Immunohistochemistry staining for DNA mismatch repair proteins in endoscopic biopsies and the corresponding surgical specimen in colorectal cancer.

Microsatellite instability is found in 15% of sporadic colorectal cancers (CRC) and 95% of hereditary CRC cases. Lynch syndrome (LS) diagnosis begins with the analysis of the surgical specimen using methods such as immunohistochemistry (IHC), which identifies changes in the nuclear expression of DNA mismatch repair (MMR) proteins. However, IHC analysis on endoscopic biopsies could provide substantial benefits. Our goal was to assess the accuracy of MMR IHC status on endoscopic biopsies in comparison to corresponding surgical specimen in a series of CRC. We retrospectively selected patients who had undergone CRC surgery between February 2011 and January 2020 and had IHC testing for MMR proteins on the surgical specimen. The study was then performed on the corresponding endoscopic biopsies and results were compared. MMR IHC staining on surgical specimens were available for 361 CRC patients and only in 154 cases for preoperative endoscopic biopsies. The concordance between MMR IHC status of the endoscopic biopsy and the surgical specimen analysis was 98.6% for the MLH1/PMS2 proteins and 100% for MSH2/MSH6. In conclusion, endoscopic biopsies of colorectal tumors serve as a suitable tissue source for the immunohistochemical analysis of DNA repair proteins. The correlation with results from the surgical specimen was notably high and discrepancies were primarily as a result of intratumoral heterogeneity within the same sample. The features of MMR protein loss in endoscopic biopsies can provide clinicians with valuable information for specific therapeutic approaches and genetic counseling.