Frequent epigenetic inactivation of RIZ1 by promoter hypermethylation in human gastric carcinoma

Abstract

The retinoblastoma protein‐interacting zinc finger gene, RIZ1 (GenBank accession number U17838), is involved in chromatin‐mediated gene expression and is also a target for frameshift mutation in microsatellite‐unstable cancers. Methylation of the RIZ1 promoter CpG island has been shown to be a common mechanism in inactivating the RIZ1 gene in human liver and breast cancers. We investigated levels of RIZ1 mRNA in 45 gastric carcinoma tissues by quantitative RT‐PCR and in gastric carcinoma cell lines by RT‐PCR. In addition, we examined CpG island methylator phenotype (CIMP) status, p53 mutation status, and the correlation between promoter methylation status and RIZ1 mRNA expression. CIMP status was investigated by examining the methylation status of MINT1, MINT2, MINT12, MINT25 and MINT31. p53 mutation status was examined by PCR‐single strand conformation polymorphism and promoter methylation status was examined by methylation‐specific PCR. Promoter hypermethylation of the RIZ1 gene was found in 31 (69%) of 45 gastric carcinoma tissues and in 3 (21%) of 14 corresponding non‐neoplastic mucosae, the incidence being significantly different (p = 0.002). None of the 12 normal gastric tissues from young non‐cancer individuals showed hypermethylation. Promoter hypermethylation was associated with reduced RIZ1 expression in gastric carcinoma tissues (p = 0.029). Promoter hypermethylation of the RIZ1 gene was significantly associated with CIMP (p = 0.002). Mutation status of the p53 gene was not associated with methylation status or RIZ1 expression in gastric carcinoma. In gastric carcinoma cell lines MKN‐28 and KATO‐III, the RIZ1 promoter was hypermethylated and RIZ1 transcription was inactive. Treatment of these cells with demethylating agent 5‐aza‐2′‐deoxycytidine restored RIZ1 transcription. Our results suggest that transcriptional inactivation of the RIZ1 gene by promoter hypermethylation may participate in stomach carcinogenesis.