Conditional Expression of a Leukemogenic Fusion Oncogene in Murine Hematopoietic Stem Cells Results in Highly Invasive AML with Origin-Related Genetic Signatures of Prognostic Significance for the Human Disease

Acute myeloid leukemia (AML) is a genetically and clinically heterogeneous disease. Chromosomal translocations causing fusions of the Mixed Lineage Leukemia (MLL) gene are associated with pediatric and adult de novo and therapy-related acute leukemia that are characterized by variable disease outcome. To date, only a limited number of genetic lesions have been implicated in AML disease variability. To address the impact of cellular origin on disease heterogeneity of AMLs, we studied AMLs originating from long-term hematopoietic stem cells (LT-HSCs) and more committed progenitors using a newly established inducible “iMLL-AF9” transgenic mouse model for the t(9:11)(p22;q23) translocation associated MLL-AF9 oncogene. Ex vivo immortalized cells displayed several origin-related growth and drug resistance characteristics and gene expression signatures. Only iMLL-AF9 expressing LT-HSCs formed novel, particularly dispersed colonies, expanded in lineage restrictive stem cell medium and were resistant to genotoxic stress. iMLL-AF9 induction in vivo resulted in fully reversible myelo-monoblastic AML in all animals. Intriguingly, induction of iMLL-AF9 in LT-HSCs caused a particularly aggressive AML phenotype in 15% of recipient mice while the remainder LT-HSCs, as well as short-term HSCs, common myeloid and granulocyte macrophage progenitors induced a more moderate AML. The aggressive LT-HSC-derived AMLs were all characterized by a drastically shorter latency (37 versus 72 days median latency), higher white blood counts, increased invasion capacity and chemo-resistance of leukemic blast, and were associated with expression of genes previously implicated in cell migration, invasion, inflammation and the epithelial-mesenchymal transition (EMT) of solid cancers. shRNA based knock-down experiments demonstrated functional importance of selected candidate genes in cell migration and invasion. Importantly, comparative gene expression analyses between mouse and human revealed that among the genes associated with aggressive AMLs in mice, elevated expression of 66, 11 and 40 human orthologous genes was significantly associated with poor overall survival of t(9;11) (n=21), 11q23-lesion positive, (n=54) and all AMLs (n=662) (p<0.05). Collectively, our data indicates that expression of MLL-AF9 in HSCs results in a particularly aggressive disease driven by expression of common MLL targets and origin-dependent targets previously associated with migration, invasion and EMT of aggressive solid cancers. Remarkably, origin-related genetic signatures associated with the aggressive murine disease revealed a large number of novel MLL-AF9 fusion targets and many highly significant genetic prognostic markers for the overall survival in human AML irrespective of the underlying genetic alterations. Our data experimentally support the previously disputed theory that human AML may also arise from stem and/or oligo-potent progenitors contributing thus to the great heterogeneity of AML including drug resistance and post therapy relapse. Validation of the novel identified target genes in a broader spectrum of human leukemia will facilitate the design of accurate personalized therapeutic interventions.