Constitutive Expression of Lef-1 in Murine Hematopoietic Progenitors Causes Myeloid and Lymphoid Leukemias Propagated by a Leukemic Stem Cell Population with Lymphoid Characteristics.

Canonical Wnt signaling is critically involved in normal hematopoietic development and the self-renewal process of hematopoietic stem cells. Deregulation of this pathway has been linked to a large variety of cancers including leukemia. Lef-1, a key factor of the Wnt / beta-catenin signaling pathway, plays pivotal roles in lymphoid development, but little is known about the role of Lef-1 in myeloid hematopoiesis and leukemogenesis. We now show that Lef-1 is expressed in murine hematopoietic stem cells (‘LSK’, Sca⁺cKit⁺Lin⁻) and both myeloid and lymphoid subpopulations as well as in different human leukemias. Using a retroviral BM transplantation model, we demonstrate that ectopic expression of wild type Lef-1 (WT) and a constitutive active mutant of Lef-1 (CA) induces a severe disturbance of normal hematopoietic development: mice transplanted with bone marrow constitutively expressing Lef-1 had a significant increase in the number of circulating myeloid cells resulting in an inverted lymphoid-myeloid ratio in the peripheral blood (ratio: 0.28 (WT), 0.10 (CA) vs. 1.07 (GFP); p<0.002). With a median latency of 12 month, transplanted mice succumbed to a lethal myeloproliferation (n=2) or acute myeloid or B-lymphoblastic leukemias (N=8). Both leukemia subtypes shared key biological characteristics such as positivity for IG DH-JH rearrangements. In addition, both subtypes were characterized by a biphenotypic cell population (B220⁺Mac1⁺Gr1⁺, BMG⁺⁺⁺) as well as varying numbers of B220⁺Mac1⁻Gr1⁻ (B⁺MG⁻) cells. In both leukemia subtypes single DJ_H–rearranged B⁺MG⁻ cells had the highest seeding efficiency and were able to give rise to both BMG⁺⁺⁺ and B⁻MG⁺ cells in vitro (mean seeding efficiency: B⁺MG⁻: 17.9%, vs. BMG⁺⁺⁺: 4.7%, B⁻MG⁺: 4.0, p=0.005). Strikingly, the frequency of leukemia-initiating cells in AML was highest in the B⁺MG⁻ population as determined by limit dilution transplantation assays (B⁺MG⁻: 1 in 433 vs. BMG⁺⁺⁺/B⁻MG⁺: 1 in 8491 cells, p<0.001). Expression analysis of malignant blast cells from both lymphoid and myeloid leukemias also revealed striking commonalities with regard to the transcription profile. Blast cells from both AML and ALL diseased mice uniformly showed expression of myelo-specific genes like C/ebpα and c-fms and lymphoid specific genes E2A and Ebf-1, but lacked expression of the B-cell differentiation regulator Pax-5. These findings demonstrate that balanced expression of Lef-1 is crucial for early normal hematopoietic differentiation and that deregulation of this factor induces the development of DJ_H-rearranged acute myeloid and lymphoid leukemias which are propagated by a leukemic stem cell with lymphoid characteristics.