Constitutive activation of FLT3 stimulates multiple intracellular signal transducers and results in transformation

Aberrant expression of FLT3 has been found in most cases of B-lineage ALL a nd AML, and subsets of T cell ALL, CML in blast crisis and CLL. In 20% of p atients with AML the receptor has small internal tandem duplications of the juxtamembrane region which appear to contitutively activate the receptor. To investigate whether FLT3 activation could play a role in leukemia, we ge nerated a constitutively activated FLT3 by fusing its cytoplasmic domain to the helix-loop-helix domain of TEL in analogy to the fusion that occurs wi th TEL-PDGFR in CMML. In vitro translation assays demonstrated oligomerizat ion and intrinsic tyrosine kinase activity of the TEL-FLT3 chimeric recepto r. Constitutively activated TEL-FLT3 conferred IL-3 independence and long-t erm proliferation to transfected Ba/F3 cells. Immunoblot analyses showed th at JAK 2, STAT 3, STAT 5a, STAT 5b and CBL were tyrosine-phosphorylated in TEL-FLT3 expressing Ba/F3 cells in the absence of IL-3. These data suggest a possible role for the JAK/STAT pathway in FLT3 signaling. Transplantation of TEL-FLT3 expressing Ba/F3 cells into syngeneic mice caused mortality in all mice by 3 weeks after injection. Histopathologic analysis demonstrated a massive infiltration of mononuclear cells in the liver, spleen and bone marrow. The mimicking of naturally occurring TEL fusions provides an approa ch to assess aspects of the biology of activated FLT3, or other receptor-ty pe tyrosine kinases (RTKs) in leukemic transformation.