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PAX7 Is a Sensitive Marker of Skeletal Muscle Differentiation in Rhabdomyosarcoma and Tumors With Rhabdomyosarcomatous Differentiation in the Female Genital Tract

In the female genital tract, rhabdomyosarcoma may occur in “pure” form or as a heterologous constituent of a biphasic neoplasm such as carcinosarcoma or adenosarcoma. Discriminating rhabdomyosarcoma from its histologic mimics relies on confirmation of skeletal muscle differentiation by morphology or immunohistochemistry (IHC), which can be challenging to interpret in some cases owing to limited expression. PAX7, a transcription factor expressed in mammalian muscle progenitor cells, has been reported in up to 86% of soft tissue rhabdomyosarcomas by IHC. To determine whether PAX7 IHC could augment current approaches to identify rhabdomyosarcoma in gynecologic malignancies, we assessed PAX7, myogenin, and MyoD1 IHC on whole tissue sections from 100 gynecologic tumors: 50 with rhabdomyosarcomatous differentiation and 50 with features mimicking rhabdomyosarcoma. PAX7 expression was present in 96% (48/50) of gynecologic tumors with rhabdomyosarcomatous differentiation and was absent in all rhabdomyosarcoma mimics; it was more diffusely expressed than myogenin in 16 cases and was positive in a greater percentage of tumor cells in 28 cases. PAX7 and myogenin were typically coexpressed, and no rhabdomyosarcoma exhibited complete absence of both markers; however, 2 myogenin-negative tumors were PAX7-postive. Morphologically, PAX7 localized to the nuclei of primitive-appearing cells, whereas myogenin was observed in maturing rhabdomyoblasts including strap cells. Our findings highlight the utility of PAX7 as a complementary diagnostic marker of rhabdomyosarcomatous differentiation in gynecologic tumors. PAX7 should be used in combination with other markers of skeletal muscle differentiation, namely myogenin, and may be particularly helpful in cases where myogenin and/or MyoD1 expression is limited.