ABSTRACT
Leukemias harboring MLL translocations are frequent in children and adults, and respond poorly to therapies. The receptor tyrosine kinase FLT3 is highly expressed in these leukemias. In vitro studies have shown that pediatric MLL-rearranged ALL cells are sensitive to FLT3 inhibitors and clinical trials are ongoing to measure their therapeutic efficacy. We sought to determine the contribution of Flt3 in the pathogenesis of MLL-rearranged leukemias using a myeloid leukemia mouse model. Bone marrow from Flt3 null mice transduced with MLL-ENL or MLL-CBP was transplanted into host mice and Flt3 (-/-) leukemias were compared to their Flt3 wild type counterparts. Flt3 deficiency did not delay disease onset and had minimal impact on leukemia characteristics. To determine the anti-leukemic effect of FLT3 inhibition we studied the sensitivity of MLL-ENL leukemia cells to the FLT3 inhibitor PKC412 ex vivo. As previously reported for human MLL-rearranged leukemias, murine MLL-ENL leukemia cells with higher Flt3 levels were more sensitive to the cytotoxicity of PKC412. Interestingly, Flt3 deficient leukemia samples also displayed some sensitivity to PKC412. Our findings demonstrate that myeloid leukemias induced by MLL-rearranged genes are not dependent upon Flt3 signaling. They also highlight the discrepancy between the sensitivity of cells to Flt3 inhibition in vitro and the lack of contribution of Flt3 to the pathogenesis of MLL-rearranged leukemias in vivo.
Subject(s)
Gene Expression Regulation, Leukemic , Histone-Lysine N-Methyltransferase/genetics , Leukemia, Myeloid/genetics , Myeloid-Lymphoid Leukemia Protein/genetics , Oncogene Proteins, Fusion/genetics , fms-Like Tyrosine Kinase 3/genetics , Animals , Bone Marrow/metabolism , Bone Marrow/pathology , Bone Marrow Transplantation , Cells, Cultured , Disease Models, Animal , Histone-Lysine N-Methyltransferase/metabolism , Leukemia, Myeloid/metabolism , Leukemia, Myeloid/pathology , Mice , Mice, Knockout , Myeloid Progenitor Cells/drug effects , Myeloid Progenitor Cells/metabolism , Myeloid Progenitor Cells/pathology , Myeloid-Lymphoid Leukemia Protein/metabolism , Oncogene Proteins, Fusion/metabolism , Protein Kinase Inhibitors/pharmacology , Signal Transduction , Staurosporine/analogs & derivatives , Staurosporine/pharmacology , fms-Like Tyrosine Kinase 3/deficiencyABSTRACT
HOX genes, MEIS1, and FLT3 are frequently up-regulated in human myeloid leukemias. Meis1 cooperates with Hox genes to induce leukemias in mice, hypothetically the consequence of Meis1-induced Flt3 overexpression. To test this, we compared the properties of Flt3(-/-) and Flt3(+/+) progenitors transduced with Hoxa9 or Hoxa9/Meis1. In a myeloid clonogenic assay, Meis1 greatly enhanced the proliferation of Hoxa9-expressing cells, massively up-regulating Flt3 protein. However, the transforming potential of Hoxa9/Meis1 was unaltered in Flt3(-/-) cells. All mice that received Hoxa9/Meis1-transduced progenitors succumbed to rapid acute myeloid leukemias regardless of Flt3 genotype. Flt3 expression levels in leukemic blasts did not correlate with parameters reflecting their proliferative rate or their impaired differentiation. Furthermore, analysis of c-Myb expression levels in Hoxa9/Meis1-transformed cells showed that the up-regulation of this critical downstream effector was independent of Flt3. Altogether, our findings demonstrate that Flt3 is dispensable to the oncogenic cooperation of Meis1 with Hoxa9.
