Complex relationship between defective DNA mismatch repair and microsatellite instability in human cancer
Institute for Clinical Research, National Kyushu Cancer Center, Fukuoka, Japan
AIM: Microsatellite instability (MSI) is an outcome of defective DNA mismatch repair (MMR), an important DNA repair removing replication errors. MSI is associated with various human malignancies including Hereditary Non-Polyposis Colorectal Cancer (HNPCC), and, consequently, is now regarded as a marker of high risk for cancer. However, despite numerous studies, reported frequencies for MSI in each malignancy differ widely in the literature. These discrepancies may derive from methodological problems left in the conventional assay technique. We have established a new fluorescent technique where these methodological problems have been overcome. Application of this system has elucidated two distinct categories of MSI, i.e. Type A and Type B, in various malignancies. To investigate relationships between each category of MSI and defective mismatch repair, MMR gene alterations were addressed in a panel of colorectal tumours exhibiting typical Type A and Type B MSI. METHODS: Alteration in microsatellite sequences was examined in detail, using the High Resolution Fluorescent Microsatellite Analysis (HRFMA). hMSH2 and hMLH1 genes were sequenced in a panel of tumours derived from colorectal cancer patients. RESULTS: The patterns of microsatellite changes were classified into two subtypes, one showing relatively small changes within 6 base pairs (Type A) and the other exhibiting drastic changes over 8 base pairs (Type B). Based upon analyses on microsatellite changes in mouse or human cell lines with a known defect in MMR genes, we conclude that Type A is a direct consequence of defective MMR. Among 12 tumours exhibiting Type A MSI, five had alterations in hMSH2 or hMLH1, while in 10 Type B tumours two harbour mutations in hMLH1. CONCLUSIONS: Use of HRFMA has shed light on previously unrecognised aspects of MSI in human cancer. Among the two subtypes of MSI, Type A has been connected to defective MMR. On the other hand, the relationship between Type B and defective MMR appears more complicated than hitherto suspected. Intriguingly, MSI that has been observed in various malignancies, including those associated with HNPCC, is predominantly Type B. Molecular abnormalities in addition to defective MMR may underlie Type B MSI and, by implication, HNPCC.
Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in oral session 796 (Genetic instability).