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The difference between CD117+ and CKIT+

Hey, everyone –

I received a request to clarify the difference between being CD117+ and CKIT+.

CD117 is a receptor on the outside of mast cells. It is normal and all mast cells are CD117+. This is how we identify them as mast cells. If you have a bone marrow biopsy done and it says no CD117 is found, this is not because there are no mast cells there. It is because the test for CD117 isn’t sensitive enough to find those few mast cells. This is called the limit of detection (LoD).

When there is more of something present, it is easier to find it. Say I am in a field and there are five tennis balls scattered. If I walk around for a long time, maybe I will find three tennis balls. But if there is only one tennis ball to be found, I may not find it. I have less of a chance of finding it because there aren’t as many so it’s harder.

Being CD117+ is NORMAL for mast cells. It just means that it’s a mast cell. But mast cells that are constantly activated have more CD117+ on their outside membranes. Think of it like the tennis balls – if there are five CD117 receptors on a mast cell, it’s easier for the test to find one. If there is only one, the test might miss it.

CD117 is also called the CKIT receptor. It is a receptor that gives mast cells the signal to stay alive and encourage more mast cells to mature. If you get a biopsy report back and it is CD117+, then it will say CD117. The reason the report doesn’t call it positive for CKIT is historical and has to do with the fact that it was identified first as CD117 and later called CKIT because of similarities with other proteins of similar names.

When mast cell patients say CKIT+, it is a misnomer. It means that they are positive for the D816V mutation in CKIT, which is a marker for systemic mastocytosis. So being CD117+ and CKIT+ are not the same. CD117+ just means mast cell. CKIT+ (D816V) means neoplastic mast cell.

The D816V mutation changes the shape of the CD117 (CKIT) receptor and tells the mast cell to stay alive and encourage other mast cells to mature even when it shouldn’t.

Being CD117+ does not affect medication profile for mast cell disease at all. It just means it’s a mast cell. Some drugs are approved only for CKIT- patients (negative for D816V).

CD117/CKIT is a tyrosine kinase, which is a kind of protein. There are hundreds of known tyrosine kinases, CD117/CKIT is just one. Tyrosine kinase inhibitors can affect cells by blocking the signal to stay alive. Tyrosine kinases do not take up tyrosine from the environment, it has literally nothing to do with tyrosine metabolism at all.

If there any questions, ask in the comments.