Prognostic Significance of Mismatch Repair Protein Expression and Its Association with Clinical Outcomes in Endometrial Cancer

Objective: This study aimed to evaluate the expression of mismatch repair (MMR) proteins in patients with endometrial cancer (EC), compare the findings with clinicopathological data, and investigate their prognostic implications. Materials and Methods: A total of 116 patients diagnosed with EC via endometrial biopsy or resection at Isparta City Hospital between March 1, 2017, and January 1, 2023, were included. Immunohistochemical analysis was performed to assess the expression of MMR proteins (MLH1, PMS2, MSH2, MSH6). Patients were classified as microsatellite stable (MSS) or microsatellite instable (MSI) based on MMR protein expression, and correlations with clinicopathological features and survival outcomes were analyzed. Results: Among the patients, 83.6% were classified as MSS and 16.4% as MSI. MSI was more frequently observed in patients under 60 years of age, with endometrioid type, low tumor grade, and hormone receptor positivity; however, these associations were not statistically significant. Estrogen receptor (ER) positivity, p53 mutation status, and microsatellite instability (MSI) were significant predictors of survival in endometrial cancer. The median overall survival (OS) was 31 months in the MSS group and 52 months in the MSI group, indicating a statistically significant difference (p = 0.002). Conclusion: MMR deficiency and MSI status are of prognostic significance in EC. MSI positivity is associated with longer survival and increased sensitivity to immunotherapy, highlighting its value as a clinically meaningful biomarker. Molecular classification can provide guidance for personalized treatment strategies.