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Background: Recently, a new founder mutation, an exon 1-6 deletion in a mismatch repair gene (MMR), MSH2, in nine kindreds with Lynch syndrome was reported. In 3 of the kindreds this mutation was traced by genealogy through 11-12 generations to a common founder, and thus termed the American Founder Mutation (AFM). Since then, 13 additional 'unrelated' kindreds with AFM were detected by a recently designed single polymerase chain reaction. This test might serve as first-line screening for Lynch syndrome mutations, provided AFM was prevalent, which is assessed in the current study., Methods: The number of current AFM carriers and the incidence of Lynch syndrome caused by AFM were estimated based on population growth of mutation carriers derived from genealogy data. For cross-checking, its annual incidence was also estimated based on published epidemiology data., Results: There are 18,981 (5th and 95th percentiles, 6038 and 34,466, respectively) expected current AFM carriers, or 160 (range 51-290) Lynch syndrome cases diagnosed per year due to AFM estimated based on genealogy data. The incidence estimate closely overlaps with that based on published epidemiology data, which is 114-400 cases per year., Conclusions: A large number of AFM carriers are likely to exist in the U.S., which harbors significant implications for cancer control. Given the ease of detection, testing for AFM not only among members of the existing AFM families, but also among all patients with Lynch syndrome in the U.S. is proposed.

Context: Hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome, is caused by mutations in the mismatch repair genes and confers an extraordinarily high risk of colorectal, endometrial, and other cancers. However, while carriers of these mutations should be identified, counseled, and offered clinical surveillance, at present the mutations are not tested for in mutation analyses., Objective: To describe the prevalence of a large genomic deletion encompassing exons 1 to 6 of the MSH2 gene that is widespread in the US population as a result of a founder effect., Design, Setting, and Patients: Ongoing genealogical and historical study conducted to assess the origin and spread of an MSH2 mutation previously identified in 9 apparently unrelated families with putative HNPCC and living in widely different geographic locations in the United States., Main Outcome Measures: Classification of family members as carriers or noncarriers of the MSH2 mutation; spread of the mutation across the continental United States., Results: To date, 566 family members of the 9 probands have been identified to be at risk and counseled; 137 of these have been tested, and 61 carry the founder mutation. Three families have been genealogically shown to descend from a German immigrant family that arrived and first settled in Pennsylvania in the early 1700s. Movements of branches of the family from Pennsylvania through North Carolina, Alabama, Kentucky, Missouri, Iowa, Nebraska, Utah, Texas, and California have been documented, and carriers of the mutation have already been diagnosed in 14 states. In contrast, the deletion was not found among 407 European and Australian families with HNPCC., Conclusion: The postulated high frequency and continent-wide geographic distribution of a cancer-predisposing founder mutation of the MSH2 gene in a large, outbred (as opposed to genetically isolated) population, and the ease with which the mutation can be detected, suggest that the routine testing of individuals at risk for HNPCC in the United States should include an assay for this mutation until more is learned about its occurrence.

In December 1997, the National Cancer Institute sponsored "The International Workshop on Microsatellite Instability and RER Phenotypes in Cancer Detection and Familial Predisposition," to review and unify the field. The following recommendations were endorsed at the workshop. (a) The form of genomic instability associated with defective DNA mismatch repair in tumors is to be called microsatellite instability (MSI). (b) A panel of five microsatellites has been validated and is recommended as a reference panel for future research in the field. Tumors may be characterized on the basis of: high-frequency MSI (MSI-H), if two or more of the five markers show instability (i.e., have insertion/deletion mutations), and low-frequency MSI (MSI-L), if only one of the five markers shows instability. The distinction between microsatellite stable (MSS) and low frequency MSI (MSI-L) can only be accomplished if a greater number of markers is utilized. (c) A unique clinical and pathological phenotype is identified for the MSI-H tumors, which comprise approximately 15% of colorectal cancers, whereas MSI-L and MSS tumors appear to be phenotypically similar. MSI-H colorectal tumors are found predominantly in the proximal colon, have unique histopathological features, and are associated with a less aggressive clinical course than are stage-matched MSI-L or MSS tumors. Preclinical models suggest the possibility that these tumors may be resistant to the cytotoxicity induced by certain chemotherapeutic agents. The implications for MSI-L are not yet clear. (d) MSI can be measured in fresh or fixed tumor specimens equally well; microdissection of pathological specimens is recommended to enrich for neoplastic tissue; and normal tissue is required to document the presence of MSI. (e) The "Bethesda guidelines," which were developed in 1996 to assist in the selection of tumors for microsatellite analysis, are endorsed. (f) The spectrum of microsatellite alterations in noncolonic tumors was reviewed, and it was concluded that the above recommendations apply only to colorectal neoplasms. (g) A research agenda was recommended.