Distinct methylation patterns of two APC gene promoters in normal and cancerous gastric epithelia

The adenomatous polyposis coli (APC) tumor suppressor gene is mutationally inactivated in both familial and sporadic forms of colorectal cancers. In a ddition, hypermethylation of CpG islands in the upstream portion of APC, a potential alternative mechanism of tumor suppressor gene inactivation, has been described in colorectal cancer. Because a subset of both gastric and c olorectal cancers display the CpG island methylator phenotype, we hypothesi zed that epigenetic inactivation of APC was likely to occur in at least som e gastric cancers, APC exhibits two forms of transcripts from exons 1A and 1B in the stomach. Therefore, we investigated CpG island methylation in the sequences upstream off sons 1A and 1B, i.e., promoters 1A and 1B, respecti vely. We evaluated DNAs from 10 gastric cancer cell lines, 40 primary gastr ic cancers, and 40 matching non-cancerous gastric mucosae, Methylated allel es of promoter 1A were present in 10 (100%) of 10 gastric cancer cell lines , 33 (82.5%) of 40 primary gastric cancers, and 39 (97.5%) of 40 non-cancer ous gastric mucosae. In contrast, promoter 1B was unmethylated in all of th ese same samples, APC transcripts from exon 1A were not expressed in nine o f the 10 methylated gastric cancer cell lines, whereas APC transcripts were expressed from exon 1B. Thus, expression from a given promoter correlated well with its methylation status. We conclude that in contrast to the colon , methylation of promoter 1A is a normal el ent in the stomach; moreover, p romoter 1B is protected from methylation in the stomach and thus probably d oes not participate in this form of epigenetic APC inactivation.