Uncertainty in the utility of immunohistochemistry in mismatch repair protein expression in epithelial ovarian cancer.

Background: Utility of immunohistochemistry (IHC) for mismatch repair (MMR) protein expression has been demonstrated in colorectal cancer but remains incompletely defined in ovarian cancer. We evaluated MMR protein expression in three population-based samples of epithelial ovarian cancers.
Materials and Methods: IHC staining was performed on full-section (FS) or tissue microarray (TMA) slides for MLH1, MSH2, and MSH6 expression.
Results: Out of 487 cases, 147 and 340 were performed through FS and TMA, respectively. Overall, Loss of Expression (LoE) of at least one MMR protein was observed in 12.7% based on an expression score of ≤3 (on a scale of 9). Notably, LoE was significantly higher in TMAs (17.9%) compared to FS cases (0.7%) (p<0.001).
Conclusion: A substantial proportion of epithelial ovarian cancers have a loss of MMR protein expression. Protein expression results vary significantly by the tissue sampling methodology utilized, raising concerns about the clinical utility of this test for ovarian tumors.