A significant component of the tumor was composed of pleomorphic eosinophilic spindle cells in whorls and interlacing fascicles that showed a strong, sharp, and diffuse positivity for desmin, thus mimicking rhabdomyosarcoma.
Expression of Kirsten rat sarcoma viral oncogene [Kras(G12V)] and disruption of cyclin-dependent kinase inhibitor 2A (CDKN2A; p16p19) in prospectively isolated satellite cells gave rise to pleomorphic rhabdomyosarcomas (MyoD-, Myogenin- and Desmin-positive), whereas introduction of the same oncogenetic hits in nonmyogenic progenitors induced pleomorphic sarcomas lacking myogenic features.
Immunohistochemical positive staining with vimentin (80% of tumour cells), desmin (20%) and titin (30%) antibodies was suggestive of a rhabdomyosarcoma.
Previous findings that the intermediate filament nestin is expressed in immature skeletal muscle cells prompted us to compare the staining patterns of nestin and desmin in rhabdomyosarcomas (RMSs) and in other small cell tumors of infancy.
In contrast to RD cells that expressed preferentially myoid and not neurofilament proteins (NFPs) upon treatment with UDP-4, differentiated RD/TE-671 cells exhibited characteristic dendritic processes and expressed NFPs (NFP68, NFP160, and NFP200), parvalbumin (calcium-binding protein), and neuron-specific enolase, as well as a small amount of vimentin and desmin.
The RMSs contained immunohistochemically the intermediate filament proteins vimentin and desmin and reacted positively with the alpha-sarcomeric actin antibody, which recognizes alpha-SK and alpha-CARD actin isoforms.
The present study indicates that both muscle-specific actin and desmin can be expressed in tumors lacking ultrastructural evidence of a rhabdomyoblastic differentiation and that the combined use of monoclonal antibodies to desmin and muscle-specific actin is of value when it comes to recognizing rhabdomyosarcomas within the group of undifferentiated small and dark cell malignancies of soft tissue tumors.
In addition, MAbs to the cytoplasmic protein desmin, myoglobin, muscle actin (alpha and gamma) and alpha-sarcomeric actin reacted with these cell lines, SCMC-RM2 and SCMC-RM2-1 being thus identified as rhabdomyosarcoma.