Background & objective: Mismatch repair (MMR) deficiency, characterized by microsatellite instability (MSI), is a major cause of hereditary nonpolyposis colorectal carcinoma (HNPCC). Our previous study showed that loss of MMR protein expression can also be seen in gastric carcinoma, but MMR gene mutation seldom occur. Promoter methylation, a main cause of suppressor gene inactivation, might also lead to defect of MMR gene function. This study was designed to investigate the relationship of hMLH1 gene promoter methylation status to MSI in gastric carcinoma, and analyze its action as a risk factor of carcinogenesis.
Methods: DNA was extracted from 52 samples of gastric carcinoma and their adjacent non-cancerous gastric mucosa. Polymerase chain reaction (PCR) was used to amplify microsatellite loci BAT-26, D17S261, D3S1283, D2S123, and D3S1611. MSI was studied by capillary electrophoresis. Methylation of hMLH1 gene promoter was detected by restrictive endonuclease digestion, expression of hMLH1 protein was detected by immunohistochemistry.
Results: Of the 52 specimens of gastric carcinoma, 13 with high MSI (MSI-H), 2 with low MSI (MSI-l), and 37 with microsatellite stability (MSS). Frequency of hMLH1 promoter methylation was significantly higher in the 13 specimens of gastric carcinoma with MSI-H than in the 39 specimens of gastric carcinoma with MSI-L or MSS (100% vs. 2.6%, P<0.01). Furthermore, frequency of hMLH1 promoter methylation was significantly higher in the 13 specimens of adjacent non-cancerous gastric mucosa of gastric carcinoma with MSI-H than in the 39 specimens of adjacent non-cancerous gastric mucosa of gastric carcinoma with MSI-L or MSS (46.2% vs. 2.6%, P<0.01). The methylation status was accordant with loss of hMLH1 protein expression. Methylation of hMLH1 gene promoter had no relation with differentiation and clinical stage of gastric carcinoma.
Conclusion: Methylation of hMLH1 gene promoter exists in gastric carcinoma tissues with MSI-H, and their adjacent non-cancerous gastric mucosa, it may be a risk factor of carcinogenesis of gastric carcinoma.