An Activating Mutation in the Kit Receptor Abolishes the Stroma Requirement for Growth of ELM Erythroleukemia Cells, But Does Not Prevent Their Differentiation in Response to Erythropoietin

Signal transduction by several KIT juxtamembrane domain mutations

Mutations of KIT receptor tyrosine kinase are found in the majority of patients with mastocytosis and in most gastrointestinal stromal tumors. Oncogenic KIT mutations in GISTs are located in the KIT juxtamembrane domain (JMD), while codon 816 in the KIT kinase domain is mutated in systemic mastocytosis. We describe and characterize a mutation in the KIT-JMD named Kdelta27. We show that Kdelta27 mutant is constitutively dimerized and phosphorylated. Kdelta27 ectopic expression renders both the Ba/F3 cell line and primary cultures of bone marrow mast cells independent of cytokines for proliferation and cell survival. The classical signaling pathways activated by wild-type KIT upon ligand stimulation are constitutively activated by Kdelta27 and other JMD mutations. However, a side-to-side comparison revealed differences between the wild-type and JMD mutations. First, in vitro kinase assays reveal a change in peptide substrate specificity. Second, STAT proteins are preferentially phosphorylated by KIT mutants. Third, inhibitors of KIT kinase are more efficient on JMD mutations than on WT KIT. We conclude that Kdelta27 is a new oncogenic KIT mutation showing constitutive activation of downstream signaling pathways, and suggest that specific pathways are activated by oncogenic KIT.

STAT3 activation is required for Asp(816) mutant c-Kit induced tumorigenicity

Activating mutations of c-kit at codon 816 (Asp(816)) have been identified in variety of malignancies, including acute myeloid leukemia (AML), mastocytosis and germ cell tumors. The mutant c-Kit receptor confers cytokine independence and induces tumorigenicity. However, the molecular mechanisms, particularly the changes in the signal transduction pathways, responsible for these biological effects induced by mutant c-Kit are largely undefined. Using the human embryonic kidney cell line, 293, we show in the current report that constitutive activation of STAT3 and STAT1 is associated with D816H mutant c-Kit. Transfection of dominant negative STAT3, but not STAT1 inhibits mutant c-Kit mediated anchorage-independent growth in vitro and tumor formation in vivo. Expression of constitutively activated STAT3 restores the mutant c-Kit receptor's transforming ability in 293 cells. These results demonstrate that activation of STAT3 by Asp(816) mutant c-Kit is required for the anchorage-independent growth and tumorigenicity induced by Asp(816) mutant c-Kit.