ARID1A is a recently identified tumor suppressor participating in chromatin remodeling. Somatic inactivating mutations of ARID1A and loss of its expression occur most frequently in ovarian clear cell and endometrioid carcinomas and uterine endometrioid carcinomas. Since endometriosis is thought to be a precursor of most ovarian clear cell and endometrioid carcinomas, we undertook an analysis of ARID1A expression of these tumors arising within an endometriotic cyst (endometrioma).
Our immunohistochemical study set consisted of 47 endometriotic cysts containing clear cell carcinoma in 24 cases, well-differentiated ovarian endometrioid carcinoma in 20 and mixed clear cell and endometrioid carcinoma in 3.
ARID1A loss was observed in 31 (66%) of 47 carcinomas and therefore these cases were informative for determining the temporal sequence of loss of ARID1A expression in tumor progression. In 16 of the 47 cases, ARID1A immunoreactivity was retained in both the endometriotic cyst and the carcinoma and thus these cases were not informative. All of the 31 informative cases showed loss of ARID1A immunoreactivity in the carcinoma and in the endometriotic cyst epithelium in direct continuity with the carcinoma but not in the cyst epithelium that was not adjacent to the tumor.
The findings in this study provide cogent evidence that loss of ARID1A function as shown by loss of expression, presumably due to mutations, is an early molecular event, occurring before malignant transformation, in the development of the majority of ovarian clear cell and endometrioid carcinomas arising in endometriomas.