PAX 2: a novel Müllerian marker for serous papillary carcinomas to differentiate from micropapillary breast carcinoma.

Ovarian serous papillary carcinoma, although rarely metastasizing to the breast, is often challenging based on morphology alone, particularly from the micropapillary variant of breast carcinoma. Gross cystic disease fluid protein-15, although a specific marker, can be negative in up to 50% of breast carcinomas. Wilm's tumor gene 1 (WT-1) has been identified as a useful marker to differentiate metastatic ovarian serous papillary carcinoma from primary breast carcinoma; however, it has recently been shown in the micropapillary variant of the primary breast carcinoma making it a less specific marker. PAX 2, a nuclear transcription factor, was recently observed in ovarian serous papillary carcinomas. In this study of 89 breast carcinoma cases, 26 micropapillary carcinoma, and 63 nonmicropapillary carcinoma types were retrieved from our pathology archives, represented on a single tissue microarray (TMA) with a 3-fold redundancy (TMA-1, TMA-2). In addition, whole tissue sections of a variety of benign and neoplastic müllerian tissues were surveyed with the PAX 2 immunostain. All cases were stained with rabbit polyclonal PAX 2 antibody and, in addition, the 5 metastatic ovarian serous carcinoma cases were stained with WT-1 as well for comparison. Only nuclear staining was considered positive. All primary breast carcinomas represented on TMA-1 and TMA-2 were entirely negative for PAX 2 100% (89/89), whereas 100% (5/5) of all metastatic ovarian serous carcinomas showed moderate-to-strong staining. PAX 2 expression was comparable with WT-1 as well in the metastatic ovarian serous carcinoma group. We therefore conclude that PAX 2 is a promising new, sensitive, and specific müllerian immunomarker for ovarian serous carcinomas (primary and metastatic).