Mismatch repair protein expression defects in endometrioid endometrial adenocarcinoma

Objectives: Endometrioid endometrial carcinoma (EEC) is the sentinel cancer in over half female patients with heritable mismatch repair (MMR) mutations as part of Lynch syndrome. Immunohistochemical testing for tumoural MMR-protein expression is the primary screening test identifying cases potentially harbouring familial cancer syndrome-related mutations and is also a predictive biomarker for immune-checkpoint blockade response. Methods: Following Data Protection and Ethical clearance by the University of Malta, 200 EEC cases were retrospectively identified and categorized into three arms: 151 cases above age 50 at diagnosis, 49 cases at or below age 50 at diagnosis and 30 controls with benign endometrial tissue sampling. H&E case slides were re-examined by an independent pathologist to confirm the diagnosis and identify the block best representing the tumour. Four new slides per case were recut and immunohistochemistry performed for MLH1, PMS2, MSH2, and MSH6 proteins. Protein expression was analysed semiquantitatively using Allred scoring. Results: 31% of the overall EEC cases were deficient for one or more MMR-proteins. Dual loss of the MLH1-PMS2 protein heterodimer was the most common deficiency, occurring in 24.5% of cases. Loss of MSH2-MSH6 protein expression represented 3.2% of MMR-deficient cases. Well differentiated tumours had a 76.5% proficiency rate as opposed to grade 2/3 disease with 53.2% and 52.9% proficiency rate respectively. There was no significant difference in MMR status when age 50 was used as a hypothetical testing threshold. After correcting for tumour grade, MLH1 and PMS2 expression was shown to be negatively correlated with age-at-diagnosis while MSH6 expression was positively correlated. Conclusion: Reflex MMR proficiency testing of all EEC cases is advisable, as using age 50 as a testing threshold would have missed 82.3% of MMR deficient cases. Prospective evidence is required to clarify the role semi-quantitative scoring plays in MMR status interpretation and patient management in the ever-evolving field of targeted therapeutics.