Hoxa9+Meis1a Efficiently Transform Hematopoietic Stem Cells but Not Committed Progenitors.

The homeobox transcription factors Hoxa9 and Meis1a induce the expansion of hematopoietic stem cells (HSC) and cooperate to induce acute myeloid leukemia (AML). We have recently shown that Hoxa9 and Meis1a are part of a self-renewal program found in leukemia stem cells (LSC) isolated from murine leukemias initiated by expression of MLL-AF9 in committed granulocyte macrophage progenitors (GMP). However, it remains unclear whether the Hoxa9-Meis1a complex is sufficient to mimic the leukemogenic effects of MLL-AF9. Therefore we assessed the clonogenic and leukemogenic activity of HSC and GMP transduced with Hoxa9-Meis1a and compared this to MLL-AF9 transduced HSC and GMP. We expressed either Hoxa9-Meis1a or MLL-AF9 in HSC or GMP and sorted single cells into 96 well plates. The data demonstrate that the clonogenic potential of single HSC or GMP expressing Hoxa9-Meis1a or MLL-AF9 are similar and both produce cells that can be replated in vitro for greater than 6 weeks. Remarkably, Hoxa9-Meis1a induces leukemia with high efficiency when expressed in HSC but not committed GMP, while MLL-AF9 can fully transform both HSC and GMP. Next, we identified a population of cells from Hoxa9-Meis1a leukemias that were enriched for LSC. Even though the leukemias were initiated from HSC, the LSC population possessed an immunophenotype and global gene expression program more consistent with differentiated myeloid cells. Further characterization of gene expression in the Hoxa9-Meis1a LSC found that only a subset of the MLL-AF9 self-renewal signature is activated by Hoxa9-Meis1a. These findings show that Hoxa9-Meis1a can efficiently induce LSC from HSC but not GMP. This suggests Hoxa9 and Meis1a induced leukemogenesis is dependent upon cellular context and require programs/pathways active in HSC in order to initiate leukemia. Thus MLL-AF9 must activate pathways critical for LSC development in addition to Hoxa9-Meis1a in order to fully transform committed progenitor cells. Identification of these cooperating pathways should provide insight into MLL-rearranged and other AML.