CpG island methylation in premalignant stages of gastric carcinoma

There are limited reports on methylation analysis of the premalignant lesio ns of gastric carcinoma thus far. This is despite the fact that gastric car cinoma is one of the tumors with a high frequency of CpG island hypermethyl ation. To determine the frequency and timing of hypermethylation during mul tistep gastric carcinogenesis, non-neoplastic gastric mucose (n = 118), ade nomas (n = 61), and carcinomas (n = 64) were analyzed for their pld, human Mut L homologue 1 (hMLH1), death-associated protein (DAP)-kinase, thromobos pondin-1 (THBS1), and tissue inhibitor of metalloproteinase 3 (TIMP-3) meth ylation status using methylation-specific PCR. Three different classes of m ethylation behaviors were found in the five tested genes. DAP-kinase was me thylated at a similar frequency in all four stages, whereas hMLH1 and p16 w ere methylated in cancer samples (20.3% and 42.2%, respectively) more frequ ently than in intestinal metaplasia (6.3% and 2.1%, respectively) or adenom as (9.8% and 11.5%, respectively). However, hMLH1 and p16 were not methylat ed in chronic gastritis. THBS-1 and TIMP-3 were methylated in all stages bu t showed a marked increase in hypermethylation frequency from chronic gastr itis (10.1% and 14.5%, respectively) to intestinal metaplasia (33.7% and 36 .7%, respectively; P < 0.05) and from adenomas (28.3% and 26.7%, respective ly) to carcinomas (48.4% and 57.4%, respectively; P < 0.05). The hMLH1, THB S1, and TIMP-3 hypermethylation frequencies were similar in both intestinal metaplasia and adenomas, but the p16 hypermethylation frequency tended to be higher in adenomas (11.5%) than in intestinal metaplasia (2.1%; P = 0.07 3). The average number of methylated genes was 0.6, 1.1, 1.1, and 2.0 per f ive genes per sample in chronic gastritis, intestinal metaplasia, adenomas, and carcinomas, respectively. This shows a marked increase in methylated g enes from non-metaplastic mucosa to intestinal metaplasia (P = 0.001) as we ll as from premalignant lesions to carcinomas (P = 0.002). These results su ggest that CpG island hypermethylation occur early in multistep gastric car cinogenesis and tend to accumulate along the multistep carcinogenesis.