Features of microsatellite instability in gastric and colorectal cancer: an assessment by High Resolution Fluorescent Microsatellite Analysis

Abstract

Microsatellite instability (MSI) is known to be associated with defective DNA mismatch repair in various human malignancies, and is regarded as a major factor in tumorigenesis. To establish an assay system where more precise and objective assessments are made feasible, we made use of fluorescence-labeled polymerase chain reaction (PCR) and laser scanning. Secondly, we cotntroled Taq polymerase-dependent modification of the amplified microsatellite sequences, by enzymatic modification with T4 DNA polymerase. Thirdly, we developed a dual fluorescence co-electrophoresis system, in which both samples derived from cancer and normal tissues are electrophoresed in the same lane, in order to minimize migration errors. These improvements remarkably facilitate precise and objective assessments of microsatellite instability. Using this new system, High Resolution Fluorescent Microsatellite Analysis (HRFMA), we examined MSI in various human malignancies. Intriguingly, patterns of microsatellite changes observed can be classified into two distinct subtypes; one showing relatively small changes within 6 base pairs (type A) and the other exhibiting drastic changes over 8 base pairs (type B). In gastric and colorectal cancer, Type A and Type B was observed in 20-25% and 3-10%, respectively. Although MSI has been uniformly connected to high risk for cancer, only type B microsatellite instability correlated with family history in gastric cancer. The risk for secondary malignancies and that for a familial predisposition may be independent, even in the same diseases.