Homeobox protein Hox-A5
negative regulation of erythrocyte differentiation
Constitutive HOXA5 expression inhibits erythropoiesis and increases myelopoiesis from human hematopoietic progenitors.
The role of the homeobox gene HOXA5 in normal human hematopoiesis was studied by constitutively expressing the HOXA5 cDNA in CD34(+) and CD34(+)CD38(-) cells from bone marrow and cord blood. By using retroviral vectors that contained both HOXA5 and a cell surface marker gene, pure populations of progenitors that expressed the transgene were obtained for analysis of differentiation patterns. Based on both immunophenotypic and morphological analysis of cultures from transduced CD34(+) cells, HOXA5 expression caused a significant shift toward myeloid differentiation and away from erythroid differentiation in comparison to CD34(+) cells transduced with Control vectors (P =.001, n = 15 for immunophenotypic analysis; and P <.0001, n = 19 for morphological analysis). Transduction of more primitive progenitors (CD34(+)CD38(-) cells) resulted in a significantly greater effect on differentiation than did transduction of the largely committed CD34(+) population (P =.006 for difference between HOXA5 effect on CD34(+) v CD34(+)CD38(-) cells). Erythroid progenitors (burst-forming unit-erythroid [BFU-E]) were significantly decreased in frequency among progenitors transduced with the HOXA5 vector (P =.016, n = 7), with no reduction in total CFU numbers. Clonal analysis of single cells transduced with HOXA5 or control vectors (cultured in erythroid culture conditions) showed that HOXA5 expression prevented erythroid differentiation and produced clones with a preponderance of undifferentiated blasts. These studies show that constitutive expression of HOXA5 inhibits human erythropoiesis and promotes myelopoiesis. The reciprocal inhibition of erythropoiesis and promotion of myelopoiesis in the absence of any demonstrable effect on proliferation suggests that HOXA5 diverts differentiation at a mulitpotent progenitor stage away from the erythroid toward the myeloid pathway.