Lineage and stage specific expression of HOX 1 genes in the human hematopoietic system

Identification of Lmo1 as part of a Hox-dependent regulatory network for hindbrain patterning

The embryonic functions of Hox proteins have been extensively investigated in several animal phyla. These transcription factors act as selectors of developmental programmes, to govern the morphogenesis of multiple structures and organs. However, despite the variety of morphogenetic processes Hox proteins are involved in, only a limited set of their target genes has been identified so far. To find additional targets, we used a strategy based upon the simultaneous overexpression of Hoxa2 and its cofactors Pbx1 and Prep in a cellular model. Among genes whose expression was upregulated, we identified LMO1, which codes for an intertwining LIM-only factor involved in protein-DNA oligomeric complexes. By analysing its expression in Hox knockout mice, we show that Lmo1 is differentially regulated by Hoxa2 and Hoxb2, in specific columns of hindbrain neuronal progenitors. These results suggest that Lmo1 takes part in a Hox paralogue 2-dependent network regulating anteroposterior and dorsoventral hindbrain patterning.

Cell type- and stage-specific changes in HOXA7 protein expression in human ovarian folliculogenesis: possible role of GDF-9

Homeobox (HOX) genes are important transcriptional regulators in development and in adult tissues. A major obstacle to the understanding of their roles in humans has been the lack of well-defined anti-human HOX antibodies. We generated a thoroughly characterized polyclonal rabbit antibody against human HOXA7 and used it to study the distribution, role, and regulation of HOXA7 in human ovarian folliculogenesis and in granulosa cell tumors. Immunohistochemically, follicles were strongly HOXA7-positive compared with stroma. Oocytes expressed little HOXA7. Granulosa cells were predominantly negative in primordial follicles, had uniformly HOXA7-positive nuclei in primary follicles, and, as follicles matured, the subcellular localization of HOXA7 changed from nuclear to predominantly cytoplasmic. HOXA7 was mainly cytoplasmic in theca interna, but completely absent in theca externa. Granulosa cell tumors were mainly HOXA7 positive and, like in preovulatory follicles and theca interna, staining was predominantly cytoplasmic. The change in HOXA7 expression from negative primordial to positive primary follicles suggested a relationship with granulosa cell proliferation. To test this hypothesis, SV40 Tag-immortalized human granulosa cells (SVOG) were double stained with anti-HOXA7 antibody and with Ki-67 as proliferation marker. HOXA7 expression was highest in mitotic cells. In addition, growth differentiation factor-9 (GDF-9), known to be secreted by oocytes in primary human follicles, up-regulated HOXA7 protein, and stimulated proliferation of SVOG, while TGF-beta1 inhibited HOXA7 expression and proliferation. This is the first report on the expression of any HOX gene in human ovarian follicles and granulosa cell tumors. It shows that HOXA7 undergoes cell type- and stage-specific changes during ovarian folliculogenesis, likely regulates granulosa cell proliferation, and in subcellular location differs between proliferating and secretory cells. The increase in HOXA7 protein in response to GDF-9 represents the first demonstration of a possible regulatory role of oocytes in ovarian follicular HOX gene expression.

HOXB6 overexpression in murine bone marrow immortalizes a myelomonocytic precursor in vitro and causes hematopoietic stem cell expansion and acute myeloid leukemia in vivo

