which was mediated by the specific DNA glycosylase OGG1. For the different 8-oxodG positions, the magnitude of this effect reflected the excision preferences of OGG1. Thus, 8-oxodG seeded in the pyrimidine-rich strand was excised with the highest efficiency and caused the most pronounced decrease of the promoter activity. Conversely, 8-oxodG in the symmetric position within the same CpG dinucleotide, was poorly excised and induced no decline of the gene expression. Of note, abasic lesions caused gene silencing in both positions. By contrast, an uncleavable apurinic lesion in the pyrimidine-rich strand enhanced the GC box activity, suggesting that the AP endonuclease step provides a switch between the active versus repressed promoter states during base excision repair.