OVEREXPRESSION OF HOXB4 ENHANCES THE HEMATOPOIETIC POTENTIAL OF EMBRYONIC STEM-CELLS DIFFERENTIATED IN-VITRO

Little is known about the molecular mechanisms controlling primitive h ematopoietic stem cells, especially during embryogenesis. Homeobox gen es encode a family of transcription factors that have gained increasin g attention as master regulators of developmental processes and recent ly have been implicated in the differentiation and proliferation of he matopoietic cells. Several Hox homeobox genes are now known to be diff erentially expressed in various subpopulations of human hematopoietic cells and one such gene, HOXB4, has recently been shown to positively determine the proliferative potential of primitive murine bone marrow cells, including cells with long-term repopulating ability. To determi ne if this gene might influence hematopoiesis at the earliest stages o f development, embryonic stem (ES) cells were genetically modified by retroviral gene transfer to overexpress HOXB4 and the effect on their in vitro differentiation was examined. HOXB4 overexpression significan tly increased the number of progenitors of mixed erythroid/myeloid col onies and definitive, but not primitive, erythroid colonies derived fr om embryoid bodies (EBs) at various stages after induction of differen tiation. There appeared to be no significant effect on the generation of granulocytic or monocytic progenitors, nor on the efficiency of EB formation or growth rate. Analysis of mRNA from EBs derived from HOXB4 -transduced ES cells on different days of primary differentiation show ed a significant increase in adult beta-globin expression, with no det ectable effect on GATA-1 or embryonic globin (beta H-1). Thus, HOXB4 e nhances the erythropoietic, and possibly more primitive, hematopoietic differentiative potential of ES cells. These results provide new evid ence implicating Hox genes in the control of very early stages in the development of the hematopoietic system and highlight the utility of t he ES model for gaining insights into the molecular genetic regulation of differentiation and proliferation events. (C) 1996 by The American Society of Hematology.