The HOX family of homeobox genes plays an important role in normal and malignant hematopoiesis. Dysregulated HOX gene expression profoundly effects the proliferation and differentiation of hematopoietic stem cells (HSCs) and committed progenitors, and aberrant activation of HOX genes is a common event in human myeloid leukemia. HOXB6 is frequently overexpressed in human acute myeloid leukemia (AML). To gain further insight into the role of HOXB6 in hematopoiesis, we overexpressed HOXB6 in murine bone marrow using retrovirus-mediated gene transfer. We also explored structure-function relationships using mutant HOXB6 proteins unable to bind to DNA or a key HOX-binding partner, pre-B-cell leukemia transcription factor-1 (PBX1). Additionally, we investigated the potential cooperative interaction with myeloid ecotropic viral integration site 1 homolog (MEIS1). In vivo, HOXB6 expanded HSCs and myeloid precursors while inhibiting erythropoiesis and lymphopoiesis. Overexpression of HOXB6 resulted in AML with a median latency of 223 days. Coexpression of MEIS1 dramatically shortened the onset of AML. Cytogenetic analysis of a subset of HOXB6-induced AMLs revealed recurrent deletions of chromosome bands 2D-E4, a region frequently deleted in HOXA9-induced AMLs. In vitro, HOXB6 immortalized a factor-dependent myelomonocytic precursor capable of granulocytic and monocytic differentiation. These biologic effects of HOXB6 were largely dependent on DNA binding but independent of direct interaction with PBX1.

HOXB6 protein is bound to CREB-binding protein and represses globin expression in a DNA binding-dependent, PBX interaction-independent process

Although HOXB6 and other HOX genes have previously been associated with hematopoiesis and leukemias, the precise mechanism of action of their protein products remains unclear. Here we use a biological model in which HOXB6 represses alpha- and gamma-globin mRNA levels to perform a structure/function analysis for this homeodomain protein. HOXB6 protein represses globin transcript levels in stably transfected K562 cells in a DNA-binding dependent fashion. However, the capacity to form cooperative DNA-binding complexes with the PBX co-factor protein is not required for HOXB6 biological activity. Neither the conserved extreme N-terminal region, a polyglutamic acid region at the protein C terminus, nor the Ser(214) CKII phosphorylation site was required for DNA binding or activity in this model. We have previously reported that HOX proteins can inhibit CREB-binding protein (CBP)-histone acetyltransferase-mediated potentiation of reporter gene transcription. We now show that endogenous CBP is co-precipitated with exogenous HOXB6 from nuclear and cytoplasmic compartments of transfected K562 cells. Furthermore, endogenous CBP co-precipitates with endogenous HOXB6 in day 14.5 murine fetal liver cells during active globin gene expression in this tissue. The CBP interaction motif was localized to the homeodomain but does not require the highly conserved helix 3. Our data suggest that the homeodomain contains most or all of the important structures required for HOXB6 activity in blood cells.

Regulation of Hoxb2 by APL-associated PLZF protein

The PLZF gene is translocated in a subset of all-trans-retinoic acid resistant acute promyelocytic leukaemia (APL) cases, encodes a DNA binding transcription factor and is expressed highly in haematopoietic progenitor cells as well-developing central nervous system (CNS). The spatially restricted and temporally dynamic pattern of PLZF expression in the developing CNS suggested that it might play a role in the circuitry regulating hindbrain segmentation. We have now identified a PLZF binding site (PLZF-RE) in an enhancer region of Hoxb2 that itself is required for directing high-level expression in rhombomers 3 and 5 of the developing hindbrain. The wild-type r3/r5 enhancer linked to a heterologous promoter was responsive to regulation by PLZF, and this activity was lost in variants containing a mutated PLZF-RE. Compared with the wild-type protein, the binding of the APL-associated reciprocal RARalpha-PLZF fusion to PLZF-RE was much stronger, suggesting that the N-terminal PLZF sequences missing from the fusion may play a role in the regulation of DNA binding. Consistent with this, the N-terminal POZ domain was required for cooperative binding of PLZF to a multimerized PLZF-RE. In the context of the r3/r5 enhancer, the PLZF-RE cooperated for PLZF binding with an additional A/T-rich motif positioned downstream of the PLZF-RE. This A/T motif was previously shown to be essential for the regulation of Hoxb2 expression in r3 and r5 in cooperation with another Krüppel-like zinc finger protein Krox 20. The presence of both the PLZF-RE and the A/T-rich motif was required for a maximal effect of PLZF on a heterologous promoter and was essential in vivo to direct the expression of a lacZ reporter in the chick neural tube. Hence, both PLZF and Krox20 cooperate with a common A/T motif in mediating in vivo activity of the Hoxb2 enhancer. Our findings indicate that Hoxb2 is a direct target for regulation by PLZF in the developing CNS and suggest that deregulation of Hox gene expression may contribute to APL pathogenesis.

