Lineage-restricted expression of homeobox-containing genes in human hematopoietic cell lines

The roles of HOXB7 in promoting migration, invasion, and anti-apoptosis in gastric cancer

BACKGROUND AND AIM

The aim of this study was to compare HOXB7 expression level between gastric cancer and non-cancerous gastric tissues. Additionally, the functional effects of HOXB7, including its pro-migration or invasion and anti-apoptosis roles, were evaluated in gastric cancer cells.

METHODS

Both gene and protein expression levels of HOXB7 were examined in gastric cancer cell lines, and HOXB7 expression was compared between primary or metastatic gastric cancer tissues and chronic gastritis or intestinal metaplasia tissues. Functional studies included a wound healing assay, a Matrigel invasion assay, and an Annexin-V assay were performed, and Akt/PTEN activity was measured by western blotting.

RESULTS

Both gene and protein expression levels of HOXB7 could be clearly detected in various gastric cancer cell lines except MKN-28 cell. HOXB7 expression was significantly higher in primary or metastatic gastric cancer tissues than in chronic gastritis or intestinal metaplasia tissues. HOXB7 knockdown led to inhibition of cell invasion and migration, had an apoptotic effect, downregulated phosphor-Akt, and upregulated PTEN in AGS and SNU-638 cells. Reinforced expression of HOXB7 caused the opposite effects in MKN-28 and MKN-45 cells.

CONCLUSION

Our study suggests that HOXB7 has an oncogenic role in gastric cancer, which might be related to the modulation of Akt/PTEN activity to induce cell migration/invasion and anti-apoptotic effects.

Transcriptional networks driving enhancer function in the CFTR gene

A critical cis-regulatory element for the CFTR (cystic fibrosis transmembrane conductance regulator) gene is located in intron 11, 100 kb distal to the promoter, with which it interacts. This sequence contains an intestine-selective enhancer and associates with enhancer signature proteins, such as p300, in addition to tissue-specific TFs (transcription factors). In the present study we identify critical TFs that are recruited to this element and demonstrate their importance in regulating CFTR expression. In vitro DNase I footprinting and EMSAs (electrophoretic mobility-shift assays) identified four cell-type-selective regions that bound TFs in vitro. ChIP (chromatin immunoprecipitation) identified FOXA1/A2 (forkhead box A1/A2), HNF1 (hepatocyte nuclear factor 1) and CDX2 (caudal-type homeobox 2) as in vivo trans-interacting factors. Mutation of their binding sites in the intron 11 core compromised its enhancer activity when measured by reporter gene assay. Moreover, siRNA (small interfering RNA)-mediated knockdown of CDX2 caused a significant reduction in endogenous CFTR transcription in intestinal cells, suggesting that this factor is critical for the maintenance of high levels of CFTR expression in these cells. The ChIP data also demonstrate that these TFs interact with multiple cis-regulatory elements across the CFTR locus, implicating a more global role in intestinal expression of the gene.

Crosstalk between leukemia-associated proteins MOZ and MLL regulates HOX gene expression in human cord blood CD34+ cells

MOZ and MLL, encoding a histone acetyltransferase (HAT) and a histone methyltransferase, respectively, are targets for recurrent chromosomal translocations found in acute myeloblastic or lymphoblastic leukemia. In MOZ (MOnocytic leukemia Zinc-finger protein)/CBP- or mixed lineage leukemia (MLL)-rearranged leukemias, abnormal levels of HOX transcription factors have been found to be critical for leukemogenesis. We show that MOZ and MLL cooperate to regulate these key genes in human cord blood CD34+ cells. These chromatin-modifying enzymes interact, colocalize and functionally cooperate, and both are recruited to multiple HOX promoters. We also found that WDR5, an adaptor protein essential for lysine 4 trimethylation of histone H3 (H3K4me3) by MLL, colocalizes and interacts with MOZ. We detected the binding of the HAT MOZ to H3K4me3, thus linking histone methylation to acetylation. In CD34+ cells, depletion of MLL causes release of MOZ from HOX promoters, which is correlated to defective histone activation marks, leading to repression of HOX gene expression and alteration of commitment of CD34+ cells into myeloid progenitors. Thus, our results unveil the role of the interaction between MOZ and MLL in CD34+ cells in which both proteins have a critical role in hematopoietic cell-fate decision, suggesting a new molecular mechanism by which MOZ or MLL deregulation leads to leukemogenesis.

