Alterations of level of total genomic DNA methylation and pattern of c-myc, c-Ha-ras oncogene methylation in human gastric carcinogenesis

To investigate the status of DNA methylation in gastric carcinogenesis. We analysed methylation pattern of c-myc, c-Ha-ras oncogenes by southern blot hybridization. DNA from cancerous, paracancerous and non-cancerous area of surgically resected samples in 21 cases of advanced gastric cancer were digested with MspI/HpaII and hybridized with the two genomic 32P labelled probes. In addition, the level of total genomic DNA methylation was measured by incubating DNA with 3H-S-Adenosylmethionine (3H-SAM) in the presence of a methylase which methylates all the cytosine residues that are in the double CpG (cytosine-guanine). The results indicated that both c-myc and c-Ha-ras oncogene fragments containing CCGG sequence were hypomethylated in DNA samples from cancerous (10/21 and 5/10) and paracancerous (13/21 and 4/10) areas. Moreover, the level of total genomic DNA methylation in cancerous tissue was significantly lower than that in non-cancerous and paracancerous mucosa tissues (P < 0.05). The results from two methods are incompletely alike. These results supported a strong correlation between DNA hypomethylation and gastric carcinogenesis, particularly methylated pattern of c-myc and c-Ha-ras oncogenes fragments containing CCGG sequence was abnormal in gastric mucosa tissue from gastric cancerous and paracancerous areas.