KIT, a receptor tyrosine kinase encoded by the gene c-kit plays a crucial role in cell growth by binding its ligand stem cell factor (SCF) in various cells, including mast cells. KIT is a large complex receptor that consists of five immunoglobulin-like domains (IgD) in the extracellular region, a transmembrane domain, and three intracellular domains that include a juxtamembrane domain and a kinase domain, which is split by a kinase insert sequence into tyrosine kinase domain 1 (an adenosine triphosphate-binding site) and tyrosine kinase domain 2 (activation loop). Normally, KIT is activated by SCF. When SCF binds to the extracellular IgD, it promotes KIT dimerization, transphosphorylation, and activation of downstream cell signaling pathways. These pathways include Ras/MAP kinase, Rac/Rho-JNK, PI3K/AKT, and JAK/STAT. Ultimately this results in cell survival, cell proliferation, cell differentiation, and gene transcription (Figure 1). The gain-of-function mutations of c-kit have been demonstrated to be closely related to the pathogenesis of several specific types of human tumors, including acute myelogenous leukemia, gastrointestinal stromal tumors, and mastocytomas. This is believed to occur by inducing constitutive ligand-independent kinase activation of KIT.

Figure 1

In canine MCTs, mutations have been found in c-kit exon 8, exon 9, exon 11, and exon 17 (Letard et al. 2008) (Figure 2). In feline MCTs, mutations have been found in c-kit exon 6, exon 8, exon 9, and exon 11 (Isotani et al. 2010) (Figure 2). Overall, c-kit mutations have been found in 50 of 191 MCTs (26.2%) in dogs. Most mutations are in exon 11 (16.8%), and of these, most are internal tandem duplications. The frequency of mutations in exon 8, exon 9, and exon 17 are 4.7%, 4.2%, and 0.5%, respectively. In cats, c-kit mutations have been found in 42 of 62 MCTs (67.7%). Mutations are most frequent in exon 8 (45.2%) followed by exon 9 (24.2%). The majority of mutations in c-kit exon 8 are internal tandem duplication mutations. Mutations in exon 6 (5.9%) and exon 11 (1.6%) are infrequent. Some cats have mutations in exon 8 and exon 9 simultaneously. All of the reported mutations are in-frame mutations that alter the amino acid composition of KIT. Mutations in exon 6, exon 8, exon 9, exon 11, and exon 17 alter the KIT amino acid composition in the fourth IgD, fifth IgD, juxtamembrane domain, and activation loop, respectively. Most of these mutations have been demonstrated to cause ligand-independent activation of KIT in cultured cells transfected with a mutated c-kit gene (Figure 3A). This suggests that these mutations are critically involved in the pathogenesis of MCTs via emergence of constitutive activation of KIT and its downstream signaling networks.

Figure 2

Imatinib is a kinase inhibitor that competes with adenosine triphosphate (ATP) for the ATP binding site of protein-tyrosine kinases, including KIT, and prevents downstream signaling. Imatinib has been shown to suppress ligand-independent phosphorylation of several types of mutant KIT found in canine and feline MCTs in vitro around the therapeutic concentration reported in humans (Cmax 400 mg/dose; approximately 3 μmol/L) (Figure 3B). These findings suggest that mutant KIT is a valuable therapeutic target for imatinib in canine as well as feline MCTs.

Figure 3

In this session, I will describe the molecular pathogenesis of canine and feline MCTs, and the molecular basis of imatinib therapy. I will also touch on the development of imatinib resistance in MCTs.

References

1.  Letard S, Yang Y, Hanssens K, et al. Gain-of-function mutations in the extracellular domain of KIT are common in canine mast cell tumors. Mol Cancer Res. 2008;6:1137–1145.

2.  Isotani M, Yamada O, Lachowicz JL, et al. Mutations in the fifth immunoglobulin-like domain of kit are common and potentially sensitive to imatinib mesylate in feline mast cell tumours. Br J Haematol. 2010;148:144–153.