Association Of MICA-TM Gene Polymorphism And Gastric Cancer Risk Among Different Ethnics In Qinghai Province

Objective : Gastric cancer is the malignant tumor of epithelial tissue. In 2002, gastric cancer was one of the most common cancer worldwide and the second leading cause of cancer-related deaths after lung carcinoma. Each year, in the worldwide were 640000 dead gastric cancer patients by estimation. It's mortality in China is 24.65 per 100 000, The mortality due to gastric cancer is 3.8-7.9 times higher than that in developed countries, such as European countries and America.Gastric carcinogenesis is a multistage process with a multifactorial aetiology and closely linked to environment factors, diet, lifestyle stress levels, immunodeficiency and inherited genetic syndromes. The aetiology of gastric cancer is multi-factorial, with contributions from exposure to biological and chemical carcinogens, such as nitrosamines in diet, H. pylori and Epstein-Barr virus, and which can lead to DNA damage and mutation of gastric epithelial cells and increase the likelihood of neoplasia.At present, many studies had been done on immune system, growth-factor, cytokines, cell adhesion molecule, polymorphism of metabolic enzymes, oncogene, suppressor gene, microsatellite instability, genetic polymorphism.These achievements provide valuable information for studying the pathogenesis of gastric cancer.By estimation, at least 9 various oncogenes, such as ras genes, vascular growth factors gene (VEGF) gene, survivin genes, human telomerase reverse transcriptase (hTRT) genes, B cell lymphoma/lewkmia-2(Bcl-2)genes, Ki-67 nuclear antigen genes, CDK4genes, MMPS genes etc. At least 13 various anti-oncogene were discovered, such as PTEN gene,p16/p27kipl gene, p21 gene,p53 gene, TGF-βⅡgene,Smad4 gene,Caspases-3 gene,Fas gene,maspin gene,KAI1 gene,Reversion-inducing cysteine-rich protein with Kazal motifs (RECK)gene,E-cadherin (E-Cad ) gene and MHC gene, etc. Many Research focus on the microsatellite instability(MSI). A standardized definition for microsatellite instability (MS) also called simple tandem repeats (STRs), refers to the DNA genome is less than 10 simple sequence repeat nucleotides. The DNA mismatch repair (mismatch repair, MMR) causes MSI, when the MMR gene is mutated, it can activate many oncogenes and deactivate anti-oncogene. MSI induce cell over proliferation and abnormal differentiation, which participate in tumorigenesis. 15% ~ 39% of MSI can be found in gastric cancer.In 1994, A nonclassical MHC genes, MHC classⅠchain-related genes families has recently been identified (Bahram S,Harvard Medical School),which on the short arm of human chromosome 6 in MHC-Ⅰdomain. MICA is structurally quite different from the HLA classⅠgenes, and the MICA proteins are also differ from HLA-Ⅰgenes. To date at least seven MIC loci have been identified. MICA and MICB are functionally expressed genes. Researchers recently identified a trinucleotide repeat polymorphism, (GCT)n, within the transmembrane (TM) segment of A5.The microsatellite repeats (GCT/AGC)n within TM region encode for a variable number of alanines (A). The different alleles are named in A4, A5, A6, A7, A8, and A9 with the number of triplets that they contain. A5.1, an allelic form with an extra insertion of G/C (GGCT/AGCC).MICA gene family have been shown to interact with the gut enriched Vδ1-bearingγδT cells and NK cells expressing the C-type lectin NKG2D activating receptor, which are relevant for triggering the innate arm of the immune system. MICA Many studies have shown that MICA microsatellite alleles were associated with several autoimmune diseases, including Behcet's disease, Ankylosing Spondylitis, Insulin-dependent diabetes mellitus (IDDM), Rectal cancer, leukemia,etc.A5.1 recently was identified play an important role in immune evasion. MICA gene A9 allele might confer the risk of gastric cancer and MICA gene A6 confer the risk of Oral squamous cell carcinoma. This valuable information is helpful for research about relationship between MICA polymorphism and tumors.Qinghai is an inland plateau province with multi-ethnic. According to statistics in 1994, gastric cancer mortality of male and female were 10.9 and 8.2 per 100000 respectively, which is the top of China. The major risk factors related to the location of cenozoic