Mismatch Repair Defect In The Population Susceptible To Gastric Cancer

Gastric cancer is one of the most common cancers and a leading cause of cancer death in China with the molecular biological mechanism of tumorigenesis remains unclear. Clinical and epidemic studies have shown that gastric cancer occurrence has a tendency of familial accumulation and several members can be attacked by gastric cancer just in one family, which is similar to hereditary nonpolyposis colorectal carcinoma (HNPCC). HNPCC is considered being caused by inherited mutation in one of DNA mismatch repair genes, resulting in the defection of MMR. The MMR system plays an important role in the maintenance of genomic stability. Genomes become rather unstable when mismatch-repair function is deficient. As errors accumulated, some will inactivate tumor suppress genes or activate cancer promoting genes. In this way, cells would reach a critical threshold of tumor promoting error. Both gastric carcinoma and colorectal carcinoma are from glandular epithelium cell of digestion tract, which have a high proliferation rate, and the younger generations of these gastric cancer families were found to develop colorectal cancer at a high frequency in recent years because of the change of food habit. So we estimated that MMR gene mutation or MMR deficiency may also associate with the coming on of gastric cancer. In order to clarify whether our estimation is correct or not and the pathway MMR system defect, we carried out the following researches. 1. Detection of mismatch repair protein hMLH1and hMSH2 in gastric carcinoma Fifty-six cancer patients from northeast China who underwent surgical excision for gastric adenocarcinoma entered the study. Specimens of carcinomas and surrounding non-cancerous gastric tissues were fixed in 10% buffered formalin respectively. To determine the protein expression of mismatch repair gene hMLH1 and hMSH2, all carcinomas were subjected to immunohistochimical analysis using the streptavidin-biotin-peroxidase complex method with 3,3'-diaminobenzidine as chromogen. Of the 56 carcinomas, 12 manifested hMLH1 expression absent and 4 manifested hMSH2 absent. Two of the carcinomas lacking hMLH1 expression also failed to express hMSH2. The difference of loss frequency of hMLH1/hMSH2 expression was not significant in different stages and in different differentiated gastric carcinoma (p=0.762, p=0.334). 2. MSI analyzing of gastric carcinoma and the relationship with MMR protein defect Fifty-six carcinomas and surrounding non-cancerous gastric tissues were all from cancer patients who underwent surgical excision for gastric adenocarcinoma as in research 1. Genomic DNAs were extracted from frozen tissues followed by PCR of 5 microsatellite loci BAT-26, D17S261, D3S1283, D2S123 and D3S1611. After denaturation by heat, single-stranded microsatellite fragments were detected by LIF and MSI was defined as the peak shift in the tumor DNA compared with the corresponding normal DNA. Based on the number of mutated MSI markers in each tumor, carcinomas were characterized as high MSI (MSI-H) if they manifested instability at two or more markers, low MSI (MSI-L) if unstable at only one marker, and microsatellite stable (MSS) if they showed no instability at any marker. Of the 56 cases of gastric carcinomas, 25% cases (14/56) showed microsatellite instability and 25% cases (14/56) showed expression loss of mismatch repair protein. In the 14 cases of the microsatellite instability, 79% cases (11/14) were accompanied by expression loss of hMLH1/hMSH2, whereas only 7% cases (3/42) were accompanied by expression loss of hMLH1/hMSH2 in the 42 cases of the microsatellite stable carcinomas. Microsatellite instability was significantly related with mismatch repair deficiency(X2=28.57 P﹤0.01). Of the 22 cases of well-differentiated carcinomas, 7 cases (32%) manifested MSI-H and 6 (27%) cases showed protein defection of MMR, Comparatively, 7 cases (21%) manifested MSI-H and 8 cases (24%) showed protein defection of MMR in 34 cases poorly-differentiated carcinomas. Of the 20 cases early stage carcinomas, only 1 case (5%) manife...