Imatinib-Resistant Activation of β-Catenin by BCR-ABL in a Murine Model of Chronic Myelogenous Leukemia Stem Cells.

[INTRODUCTION] Chronic myelogenous leukemia (CML) is effectively treated with imatinib, a small molecule inhibitor of the BCR-ABL tyrosine kinase that is expressed in the hematopoietic compartment including stem and progenitor cells in CML patients. While imatinib induces disease remission, it does not eradicate BCR-ABL-positive stem cells. Recently, granulocyte-macrophage progenitors (GMP) with nuclear β-catenin and an aberrant potential for self-renewal were detected in CML blast crisis (Jamieson et al, NEJM, 2004). We have established a murine model that generates BCR-ABL-positive GMP and a transplantable CML. In this model, BCR-ABL activates β-catenin through an imatinib-resistant pathway.

[METHODS] Retroviral mediated gene transfer was employed to express BCR-ABL (p210) and/or GFP in a pluripotent murine hematopoietic progenitor cell line (mHPC) (Ikawa et al, Immunity, 2004). Infected cells were isolated using GFP-sorting and characterized ex vivo for leukemogenic phenotypes and gene expression profiles. The GFP-sorted cells were transplanted into lethally irradiated congenic mice. The distribution of GFP-positive cells in the hematopoietic compartments was determined by FACS.

[RESULTS] BCR-ABL expression induced leukemogenic traits in mHPC, i.e., stroma- and cytokine-independent survival and proliferation. BCR-ABL also induced the expression of myeloid lineage markers, a process that was inhibited by imatinib. By contrast, imatinib did not abolish the stabilization of β-catenin in BCR-ABL transduced mHPC. Transplantation of BCR-ABL-transduced mHPC, but not GFP-transduced mHPC, induced in mice a CML-like disease presented with increased white blood cell counts and splenomegaly. The bone marrow cells from these mice reproducibly transferred CML phenotypes to secondary recipient mice. The diseased bone marrow contains an expanded population of GFP-positive GMP. The effect of imatinib on the leukemogenic potential of these BCR-ABL-positive GMP in vivo is under investigation.

[CONCLUSIONS] Expression of BCR-ABL in the context of mHPC induces leukemogenic traits, including the stabilization of β-catenin, without requirement for additional genetic events. The BCR-ABL-transduced mHPC generates CML-like disease in mice accompanied by the expansion of a BCR-ABL-positive GMP population, similar to that observed in blast crisis CML patients. The stabilization of β-catenin by BCR-ABL is not inhibited by imatinib; consistent with the notion that imatinib cannot eradicate CML stem cells. This murine model can be used to identify therapeutics that targets the CML stem cells.