Down-regulation of the myeloid homeobox protein Hex is essential for normal T-cell development

The haematopoietic homeobox gene Hex (also called Prh) is expressed in myeloid cells and B cells but not T cells. To investigate whether Hex levels might play a role in myeloid versus T-cell development, two types of transgenic mouse lines were constructed, each with ectopic expression of Hex in T cells (CD11a/Hex and Lck/Hex). Both these types of transgenic mouse had the same defects in T-cell maturation, indicating that proper T-cell development may be dependent not just on the up-regulation of lymphoid-specific transcriptional regulators but also on the co-ordinated down-regulation of myeloid-specific transcriptional regulators such as Hex. In addition, Hex over-expression significantly increased myeloid progenitor cycling, which may explain its role in retrovirally induced murine leukaemia.

Enforced expression of HOX11 is associated with an immature phenotype in J2E erythroid cells

The HOX11 gene encodes a homeodomain transcription factor that is essential for spleen development during embryogenesis. HOX11 is also leukaemogenic, both through its clinical association with childhood T-cell acute lymphoblastic leukaemia, and its ability to immortalize other haematopoietic cell lineages experimentally. To examine the pathological role of HOX11 in tumorigenesis, we constitutively expressed HOX11 cDNA in J2E murine erythroleukaemic cells, which are capable of terminal differentiation. Enforced HOX11 expression was found to induce a profound alteration in J2E cellular morphology and differentiation status. Our analyses revealed that HOX11 produced clones with a preponderance of less differentiated cells that were highly adherent to plastic. Morphologically, the cells overexpressing HOX11 were larger and had decreased globin levels, as well as a reduction in haemoglobin synthesis in response to erythropoietin (EPO). Immunocytochemical analysis confirmed the immature erythroid phenotype imposed by HOX11, with clones transfected with HOX11 demonstrating expression of the c-Kit stem cell marker, while retaining EPO receptor expression. Taken together, these results show that HOX11 alters erythroid differentiation, favouring a less mature progenitor-like stage. This supports the notion that disrupted haematopoietic cell differentiation is responsible for pre-leukaemic immortalization by the HOX11 oncoprotein.

Absence of mutations in the HoxA10, HoxA11 and HoxD11 nucleotide coding sequences in thrombocytopenia with absent radius syndrome

Recent studies have suggested the HoxA10, HoxA11 and HoxD11 homeobox genes as candidate loci for the thrombocytopenia with absent radius (TAR) syndrome. For example, targeted disruptions of these Hox genes result in abnormal development of the mouse radius, while overexpression of HoxA10 stimulates mouse megakaryocyte (MK) development in vitro. To examine the expression of Hox genes in human MK cells, we utilized reverse transcription polymerase chain reaction with degenerate oligonucleotides to study megakaryocytic cell lines (MEG-01, DAMI), and primary human MK purified from adult and cord blood. Using this approach, 13 out of 40 clones isolated from cell lines, 10 out of 21 from cord MK, and 11 out of 21 from adult MK were identified as HoxA10, while HoxA11 and HoxD11 sequences were not detected. The normal genomic sequences for the human HoxA10, -A11, and -D11 genes were then determined and sequenced in 10 unrelated individuals with TAR syndrome. In all patients the derived amino acid sequence for the three Hox genes was identical to normal controls. Southern blotting did not reveal genomic rearrangements or deletions at these loci, and in two patients intact HoxA10 transcripts were detected by amplification in myeloid cells. Although these studies cannot completely exclude the possibility that the TAR syndrome results from non-coding mutations that affect the level of Hox gene expression in megakaryocytes, mutations in the coding sequence of the Hox genes known to affect radial development are not a common cause of TAR syndrome.

