8-Oxo-2'-deoxyguanosine (8-oxo-dG) is an oxidized derivative of deoxyguanosine. 8-Oxo-dG is one of the major products of DNA oxidation. Concentrations of 8-oxo-dG within a cell are a measurement of oxidative stress.
Steady-state levels of DNA damages represent the balance between formation and repair. Swenberg et al. measured average frequencies of steady state endogenous DNA damages in mammalian cells. The most frequent oxidative DNA damage normally present in DNA is 8-oxo-dG, occurring at an average frequency of 2,400 per cell.
When 8-oxo-dG is induced by a DNA damaging agent it is rapidly repaired. For example, 8-oxo-dG was increased 10-fold in the livers of mice subjected to ionizing radiation, but the excess 8-oxo-dG was rapidly removed with a half-life of 11 minutes.
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 shown in this section, the colonic epithelium from a mouse on a normal diet has 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) has 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, and this leads to tumorigenesis and carcinogenesis. Of 22 mice fed the diet supplemented with deoxycholate, 20 (91%) developed colonic tumors after 10 months on the diet, and the tumors in 10 of these mice (45% of mice) included an adenocarcinoma (cancer).
8-oxo-dG increases with age in DNA of mammalian tissues. 8-oxo-dG increases in both mitochonndrial DNA and nuclear DNA with age. Fraga et al. estimated that in rat kidney, for every 54 residues of 8-oxo-dG repaired, one residue remaines unrepaired. (See also DNA damage theory of aging.)
Valavanidis et al. pointed out that oxidative DNA damage, such as 8-oxo-dG, likely contributes to carcinogenesis by two mechanisms. The first mechanism involves modulation of gene expression, whereas the second is through the induction of mutations.
Epigenetic alteration, for instance by methylation of CpG islands in a promoter region of a gene, can repress expression of the gene (see DNA methylation). In general, epigenetic alteration can modulate gene expression. As reviewed by Bernstein and Bernstein, the repair of various types of DNA damages can, with low frequency, leave remnants of the different repair processes and thereby cause epigenetic alterations. 8-Oxo-dG is primarily repaired by base excision repair (BER). Li et al. reviewed studies indicating that one or more BER proteins also participate(s) in epigenetic alterations involving DNA methylation, demethylation or reactions coupled to histone modification. Nishida et al. examined 8-oxo-dG levels and also evaluated promoter methylation of 11 tumor suppressor genes (TSGs) in 128 liver biopsy samples. These biopsies were taken from patients with chronic hepatitis C, a condition causing oxidative damages in the liver. Among 5 factors evaluated, only increased levels of 8-oxo-dG was highly correlated with promoter methylation of TSGs (p<0.0001). This promoter methylation could have reduced expression of these tumor suppressor genes and contributed to carcinogenesis.
Yasui et al. examined the fate of 8-oxo-dG when this oxidized derivative of deoxyguanosine was inserted into the thymidine kinase gene in a chromosome within human lymphoblastoid cells in culture. They inserted 8-oxo-dG into about 800 cells, and could detect the products that occurred after the insertion of this altered base, as determined from the clones produced after growth of the cells. 8-Oxo-dG was restored to G in 86% of the clones, probably reflecting accurate base excision repair or translesion synthesis without mutation. G:C to T:A transversions occurred in 5.9% of the clones, single base deletions in 2.1% and G:C to C:G transversions in 1.2%. Together, these more common mutations totaled 9.2% of the 14% of mutations generated at the site of the 8-oxo-dG insertion. Among the other mutations in the 800 clones analyzed, there were also 3 larger deletions, of sizes 6, 33 and 135 base pairs. Thus 8-oxo-dG, if not repaired, can directly cause frequent mutations, some of which may contribute to carcinogenesis.