Effects of HOX homeobox genes in blood cell differentiation.

The burgeoning number of articles concerning the role of HOX genes and hematopoiesis ensures that this will continue to be an area of very active research. It seems clear that HOX genes are expressed in stage- and lineage-specific patterns during early stages of hematopoietic development and differentiation. Several lines of evidence suggest that multiple genes of the HOXB (B2, B4, B6-B9), HOXC (C6, C8), and HOXA (A5) are involved in erythropoiesis. Similarly, a number of genes of the HOXA, HOXB, and HOXC appear to play a role in lymphoid cells. Furthermore, several genes, such as A9, A10, B3, B7, and B8, may control myelomonocytic differentiation. The question arises as to whether such a multiplicity of HOX genes reflects redundancy or indicates subtlety of the regulatory machinary. A similar complexity has been observed for hematopoietic cytokines, and the current view is that, although multiple molecules may have similar or overlapping effects, each factor has a specific function and regulatory combinations appear to play a critical role in controlling hematopoietic cell processes (99). One challenge for the future is to delineate in more detail the precise expression patterns of these genes in the many distinct subpopulations of blood cells and during fetal development. Overexpression of HOX genes in hematopoietic cells can dramatically perturb the differentiation of various cell lineages and can contribute to leukemogenesis. Future studies may involve the overexpression of alternatively spliced versions of different HOX genes or of truncated versions of HOX genes to ascertain the functional domains of the proteins that mediate the biologic effects. The findings in HOX knockout mice confirm a role for these genes in normal blood cell development. Further work in this area will require careful examination of fetal hematopoiesis and of animals bearing multiple HOX gene knockouts. Involvement of HOX genes in leukemia is just beginning to be appreciated. Establishing the true extent of HOX gene mutations in human disease will require strategies such as comparative genomic hybridization (100) and analysis of high density oligonucleotide arrays (101). The holy grail of homeobox work is to discover the physiologic processes and specific target genes regulated by HOX proteins. Given the broad range of tissues in which HOX genes are expressed, they would appear to be involved in very basic cellular processes, e.g., cell proliferation and death, adhesion, and migration, etc., rather than the direct regulation of tissue-specific genes. The search for target genes may be made easier by the further characterization of cooperative DNA binding between HOX proteins and other transcription factors. We speculate that HOX proteins do not behave as conventional transcriptional activators or inhibitors but rather may mark genes for potential future activation, i.e., they may establish competency to execute specific differentiation programs, with the actual activation being accomplished by transcriptional pathways triggered by exogenous signals. This proposed function may be an architectural one, involving changes in the conformation of DNA and/or altering interactions between DNA and histones, thus making areas of the genome more or less accessible to other protein factors (102). If this is the case, we may need to develop new assays to discern the molecular action of HOX proteins. The ease of manipulating the hematopoietic systems would appear to make it a very attractive model for explicating the general functions of this remarkable family of genes.