Targeted down-regulation of MLL-AF9 with antisense oligodeoxyribonucleotide reduces the expression of the HOXA7 and -A10 genes and induces apoptosis in a human leukemia cell line, THP-1

The MLL gene is frequently rearranged in leukemias, and MLL chimeric proteins generated by chromosomal translocations play crucial roles in leukemogenesis. Targets of murine Mll include HOX proteins that regulate body pattern formation and hematopoiesis. However, it is not known whether or not the MLL chimeric proteins regulate the HOX gene expression in human leukemia. To address this issue, THP-1 cells, a human leukemia cell line expressing MLL-AF9, were treated with antisense oligodeoxyribonucleotide (ODN) complementary to the coding sequence of the MLL-AF9 junction. Down-regulation of the MLL-AF9 transcript was accompanied by the reduced expression of the HOXA7 and -A10 genes, but not of the HOXA2, -A4, -A5, and -A9 genes. The number of viable cells cultured with 20 microM antisense ODN for 5 days was 10-fold lower than that of the sense ODN-treated control. And the number of the annexin V-/propidium iodide- apoptotic cells in the antisense ODN-treated cells after 3 days of culture was two-fold higher than that in the control. Staining of the antisense ODN-treated cells with Hoechst 33258 showed the morphology characteristic to apoptosis. These results indicate that MLL-AF9 regulates the expression of the selected HOX genes as well as prevents the leukemic cells from apoptosis.

NUP98 gene fusions in hematologic malignancies

Acute leukemia is associated with a wide spectrum of recurrent, non-random chromosomal translocations. Molecular analysis of the genes involved in these translocations has led to a better understanding of both the causes of chromosomal rearrangements as well as the mechanisms of leukemic transformation. Recently, a number of laboratories have cloned translocations involving the NUP98 gene on chromosome 11p15.5, from patients with acute myelogenous leukemia (AML), myelodysplastic syndrome (MDS), chronic myelogenous leukemia (CML), and T cell acute lymphoblastic leukemia (T-ALL). To date, at least eight different chromosomal rearrangements involving NUP98 have been identified. The resultant chimeric transcripts encode fusion proteins that juxtapose the N-terminal GLFG repeats of NUP98 to the C-terminus of the partner gene. Of note, several of these translocations have been found in patients with therapy-related acute myelogenous leukemia (t-AML) or myelodysplastic syndrome (t-MDS), suggesting that genotoxic chemotherapeutic agents may play an important role in generating chromosomal rearrangements involving NUP98.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

Disruption of the homeobox gene Hoxb-6 in mice results in increased numbers of early erythrocyte progenitors

Hox genes encode transcription factors that are required for proper development of certain tissues and for patterning of the hindbrain, the limbs, and skeleton. They are also expressed in the hematopoietic system with a preference for specific cell lineages. To determine the role of Hoxb-6 in normal hematopoiesis, mice with a targeted disruption in the Hoxb-6 gene were generated. Mature hematopoietic cell types and immune responses are normal in homozygous Hoxb-6 mutants. Clonogenic progenitor cell assays demonstrate an increased number of early erythroid progenitor cells in the bone marrow and fetal liver of mutants, while differentiation of other cell lineages is unaffected. These results suggest that Hoxb-6 controls the generation, proliferation, or survival of erythroid progenitor cells.

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 theHOXA5 cDNA in CD34+ and CD34+CD38− cells from bone marrow and cord blood. By using retroviral vectors that contained both HOXA5and 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, HOXA5expression 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 HOXA5expression 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.

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 theHOXA5 cDNA in CD34+ and CD34+CD38− cells from bone marrow and cord blood. By using retroviral vectors that contained both HOXA5and 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, HOXA5expression 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 &lt; .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 HOXA5expression 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.

