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HOX homeobox proteins are key oncogenic drivers in hematopoietic malignancies. Here we demonstrate that HOXA1, HOXA6 and predominantly HOXA9 are able to induce the production of insulin-like growth factor 1 (Igf1). In chromatin immunoprecipitations, HOXA9 bound directly to the putative promoter and a DNase-hypersensitive region in the first intron of the Igf1 gene. Transcription rates of the Igf1 gene paralleled HOXA9 activity. Primary cells transformed by HOXA9 expressed functional Igf1 receptors and activated the protein kinase Akt in response to Igf1 stimulation, suggesting the existence of an autocrine signaling loop. Genomic deletion of the Igf1 gene by Cre-mediated recombination increased sensitivity toward apoptosis after serum starvation. In addition, the leukemogenic potential of Igf1-negative, HOXA9-transformed cells was impaired, leading to a significant delay in disease development on transplantation into recipient animals.

Mixed-lineage leukemia fusion proteins activate their target genes predominantly by stimulating transcriptional elongation. A core component necessary for this activity is cyclin-dependent kinase 9. Here we explored the effectiveness of small molecules targeting this enzyme as potential therapeutics. A screen of seven compounds with anti-CDK9 activity applied to a panel of leukemia cell lines identified flavopiridol and the experimental inhibitor PC585 as superior in efficacy with inhibitory concentrations in the submicromolar range. Both substances induced rapid dephosphorylation of the RNA polymerase II C-terminal domain, accompanied by downregulation of CDK9-dependent transcripts for MYC and HOXA9. Global gene expression analysis indicated the induction of a general stress response program, culminating in widespread apoptosis. Importantly, colony-forming activity in leukemia lines and primary patient samples could be completely inhibited under conditions that did not affect native precursors from bone marrow. In vivo application in a mouse transplant model significantly delayed disease with PC585 showing also oral activity. These results suggest CDK9 inhibition as novel treatment option for mixed-lineage leukemia. [ABSTRACT FROM AUTHOR]

Mixed-lineage leukemia (MLL) fusion proteins are associated with a unique class of leukemia that is characterized by the simultaneous expression of lymphoid-specific as well as myeloid-specific genes. Here we report the first experimental model of MLL. Murine bone marrow cells were retrovirally transduced to express the MLL-eleven nineteen leukemia (MLL-ENL) fusion protein. When cultivated in flt-3 ligand, stem cell factor and interleukin-7 (IL-7) in a stroma-free culture system MLL-ENL-transduced as well as control cells showed a wave of B-lymphopoiesis. Whereas the controls exhausted their proliferative capacity in a CD19+/B220+ state, a continuously proliferating CD19-/B220+ cell population emerged in the MLL-ENL-transduced cultures. Despite the lymphoid surface marker, these cells were of monocytoid morphology. The immortalized cells contained unrearranged retrovirus, expressed MLL-ENL mRNA and were able to grow in syngenic recipients. From the diseased animals an MLL-ENL positive, B220+/CD19- cell type could be reisolated and cultivated in vitro. In analogy to human MLL, MLL-ENL-transformed cells not only coexpressed lymphocyte-specific (rag1, rag2, pax5, Tdt) and monocyte-specific genes (lysozyme, c-fms), but also showed rearrangements of the genomic immunoglobulin locus. This model shows that MLL-ENL influences events of early lineage determination and it will enable the investigation of the underlying molecular processes.Oncogene (2003) 22, 1629-1637. doi:10.1038/sj.onc.1206104 [ABSTRACT FROM AUTHOR]