Mismatch repair deficiency in bilateral breast cancer

Abstract Background Since the FDA approved immune-enhancing therapies for patients with high microsatellite instability (MSI-H) and/or mismatch repair deficiency (dMMR), recognizing these biomarkers in solid tumors has gained clinical importance. Although MSI-H and dMMR are considered uncommon in breast cancer, previous studies on bilateral breast cancer (biBC) identified a surprisingly high frequency of MSI. Methods In this study, we aimed to describe the prevalence of dMMR and its association with clinicopathologic parameters in biBC. We performed immunohistochemistry with anti-MMR proteins on tissue microarrays (TMAs) with 58 bilateral breast cancer cases. The biomarkers used were MLH1, PMS2, MSH2, MSH6, ER, PR, HER2 and Ki67. SPSS was used for data analysis. Results Four (6.9%) cases showed dMMR on TMAs. Three (75%) of the dMMR cases were luminal and one (25%) was triple negative. Two biBC cases presented unilateral dMMR. No association between dMMR status and clinicopathologic parameters was found. Conclusions This work highlights a noticeable frequency of dMMR in bilateral breast cancer and builds upon previous research in this area, suggesting routine MMR protein testing as part of the immunohistochemical panel for biBC to identify candidates for immune-enhancing therapies.