E-CADHERIN GENE-MUTATIONS IN HUMAN GASTRIC-CARCINOMA CELL-LINES

Reduced expression of E-cadherin has been regarded as one of the main molecular events involved in dysfunction of the cell-cell adhesion sys tem, triggering cancer invasion and metastasis. However, even with a s ufficient amount of E-cadherin, cell-cell adhesion is sometimes lost i n ''diffusely invasive'' human carcinomas. Ten human cancer cell lines , showing growth characterized morphologically by loose cell-cell adhe sion, were analyzed for possible structural abnormalities of their exp ressed E-cadherin. Four of the cell lines showed strong mRNA and prote in expression with no nucleotide sequence abnormalities, and mRNA was absent in four other cell lines. mRNA sequence was abnormal in the rem aining two gastric carcinoma cell lines. In MKN45 (poorly differentiat ed adenocarcinoma), this involved a 12-bp in-frame deletion with stron g expression of mRNA and protein. In KATO-III (signet ring cell carcin oma), there were four mRNA species with insertions of different sizes, among which the major transcripts (with a 7-bp insertion) caused a fr ameshift, and expression of both mRNA and protein was markedly reduced . In these two cell lines, DNA mutations were detected around exon-int ron junctions, revealing that aberrant RNA splicing was the cause of t he mRNA abnormalities. In addition, the wild-type allele of the E-cadh erin locus was lost, suggesting that the E cadherin gene had been inac tivated by two hits (mutation and allele loss), similar to the mechani sm for inactivation of tumor suppressor genes.