PLAG1 expression in mesenchymal tumors: An immunohistochemical study with special emphasis on the pathogenetical distinction between soft tissue myoepithelioma and pleomorphic adenoma of the salivary gland

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Abstract
PLAG1, a proto‐oncogene activated in several types of tumors including pleomorphic adenoma of the salivary gland and lipoblastoma, is usually overexpressed because of chromosomal aberrations resulting in fusion genes. Myoepithelial tumors in soft tissue are morphologically similar to pleomorphic adenoma, but the genetic profiles of these tumors have not been fully examined. In the present study, we immunohistochemically evaluated the expression of PLAG1 in a series of 243 mesenchymal tumors. We determined that 14 tumors, including eight of 10 lipoblastomas, two of seven gastrointestinal stromal tumors, one of two angiomyofibroblastomas, one of five synovial sarcomas, one of seven leiomyomas and one of 12 myxofibrosarcomas were positive for PLAG1, whereas all seven soft tissue myoepitheliomas were PLAG1 negative. We examined two soft tissue myoepitheliomas, whose paraffin blocks were available, for fusion gene transcripts involving PLAG1 or HMGA2 specific for pleomorphic adenoma by a reverse transcription‐polymerase chain reaction assay, and no fusion transcripts were detected. Our results suggest that soft tissue myoepithelioma may be a pathogenetically distinct tumor entity from pleomorphic adenoma based on the absence of PLAG1 overexpression and characteristic fusion genes. On the other hand, PLAG1 immunohistochemistry is useful for distinguishing lipoblastoma from other lipomatous tumors including liposarcoma.

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