Actin smooth muscle – Antibody Lexicon

Synonyms: ASMA, α-Smooth Muscle Actin, Alpha-Smooth Muscle Actin, α-SMA, α-Actin, SMA

by Jan Klos

Background

Actin is a contractile microfilament important for cellular growth, form and contractility present in cytoplasm. Actin has 6 different isoforms and presence of isoforms may vary between different cell types. The most commonly used in histopathology is α-Smooth Muscle Actin.

Staining in normal tissues

Staining in normal tissues: smooth muscle cells, cardiac muscle, myoepithelial cells, myofibroblasts, hepatic perisinusoidal cells, decidua and pericytes but is not demonstrated in normal striated muscle cells. Antibodies to α-Smooth Muscle Actin do not detect the other actin isoforms.

Staining in tumors

There is a long list of positively stained tumors including these with differentiation towards smooth muscles, myoepithelial cells or myofibroblasts. Rhabdomyosarcoma, malignant mesothelioma especially sarcomatoid variant, metastatic melanoma are positive in a half of the cases or more. Almost 50% of GIST are also reported positive. A minority of carcinomas especially with poorly differentiated or sarcomatoid morphology show often focal staining.

Staining pattern

Staining patternis cytoplasmic. In myofibroblastic cells often condensated under cell membrane forming “tram track” pattern.

Application

Important component of antibody panel for classification of spindle cell proliferations.
Identification of mainly smooth muscle differentiation in tumors (tumors of striated muscles show heterogeneous positivity in about 50% of cases).
Identification of myofibroblastic and myoepithelial proliferations as well as myoepithelial components in tumors.
Identification of myoepithelial cells in epithelial proliferations i.e . differential diagnosis of non invasive and invasive epithelial lesions in many organs.
Together with Actin HHF35 is present in ~90% of granulosa cell tumors and thecomas, but in lower percentage in other sex cord tumors.
GIST show heterogeneous positivity in ~50 % of cases.
Some papers show correlation between expression of α-Smooth Muscle Actin in stromal myofibroblasts and prognosis of tumors.
Some other proliferations may also exhibit reactivity with α-SMA antibodies including spindle cell and metaplastic carcinomas as well as melanomas with aberrant phenotype.

Selected references

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