Specialties: Pediatric Pathology Soft Tissue Pathology

The paired-box (PAX) family of proteins are key transcriptional regulators involved in early critical development.¹ The PAX-7 transcription factor has important functions in mammalian myogenesis and early neural development, with a particularly crucial role in specification and self-renewal of skeletal muscle tissue.^2,3 The expression of PAX-7 is highly restricted in normal adult tissues as demonstrated by nuclear immunoreactivity only being identified in rare, scattered satellite cells of the skeletal muscle and absent in both visceral smooth muscle and cardiac muscle as well as in a broad anatomical range of other non-neoplastic tissues. The assessment of soft tissue tumors and small round cell tumors is a persistent diagnostic challenge because of overlapping morphological features and insufficient molecular characterization in these groups of neoplasms. A high frequency of strong PAX-7 nuclear expression has been identified predominantly in rhabdomyosarcomas, preferentially in the embryonal subtype, and Ewing’s sarcoma, with reactivity being absent in related malignancies such as leiomyosarcoma, lymphoblastic lymphoma, neuroblastoma, carcinoid tumor, gastrointestinal stromal tumor, and small cell lung carcinoma. Immunohistochemical detection of PAX-7 protein can be used as a tool in distinguishing embryonal rhabdomyosarcoma and Ewing’s sarcoma from histologic mimics.^4,5

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