Leukemias harboring chromosomal translocations involving the Mixed Lineage Leukemia (MLL) gene express high levels of the receptor tyrosine kinase FLT3. Activation of FLT3 signaling is very frequent in human acute myeloid leukemias and its role in the transformation of myeloid cells was demonstrated experimentally. To study what role FLT3 activation played in the cellular transformation leading to MLL-rearranged leukemias we used retroviral vectors to express MLL fusion genes in flt3+/+ and flt3−/− bone marrow cells. Cohorts of mice transplanted with flt3+/+ and flt3−/− bone marrow expressing either MLL-ENL or MLL-CBP were established. All mice succumbed to AML within 60 days (MLL-ENL) or 120 days (MLL-CBP) regardless of flt3 status.

Parameters reflecting cellular proliferation (leukocytosis, spleen weights) or maturation (morphology, immunophenotype) were identical in flt3−/− versus flt3+/+ leukemic blasts. Transplantability and clonality of the leukemias were also unchanged. We showed that the FLT3 protein was expressed in flt3+/+ MLL-ENL leukemic blasts and to explore its role in cell growth we studied the sensitivity of the cells to various FLT3 inhibitors ex vivo. We found that PKC412, AG1296 and Herbymicin A had growth inhibitory activity on MLL-ENL leukemic cells in culture. However the IC50 of these drugs was much higher than those determined on cell lines expressing FLT3 mutants that constitutively activate the FLT3 signaling pathway. Furthermore, flt3−/− MLL-ENL leukemic cells were equally sensitive to the growth inhibitory activity of these drugs than flt3+/+ leukemic cells. This demonstrates that PKC412, AG1296 and Herbymicin A impair the growth of MLL cells through the inhibition of tyrosine kinases other than FLT3. Altogether our data question the value of FLT3 as a therapeutic target to fight MLL-rearranged leukemias and suggest that the efficacy of FLT3 inhibitors is in fact mediated by the interference with another pathway.

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