DNA methylation of microRNA genes in gastric mucosae of gastric cancer patients: Its possible involvement in the formation of epigenetic field defect

Accumulation of aberrant DNA methylation in normal-appearing gastric mucosae, mostly induced by H. pylori infection, is now known to be deeply involved in predisposition to gastric cancers (epigenetic field defect), and silencing of protein-coding genes has been analyzed so far. In this study, we aimed to clarify the involvement of microRNA (miRNA) gene silencing in the field defect. First, we selected three miRNA genes as methylation-silenced after analysis of six candidate "methylation-silenced" tumor-suppressor miRNA genes. Methylation levels of the three genes (miR-124a-1, miR-124a-2 and miR-124a-3) were quantified in 56 normal gastric mucosae of healthy volunteers (28 volunteers with H. pylori and 28 without), 45 noncancerous gastric mucosae of gastric cancer patients (29 patients with H. pylori and 16 without), and 28 gastric cancer tissues (13 intestinal and 15 diffuse types). Among the healthy volunteers, individuals with H. pylori had 7.8-13.1-fold higher methylation levels than those without (p < 0.001). Among individuals without H. pylori, noncancerous gastric mucosae of gastric cancer patients had 7.2-15.5-fold higher methylation levels than gastric mucosae of healthy volunteers (p < 0.005). Different from protein-coding genes, individuals with past H. pylori infection retained similar methylation levels to those with current infection. In cancer tissues, methylation levels were highly variable, and no difference was observed between intestinal and diffuse histological types. This strongly indicated that methylation-silencing of miRNA genes, in addition to that of protein-coding genes, contributed to the formation of a field defect for gastric cancers.