Impact of microsatellite testing and mismatch repair protein expression on the clinical interpretation of genetic testing in hereditary non-polyposis colorectal cancer

Purpose: Identification of germline mutations in mismatch repair genes is increasingly being used to guide clinical practice in hereditary non-polyposis colon cancer. The aim of this study was to retrospectively assess the clinical utility of immunostaining and microsatellite instability testing in a group of individuals in whom germline testing of hMSH2 and hMLH1 had already been performed. Methods: Individuals were identified from the records of family cancer clinics. A total of thirty-eight tumour blocks were retrieved from 28 kindreds. DNA was extracted and PCR amplification of six microsatellite markers was performed. Immunostaining was used to examine the expression of hMSH2 and hMLH1 protein. Results: Of the 32 assessable tumours, 24 (75%) showed microsatellite instability. Most of the MSI-H cancers (92%) failed to express either hMLH1 or hMSH2. Deleterious germline mutations were identified in the proband in 12 of 28 families. Missense mutations were identified in 11 cases and no mutations in six probands. Conclusions: The use of germline genetic testing is indicated for a highly selected group of individuals. MSI testing and immunostaining are extremely useful tools which significantly improve the clinical interpretation of germline results. Ambiguity regarding the significance of missense mutations in hereditary bowel cancer suggests that these findings should be interpreted with caution.