The Role of Microsatellite Instability in Endometrial Hyperplasia and Risk of Carcinoma Development

Background: Endometrial hyperplasia (EH) represents a precursor lesion in the development of endometrial carcinoma, particularly the endometrioid subtype. Among the molecular pathways involved, microsatellite instability (MSI) resulting from DNA mismatch repair (MMR) deficiency has gained increasing attention as an early event in endometrial carcinogenesis. Objective: This study aimed to evaluate the expression of key MMR proteins (MLH1, PMS2, MSH2, and MSH6) in endometrial hyperplasia without atypia and endometrial atypical hyperplasia/endometrioid intraepithelial neoplasia (EAH/EIN) to determine the prevalence and potential implications of MMR deficiency at the precancerous stage. Methods: Fifty-six cases of EH were analyzed, including 28 endometrial hyperplasia without atypia and 28 EAH/EIN. Immunohistochemical (IHC) analysis was performed to assess the nuclear expression of MMR proteins. Loss of expression was defined as complete absence of epithelial nuclear staining with retained stromal positivity. Results: MMR protein expression was retained in all cases of endometrial hyperplasia without atypia, while total loss of one or more MMR proteins was observed in 3 of 28 (10.7%) EAH/EIN. The most frequent pattern involved concurrent MLH1/PMS2 loss, consistent with sporadic MLH1 promoter hypermethylation. One case exhibited isolated MSH6 loss, suggesting a potential Lynch syndrome, and another showed combined MSH6/PMS2 loss. Conclusions: MMR deficiency appears confined to atypical EH, supporting its role as an early molecular alteration in the neoplastic sequence leading to endometrioid carcinoma. Identification of abnormal MMR expression in EH may facilitate risk stratification, guide reflex testing for MLH1 methylation, and prompt genetic counseling for hereditary cancer predisposition.