Tyrosinase expression in malignant melanoma, desmoplastic melanoma, and peripheral nerve tumors / In reply

Archives of Pathology & Laboratory Medicine, Sep 2003 by Xu, Xiaowei, Zhang, Paul J, Elder, David E, Haupt, Helen M, Et al

To the Editor-We read with interest the article "Tyrosinase Expression in Malignant Melanoma, Desmoplastic Melanoma, and Peripheral Nerve Tumors" by Boyle et al.1 In the article, Boyle et al reported that 100% (5/5) of their desmoplastic melanomas stained with antibodies to tyrosinase and HMB-45 using a microwave-based antigen-retrieval immunohistochemical technique. This result surprised us because our immunohistochemical laboratory has been using microwave-based antigen-retrieval techniques for many years and we have never seen consistent staining of desmoplastic melanoma with tyrosinase and HMB-45, except focally in superficially located or in situ epithelioid or nevoid components of the lesions. In fact, our previous study and many others in the literature demonstrated rare if any staining of the spindle cell component of desmoplastic melanoma with tyrosinase and HMB-45.2,3

Most desmoplastic melanomas are positive for S100, but frequently lack the melanocytic differentiation markers such as tyrosinase, gp100 (HMB-45), melan-A, and MITF. In an effort to resolve the difference, we performed tyrosinase immuno-staining on 5 well-characterized desmoplastic melanomas using microwave-based antigen-retrieval techniques and the same tyrosinase antibody (T311) as was used in the mentioned article. In 4 of 5 cases (80%), the tumor cells were completely negative; in 1 case (20%), only superficial more epithelioid tumor cells were positive; however, the deeper more spindled cells were negative for tyrosinase. In all of these cases, internal positive controls stained properly.

We disagree with the conclusion of Boyle et al and would also like to raise 3 questions. First, a conventional microwave oven has no device to control the buffer temperature and can only be estimated at about 100[degrees]C when the buffer is boiling; however, the authors stated that their antigen-retrieval technique was performed at 121[degrees]C, a temperature that likely required a pressure cooker or specially designed microwave oven. In the article, there was no detail about how 121[degrees]C was achieved in a microwave oven. Second, the antigen retrieval condition used in the paper (121[degrees]C, 20 minutes) is very harsh to the tissue. In our experience, heating tissue sections in a pressure cooker more than 5 minutes will often result in dissolution of the tissue section. In addition, we wonder whether the authors used other means to validate their findings so as to exclude the possibility that the staining was due to cross-binding of the antibody to some unrelated epitopes generated by the harsh antigen-retrieval technique? Third, because the photomicrographs of the desmoplastic melanoma showed aggregates of epithelioid to spindled tumor cells, unlike typical desmoplastic melanoma, in which individual tumor cells are embedded in a fibrotic stroma, we question whether these tumors may represent nondesmoplastic spindle cell melanomas, which might account for the discrepancy in the reported findings.

We believe it is misleading to represent desmoplastic melanomas, at least in their predominant spindle cell components, as reacting with HMB-45 and tyrosinase. It is well known that desmoplastic melanoma can mimic scar and benign neural lesions because of its bland spindle cell morphology; and it is dangerous to assume that negative HMB-45 and tyrosinase stains are sufficient to rule out desmoplastic melanoma.

XIAOWEI XU, MD, PhD

PAUL J. ZHANG, MD

DAVID E. ELDER, MB,ChB

Department of Pathology and Laboratory Medicine

Hospital of University of Pennsylvania

Philadelphia, PA 19104

1. Boyle JL, Haupt HM, Stern JB, Multhaupt HA. Tyrosinase expression in malignant melanoma, desmoplastic melanoma, and peripheral nerve tumors. Arch Pathol Lab Med 2002;126:816-822

2. Xu X, Chu AY, Pasha TL, Elder DE, Zhang PJ. Immunoprofile of MITF, tyrosinase, melan-A, and MAGE-1 in HMB45-negative melanomas. Am J Surg Pathol. 2002;26:82-87.

3. Busam KJ, Iversen K, Coplan KC, Jungbluth AA. Analysis of microphthalmia transcription factor expression in normal tissues and tumors, and comparison of its expression with S-100 protein, gp100, and tyrosinase in desmoplastic malignant melanoma. Am J Surg Pathol. 2001;25:197-204.

In Reply.-We wish to thank the authors for their interest in our paper.1 It is not the first time a laboratory claiming to use the same techniques does not reproduce results of another laboratory. Although our finding of positive staining for tyrosinase in desmoplastic melanomas clearly differs from the authors' prior conclusions, our results are similar to previously published studies of Jungbluth et al,2 Orchard,3 and Busam et al,4 who found a proportion of desmoplastic melanomas to stain positively for tyrosinase. In particular, the study by Busam et al,4 cited by these authors as evidence against tyrosinase staining, demonstrated 11 of 20 desmoplastic melanomas to be positive for tyrosinase.

The antigen retrieval and immunohistochemical staining in our study was performed using a Ventana Discovery (Ventana Medical Systems, Tucson, Ariz), which allows the precise setting of 121[degrees]C for 20 minutes. The immunohistochemical technologist from the laboratory that submitted this letter contacted our laboratory and we explained in detail our method of performing the immunohistochemistry. She indicated that the laboratory did not have access to a Ventana Discovery at the time these studies were performed.