PAX2 loss by immunohistochemistry occurs early and often in endometrial hyperplasia.

Immunohistochemical markers to assist in the diagnosis and classification of hyperplastic endometrial epithelial proliferations would be of diagnostic use. To examine the possible use of PAX2 as a marker of hyperplastic endometrium, cases of normal endometrium, simple and complex hyperplasia without atypia, atypical hyperplasia, and International Federation of Gynecology and Obstetrics (FIGO) grade 1 endometrioid carcinomas were stained for PAX2. Two hundred and six endometrial samples were available for interpretation of PAX2 staining. The percentage of cases with complete PAX2 loss (0% of cells staining) increased with increasing severity of hyperplasia: 0% of normal proliferative and secretory endometrium (n=28), 17.4% of simple hyperplasia (n=23), 59.0% of complex hyperplasia (n=83), 74.1% of atypical hyperplasia (n=54), and 73.3% of FIGO grade 1 endometrioid cancers (n=15). Partial loss of PAX2 expression did occur in normal endometrium (17.9%) but in smaller proportions of tissue and was less frequent than in simple hyperplasia (47.8% with partial loss), complex hyperplasia (32.5%), atypical hyperplasia (22.2%), and FIGO grade 1 carcinomas (20.0%). Uniform PAX2 expression was rare in complex (8.4%) and atypical hyperplasia (3.7%) and carcinoma (6.7%). When evaluating loss of PAX2 in histologically normal endometrium adjacent to lesional endometrium in a given case, statistically significant differences in staining were observed for simple hyperplasia (P=0.011), complex hyperplasia (P<0.001), atypical hyperplasia (P<0.001), and FIGO grade 1 endometrioid cancer (P=0.003). In summary, PAX2 loss seems to occur early in the development of endometrial precancers and may prove useful in some settings as a diagnostic marker in determining normal endometrium from complex and atypical hyperplasia and low-grade carcinomas. However, it is not useful in distinguishing between these diagnostic categories.