Microsatellite instability analysis: a multicenter study for reliability and quality control.

The molecular biology section of the Hereditary Non-Polyposis Colorectal Cancer study group-Germany, instituted a multicenter study to test the reliability and quality of microsatellite instability (MSI) analysis. Eight laboratories compared MSI analyses performed on 10 matched pairs of normal and tumor DNA from patients with colorectal carcinomas. A variety of techniques were applied to the detection of microsatellite changes: (a) silver and ethidium bromide staining of polyacrylamide gels; (b) radioactive labeling; and (c) automated fluorescence detection. The identification of highly unstable tumors and tumors without MSI was achieved in high concordance. However, the interpretation of the band patterns resulted in divergent classifications at several microsatellite marker loci for a large fraction of this tumor/normal panel. The data on more than 30 primers per case suggest that the enlargement of the microsatellite panel to more than 10 loci does not influence the results. In this study, cases with MSI in less than 10% of loci were classified as microsatellite stable, whereas MSI was diagnosed in cases with more than 40% of all markers unstable. We propose that a panel of five microsatellite loci consisting of repeats with different lengths should be analyzed in an initial analysis. When less than two marker loci display shifts in the microsatellite bands from tumor DNA, the panel should be enlarged to include an additional set of five marker loci. The number of marker loci analyzed as well as the number of unstable marker loci found should always be identified. These criteria should result in reports of MSI that are more comparable between studies.