OVEREXPRESSION OF HOXB4 IN HEMATOPOIETIC-CELLS CAUSES THE SELECTIVE EXPANSION OF MORE PRIMITIVE POPULATIONS IN-VITRO AND IN-VIVO

Hox genes were first recognized for their role in embryonic developmen t and may also play important lineage-specific functions in a variety of somatic tissues including the hematopoietic system. We have recentl y shown that certain members of the Hox A and B clusters, such as HOXB 3 and HOXB4, are preferentially expressed in subpopulations of human b one marrow that are highly enriched for the most primitive hematopoiet ic cell types. To assess the role these genes may play in regulating t he proliferation and/or differentiation of such cells, we engineered t he overexpression of HOXB4 in murine bone marrow cells by retroviral g ene transfer and analyzed subsequent effects on the behavior of variou s hematopoietic stem and progenitor cell populations both in vitro and in vivo. Serial transplantation studies revealed a greatly enhanced a bility of HOXB4-transduced bone marrow cells to regenerate the most pr imitive hematopoietic stem cell compartment resulting in 50-fold highe r numbers of transplantable totipotent hematopoietic stem cells in pri mary and secondary recipients, compared with serially passaged neo-inf ected control cells. This heightened expansion in vivo of HOXB4-transd uced hematopoietic stem cells was not accompanied by identifiable anom alies in the peripheral blood of these mice. Enhanced proliferation in vitro of day-12 CFU-S and clonogenic progenitors was also documented. These results indicate HOXB4 to be an important regulator of very ear ly but not fate hematopoietic cell proliferation and suggest a new app roach to the controlled amplification of genetically modified hematopo ietic stem cell populations.