Environmental Factors Determine Lineage Fate in a Human Model of MLL-AF9 Leukemia.

The MLL gene is fused to over 30 different fusion partners by reciprocal translocations in human acute leukemias. Some fusion partners are associated almost exclusively with myeloid or lymphoid leukemias while others are found in both. The degree to which the fusion partner contributes to the lineage of the resulting leukemia remains a matter of controversy. Using a novel model system, we demonstrate that myeloid vs lymphoid differentiation of hematopoietic progenitors transformed by MLL-AF9 can be predictably driven by cytokine combinations in vitro and in vivo. The t(9;11)(p22;q23) MLL-AF9 fusion gene is commonly associated with M5 myeloid leukemia but approximately 5% of MLL-AF9 leukemia is B-lymphoid. Expression of MLL-AF9 in human CD34+ cells enables efficient modeling of acute myeloid, B-lymphoid and biphenotypic leukemia. The lineage of the resulting leukemia can be readily manipulated in vitro (by altering the growth factors) or in vivo (using B-lymphoid-biased NOD/SCID mice or myeloid-biased NOD/SCID that are transgenic for human SCF, GM-CSF and IL-3). The cytokines IL-3, IL-7 and FLT3L appear to exert the major effects on lineage fate determination in vitro. Through limiting dilution and clonality analyses, we find a complex relationship between different leukemia stem cell compartments, with some LSC demonstrating multipotentiality and others showing strict lineage commitment. Data indicate that these differences are primarily due to microenvironment effects, with the identity of the initial cell that is targeted by MLL-AF9 possibly playing a role. These results would argue against a deterministic role for the fusion partner in MLL leukemia. This human-based system should prove useful in addressing the mechanism of lineage promiscuity of MLL leukemias. It also affords us the unique ability to determine the susceptibility of the different LSC to standard chemotherapeutic compounds, in addition to identifying novel therapeutic strategies that may be effective in treating MLL leukemia.