Hereditary nonpolyposis colon cancer mismatch repair genes

PMSl). The DNA located between the nick and the mismatch is removed by 3 to 5 or 5 to 3 exonucleases depending on whether the nick is 3 or 5 to the mismatch. The gap is then filled by DNA Pol III in E. coli (Fig. 10) by DNA Pol 5 in humans and is ligated. Mismatch repair increases the spontaneous mutation rate in E. coli and yeast. Humans defective in mismatch repair exhibit a cancer-prone syndrome called hereditary nonpolyposis colon cancer (HNPCC), and sporadic mutations of mismatch repair genes are also associated with up to 50% of all human cancers (42 4). 

Several human DNA mismatch repair genes are associated with hereditary nonpolyposis colon cancer (HNPCC). One of these mismatch repair genes (hMSH2) is located on the short arm of chromosome 2 others are located on chromosome 3. Defects in any of these DNA repair genes predispose individuals to colon cancer as well as to other cancers. 

As discussed earlier in this chapter, Lynch syndrome or hereditary nonpolyposis colon cancer syndrome (HNPCC) is a cancer predisposition syndrome caused by mutation of a mismatch repair gene. Immunohistochemistry can be used to screen for mutations in... 

Without regard to therapy, potentially valuable diagnostic tests are available for presymptomatic evaluation of risk of breast cancer due to predisposition from BRCA 1 or BRCA 2 and of colon cancer related to familial adenomatous polyposis (APC gene) or hereditary nonpolyposis mismatch repair genes (MSH 2). Genetic predisposition to Alzheimer disease associated with ApoE4 is neither sufficient nor necessary to lead to the clinical condition, and no definitive therapy is available. 

A second group of inherited colon cancers are termed hereditary nonpolyposis colorectal cancer (HNPCC). HNPCC may account for 5% of all colon cancer cases and can be caused by mutations in any of five different genes. All of these genes encode proteins involved in DNA mismatch repair (Fig II-5-3). As with inherited breast cancer, fiiulty DNA repair leads to mutated cells capable of producing tumors. 

There are two major forms of hereditary susceptibility to colon cancer.00 Familial adenomatous polyposis is caused by defects in the APC gene (see Chapter 32). The more common hereditary nonpolyposis colorectal cancer (HNPCC), which includes many endometrial, stomach, and urinary tract tumors, results from defects in DNA mismatch repair. -)) The proteins hMSH2 and hMSLl are homologs of the E.coli MutS and MutL (main text). 

Colon cancer is one of the main causes of cancer mortality in Western societies [150]. About 15-20% of colorectal tumors are causally determined by inheritance of genetic alterations such as the hereditary nonpolyposis colorectal cancer (HNPCC) and the syndrome familial adenomatous polyposis (FAP) [151,152]. Microsatellite instability, a characteristic of HNPCC, is caused by mutations in the genes essential for mismatch repair. The loss of mismatch repair has several consequences most crucially, the loss of proofreading and correction of small deletions and insertions. FAP is a rare autosomal dominant syndrome caused by an inherited mutation in the APC gene. The disease is characterized by the development of multiple colorectal adenomas, numbering from a few polyps to several thousands. 

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