Germline mutation screening of DNA repair genes MLH3 and MBD4 in typical and incomplete HNPCC families

Q Wang,PhD,a, G Montmain,a, C Lasset, MD,a, V Bonadona, MD, a, F Desseigne, MD, a, JC Saurin, MD, b, A Puisieux, PhD,a

aCentre Leon Berard, Lyon, France, bHopital Edouard Herriot, Lyon, France

AIM. Hereditary Nonpolyposis Colon Cancer (HNPCC) is an autosomal dominant disorder defined by clinical criteria (known as Amsterdam criteria), characterized by a hereditary predisposition to early onset colorectal cancer and extra-colonic cancers of the gastrointestinal, urological and female reproductive tracts. The syndrome is caused by germline defects in one of at least five DNA mismatch repairs (MMR) genes: hMLH1, hMSH2, hPMS1, hPMS2 and hMSH6 (GTBP). Germline mutations of hMSH2 and hMLH1 are also frequently identified in families who fail to fulfill all Amsterdam criteria, demonstrating that the involvement of these genes is not confined to typical HNPCC. In contrast, 30% of typical HNPCC families do not harbor any mutation of those genes, suggesting the involvement of other DNA repair component genes, such as hMLH3 and BMD4. Both proteins encoded by these genes interact with hMLH1 protein. We thus conducted a germline mutation screening in a series of 112 typical or incomplete HNPCC families. METHODS. A genomic DNA-based heteroduplex screening method was performed. All aberrant bands revealed by electrophoresis were subjected to subsequent sequencing for characterization. RESULTS. We identified 1 variant in BMD4 and 10 variants in hMLH3. These variants were not observed in more than 100 control chromosomes. All generated amino-acid substitutions. No truncation mutation was identified. Interestingly, most variants were found in incomplete HNPCC families. CONCLUSION. Our large scale study demonstrates that molecular alterations in BMD4 and hMLH3 are not a major cause for HNPCC. However, the identification of variants in incomplete HNPCC families may suggest a low penetrant role of hMLH3 abnormalities.

KEY WORDS: Germline mutations, Predisposition, Colon cancer, Mismatch repair, Cancer syndrome.

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Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Novel Therapies, Part 1.