Neoplastic transformation of mast cells through activating mutations of c-kit receptor

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The c-kit proto-oncogene encodes a receptor tyrosine kinase (KIT). Although the enzymatic activity of KIT is regulated by its ligand, stem cell factoe (SCF), the substitution of varine or tyrosine for aspartic acid-814 at the kinase domain lead to constitutive avtivation of KIT.To examine the transforming potential of the mutant KIT,the c-kit^<Val814> cDNA was introduced into the murine interleukin-3-dependent IC-2 mast cell line, which is of cultured mast cell origin but does not express KIT,and normal hematopoietic progenitor cells with retroviral vector. IC-2 cells expressing KIT^<Val814> showed factor-independent growth in suspension culture and produced tumors in nude athymic mice. From bone marrow cells infected with KIT^<Val814>, granulocyte/macrophage, mast-cell colonies, and mixed erythroid/myeloid colonies developed without the addition of exogenous growth factors. Transplantation of KIT^<Val814>-infected bone marrow cells led to development of acute leukemia in transplanted mice. Furthermore, transgenic mice expressing KIT^<Val814> developed acute leukemia or malignant lymphoma. We also investigated the molecular mechanism of constitutive activation of KIT in the FMA3 mouse mastocytoma cell line. Sequencing of the whole coding region of the c-kit showed that the Val^<814> or Tyr^<814> mutation was absent in FMA3 cells and that the c-kit cDNA of FMA3 cells carried an in-frame deletion of 21 base pairs encoding Thr-Gln-Leu-Pro-Tyr-Asp-His at the juxtamembrane domain. In IC-2 cells introduced with the FMA3-type c-kit cDNA with 21 bp deletion, KIT was constitutively activated and dimerized without the stimulation by SCF.IC-2 cells expressing the FMA3-type KIT grew in suspension culture without IL-3 and SCF and became leukemic in nude athymic mice. These results demonstrate a direct role of the mutant KITs in tumorigenesis of mast cells and hematopoietic stem cells.