| Summary: | DNA modifications can be used to monitor pathological processes. We have previously shown that estimating the amount of the main DNA epigenetic mark, 5-methylcytosine (m<sup>5</sup>C), is an efficient and reliable way to diagnose brain tumors, hypertension, and other diseases. Abnormal increases of reactive oxygen species (ROS) are a driving factor for mutations that lead to changes in m<sup>5</sup>C levels and cancer evolution. 8-oxo-deoxyguanosine (8-oxo-dG) is a specific marker of ROS-driven DNA-damage, and its accumulation makes m<sup>5</sup>C a hotspot for mutations. It is unknown how m<sup>5</sup>C and 8-oxo-dG correlate with the malignancy of gliomas. We analyzed the total contents of m<sup>5</sup>C and 8-oxo-dG in DNA from tumor tissue and peripheral blood samples from brain glioma patients. We found an opposite relationship in the amounts of m<sup>5</sup>C and 8-oxo-dG, which correlated with glioma grade in the way that low level of m<sup>5</sup>C and high level of 8-oxo-dG indicated increased glioma malignancy grade. Our results could be directly applied to patient monitoring and treatment protocols for gliomas, as well as bolster previous findings, suggesting that spontaneously generated ROS react with m<sup>5</sup>C. Because of the similar mechanisms of m<sup>5</sup>C and guanosine oxidation, we concluded that 8-oxo-dG could also predict glioma malignancy grade and global DNA demethylation in cancer cells.
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