CpG island promoter hypermethylation of Ras association domain family 1A gene contributes to gastric carcinogenesis

Abstract

Methylation rates of the Ras association domain family 1A gene (RASSF1A) have been variously reported as between 7.5 and 66.7% in gastric carcinoma tissues. The role of this gene in gastric cancer also remains to be fully elucidated. The present study aimed to investigate whether promoter hypermethylation of RASSF1A occurs in gastric adenocarcinoma tissues and gastric cancer cell lines, and to determine the effects of RASSF1A in gastric carcinoma cell lines. The results showed a methylation-specific band only in SNU-719, MKN28 and AGS human gastric cancer cells (indicating full methylation), none of which exhibited RASSF1A expression. By contrast, SNU-16, MKN-45 and KATO-III human gastric carcinoma cells exhibited methylation as well as unmethylation-specific bands (indicating partial methylation), and all displayed positive or weakly positive expression of RASSF1A. Bisulfite sequencing in AGS and SNU-719 cells revealed that virtually all CpG sites were densely methylated. When SNU-719, MKN-28 and AGS cells were treated with the demethylating agent 5-aza-2-deoxycytidine, RASSF1A gene expression was restored and the methylation-specific polymerase chain reaction pattern was altered in all three cell lines. Transfection of a plasmid expressing RASSF1A into AGS and SNU-719 cells significantly inhibited cell proliferation. Exogenous RASSF1A also reduced the expression of cyclin D1 and phospho-retinoblastoma protein, and increased that of p27 as demonstrated by western blot analysis. Furthermore, RASSF1A expression was significantly reduced (P=0.048) and the methylation rate was elevated in gastric adenocarcinoma tissues, compared with those in adjacent healthy intestinal metaplasia (34.6 vs. 66.7%, P=0.029). The present study indicated that epigenetic silencing of RASSF1A is frequently caused by promoter hypermethylation in gastric cancer cell lines as well as in gastric adenocarcinoma tissues, which may contribute to gastric carcinogenesis.