Abstract

392 Background: Data from studies performed at tertiary referral centers suggest that the yield of patients with the underlying defect in hereditary non-polyposis colon cancer (HNPCC) could be improved by screening every colon cancer for the phenotypic expression of the MMR defect by immunohistochemistry (IHC) and/or an assay for microsatellite instability (MSI). We propose to determine the incidence of microsatellite unstable colon cancers and the incidence of absence of expression of DNA MMR enzymes in de-identified colorectal cancer specimens from patients under the age of 50 who have presented to Winthrop University Hospital over the past ten years. This incidence will be compared with those suggested by recent large retrospective studies in tertiary care centers. Methods: Immunoperoxidase staining for MLH1, MSH2, MSH6 and PMS2 were performed on formalin-fixed tissue. MSI assays were performed on microdissected DNA from paraffin-embedded tissue blocks. All cases were tested with five mononucleotide repeat markers (BAT-25, BAT-26, NR-21, NR-24 and MONO-27) and two pentanucleotide repeat markers (Penta C and Penta D). Tumor samples in which two or more altered monoclonal repeat markers were found out of five were classified as MSI-H. Results: Screening for expression of DNA mismatch repair enzymes and MSI in an enriched selected (by age <50) population of colorectal cancer patients resulted in detection of 7/51 (14%) specimens that were MSI-H and 7/61 (12%) that were “positive” for lack of expression of at least one DNA mismatch repair enzyme. Conclusions: Our results are similar to those reported in the literature in unselected series of patients with colorectal cancer. IHC staining or MSI analysis alone may be insufficient in selecting those colorectal cancer patients that should be referred for genetic testing for HNPCC. However, screening of an unselected population with both these modalities should have a low but clinically significant yield. Since other research have shown that traditional criteria such as the Bethesda guidelines are inadequate, we support the use of both measures prospectively in all colorectal cancer patients. No significant financial relationships to disclose.

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