Differentiation arrest by HoxA9, Meis1, Nup98-HoxA9 and E2a-Pbx1 homeodomain transcription factors: The pathogenesis of a subset of acute leukemia

Homeodomain proteins regulate gene expression during development and contribute to lineage-specific differentiation during hematopoiesis. HoxA9, Pbx1 and Meis1 are homeodomain heterodimer partners that form transcription factor complexes on DNA and are targets of chromosomal translocations, or genetic events leading to their persistent transcriptional activation in leukemia. The goal of this thesis is to elucidate aspects of the mechanism of differentiation arrest induced in hematopoietic progenitors by HoxA9 and Meis1, and by the chimeric oncoproteins E2a-Pbx1, and Nup98-HoxA9.; E2a-Pbx1 is created by the t(1:19) translocation in acute lymphoblastic leukemia (ALL), whereby sequences encoding the transactivation domains of E2a are fused to sequences encoding Pbx1. E2a-Pbx1 immortalizes murine progenitors in vitro and induces murine AML in vivo. It is logical to hypothesize that E2a-Pbx1 induces aberrant gene transcription and differentiation arrest by persistently activating genes normally regulated by Pbx1 and Hox cofactors. Structure-function analysis was performed on E2a-Pbx1. Chapter 2 describes an inhibitory switch residing upstream of the Pbx1 HD that prevents DNA binding by the HD and requires a partner-mediated derepression mechanism to permit DNA binding. This switch module was dispensable for myeloid differentiation arrest by E2a-Pbx1.; HoxA9 and Meis1 are persistently activated in a majority of human AML subtypes, and HoxA9 expression in AML portends a poor outcome. In murine marrow reconstitution experiments, coexpression of Hoxa9 and Meis1 induce rapid leukemia while expression of either gene alone fails to induce leukemia. Chapter 4 reveals that Hoxa9 immortalizes a GM-CSF-dependent promyelocyte. Strikingly G-CSF and M-CSF still induce granulocytic or monocytic differentiation, respectively, of these Hoxa9-immortalized progenitors.; If Meis1 is not required for immortalization by Hoxa9, how does Meis1 cooperate in leukemogenesis? Chapter 5 demonstrates that Meis1 coexpression in Hoxa9-immortalized progenitors adds the additional transforming property of blocking G-CSF-induced differentiation, preventing expression of terminal granulocytic differentiation genes. These results reveal a novel cooperative mechanism between Hoxa9 and Meis1 involving differentiation arrest through independent, parallel, cytokine-mediated pathways.; Nup98-HoxA9 results from the t(7:11) in AML. Chapter 6 demonstrates that Nup98-HoxA9 immortalizes IL-3-dependent progenitors and maintains expression of Hoxa9 and Meis1 at levels sufficient for their independent transforming functions.