[541] PAX8 Is a Sensitive Marker for Primary and Metastatic Merkel Cell Carcinoma

Ankur R Sangoi, David Cassarino. El Camino Hospital, Mountain View, CA; Southern California Permanente Medical Group, Los Angeles, CA

Background: PAX8, a nephric cell-lineage transcription factor initially characterized in renal cell carcinomas, is also well-recognized as a marker of Mullerian tract and thyroid tumors. From a previous tissue microarray study of non-renal neoplasms including a variety of skin tumors, we identified PAX8 staining in a small set of Merkel cell carcinomas, a finding not previously described. Herein, we explore PAX8 immunoreactivity in a larger series of whole-section primary and metastatic Merkel cell carcinomas to determine potential diagnostic utility.
Design: Whole-section slides from 25 Merkel cell carcinomas were stained with polyclonal PAX8 (pre-diluted, Cell Marque, Rocklin, CA) and two varieties of monoclonal PAX8 (prediluted clone MRQ-50, Cell Marque, Rocklin, CA; 1:100 dilution clone BC12, Biocare, Concord, CA). Nuclear staining intensity (I) and extent (E) was semi-quantitatively analyzed (range 0-3) with a comparison of staining thresholds required for a "positive" result (≥1+ vs ≥2+).
Results: The 25 Merkel cell carcinomas included 18 primary and 7 metastatic tumors from 17 males and 8 females (51-91 years, mean 75.6 years). 25/25 (100%) of cases showed positive polyclonal and monoclonal PAX8 staining with the Cell Marque antibodies (polyclonal: average I=2.4, average E=2.6, monoclonal: average I=2.0, average E=2.1). Utilizing a ≥2+ staining I and E requirement, 21/25 (84%) and 19/25 (76%) of cases were positive with the PAX8 antibodies (polyclonal and monoclonal, respectively) from Cell Marque. The Biocare monoclonal PAX8 antibody was negative in all cases.
Conclusions: PAX8 staining in Merkel cell carcinoma, a novel finding, expands the spectrum of tumors showing immunoreactivity for this marker. Both polyclonal/monoclonal PAX8 antibodies from Cell Marque show high (100%) sensitivity for Merkel cell carcinoma and may prove useful additions to a panel of diagnostic markers. Interestingly, the Biocare monoclonal PAX8 antibody does not show any staining of Merkel cell carcinoma. Both PAX8 antibodies from Cell Marque maintain strong sensitivity using a ≥2+ staining cut-point for a "positive" result (polyclonal>monoclonal at 84% vs 76%), an important finding in cases with scant tissue and/or background staining.
Category: Dermatopathology

Tuesday, March 20, 2012 9:30 AM

Poster Session III # 117, Tuesday Morning


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