A ten markers panel provides a more accurate and complete microsatellite instability analysis in mismatch repair-deficient colorectal tumors

Tumour microsatellite instability (MSI) is useful in identifying patients with hereditary non-polyposis colorectal cancer (HNPCC) with defective DNA mismatch repair (MMR} genes. A reference Bethesda panel has limitations resulting from the inclusion of dinucleotide markers, which are less sensitive and specific for detection of tumours with MMR deficiencies. We developed a multiplex PCR assay with additional four mononucleotide markers and one dinucleotide marker (NR-21, NR-24, BAT-40, TGF-BetaR and D18S58) for a rapid and proper classification of MSI-H, MSI-L and MSS colorectal cancers. Two tetranucleotide markers were added to identify sample mix-ups and/or contamination. Results: all the 44 cases test cases were in agreement with previous classification except for three cases: one case MSI-H-Bethesda unstable only for dinucleotides markers shifted to MSI-L category and two cases MSI-L-Bethesda unstable for mononucleotide markers shifted to MSI-H category. Immunohistochemistry analysis revealed that these two MSI-H cases did not expressed hMLH1 and they were found to be methylated at the MLH1 promoter, while the first one that shifted to MSI-L showed MMR protein expression. Conclusion: a complete panel of ten markers including four dinucleotide and six mononucleotide microsatellites allows accurate evaluation of tumor MSI status.