Clustering of activating mutations in c-KIT's juxtamembrane coding region in canine mast cell neoplasms

The proto-oncogene c-KIT encodes a growth factor receptor, KIT, with Ligand -dependent tyrosine kinase activity that is expressed by several cell types including mast cells. c-KIT juxtamembrane coding region mutations causing constitutive activation of KIT are capable of transforming cell lines and h ave been identified in a human mast cell line and in situ in human gastroin testinal stromal tumors, but have not been demonstrated in situ in neoplast ic mast cells from any species. To determine whether c-KIT juxtamembrane mu tations occur in the development of mast cell neoplasms, we examined canine mastocytomas, which are among the most common tumors of dogs and which oft en behave in a malignant fashion, unlike human solitary mastocytomas. Seque ncing of c-KIT cDNA generated from tumor tissues removed from seven dogs re vealed that three of the tumors contained a total of four mutations in an i ntracellular juxtamembrane coding region that is completely conserved among vertebrates. In addition, two mutations were found in three mast cell line s derived from two additional dogs. One mutation from one line matched that found in situ in one of the tumors. The second was found in two lines deri ved from one dog at different times, indicating that the mutation was prese nt in situ in the animal. All five mutations cause high spontaneous tyrosin e phosphorylation of KIT. Our study provides in situ evidence that activati ng c-KIT juxtamembrane mutations are present in, and may therefore contribu te to, the pathogenesis of mast cell neoplasia. Our data also suggest an in hibitory role for the KIT juxtamembrane region in controlling the receptor kinase activity.