Intermediate filament proteins and actin isoforms as markers for soft tissue tumor differentiation and origin: II. Rhabdomyosarcomas


A series of 15 rhabdomyosarcomas was examined by light microscopy, transmission electron microscopy, two-dimensional gel electrophoresis (2D-GE) and indirect immunofluorescence, the latter using monoclonal or affinity-purified polyclonal antibodies to desmin, vimentin, α-smooth muscle and α-sarcomeric (α-sr) actins. By light microscopy, the authors diagnosed 1 botrioid, 1 alveolar, and 7 embryonal rhabdomyosarcomas, 4 pleomorphic spindle cell sarcomas, and 2 spindle cell sarcomas, one nondistinct, the other with a hemangiopericytomatous pattern. By transmission electron microscopy, 13 neoplasms disclosed rhabdomyoblastic differentiation; the remaining 2, myogenic differentiation. By immunofluorescence microscopy, all neoplasms expressed vimentin and α-sr actin, 12 expressed, in addition, desmin, and 1 expressed α-smooth muscle actin. Among the 11 neoplasms studied by means of 2D-GE, 7 demonstrated an α-actin spot, while 4 showed only β and γ spots. One tumor disclosed, in addition to α, β, and γ spots, a spot with a molecular weight corresponding to actin, but more acidic than α-actins. This study demonstrates that α-sr actin antibody represents a valuable marker for the diagnosis of rhabdomyosarcoma, because it was present in all neoplasms, including the one negative for desmin. This antibody further allowed the recognition of pleomorphic variants and morphologically atypical forms of rhabdomyosarcomas. The presence of α-smooth muscle actin in 1 case of rhabdomyosarcoma suggests that this actin isoform may be expressed during skeletal muscle differentiation.

Publication Title

American Journal of Pathology

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