Transcription factors GATA-1 and Fli-1 regulate human HOXA10 expression in megakaryocytic cells

HOXA10 is a member of the HOX family of regulatory genes that are involved in hematopoiesis. Its role in megakaryopoiesis has been suggested by its expression in immature megakaryocytes and by the proliferation of megakaryocyte-primitive blast colonies upon HOXA10 overexpression. We sought to understand the role of HOXA10 in megakaryopoiesis better, by investigating its transcriptional regulation. Analysis of the 5' untranslated region and transfection of promoter/plasmids into human tissue culture cell lines identified transcriptionally active sequences that contain GATA-1 and Ets-1 sites and a putative binding site for its neighboring gene, HOXA11. Gel shift assays confirmed protein-DNA interactions at these sites. Mutation of the GATA-1 and the Ets-1 motifs amplified the expression of HOXA10 in HEL and K562 cells, confirming the importance of these cis-acting elements in regulating HOXA10 expression in megakaryocytic cells. Chromatin immunoprecipitation (ChIP) and chloramphenicol acetyl transferase (CAT) assays confirm that HOXA11 binds to the putative binding site, resulting in repression of HOXA10 expression. These data taken together give insight into the regulation of HOXA10 expression in megakaryocytic differentiation.

De novo t(12;17)(p13.3;q21.3) translocation with a breakpoint near the 5′ end of the HOXB gene cluster in a patient with developmental delay and skeletal malformations

A boy with severe mental retardation, funnel chest, bell-shaped thorax, and hexadactyly of both feet was found to have a balanced de novo t(12;17)(p13.3;q21.3) translocation. FISH with BAC clones and long-range PCR products assessed in the human genome sequence localized the breakpoint on chromosome 17q21.3 to a 21-kb segment that lies <30 kb upstream of the HOXB gene cluster and immediately adjacent to the 3' end of the TTLL6 gene. The breakpoint on chromosome 12 occurred within telomeric hexamer repeats and, therefore, is not likely to affect gene function directly. We propose that juxtaposition of the HOXB cluster to a repetitive DNA domain and/or separation from required cis-regulatory elements gave rise to a position effect.

Conditional MLL-CBP targets GMP and models therapy-related myeloproliferative disease

Chromosomal translocations that fuse the mixed lineage leukemia (MLL) gene with multiple partners typify acute leukemias of infancy as well as therapy-related leukemias. We utilized a conditional knockin strategy to bypass the embryonic lethality caused by MLL-CBP expression and to assess the immediate effects of induced MLL-CBP expression on hematopoiesis. Within days of activating MLL-CBP, the fusion protein selectively expanded granulocyte/macrophage progenitors (GMP) and enhanced their self-renewal/proliferation. MLL-CBP altered the gene expression program of GMP, upregulating a subset of genes including Hox a9. Inhibition of Hox a9 expression by RNA interference demonstrated that MLL-CBP required Hox a9 for its enhanced cell expansion. Following exposure to sublethal gamma-irradiation or N-ethyl-N-nitrosourea (ENU), MLL-CBP mice developed myelomonocytic hyperplasia and progressed to fatal myeloproliferative disorders. These represented the spectrum of therapy-induced acute myelomonocytic leukemia/chronic myelomonocytic leukemia/myelodysplastic/myeloproliferative disorder similar to that seen in humans possessing the t(11;16). This model of MLL-CBP therapy-related myeloproliferative disease demonstrates the selectivity of this MLL fusion for GMP cells and its ability to initiate leukemogenesis in conjunction with cooperating mutations.

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.