Characterization of HOX Gene Expression During Myelopoiesis: Role of HOX A5 in Lineage Commitment and Maturation

During the process of normal hematopoiesis, proliferation is tightly linked to maturation. The molecular mechanisms that lead to production of mature effector cells with a variety of phenotypes and functions from a single multipotent progenitor are only beginning to be elucidated. It is important to determine how these maturation events are regulated at the molecular level, because this will provide significant insights into the process of normal hematopoiesis as well as leukemogenesis. Transcription factors containing the highly conserved homeobox motif show considerable promise as potential regulators of hematopoietic maturation events. In this study, we focused on identification and characterization of homeobox genes of the HOX family that are important in regulating normal human myeloid differentiation induced by the hematopoietic growth factor, granulocyte-macrophage colony-stimulating factor (GM-CSF). We have identified three homeobox genes, HOX A5, HOX B6, and HOX B7, which are expressed during early myelopoiesis. Treating bone marrow cells with antisense oligodeoxynucleotides to HOX A5 resulted in inhibition of granulocytic/monocytic hematopoiesis and increased the generation of erythroid progenitors. Also, overexpression of HOX A5 inhibited erythroid differentiation of the K562 cell line. Based on these observations, we propose that HOX A5 functions as an important regulator of hematopoietic lineage determination and maturation.

Sequence characterisation and expression of homeobox HOX A7 in the multi-potential erythroleukaemic cell line TF-1

Homeobox gene expression was examined in the erythroleukaemic cell line TF-1. Expression of a number of HOX A, B and C genes, including HOX A7 was detected. Expression of this gene has not previously been reported in erythroleukaemic cell lines. A 2.1 kb full length cDNA of the HOX A7 gene was cloned. The predicted amino acid sequence C-terminal to the homeodomain consists of an alanine-rich region and a strongly negatively charged domain consisting entirely of aspartic and glutamic acid residues.

Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a rare, congenital, hypoplastic anemia that usually presents in early infancy. Congenital anomalies, particularly of the head and upper limbs, are present in about a quarter of reported patients. The disease is characterized by a moderate-to-severe macrocytic anemia, occasional neutropenia or thrombocytosis, a normocellular bone marrow with erythroid hypoplasia, and an increased risk of developing leukemia. The pathogenesis is unknown. The majority of patients respond to prednisone, and often erythropoiesis can be maintained with low doses of the drug. Both remissions and increased resistance to steroid treatment can occur. Nonresponders usually are transfusion dependent, although responses to high dose steroid, androgen, and interleukin-3 have been observed. Bone marrow transplantation can be curative.

Hox homeobox genes as regulators of normal and leukemic hematopoiesis

Hox genes, first recognized for their role in embryonic development, may also play lineage-specific functions in a variety of somatic tissues including the hematopoietic system. Expression of these transcription factors has been demonstrated both in normal and leukemic human and hematopoietic cells, suggesting functional roles in hematopoietic cell growth and differentiation. Several recent studies have shown that Hox proteins are involved in controlling proliferation of primitive bone marrow cells and also in altering differentiation of myeloid as well as lymphoid progenitors, alterations that also can contribute to leukemic transformation. Hox genes, together with their upstream regulators and downstream target genes, may play key roles in fundamental processes controlling hematopoietic stem cell properties.

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.

Mammalian Homeobox B6 Expression Can Be Correlated With Erythropoietin Production Sites and Erythropoiesis During Development, But Not With Hematopoietic or Nonhematopoietic Stem Cell Populations

