Retrospective review of colorectal cancer specimens in individuals younger than age 50 for microsatellite instability testing and DNA mismatch repair enzyme expression.

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 re...