ORIGINAL ARTICLE Apo D in Soft Tissue Tumors A Novel Marker for Dermatofibrosarcoma Protuberans Robert B. West, MD, PhD,* Jeff Harvell, MD,* Sabine C. Linn, MD, PhD,* Chih Long Lui,¶ Wijan Prapong,¶ Tina Hernandez-Boussard, PhD,¶ Kelli Montgomery,* Torsten O. Nielsen, MD, PhD,† Brian P. Rubin, MD, PhD,‡ Rajiv Patel, MD,§ John R. Goldblum, MD,§ Patrick O. Brown, MD, PhD,¶ and Matt van de Rijn, MD, PhD* Abstract: Using gene microarray expression profiling, we previ- ously found that apolipoprotein D (Apo D) was highly expressed in dermatofibrosarcoma protuberans (DFSP). In this study, we confirm that Apo D is highly and relatively specifically expressed in DFSP using immunohistochemistry. A tissue microarray containing 421 soft tissue tumors was constructed and stained with antibodies against Apo D and CD34. Cytoplasmic immunostaining for Apo D was found in 9 of 10 typical DFSPs. In addition, 3 of 3 Bednar tumors and 2 of 3 giant cell fibroblastomas stained in conventional sections. In contrast, Apo D was immunoreactive in only a very small subset of a diverse collection of other soft tissue tumors, including Malignant Fibrous Histiocytoma (MFH), glomus tumor, neurofibroma, and malignant peripheral nerve sheath tumors. Immunostains for Apo D were nega- tive in conventional sections of 16 fibrous histiocytomas, and an ad- ditional 12 variants of fibrous histiocytoma. Digital images of all im- munohistochemical and hematoxylin and eosin tissue microarray stains are available at the accompanying website (http://microarray- pubs.stanford.edu/tma_portal/apod/). We conclude that Apo D is strongly expressed in DFSPs and neural lesions and may be useful in differentiating DFSP from fibrous histiocytoma. Key Words: dermatofibrosarcoma protuberans, soft tissue tumors, gene microarray expression, apolipoprotein D (Am J Surg Pathol 2004;28:1063–1069) D ermatofibrosarcoma protuberans (DFSP) is a spindle-cell tumor of uncertain histogenesis. The clinical behavior of this lesion is characterized by multiple, often aggressive, local recurrences. A translocation of chromosomes 17 and 22, re- sulting in fusion of the collagen type I alpha I (COL1A1) and platelet-derived growth factor beta (PDGF-) genes is thought to be the transforming event for DFSPs. 16,19 Because of this translocation, the high activity of the promoter of COL1A1 leads to the high expression of PDGF-. 4,10,18 The main lesion in the differential diagnosis is fibrous histiocytoma (FH), a be- nign spindle cell proliferation. These uncommon tumors can have similar and overlapping morphologic findings, but the histologic distinction between the two entities is clinically rel- evant. The most commonly used histochemical marker to aid in this distinction is CD34, which reacts with DFSP but not with FH. In normal tissue, CD34 reacts with a number of dif- ferent cell types, including endothelial cells and interstitial dendritic cells found within the gastrointestinal tract and in the skin. 20,21 However, CD34 does not react with all DFSPs and, more importantly, CD34 is also expressed in a number of other soft tissue tumors, such as gastrointestinal stromal tumor, soli- tary fibrous tumor, and a variety of vascular and neural le- sions. 23 Recently, several reports have described molecular char- acterization of soft tissue tumors using gene arrays. 1,8,12,15 These techniques determine mRNA expression on a genome- wide basis and can discern patterns of gene expression that are specific for certain histologic entities. Antisera or monoclonal antibodies reacting with the gene products of interest can then be used in immunohistochemical studies to search for practical markers in surgical pathology practice. Tissue microarrays (TMAs) allow large numbers of tissues to be stained with a novel antibody on a single glass slide. 9 This method can evalu- ate antibodies with potential for diagnostic 5,13 or prognostic 22 use. Recently, DNA microarrays were used to profile gene expression in DFSP. 10 In analysis of a series of 36 soft tissue tumors, several genes were highly expressed in 9 cases of DFSP when compared with other spindle cell neoplasms. One From the Departments of *Pathology and ¶Biochemistry and Howard Hughes Medical Institute, Stanford University Medical Center, Stanford, CA; †Department of Pathology and Genetic Pathology Evaluation Centre, Vancouver General Hospital, Vancouver, British Columbia, Canada; ‡De- partment of Anatomical Pathology, University of Washington Medical Center, Seattle, WA; and §Department of Anatomic Pathology, Cleveland Clinic Foundation, Cleveland, OH. Supported by NIH grants CA85129 and CA84967 and the Howard Hughes Medical Institute. P.O.B. is an Associate Investigator of the Howard Hughes Medical Institute. S.C.L. was a recipient of a Dutch Cancer Soci- ety Postdoctoral Research Fellowship. Reprints: Matt van de Rijn, Department of Pathology, Stanford University Medical Center, 300 Pasteur Drive, Stanford, CA 94305 (e-mail: mrijn@ stanford.edu). Copyright © 2004 by Lippincott Williams & Wilkins Am J Surg Pathol • Volume 28, Number 8, August 2004 1063