| Symbol: | OGG_N |
| 8-oxoguanine DNA glycosylase, N-terminal domain | |
| Pfam: | PF07934 |
| Pfam Clan: | CL0407 |
| Interpro: | IPR012904 |
| Scop: | 1ebm |
8-Oxoguanine glycosylase, also known as OGG1, is a DNA glycosylase enzyme that, in humans, is encoded by the OGG1 gene. It is involved in base excision repair. It is found in bacterial, archaeal and eukaryotic species.
OGG1 is the primary enzyme responsible for the excision of 8-oxoguanine (8-oxoG), a mutagenic base byproduct that occurs as a result of exposure to reactive oxygen species (ROS). OGG1 is a bifunctional glycosylase, as it is able to both cleave the glycosidic bond of the mutagenic lesion and cause a strand break in the DNA backbone. Alternative splicing of the C-terminal region of this gene classifies splice variants into two major groups, type 1 and type 2, depending on the last exon of the sequence. Type 1 alternative splice variants end with exon 7 and type 2 end with exon 8. One set of spliced forms are designated 1a, 1b, 2a to 2e. All variants have the N-terminal region in common. Many alternative splice variants for this gene have been described, but the full-length nature for every variant has not been determined. In eukaryotes, the N-terminus of this gene contains a mitochondrial targeting signal, essential for mitochondrial localization. However, OGG1-1a also has a nuclear location signal at its C-terminal end that suppresses mitochondrial targeting and causes OGG1-1a to localize to the nucleus. The main form of OGG1 that localizes to the mitochondria is OGG1-2a. A conserved N-terminal domain contributes residues to the 8-oxoguanine binding pocket. This domain is organised into a single copy of a TBP-like fold.
Despite the presumed importance of this enzyme, mice lacking Ogg1 have been generated and found to have a normal lifespan, and Ogg1 knockout mice have a higher probability to develop cancer, whereas MTH1 gene disruption concomitantly suppresses lung cancer development in Ogg1-/- mice. Mice lacking Ogg1 have been shown to be prone to increased body weight and obesity, as well as high-fat-diet-induced insulin resistance. There is some controversy as to whether deletion of Ogg1 actually leads to increased 8-Oxo-2'-deoxyguanosine (8-oxo-dG) levels: high performance liquid chromatography with electrochemical detection (HPLC-ECD) assay suggests the deletion can lead to an up to 6 fold higher level of 8-oxo-dG in nuclear DNA and a 20-fold higher level in mitochondrial DNA, whereas DNA-fapy glycosylase assay indicates no change in 8-oxo-dG levels.
Increased oxidant stress temporarily inactivates OGG1, which recruits transcription factors such as NFkB and thereby activates expression of inflammatory genes.[1]
Mice without a functional OGG1 gene have about a 5-fold increased level of 8-oxo-dG in their livers compared to mice with wild-type OGG1. Mice defective in OGG1 also have an increased risk for cancer. Kunisada et al. irradiated mice without a functional OGG1 gene (OGG1 knock-out mice) and wild-type mice three times a week for 40 weeks with UVB light at a relatively low dose (not enough to cause skin redness). Both types of mice had high levels of 8-oxo-dG in their epidermal cells three hours after irradiation. After 24 hours, over half of the initial amount of 8-oxo-dG was absent from the epidermal cells of the wild-type mice, but 8-oxo-dG remained elevated in the epidermal cells of the OGG1 knock-out mice. The irradiated OGG1 knock-out mice went on to develop more than twice the incidence of skin tumors compared to irradiated wild-type mice, and the rate of malignancy within the tumors was higher in the OGG1 knock-out mice (73%) than in the wild-type mice (50%).
As reviewed by Valavanidis et al., increased levels of 8-oxo-dG in a tissue can serve as a biomarker of oxidative stress. They also noted that increased levels of 8-oxo-dG are frequently found during carcinogenesis.
In the figure showing examples of mouse colonic epithelium, the colonic epithelium from a mouse on a normal diet was found to have a low level of 8-oxo-dG in its colonic crypts (panel A). However, a mouse likely undergoing colonic tumorigenesis (due to deoxycholate added to its diet) was found to have a high level of 8-oxo-dG in its colonic epithelium (panel B). Deoxycholate increases intracellular production of reactive oxygen resulting in increased oxidative stress,