Hox genes in early hematopoietic development and leukemia

Hematopoiesis is a complex process by which billions of mature cells are produced from a small pool of primitive cells found in the bone marrow (BM) known as hematopoietic stem cells (HSC). The Hox homeobox gene family of transcription factors (TF) first recognized as key regulators of embryogenesis has recently been implicated in the regulation of normal hematopoiesis and in leukemogenesis. To gain further insights into possible roles of Hox genes in hematopoiesis, three line of investigation were pursued in this thesis work. First, Hox gene expression and that of their co-factors Pbx1 and Meis1 was assessed at different stages of hematopoietic development in functionally distinct hematopoietic subpopulations. Hox genes were preferentially expressed in HSC-enriched subpopulations in both adult and fetal stages of hematopoiesis, and down-regulated following differentiation and maturation. The Pbx1 and Meis1 genes had important differences in their expression pattern, but were both detected in Hox expressing subpopulations. Together, these results further support the notion that Hox genes are involved in the regulation of early hematopoietic cells at all stages of hematopoietic ontogeny. The transcriptional regulation of Hox genes in hematopoietic cells was investigated by taking advantage of a recently cloned novel human cDNA for HOXB3 from purified CD34⁺ BM cells. Functional characterization of the upstream genomic DNA led to the discovery of a novel HOXB3 transcriptional regulatory element in intron 2 with enhancerlike properties when tested in hematopoietic cell lines. These results point to the existence of specific hematopoietic-active transcriptional regulatory elements important for regulating Hox gene expression in primitive hematopoietic cells. The recent identification of a second translocation involving a Hox gene and the NUP98 gene in acute myeloid leukemia (AML) further suggested that deregulated and/or mutant Hox genes might be directly involved in the leukemic process. To test this hypothesis, the leukemogenic potential of the NUP98-HOXD13 t(2;11) fusion gene, alone or in concert with a candidate collaborating gene, Meis1, was studied in the murine BM transplantation model. Overexpression of NUP98-HOXD13 in vivo led to mild myeloproliferation, whereas co-expression with Meis1 resulted in the development of myelomonocytic leukemia, closely re-capitulating human myelomonocytic NUP98-associated leukemia.Medicine, Faculty ofMedical Genetics, Department ofGraduat