Bmi-1 Overexpression Synergizes with p210-BCR-ABL to Induce Stem Cell and Progenitor Transformation.

Abstract 3251

Poster Board III-1

Chronic myelogenous leukemia (CML) is a hematopoietic stem cell (HSC) malignancy induced by p210-BCR-ABL and characterized by myeloproliferation followed by poor-prognosis acute blastic transformation. Persistence of BCR-ABL⁺ HSCs in patients under tyrosine kinase inhibitor therapy suggests that inhibition of ABL-kinase alone is not sufficient to completely eliminate the leukemic stem cells and progenitor (LSC/P) population and a group of patients continue developing accelerated/blastic phase despite therapy. Expression of p210-BCR-ABL is necessary and sufficient to develop CML but the molecular mechanisms necessary for its blastic transformation remain elusive. The polycomb group gene Bmi1 plays an essential role in regulating the proliferative capacity of both normal and leukemic stem cells (Lessard J, et al. Nature 2003). Recently, expression of Bmi1 has been found significantly elevated in CD34⁺ cells of patients with advanced phases compared with patients in chronic-phase CML (Mohty M et al. Blood 2007). We therefore intend to determine whether Bmi1 expression in CML stem cells and progenitors, isolated from Scl/p210-BCR-ABL-expressing mice, is sufficient to accelerate significantly the development of blastic phase. Since simultaneous co-expression of Bmi1 and BCR-ABL in normal HSC/P may not faithfully recapitulate the progression events in CML transformation, we have over-expressed Bmi1 in a model of stem cell-dependent chronic phase CML. This model is based on the tetracycline-dependent expression of p210-BCR-ABL driven by the Scl promoter (Scl-tTA x TRE-BCR-ABL, Koschmieder S et al. Blood 2005), where expression of BCR-ABL is restricted to the HSC/P compartment. Scl-driven expression of BCR-ABL is predominantly active in HSC (Lin^-/Sca1⁺/c-kit⁺; LSK) and progenitors (Lin^-/c-kit⁺; LK) and is transplantable into secondary recipients (Sengupta A et al., ASH 2008). Expression of Bmi1 into Scl/p210-BCR-ABL-expressing cells significantly increases the HSC/P proliferation (2.5 fold) and also increases the frequency of colony forming cells (CFU-Cs) after serial propagation in liquid culture, compared to Bmi1 (4.6-fold) or BCR-ABL-expressing CML cells alone (517-fold). Furthermore, Bmi1 expression into Scl/p210 leukemic progenitors increases the hypermigratory phenotype of leukemic progenitors (3-fold increase over 5.5-fold increase induced by BCR/ABL alone; P<0.005) in response to CXCL12. Although, Bmi1 overexpression in Scl/p210 cells does not decrease further the reduced adhesion to fibronectin of BCR/ABL-transformed CML HSC/P, leukemic progenitors co-expressing Bmi1 and SCL/p210 have significantly reduced homing in bone marrow compared to Bmi1-expressing HSC/P (7.7 fold, P≤0.005). Altogether, these data indicate that Bmi-1 synergistically enhances the transformation phenotype of p210-BCR-ABL-expressing HSC/P and emphasize the role of epigenetic changes inducing overexpression of self-renewal genes in the pathogenesis of CML. These data suggest that Bmi-1 may represent a novel therapeutic target for CML LSC/P.