Human Gene Expression Regulated by Epigenetic-Like Oxidative DNA Modification.

The DNA repair gene is induced in cells or animal models experiencing oxidative or inflammatory stress along with oxidation of guanine (G) to 8-oxo-7,8-dihydroguanine (OG) in the genome. We hypothesize that a G-rich promoter element that is a potential G-quadruplex-forming sequence (PQS) in is a site for introduction of OG with epigenetic-like potential for gene regulation. Activation occurs when OG is formed in the PQS located near the transcription start site. Oxidative stress either introduced by TNFα or synthetically incorporated into precise locations focuses the base excision repair process to read and catalyze removal of OG via OG-glycosylase I (OGG1), yielding an abasic site (AP). Thermodynamic studies showed that AP destabilizes the duplex, enabling a structural transition of the sequence to a G-quadruplex (G4) fold that positions the AP in a loop facilitated by the PQS having five G runs in which the four unmodified runs adopt a stable G4. This presents AP to apurinic/apyrimidinic endonuclease 1 (APE1) that poorly cleaves the AP backbone in this context according to in vitro studies, allowing the protein to function as a trans activator of transcription. The proposal is supported by chemical studies in cellulo and in vitro. Activation of expression via the proposed mechanism allows cells to respond to mutagenic DNA damage removed by NEIL3 associated with oxidative or inflammatory stress. Lastly, inspection of many mammalian genomes identified conservation of the PQS, suggesting this sequence was favorably selected to function as a redox switch with OG as the epigenetic-like regulatory modification.