Embryonal Rhabdomyosarcoma : MyoD1
MYOGENIC REGULATORY PROTEINS: MyoD1 and myogenin are DNA-binding nuclear regulatory proteins that play a major role in the expression and maintenance of skeletal muscle phenotype. They interact with muscle-specific genes at early stages of skeletal muscle differentiation. Expression of both MyoD1 (shown here) and myogenin (next image) is seen in more than 95% of rhabdomyosarcomas (RMS) of all subtypes with high specificity. No other round blue cell tumor has shown nuclear positivity for these markers. The level of expression appears to be inversely related to the degree of differentiation. Primitive-appearing cells are strongly positive. Well-differentiated rhabdomyoblasts have weak or no staining. Strong diffuse myogenin expression (>80% of cells) appears to correlate with poor prognosis in pediatric RMS. The spindle cell/sclerosing variant of RMS shows strong MyoD1 positivity but variable expression of myogenin. Pleomorphic RMS show MyoD1 and myogenin positivity in only about 50% of cases. In addition, the immunoreactivity is focal. Only nuclear positivity should be considered as true evidence of skeletal muscle differentiation. Non-specific cytoplasmic immunoreactivity for MyoD1 has been observed in non-muscle tumors such as primitive neuroectodermal tumor, Wilms tumor, and undifferentiated sarcoma. Myogenin antibody is technically superior and does not show any non-specific cytoplasmic staining. Caution: Notwithstanding the high specificity, the immunoreactivity for MyoD1 and myogenin does not automatically warrant a diagnosis of rhabdomyosarcoma. There are rare non-muscle tumors which can show rhabdomyoblastic differentiation and positivity for MyoD1 and myogenin. They include Wilms tumor, neuroendocrine carcinoma, malignant glial tumors, malignant peripheral nerve sheath tumors, and teratomas. Mesenchymal chondrosarcomas can also show MyoD1 and myogenin expression.