There has been increasing interest in the involvement of mammalian homeobox (HOX) genes in hematopoietic regulation. The HOX genes are clustered in 4 chromosomes in mice and humans. In general, 5′ end HOX gene expression is predominant in hematopoietic stem cell populations, whereas 3′ end HOX gene expression are primarily found in committed progenitor cells. Furthermore, HOX genes of the A cluster are generally found in myelomonocytic cells, B cluster genes in erythropoietic cells, and C cluster genes in lymphoid cells. The results presented here concentrate on a single gene, namely HOX B6. Preliminary observations using whole mount in situ hybridization showed that both HOX B6 and erythropoietin (EPO) gene expression occurred in exactly the same areas of the 8.5-day mouse embryo. As a consequence, we studied the expression of HOX B6 and EPO gene expression from 6.5 to 19.5 days of gestation, in the neonate, and in the adult. It was found that the sequential transfer of erythropoiesis in different organs during development was followed by a similar transfer of HOX B6 and EPO gene expression. Between days 16.5 and 17.5, both HOX B6 and EPO gene expression decrease in the fetal liver, even though hepatic erythropoiesis continues to decline and is transferred to the fetal spleen. Precisely at this time point, HOX B6 and EPO gene expression are transferred to both the fetal spleen and fetal kidney. However, surprisingly, expression of both genes increases again in the fetal liver just before birth. HOX B6 is expressed in cells from in vitro erythropoietic colonies (colony-forming unit-erythroid and burst-forming unit-erythroid) and TER-119+ erythroid cells but not in hematopoietic or nonhematopoietic stem cell populations. When the latter two populations are allowed to differentiate into erythropoietic cells, HOX B6 and erythroid-relevant markers are expressed. The results indicate that HOX B6 is intimately involved in the regulation of the erythropoietic system and could be a marker for this lineage.

Overexpression of HOXA10 in murine hematopoietic cells perturbs both myeloid and lymphoid differentiation and leads to acute myeloid leukemia

Multiple members of the A, B, and C clusters of Hox genes are expressed in hematopoietic cells. Several of these Hox genes have been found to display distinctive expression patterns, with genes located at the 3' side of the clusters being expressed at their highest levels in the most primitive subpopulation of human CD34+ bone marrow cells and genes located at the 5' end having a broader range of expression, with downregulation at later stages of hematopoietic differentiation. To explore if these patterns reflect different functional activities, we have retrovirally engineered the overexpression of a 5'-located gene, HOXA10, in murine bone marrow cells and demonstrate effects strikingly different from those induced by overexpression of a 3'-located gene, HOXB4. In contrast to HOXB4, which causes selective expansion of primitive hematopoietic cells without altering their differentiation, overexpression of HOXA10 profoundly perturbed myeloid and B-lymphoid differentiation. The bone marrow of mice reconstituted with HOXA10-transduced bone marrow cells contained in high frequency a unique progenitor cell with megakaryocytic colony-forming ability and was virtually devoid of unilineage macrophage and pre-B-lymphoid progenitor cells derived from the transduced cells. Moreover, and again in contrast to HOXB4, a significant proportion of HOXA10 mice developed a transplantable acute myeloid leukemia with a latency of 19 to 50 weeks. These results thus add to recognition of Hox genes as important regulators of hematopoiesis and provide important new evidence of Hox gene-specific functions that may correlate with their normal expression pattern.

The role of HOX homeobox genes in normal and leukemic hematopoiesis

A sizable amount of new data points to a role for the HOX family of homeobox genes in hematopoiesis. Recent studies have demonstrated that HOXA and HOXB genes are expressed in human CD34+ cells, and are downregulated as cells leave the CD34+ compartment. In addition, expression of certain genes, including HOXB3 and HOXB4, is largely restricted to the long-term culture-initiating cell enriched pool, containing the putative stem cell population. Studies have also shown that HOX genes appear to be important for normal T lymphocyte and activated natural killer cell function. Overexpression of Hox-b4 in transplanted murine marrow cell results in a dramatic expansion of stem cells, while maintaining normal peripheral blood counts. In contrast, overexpression of Hox-a10 resulted in expansion of progenitor pools, accompanied by unique changes in the differentiation patterns of committed progenitors. Overexpression of Hox-a10 or Hox-b8 led to the development of myeloid leukemias, while animals transfected with marrow cells overexpressing Hox-b4 do not appear to develop malignancies. Blockade of HOX gene function using antisense oligonucleotides has revealed that several HOX genes appear to influence either myeloid or erythroid colony formation. Mice homozygous for a targeted disruption of the HOX-a9 gene show reduced numbers of granulocytes and lymphocytes, smaller spleens and thymuses, and reduced numbers of committed progenitors. These studies demonstrate that HOX homeobox genes play a role in both the early stem cell function as well as in later stages of hematopoietic differentiation, and that perturbations of HOX gene expression can be leukemogenic.

