The mismatch repair gene MSH3 is frequently lost but rarely mutated in colorectal cancer with low-level microsatellite instability
Department of Surgical Research, Dresden University of Technology, Dresden, Germany
AIM: High level of microsatellite instability (MSI-H) in colorectal cancer (CRC) is due to the inactivation of various mismatch repair (MMR) genes, specifically MLH1, MSH2 or MSH6. In contrast, it is not known whether low level MSI (MSI-L; affecting less than 30% of analyzed markers) is induced by a similar deficiency in DNA repair or whether it is merely due to background instability. Di- and tetranucleotide repeats, rather than mononucleotide repeats, are often affected in MSI-L CRC. Therefore, the MSH3 protein is a candidate for the MSI-L phenotype, as it preferentially recognizes insertion/deletion-loops of 2-8 bases. The objective was to determine whether the MSH3 gene is mutated in CRC with the MSI-L phenotype. METHODS: By applying 6 mono-, 8 di- and one tetranucleotide repeat marker to matched tumor and control DNA 15 MSI-L CRCs were selected. These tumors were first analyzed to determine the expression of MLH1, PMS2, MSH2 and MSH6 via immunohistochemistry. The complete coding region of MSH3, including the promoter and flanking intronic sequences, were analyzed by sequencing of tumor DNA. In addition, sequence variants were utilized in the detection of loss of heterozygosity (LOH). RESULTS: All 15 tumors presented with normal expression of MLH1, PMS2, MSH2 and MSH6. Sequencing of the MSH3 gene revealed 29 different sequence variants in the coding, intronic, promoter and 5’ and 3’ untranslated regions. All sequence variants have also been found in germline-derived DNA and none of the variants were predicted to result in a truncated protein. However, LOH was evident for approximately half of the tumors. CONCLUSIONS: The MSI-L phenotype in colorectal cancer seems not to be due to mutations in the MSH3 gene, nor is it associated with protein depletion of MLH1, PMS2, MSH2 or MSH6. In contrast however, the high level of LOH affecting the MSH3 locus, which is similar to microsatellite stable (MSS) CRC, may indicate an involvement of MSH3 in the tumorigenesis of CRC not necessarily resulting in MSI.
Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in oral session 796 (Genetic instability).