Objectives: Inactivating somatic mutations of ARID1A, a chromatin remodeling gene, are common in endometrioid endometrial carcinoma (EEC) but rare in complex atypical hyperplasia (CAH). Our objectives were to determine the clinical significance of ARID1A loss during tumor progression from CAH to EEC and to assess its role as a predictive cancer biomarker. Methods: In cohort A, ARID1A immunoreactivity was evaluated in endometrial sampling (biopsy/curettage) specimens showing CAH to determine whether ARID1A expression correlates with the presence of EEC at subsequent hysterectomy. In cohort B, ARID1A immunoreactivity was evaluated in the hysterectomy specimens with concurrent CAH and EEC to assess for the concordance of ARID1A expression in both components. Results: In cohort A, loss of ARID1A immunoreactivity was identified in the endometrial sampling specimen of 31% of patients undergoing hysterectomy for a preoperative diagnosis of CAH. EEC was identified in the hysterectomy specimen of 94% of patients with loss of ARID1A in the endometrial sampling specimen while only 15% of patients with retained ARID1A expression (P < 0.0001). No association was observed between ARID1A expression and demographic characteristics. In cohort B, 14 (31%) of 45 patients with concurrent CAH/EEC in their hysterectomy specimens had complete loss of ARID1A expression in the EEC components. Among these 14 patients, 50% also had loss of ARID1A immunoreactivity in the CAH component. Conclusions: ARID1A immunostaining may correlate with malignant transformation and the presence of concurrent EEC in patients with CAH identified at pre-hysterectomy endometrial sampling. Further investigation to determine the potential utility of ARID1A expression as a tissue biomarker is warranted.
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