The t(7;11)(p15;p15) translocation in acute myeloid leukaemia fuses the genes for nucleoporin NUP98 and class I homeoprotein HOXA9

The t(7;11)(p15;p15) translocation is a recurrent chromosomal abnormality associated primarily with acute myeloid leukaemia (FAB M2 and M4). We present here the molecular definition of this translocation. On chromosome 7 positional cloning revealed the consistent rearrangement of the HOXA9 gene, which encodes a class I homeodomain protein potentially involved in myeloid differentiation. On chromosome 11 the translocation targets the human homologue of NUP98, a member of the GLFG nucleoporin family. Chimaeric messages spliced over the breakpoint fuse the GLFG repeat domains of NUP98 in-frame to the HOXA9 homeobox. The predicted NUP98-HOXA9 fusion protein may promote leukaemogenesis through inhibition of HOXA9-mediated terminal differentiation and/or aberrant nucleocytoplasmic transport.

Transcription factor GATA-1 regulates human HOXB2 gene expression in erythroid cells

The human HOXB2 gene is a member of the vertebrate Hox gene family that contains genes coding for specific developmental stage DNA-binding proteins. Remarkably, within the hematopoietic compartment, genes of the HOXB complex are expressed specifically in erythromegakaryocytic cell lines and, for some of them, in hematopoietic progenitors. Here, we report the study of HOXB2 gene transcriptional regulation in hematopoietic cells, an initial step in understanding the lineage-specific expression of the whole HOXB complex in these cells. We have isolated the HOXB2 5'-flanking sequence and have characterized a promoter fragment extending 323 base pairs upstream from the transcriptional start site, which, in transfection experiments, was sufficient to direct the tissue-specific expression of HOXB2 in the erythroid cell line K562. In this fragment, we have identified a potential GATA-binding site that is essential to the promoter activity as demonstrated by point mutation experiments. Gel shift analysis revealed the formation of a specific complex in both erythroleukemic lines K562 and HEL that could be prevented by the addition of a specific antiserum raised against GATA-1 protein. These findings suggest a regulatory hierarchy in which GATA-1 is upstream of the HOXB2 gene in erythroid cells.

Transcription factors in early T-cell development

Characterization of the transcription control regions for most genes encoding the TCR-CD3 complex has been followed by identification of DNA-binding factors, some of which are restricted to lymphoid or T-lineage cells. The sequential expression of these factors is linked with the appearance of stage-specific T-cell markers, thus controlling the commitment of the elusive lymphoid progenitor cell and its progression along the T-lineage. Here, Hans Clevers, Mariëtte Oosterwegel and Katia Georgopoulos discuss recent progress made in the recognition of transcription factors that regulate these early events in T-cell ontogeny.

Identification of homeobox-containing genes expressed in hematopoietic blast cells

Among putative candidates involved in commitment and differentiation of hematopoietic cells are homeoproteins, a large family of transcription factors playing a major role during development. Using a polymerase chain reaction (PCR)-derived protocol, we have investigated for Antennapedia-like homeobox-containing (HOX) gene expression in an enriched population of human hematopoietic progenitors. Nine members of HOX 1 and HOX 2 loci were isolated. Together with recent studies using established cell lines, this indicates a large representation of HOX genes in the hematopoietic compartment and suggests a participation of this class of nuclear proteins to early steps of hematopoiesis.