Global genome hypomethylation as an epigenetic phenomenon may induce (pre)neoplastic transformation through inducing chromosomal and genomic instability and activating oncogenes. Global genome hypomethylation has a fundamental role in early stages of tumorigenesis but little is known about this epigenetic event in gastric precancerous lesions such as gastritis. Therefore, we decided to evaluate this issue in gastritis lesion for obtaining new insight toward molecular biology of gastric cancer. Here we used a technique composed of restriction enzyme digestion and pyrosequencing known as luminometric methylation assay to evaluate this issue. DNA obtained from normal and gastritis lesions was digested with HpaII (sensitive to methylation in its cut site) and MspI (insensitive). Overhangs resulting from these enzymes then fill in by polymerase extension assay using pyrosequencing instrument. Nucleotide incorporation during polymerase extension generates light, which expresses as pick in the pyrogram. By comparing the height of picks obtained form both enzymes it can be possible to evaluate and compare global genome methylation level of gastritis and normal tissues. If the target site is fully methylated, the HpaII/MspI (their pick height) will approach zero. If not, this ratio will be around 1. In the other conditions this ratio varies between 0 and 1. Comparing the ratio of normal and gastritis sample, it can be inferred whether or not gastritis is hypomethylated. This study was performed on 83 gastritis and normal adjacent tissues. The patients included 34 male and 49 female and were 15 to 83 years old. According to our study, gastritis tissue was hypomethylated more than the normal tissue (p = 0.028). Global genome methylation has no significant correlation with MSI, pathological findings, age, and gender. We conclude that global genome hypomethylation occurs in the gastritis level. This reduction probably continues in the next steps toward gastric cancer and may induce other epigenetic and/or genetic changes (such as MSI) that promote carcinogenesis.
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