Molecular genetics of colorectal and gastric cancers

We screened for germline mutations of the mismatch repair genes and related genes in 37 Japanese hereditary nonpolyposis colorectal cancer (HNPCC) cases. Fifteen kindreds fulfilled the Amsterdam criteria; and 22 other kindreds, the less stringent "Japanese clinical criteria." When we examined the entire coding sequences and flanking intronic sequences of the hMLH1 and hMSH2 mismatch repair genes by PCR-SSCP, 9 of the 15 (60%) cases satisfying the Amsterdam criteria and 3 of the 22 (13.6%) cases according to the Japanese clinical criteria showed germline mutations, indicating a significant difference in the detection rate. Interestingly, the mutation frequency of hMSH2 (11/12, 91.7%) was much higher than that of hMLH1 (1/12, 8.3%). We also analyzed colorectal cancer-related genes, e.g., the transforming growth factor-beta type 11 receptor (TGF-betaRII) gene, in several microsatellite instability (MSI)negative HNPCC cases. A substitution of methionine for threonine at codon 315 in the kinase domain of TGF-betaRII gene was found in one kindred with the Japanese clinical criteria. Thus, it is likely that causative genes of HNPCC are heterogeneous. NISI has been reported in familial gastric cancers (FGC). However, genetic defects responsible for this phenotype, that is, mutations in mismatch repair genes such as hMLH1 and hMSH2 have not been detected in most FGC cases. Earlier studies have shown that the promoter region of the hMLH1 gene was methylated in some sporadic colorectal cancers. To determine how FGC acquire MSI. we examined the MSI status, hMLH1 protein expression. and methylation status of the hMLH1 promoter region in FGC cases. Out of 9 cancers from 8 FGC kindreds, 6 showed NISI at one or more loci; no germline mutations in the hMLH1 or hMSH2 genes were detected; 4 cancers exhibiting MSI displayed aberrant hMLH1 expression. Methylation in the hMLH1 promoter region was found in these 4 cases. In contrast, the cancers displaying hMLH1 protein expression were not methylated in the hMLH1 promoter region. Our data show a significant association between the absence of hMLH1 expression and methylation of its promoter in FGC cases with MSI. This suggests that the mechanism of inactivation of hMLH1 is epigenetic and that there are other genes responsible for FGC.