DIFFERENTIAL EXPRESSION OF HOMEOBOX GENES IN FUNCTIONALLY DISTINCT CD34(+) SUBPOPULATIONS OF HUMAN BONE-MARROW CELLS

Class I homeobox (Hox) genes encode a major group of transcription factors controlling embryonic development and have been implicated in the continuing process of hematopoietic cell differentiation. They are clustered on four chromosomes and, in early development, exhibit spatially restricted expression with respect to their 3' --> 5' chromosomal position. By using an improved PCR-based method for amplifying total cDNA derived from limited cell numbers, we now describe the expression of class I Hox genes in highly purified CD34(+) cell subpopulations isolated from normal human bone marrow that represent functionally distinct stem and progenitor cell compartments. Our data indicate that at least 16 different Hox genes, mainly from the A and the B clusters, are expressed in one or more of these subpopulations of human hematopoietic cells. Moreover, markedly elevated expression of some of the Hox genes found at the 3' end of the A and B clusters (e.g., HoxB3) was a unique feature of the subpopulations that contained the most primitive functionally defined cells, whereas genes located in the 5' region of each cluster (e.g., HoxA10) were found to be expressed at nearly equal levels in the CD34(+) subpopulations analyzed. In contrast to the findings for CD34(+) cells, expression of two selected Hox genes, HoxB3 and HoxA10, was virtually extinguished in the CD34(-)fraction of bone marrow cells. These results demonstrate the expression of a broad range of Hox genes in primitive hematopoietic cells and point to the existence of a regulated program of Hox gene expression during their normal development.