Oxidative stress and OGG1 : beyond OGG1-initiated BER

Oxidative stress and subsequent oxidative DNA damage play an important role in the process of ageing and are also linked to numerous different diseases. A prevalent type of oxidative DNA damage is the oxidation of guanine, generating 8-oxoguanine. It is therefore unsurprising that 8-oxoguanine has been found to be elevated in the genome in a range of pathological conditions, including multiple cancers. However, whether 8-oxoguanine drives the progression of these diseases or is a result of the disease process itself is often unclear. 8-oxoguanine is recognised in genomic DNA by the glycosylase, 8-oxoguanine glycosylase-1 (OGG1), which initiates removal by the base excision repair pathway. To study cellular function, OGG1-null mouse models and in vitro cell models have been previously generated. Primarily, this thesis is an investigation into the cellular role of OGG1 and functional changes in cells that lack OGG1-activity. We show that OGG1-null cells exist in a state of oxidative stress, possessing a persistent elevation of intracellular reactive oxygen species. Further increasing the oxidative stress burden by the depletion of the antioxidant, glutathione, induces apoptosis in OGG1-null cells. Surprisingly, however we show that induction of apoptosis is not accompanied by an accumulation of genomic 8-oxoguanine. This finding highlights the importance of backup repair mechanism, which are able to repair 8-oxoguanine in the absence of OGG1. Using gene expression and biochemical studies, we have identified two potential backup mechanisms: components of the nucleotide excision repair pathway (ERCC1, ERCC4 and ERCC5) and the Nei-like DNA glycosylase-3 (NEIL3). Lastly, we have confirmed that OGG1 possesses an additional cellular role, acting as a signalling molecule to activate small GTPases, activating multiple downstream pathways. This stresses the importance of not only considering OGG1 as a glycosylase, with multiple additional functions having already been investigated.