Homeobox genes in normal hematopoiesis and leukemogenesis

Homeobox genes are broadly classified into two subclasses: HOX and non-HOX homeobox genes. A number of genes in both classes are expressed in a variety of hematopoietic cells. Two major categories of evidence implying the involvement of these genes in normal hematopoiesis have been demonstrated. First, the expression pattern of the homeobox genes in hematopoietic cells is lineage- and differentiation-stage specific. Second, enforced and suppressed expression of various homeobox genes cause defects in the hematopoietic cells of specific lineages. The reduction in myeloid, erythroid and B cell progenitors is found in mice with a disrupted HOXA9 gene. The thymuses of HOXB3-overexpressed marrow recipients contain a markedly decreased number of CD4/CD8 double-positive T cells. These examples suggest that the proper level of expression and timely down-regulation of some homeobox genes are necessary for normal hematopoiesis. Homeobox genes are also implicated in human and mouse leukemias. In human leukemias, a HOX gene (HOXA9) and two non-HOX homeobox genes (PBX1 and HOX11) are involved in chromosomal translocations. In mouse leukemias, provirus integrations cause aberrant expression of several HOX and non-HOX genes. Currently available information will be discussed separately on HOX and non-HOX genes, in normal and leukemic hematopoiesis, respectively.