Evaluation of PAX8 Expression and Its Potential Diagnostic and Prognostic Value in Renal and Extra-Renal Ewing Sarcomas/PNETs.

PAX8 is a transcription factor involved in the regulation of organogenesis of the thyroid gland, kidney, and Müllerian system. It is commonly expressed in epithelial tumors of thyroid and parathyroid glands, kidney, thymus, and female genital tract. PAX8 is increasingly used in the establishment of tissue of origin in carcinomas and has recently been identified in a subset of small blue round cell tumors including Ewing sarcomas/PNETs. However, it is unclear if this association in ES/PNETs is due to renal origin or is PNET specific. In this study we investigated the PAX8 staining pattern of primary renal and extra-renal ES/PNETs to explore its potential diagnostic and prognostic role. A tissue microarray (TMA) of 22 cases of extra-renal Ewing/PNETs and two separate cases of primary renal PNET whole slide sections were immunohistochemically stained with rabbit polyclonal PAX8 antibody. PAX8 was positive in 2 of 2 primary renal PNETs and in 14 (64 %) cases of the extra renal PNETs. The association between PAX8 immunoreactivity and Ewing/PNET was identified in both primary renal and extra-renal Ewing/PNETs for the first time. Further studies are warranted to verify these findings and to shed light in the tumorigenesis of Ewing/PNET. However, PAX8 is not useful in establishing a diagnosis of Ewing/PNET due to its presence in different tumors like carcinomas, lymphomas and sarcomas. PAX8 does not seem to have prognostic value.