GFAP (glial fibrillary acidic protein)

GFAP stands for glial fibrillary acidic protein. It is a protein found in specific cells of the nervous system and in the salivary glands. GFAP is important because it helps maintain the structure and supports the function of these cells.

What types of normal cells and tissues express GFAP?

Examples of cell types that normally express GFAP:

• Astrocytes: These are star-shaped cells located in the brain and spinal cord. Astrocytes support and protect nerve cells, also known as neurons.
• Other glial cells: Glial cells provide support, protection, and insulation for neurons throughout the central nervous system.
• Myoepithelial cells: These specialized cells are found in the salivary glands, where they assist in glandular contraction and fluid release, such as saliva.

How do pathologists test for GFAP?

Pathologists test for GFAP using a method called immunohistochemistry (IHC). This technique involves applying antibodies to a tissue sample. These antibodies specifically attach to the GFAP protein if it is present. When the antibodies attach to GFAP, they cause a visible color change that can be observed under a microscope. The pathologist then examines the stained tissue sample, determining whether GFAP is present, the number of cells that contain it, and the strength of its expression. Cells that express GFAP are described as positive (or reactive) while those that do not express GFAP are described as negative (or nonreactive).

What types of tumors express GFAP?

Both benign (noncancerous) and malignant (cancerous) tumors can express GFAP.

Benign (noncancerous) tumors:

• Pilocytic astrocytoma: A slow-growing brain tumor often found in children and young adults.
• Subependymal giant cell astrocytoma: A rare brain tumor linked to a genetic condition known as tuberous sclerosis.
• Pleomorphic adenoma: A common noncancerous salivary gland tumor.
• Myoepithelioma: A rare, benign tumor originating from myoepithelial cells in salivary glands.

Malignant (cancerous) tumors:

• Glioblastoma: An aggressive form of brain cancer arising from astrocytes.
• Astrocytoma: A brain cancer arising from astrocytes, typically less aggressive than glioblastoma but more aggressive than pilocytic astrocytoma.
• Myoepithelial carcinoma: A type of salivary gland cancer derived from myoepithelial cells.
• Epithelial-myoepithelial carcinoma: Another salivary gland cancer, often expressing GFAP in areas containing myoepithelial cells.

Why is GFAP important in a pathology report?

Detecting GFAP helps pathologists accurately diagnose specific types of tumors.

For example:

• Brain tumors: Identifying GFAP can help distinguish between various types of tumors that arise from astrocytes.
• Salivary gland tumors: GFAP expression can confirm a diagnosis of certain tumors involving myoepithelial cells.

Understanding whether a tumor expresses GFAP helps pathologists and doctors choose the most effective treatment strategy and provides valuable information about the tumor’s likely behavior and prognosis.