Mutation Study Of Mismatch Repair Genes In Two Nch Syndrome Families

Background:DNA mismatch repair is a system for recognizing and repairing erroneous insertion, deletion of microsatellite repeats during DNA replication, as well as preventing uncoiled DNA sequence recombination, so guarantying human genetic information of conservatism and stability. MMR gene mutations disrupt the function of the MMR protein, causing repetitive DNA sequence generation and DNA repair error, resμlting in patients with DNA microsatellite instability (MSI). MSI leads to the activation of oncogenes, inactivation of tumor suppressor genes, and accumμlation of tumor genes mutation, inducing carcinogenesis.Lynch syndrome is an autosomal dominant genetic disease caused by mismatch repair gene (MMR) mutationthis. Colorectal cancer, endometrial cancer and other malignant tumors are typical symptoms in this syndrome. The main clinical features of this syndrome are earlier age of onset in patients, predisposition to colorectal cancer, and risk of other tumors increased, such as:Endometrial cancer, gastric cancer, ovarian cancer, resection tumor, hepatobiliary tumor, pancreatic cancer and small intestine cancer. Gene mutation frequency of Lynch syndrome patient rises after the MMR gene function loss, and mutation accumμlation is speeding up. Therefore, the time from adenoma to carcinoma is shortening. In Comparison, normal people from adenoma to carcinoma take8to10years, but patients with Lynch syndrome take only2to3years. As one of the intestinal malignant tumor, the genetic characteristic of Lynch syndrome is distinctive. Lynch syndrome accounting for about2%～5% of all colorectal cancer. So Lynch named this syndrome hereditary non polyposis colorectal cancer (HNPCC).In addition to colorectal cancer, Lynch syndrome is often accompanied with various malignant tumors of extraintestinal organs, and the most common extraintestinal tumor type is endometrial cancer. As one of the most common malignancies in female genital tract, endometrial cancer is a malignant tumor of the endometrial epithelial tissue, seriously endangering the health of women. Endometrial cancer is also the most common malignant tumor of extraintestinal organs in Lynch syndrome families.20%to30%Lynch syndrome female patients are suffering endometrial cancer. And women with Lynch syndrome have a60%lifetime risk of endometrial cancer, the general population only2.3%. The nosogenesis of endometrial tumor is not clear until now; about20%patients show family medical history and microsatellite instability. The disease is caused by a defect in the DNA mismatch repair system.Germline mutations in five genes involved in the human DNA mismatch repair mechanism have been identified:MSH2、MSH3、MSH6、 MLH1and MLH3. Among the mismatch repair gene mutations have been identified, MLH1accounts for50%, MSH2accounts for39%, MSH6accounts for7%.Objective:We use a variety of molecμlar genetic methods to study an endometrial cancer family and a colorectal cancer family respectively, defining the disease-causing gene, providing accurate diagnosis and genetic counseling for patient and his family. The goal of this study is to provide the opportunity of early clinical diagnosis and early intervention.Methods:Extracting peripheral blood DNA by phenol chloroform in a Hunan Dongkou colorectal cancer family and a Hunan Guiyang endometrial cancer family. Then using polymerase chain reaction and directly sequencing technology to detect MLH1、MSH2and MSH6genes in the endometrial cancer family and the colorectal cancer family respectively. Once variation detected, adopting directly sequencing technology to identify it whether a mutation.Results:A novel heterozygotic mutation, c.1344insG locating in exon12of MLH1gene was detected in the colorectal cancer family. This mutation causes premature termination of encoding protein, producing a truncated protein which has only477amino acids, influencing normal recognization and repairing function.Another novel heterozygotic mutation, c.2297delA locating in the fourth exon of MSH6gene was identified by direct DNA sequencing in the endometrial cancer family. Functionally, this mutation causes premature termination of encoding protein. The truncated protein has only773amino acids, lost a very important domain called Walker-A adenine nucleotide binding motif. This defect incapacitates MSH6for mismatch repairing.Reference controls prove, c.2297delA or c.1344insG mutation only exist in patients of two Lynch syndrome families, but not in normals. Screening the Human Gene Mutation Datebase(HGMD) and International Society for Gastrointestinal Hereditary Tumours(InSiGHT), we identified them be two novel mutations.Conclusion:It is concluded that MLH1gene with novel mutation c.1344insG is the disease-causing gene in the colorectal cancer family according to the analysis with reference controls. And MSH6gene with novel mutation c.2297delA is the disease-causing gene in the endometrial cancer family.