CD117


CD117 (also known as c-kit) is a protein found on the surface of many types of normal and abnormal cells. It plays an important role in helping cells grow and survive. Pathologists often look for CD117 in tissue samples when diagnosing certain types of cancer, especially gastrointestinal stromal tumors (GISTs).

What does CD117 do?

CD117 is a type of protein called a receptor tyrosine kinase, which means it sits on the surface of cells and sends signals inside the cell when activated. These signals help control how cells grow, divide, and survive. CD117 is especially important in the development and maintenance of specific cells, including:

  • Mast cells (involved in allergic responses).

  • Melanocytes (pigment-producing cells in the skin).

  • Hematopoietic stem cells (blood-forming cells in the bone marrow).

  • Germ cells (precursors to sperm and egg).

  • Interstitial cells of Cajal (specialized cells in the digestive tract that help control movement).

What types of cells normally express CD117?

In healthy tissues, CD117 is found on:

  • Mast cells.

  • Melanocytes.

  • Blood-forming stem cells.

  • Germ cells in the testis and ovary.

  • Glandular cells in the breast.

  • Cells in the developing brain and certain blood vessels.

  • Interstitial cells of Cajal in the wall of the stomach and intestines.

How do pathologists test for CD117?

Pathologists test for CD117 using a method called immunohistochemistry (IHC). This test uses special antibodies to detect CD117 in a tissue sample. When CD117 is present, the tumor cells show cytoplasmic staining under the microscope, meaning the inside of the cell stains positive. The results are reported as either positive (CD117 is present) or negative (CD117 is not detected), and the pathologist uses this information to help make or confirm a diagnosis.

What types of tumors show CD117 expression?

CD117 is very helpful in diagnosing certain types of tumors.

Some tumors that often show CD117 include:

  • Gastrointestinal stromal tumor (GIST) – This is the most common tumor associated with CD117. Most GISTs have mutations in the KIT gene, which causes CD117 to remain constantly active and promotes tumor growth.

  • Mast cell tumors – CD117 is expressed in both normal and abnormal mast cells, and it helps in diagnosing conditions such as mastocytosis.

  • Acute myeloid leukemia (AML) – CD117 can be found on immature blood cells, and helps estimate the number of abnormal stem cells (called blasts) in the bone marrow.

  • Seminomas and dysgerminomas – These are tumors that arise from germ cells in the testis or ovary.

  • Melanoma – A skin cancer that sometimes shows CD117 positivity.

  • Salivary gland tumors, small cell lung cancer, and thymic carcinoma – These may also show CD117 staining.

  • Other sarcomas – CD117 may be seen in some cases of Ewing sarcoma, osteosarcoma, and rhabdomyosarcoma.

  • Adenoid cystic carcinoma – A cancer of the salivary glands that often shows strong CD117 staining.

  • Angiomyolipoma and solid pseudopapillary tumors – These rare tumors can also be CD117 positive.

Not all tumors show CD117 expression. For example, CD117 is usually negative in schwannomas, leiomyomas, myxomas, and glomus tumors.

Why is CD117 important in a pathology report?

CD117 is especially important in diagnosing GIST because most of these tumors are driven by mutations in the KIT gene. If CD117 is present, it not only confirms the diagnosis but may also help predict response to treatment with targeted therapy, such as imatinib (Gleevec), which blocks the abnormal activity of the CD117 protein. CD117 can also be used to help identify mast cell disorders, germ cell tumors, and certain types of leukemia.

What happens when there is a mutation in the KIT gene?

In many GISTs, the KIT gene (which makes CD117) has a mutation that causes the protein to be permanently turned on. This leads to uncontrolled cell growth. These mutations are most commonly found in a part of the gene called exon 11, but can also occur in exons 9 or 13. Mutated CD117 proteins can be targeted with drugs like imatinib, which block the signals and stop the tumor from growing.

Questions to ask your doctor

  • Was CD117 tested in my biopsy or tumor?

  • What does the CD117 result mean for my diagnosis?

  • Does my tumor have a KIT mutation?

  • Could targeted therapy with a drug like imatinib be helpful for my condition?

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