Smooth muscle actin (SMA)

Smooth muscle actin (SMA) is a protein found in the cells of smooth muscle, which is responsible for involuntary movements in the body, such as the contraction of blood vessels and the gastrointestinal tract. Pathologists use special tests to detect SMA in tissues, which can help them diagnose certain types of tumours and conditions that involve smooth muscle or cells with similar properties.

What types of cells normally express SMA?

SMA is primarily found in smooth muscle cells, which are located in areas such as blood vessels, the walls of the intestines, and the uterus. It is also found in myoepithelial cells, specialised cells in tissues like the salivary glands and the breast. Additionally, SMA can be expressed in certain other cell types, such as fibroblasts and myofibroblasts, which play a role in wound healing and tissue repair.

What types of tumours express SMA?

SMA is expressed in various tumours that arise from smooth muscle, myoepithelial cells, and other cell types with smooth muscle-like properties. These tumours can be benign (non-cancerous) or malignant (cancerous).

Benign (non-cancerous) tumours that express SMA:

• Leiomyoma: A benign tumour made up of smooth muscle, often found in the uterus or gastrointestinal tract.
• Myoepithelioma: A benign tumour that arises from myoepithelial cells commonly found in the salivary glands.
• Angioleiomyoma: A benign tumour composed of both smooth muscle and blood vessels.
• Pleomorphic adenoma: A benign salivary gland tumour that often contains myoepithelial cells expressing SMA.
• Fibromatosis: A benign growth of fibroblasts and myofibroblasts that can express SMA.
• Angiomyolipoma (PEComa): A benign tumour typically found in the kidneys, made up of blood vessels, smooth muscle, and fat.
• Nodular fasciitis: A rapidly growing but benign tumour of fibroblasts and myofibroblasts that can express SMA.

Malignant (cancerous) tumours that express SMA:

• Leiomyosarcoma: A cancer that arises from smooth muscle cells, typically found in the uterus or soft tissues.
• Myoepithelial carcinoma: A malignant tumour that arises from myoepithelial cells commonly found in salivary glands or skin.
• Adenoid cystic carcinoma: A malignant tumour that often develops in the salivary glands and can express SMA in the myoepithelial component.
• Epithelial-myoepithelial carcinoma: A rare cancer that arises from both epithelial and myoepithelial cells, often found in the salivary glands.
• Inflammatory myofibroblastic tumour (IMT): A tumour made up of myofibroblasts that can express SMA and may have malignant potential.
• Gastrointestinal stromal tumour (GIST): A tumour of the gastrointestinal tract that can express SMA, especially in specific subtypes.
• Synovial sarcoma: A rare cancer that often develops near joints and can express SMA in some instances.

How do pathologists test for SMA?

Pathologists use a test called immunohistochemistry (IHC) to look for SMA in tissue samples. In this test, a small piece of tissue is treated with special antibodies that bind to the SMA protein. If the tissue contains SMA, the antibodies will cause the cells to change colour, which the pathologist can see under a microscope. A positive result means that the cells are expressing SMA, while a negative result means they are not. This test helps the pathologist determine if the tumour has smooth muscle or smooth muscle-like cells.

When do pathologists perform this test?

Pathologists perform this test to confirm the presence of smooth muscle or myoepithelial cells in a tumour. It is often used to distinguish between different types of tumours, especially when the diagnosis is unclear. For example, SMA testing is helpful when diagnosing tumours like leiomyoma, leiomyosarcoma, or myoepithelial tumours, as it helps confirm the type of cells that make up the tumour.

About this article

Doctors wrote this article to help you read and understand your pathology report. Contact us if you have questions about this article or your pathology report. For a complete introduction to your pathology report, read this article.

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