A Novel Conditional Mouse Model For MLL-ENL Induced Acute Leukemia

The t(11;19) translocation leading to the MLL-ENL fusion is recurrently found in pediatric and adult de novo and therapy related mixed-lineage acute leukemia and is often associated with a poor prognosis. Previous studies have shown that (retroviral) overexpression of MLL-ENL potently immortalizes bone marrow cells in vitro and induces a lethal acute myeloid leukemia (AML) in mice. To establish a mouse model that phenocopies more closely the human disease, we generated conditional transgenic mice in which the expression of MLL-ENL is controlled by doxycycline (DOX) through a stably integrated reverse tet-responsive transactivator (rtTA). Induction of MLL-ENL expression in newborn or adult mice resulted in a leukemic phenotype that phenocopied pre-B- and myeloid mixed lineage leukemia as observed in most patients with MLL-ENL. The diseased mice displayed excessive splenomegaly, massive lymph node as well as multiple organ infiltration by two co-existing types of blasts mostly expressing higher or lower levels of B220 and Gr1/Mac1 and similar levels of c-kit. Expression of the fusion gene and disease induction was DOX dosage dependent and reversible upon DOX removal. Despite significantly lower fusion gene expression levels as we observed in retroviral systems the median latency for the development of the disease in this model (104.3±16.9 days) was comparable to them (62±10.4 days) and much shorter than any of the previously reported MLL-ENL knock-in mouse models (> 1 year). Continuous ex vivo expression of MLL-ENL provided bone marrow and fetal liver hematopoietic cells with a strong self-renewal capacity and caused the accumulation of immature blast-like cells upon serial replating in methylcellulose cultures. In the presence of factors favoring myelopoiesis, like IL-3, DOX removal resulted in a complete differentiation towards the granulocytic-monocytic, lineages expressing high levels of Mac1/Gr-1, whereas IL-7 favored differentiation towards the B-cell lineage characterized by the expression of high levels of B220. In addition, MLL-ENL induced a DOX dependent aberrant self renewal capacity and a differentiation block in methylcellulose cultures of hematopoietic stem cells (Lin^- c-kit⁺ Sca-1⁺, LSK) and various progenitors including common lymphoid progenitor (CLP) and granulocyte-macrophage progenitor (GMP) cells. Interestingly, MLL-ENL expression preferentially expanded LSK- rather than GMP-derived cells as assayed by growth curves in long-term (> 1 month) liquid cultures in the presence of cytokines in vitro. In line with this observation, in vivo, expression of MLL-ENL in long-term hematopoietic stem cells (LT-HSC) induced an aggressive mixed lineage leukemia characterized by the presence of two distinguishable populations of blasts, whereas induction in GMPs never induced a disease. These data suggest that MLL-ENL preferentially transforms hematopoietic stem cells rather than more differentiated progenitors. Thus, this novel transgenic mouse model for MLL-ENL induced acute leukemia closely recapitulates the human disease and combines the advantages of the existing knock-in and retroviral models. This model allowed us to demonstrate that in contrast to the MLL-AF9 fusion, that preferentially immortalizes GMPs, MLL-ENL preferentially transforms HSCs. We anticipate that our model will be a valuable tool to study the cellular origin and to search for and/or validate novel therapeutic targets for MLL-ENL induced acute leukemia.