Standardized Approach for Microsatellite Instability Detection in Colorectal Carcinomas

Background: Ubiquitous mutations in microsatellite DNA sequences define a specific type of genetic instability, termed microsatellite instability (MSI). Various approaches have been used to identify the presence and degree of MSI. To define standard diagnostic criteria for MSI, we developed and tested a mathematical model. Methods: We designed an algorithm for the efficient characterization of MSI and used it to analyze data on six microsatellite markers in colorectal carcinoma and normal tissues from 415 patients. Theoretical models considering one, two, or three populations were tested against the data collected. Results: The observed frequencies of MSI in our series of samples best fit a two-population model, stable and unstable, defined by instability in two or more of four to six markers analyzed. MSI was observed in 7.5% of the tumors. The misclassification rate was less than 5% when any four loci were analyzed and less than 1% when the six markers were used. A stepwise strategy, consisting first of a bulk screening of two loci and then a second screening of two to four additional markers, provided excellent sensitivity (≥97%) and specificity (100%). Tumors with MSI had distinctive genetic and clinicopathologic features, including better patient survival. Conclusion: To assess the presence of MSI in colorectal cancer, we have developed a simple, sensitive, and specific approach based on the apparent good fit of the data to a two-population model. Its application to a prospective series of patients with colorectal carcinomas demonstrates that the presence of MSI characterizes a subset of less aggressive tumors.