c-kit protooncogene encodes a type III transmembrane receptor kinase, the s
tem cell factor receptor, or KIT. The ligand of the KIT, stem cell factor,
is a cytokine that stimulates mast cell growth and differentiation. We have
studied immunohistochemically KIT expression in 23 canine mast cell tumors
(MCTs), 10 histiocytomas, 5 malignant melanomas, and in 2 cell lines deriv
ed from mast cells (HMC-1, human and C2, canine). As expected, KIT was dete
cted both in the human mast cell leukemia cell line (HMC-1) and in the cani
ne mastocytoma cell line C2. In normal canine skin, KIT expression was conf
ined to mast cells. All canine MCTs expressed KIT, although the intensity o
f the staining reaction varied considerably among the 23 neoplasms. Grade I
II tumors showed the highest expression of KIT, whereas grade I tumors show
ed the lowest expression of KIT. Two patterns of KIT expression were detect
ed in mast cells. In normal canine mast cells and in some neoplastic mast c
ells, KIT appeared mainly on the cell membrane. However, in many canine MCT
s, KIT is accumulated in the cytoplasm, usually near the cell nucleus. The
meaning of these two patterns is not clear. Expression of KIT could not be
detected immunohistochemically in any of the other neoplasias investigated.
According to our results, it can be concluded that most, if not all, canin
e MCT express KIT. Furthermore, there is an inverse correlation between the
degree of differentiation and the expression of KIT. Moreover, according t
o our results, KIT can be used as a reliable immunohistochemical marker for
canine mast cells and undifferentiated mast cell tumors.