volcanic belt. Chinese gastric cancer mortality concentrated in the north- western and southeastern China, which is higher than the other places. Secondly,plateau climate is important. The average altitude of Qinghai-Tibet Plateau is 2300～3000M. The residents live in hypoxia circumstance. It was proved that gastric ulcer exponential in hypoxia group was higher than control group in animal experiment. Probably, gastric cancer mortality relate to typical diet. Native persons addict to smoking, alcohol, pickled vegetables, salt-preserved food, smoked food, fried food, which contain much more nitrite and carcinogenic polycyclic. Besides ,ethnic genetic characteristics is a great breakthrough for gastric cancer study in multi-ethnic domains.Based above citations, we collect in this research the most population of three ethnic groups of Han, Hui and Tibetan in Qinghai province as research objects. In candidate genes, we selected the MICA exon5 which relate to immune regulation located in MHC-Ⅰregion as target gene. MCIA exon5 can be regard as genetic markers, because of the special microsatellite structure, moreover it can imply the relationship between diseases and MICA. So we expect provide the evidences for mechanism and treatment through investigating the MICA*A5 in three ethnic in Qinghai province.Methods One hundred and thirty eight patients were pathologically diagnosed with gastric cancer, were enrolled in this study, including Han(n=42, 57+7.21years), Hui(n=42, 56+9.64 years), Tibetan(n=54, 59+8.77 years). A total of 166 control subjects were selected from people who came for routine physical check up, including Han(n=32,51+7.44years),Hui(n=44,51+10.63years),Tibetan(n=90,54+9.25years). Those with autoimmune disorders, blood disease and previous malignancy were excluded. Gender did not affect the MICA alleles distribution.The patients recruited at Qinghai people's hospital (Han n=13,Hui n=11,Tibetan n=17),Qinghai university medical college affiliated hospital (Han n=21,Hui n=16,Tibetan n=27),Qinghai Red cross hospital (Han n=8,Hui n=5,Tibetan n=10).The controls were obtained from Qinghai people's hospital (Han n=10,Hui n=20,Tibetan n=23),Qinghai university medical college affiliated hospital (Han n=12,Hui n=19,Tibetan n=39),Qinghai Red cross hospital (Han n=10,Hui n=5,Tibetan n=28). Peripheral blood (5mL) was drawn from each of the controls or from the patients. The DNA samples were purified by DNA extraction kit according to manufacturer's protocol. MICA exon5 polymorphisms were studied by using polymerase chain reaction sequencing based typing(PCR-SBT).To determine statistical differences between the two groups,χ~2and Fisher's tests were used. The accepted level of significance was P < 0.05. SPSS software was used for all statistical analyses.Results It was determined that A5 alleles polymorphism in both normal controls and gastric cancer patients. A4, A5, A5.1,A6 and A9 five alleles were detected in control groups. The analyses concluded that A4 is 9.4%, A5 is 28.1% , A5.1 is 25.0%, A6 is 15.6% and A9 is 21.9% in Han; 9.1%, 31.8%, 25.0%, 15.9% and 18.2% in Hui; 10.0%,34.4%,10.0%,17.8% and 17.8% in Tibetan. No significant difference of frequency of A4, A5, A5.1, A6 and A9 alleles was found among tree ethnics in control. A4, A5, A5.1, A6, A7, A8 and A9 were detected in patients. In patients, seven alleles were found A4, A5, A5.1, A6, A7, A8 and A9. A5 (42.9%)allele frequency is highest in Han, A5.1(57.1%) in Hui, A5 (40.7%)in Tibetan. We found frequency of A5,A5.1 are significant difference between Hui and Tibetan(χ~2=8.003,P<0.05;χ~2=23.300,P<0.05),also between Hui and Han(χ~2=8.400,P<0.05;χ~2=9.685,P<0.05). The frequency of A5.1 in Hui patients were higher than those in Hui controls(χ~2=9.181,P=0.002;OR=4.000,95%CI=1.6000~9.997).Conclusion MICA*A5 and MICA*A5.1 alleles are common high frequency in Han, Hui and Tibetan population. The result was similar with others result. No significant difference of frequency of A4, A5, A5.1, A6 and A9 alleles was found among three ethnics in control, the finding was not inconsistent to other results. The A7, A8 were detected in Hui and Tibetan patients for the first time. he frequency of A5.1 in Hui patients were higher than those in Hui controls.We considered that MCIA A5.1 was a possible gastric cancer susceptibility gene.