Myogenic regulatory protein expression in adult soft tissue sarcomas. A sensitive and specific marker of skeletal muscle differentiation.

Myogenic regulatory protein (MyoD1) is a DNA-binding nuclear protein that initiates myogenesis in mesenchymal stem cells. Its expression has proved a very sensitive marker of myogenic differentiation in malignant tumors of childhood. In this study, the reliability of MyoD1 expression as marker of skeletal muscle differentiation has been tested in 38 cases of adult-type sarcomas. Frozen sections were stained with the monoclonal antibody 5.8A (MAb 5.8A) developed against recombinant wild-type murine MyoD1. Routinely processed formalin-fixed sections from the same cases were reviewed and immunostained with a panel of antibodies commonly used to detect muscle differentiation: myoglobin, fast-myosin, desmin, muscle-specific actin, alpha-smooth muscle actin. Four cases positively stained with MAb 5.8A: three were high-grade spindle cell sarcomas, the fourth case was an alveolar soft-part sarcoma. Only tumors in which skeletal muscle histogenesis was inferred on the basis of histology and/or immunohistochemistry with conventional muscle markers stained positively for MAb 5.8A. All the other tumors tested, including leiomyosarcomas (11), liposarcomas, (10, including four dedifferentiated liposarcomas), fibrosarcomas (three), malignant fibrous histiocytomas (two), synovial sarcomas (two), and malignant peripheral nerve sheath tumors (two), were negative for MAb 5.8A. The high level of sensitivity and specificity of MyoD1 expression indicates the value of this marker in the diagnosis of soft tissue sarcomas.