The relationship between 8-oxo-7,8-dihydro-2'-deoxyguanosine level and extent of cytosine methylation in leukocytes DNA of healthy subjects and in patients with colon adenomas and carcinomas

It has been known for a long time that DNA hypomethylation occurs in many human cancers and precancerous conditions. However, the mechanisms of hypomethylation are largely unknown. It is possible that endogenous 8-oxo-7,8-dihydroguanine (8-oxoGua) level may be linked to aberrant DNA methylation of adjacent cytosine and in this way influences carcinogenesis. Therefore, the aim of the present study was to assess a possible link between 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) background level and 5-methylcytosine content in DNA from human leukocytes of healthy subjects (n = 105) as well as in patients with colon adenomas (n = 39) and carcinomas (n = 50).

Our results demonstrated statistically significant negative correlation between background level of 8-oxodG and 5-methylcytosine content in DNA isolated from leukocytes of healthy donors (r = ?0.3436, p = 0.0003). The mean content of 5-methylcytosine was significantly lower, while 8-oxodG level was significantly higher in leukocytes DNA of patients with colon adenomas and carcinomas in comparison with healthy subjects. The mean values for 5-methylcytosine were: 3.59 ± 0.173% (healthy subjects), 3.38 ± 0.128% (patients with adenomas), 3.40 ± 0.208% (colon cancer patients). The mean values of 8-oxodG in DNA were, respectively: 4.67 ± 1.276, 5.72 ± 1.787, 5.76 ± 1.884 8-oxodG per 10⁶ dG molecules. DNA from affected tissue (colon) suffered from significant, about 10% reduction in cytosine methylation in comparison with leukocytes of the paired subjects.

Our work provides the first in vivo evidence suggesting that increased levels of 8-oxodG in DNA may lead to carcinogenesis not only via mispair/mutagenic potential of the modified base but also through its ability to influence gene expression by affecting DNA methylation.