Microsatellite instability and loss of mismatch-repair protein expression in gastric carcinoma

hMLH1 and hMSH2 gene mutations in a population susceptible to gastric cancer

AIM: To analyze human mutL homolog 1 (hMLH1) and hMSH2 gene mutations in Chinese familial gastric cancer kindreds and sporadic gastric cancer patients.

METHODS: Blood samples from 16 gastric cancer patients and 114 healthy members with familial clustering of gastric cancer and 56 sporadic gastric cancer patients were collected. After DNA extraction, the fragments of exons 3, 8, 12, 13 and 16 of the hMLH1 gene and exons 5 and 7 of the hMSH2 gene were amplified by polymerase chain reaction (PCR), followed by single-stranded conformation polymorphism/capillary electrophoresis (SSCP-CE) genotyping assay. The fragments suspected to harbor mutations were sequenced.

RESULTS: Mutations were detected in exon 8, 12 and 16 of the hMLH1 gene, whereas no mutation was detected in exons 5 and 7 of the hMSH2 gene. In subjects with familial clustering of gastric cancer, the mutation frequency was 37% (6/16) in gastric cancer patients and 27% (31/114) in healthy members, both significantly higher than that in controls (5%; P = 0.001 and 0.000, respectively). The mutation frequency was 11% in gastric patients without familial clustering, significantly lower than that in subjects with familial clustering. The mutation at codon 219 in exon 8 of the hMLH1 gene (A→G) results in a conversion of Ile→Val (ATC→GTC), whereas the mutation at codon 384 in exon 12 of the hMLH1 gene (T→A) results in a conversion of Asp→Val (GTT→GAT). The mutation at codon 553 in exon 16 of the hMLH1 gene (T→G) results in a conversion of Ser→Arg (AGT→AGG), which represents a novel mutation of the hMLH1 gene that has not been reported before.

CONCLUSION: Familial gastric cancer kindreds may have similar hMLH1 gene mutations to those observed in hereditary nonpolyposis colorectal carcinoma.

Relationship between Helicobacter pylori and DNA mismatch repair system

Helicobacter pylori infection causes the defect of mismatch repair (MMR) in eucaryotes, and microsatellite instability (MSI) in gastric epithelial cells, which increases the spontaneous mutation of relative genes, and finally results in the susceptibility to tumors in human bodies. DNA MMR can prevent the excessive proliferation of mutant cells and development of tumors by correcting mismatched bases, which occur in the process of duplication and recombination, and inducing apoptosis of the cells in which DNA had been seriously impaired. The majority of the mutation in mismatch repair gene is the gene mutation in which substitution exceeds the absence or insertion of the single basic group. In MMR, hMLH1 and hMSH2 gene are the main controlling genes. Current research showed that H pylori infection might lead to functional defect of MMR, which had played an important role in the development of gastric carcinomas. In this article, we reviewed the correlations between H pylori infection and DNA mismatch repair system.