Expression profiles of 39 HOX genes in normal human adult organs and anaplastic thyroid cancer cell lines by quantitative real-time RT-PCR system

HOX genes are well known as master control genes in embryonic morphogenesis. We hypothesized that HOX genes give cells spatial information to maintain tissue- or organ-specificity in adult body and that the deregulated expression of HOX genes results in tumor development. We established a comprehensive analysis system to quantify expression of 39 human HOX genes based on the real-time reverse transcription PCR (RT-PCR) method. Analysis of 39 HOX genes of 20 normal adult organs by this system revealed that 5' HOX genes were expressed in organs in the caudal parts of the body, and that the more caudal regions the more numbers of HOX genes were expressed. It was also found that the expression patterns of HOX genes were more similar in the adjacent genes on the same cluster rather than in those belonging to the same paralogs. Compared with normal thyroid tissues, thyroid cancer cell lines showed the altered expression of some HOX genes, especially Abd-B homeobox family genes. Our results showed that HOX genes were organ-specifically expressed in adult body and that the deregulated expressions of Abd-B family genes were implicated in thyroid tumor development.

Down-regulation of Hox A7 is required for cell adhesion and migration on fibronectin during early HL-60 monocytic differentiation

Hox genes, which are key regulators of cell fate and pattern formation during embryogenesis, are also important regulators of hematopoiesis, and different combinations of Hox gene products are involved in lineage commitment or maturation. However, their molecular and cellular modes of action are not yet completely understood. Recent studies have indicated that Hox genes are involved in the regulation of cell-extracellular matrix (ECM) interactions and cell migration. Here, we report that Hox A7, a gene frequently overexpressed in acute myeloid leukemia, is down-regulated during HL-60 monocytic differentiation. Using a model in which HL-60 cells are induced to differentiate toward the monocytic lineage with bone marrow stromal-like cells, we demonstrate that Hox A7-sustained expression disturbs the regulation of cell adhesive and migratory capacities on fibronectin during early differentiation. We show that this is accompanied by a partial blockage of the transcriptional induction of proline-rich tyrosine kinase 2, a gene coding for a focal adhesion kinase active in monocytes, and of tissue transglutaminase, a gene coding for a fibronectin coreceptor in monocytes. This is the first report that demonstrates the involvement of a Hox gene in the regulation of adhesion and migration of hematopoietic cells and that links it to the deregulation of genes involved in cell-ECM interactions and downstream signaling pathways.

In vitro expansion of human cord blood CD36+ erythroid progenitors: temporal changes in gene and protein expression

OBJECTIVE

Erythropoiesis involves proliferation and differentiation of committed erythroid progenitors to mature red blood cells. The objective of this study was to characterize growth characteristics of human CD36+ erythroid progenitors and to profile temporal expression of lineage-specific transcription factors, structural proteins, and growth factor receptors involved in erythropoiesis.

MATERIALS AND METHODS

Erythropoietin-induced differentiation of human cord blood CD36+ erythroid progenitors was profiled for GATA-1, GATA-2, NFE2, EKLF, SCL, PU.1, Id1, Evi-1, c-myb, Hox2.2, c-kit, EpoR, glycophorin A (GPA), CD71, beta- and gamma-globin, and protein 4.2 gene and/or protein expression and DNA content analysis on days 4, 7, and 15 of culture.

RESULTS

Real-time RT-PCR analysis revealed upregulation of GATA-1, Id1, glycophorin A, and protein 4.2 mRNA expression on day 7 when compared to day 4 and decreased expression on day 15. EKLF, GATA-2, Hox2.2, c-myb, Evi-1, c-kit, and PU.1 mRNA expression decreased on days 7 and 15. NFE2, CD71, SCL, and EPO-R mRNA expression remained similar on days 4 and 7 but decreased on day 15. Expression of globin genes beta- and gamma-globin increased on both day 7 and day 15 compared to day 4. Values from flow cytometric quantitation of glycophorin A, transferrin receptor (CD71), and hemoglobin A proteins correlated with gene expression results. DNA analysis demonstrated that most cells lacked DNA content by day 15, a finding consistent with enucleation and terminal erythroid differentiation.

CONCLUSION

These data indicate that in vitro liquid cultures of committed CD36+ erythroid progenitor cells retain, in part, many features of erythropoiesis at the cellular and molecular level and may provide a useful model for assessment of disease-related or drug-induced erythropoietic abnormalities.

Global down-regulation of HOX gene expression in PML-RARalpha + acute promyelocytic leukemia identified by small-array real-time PCR

Acute promyelocytic leukemia (APL) is associated with a reciprocal and balanced translocation involving the retinoic acid receptor-alpha (RARalpha). All-trans retinoic acid (ATRA) is used to treat APL and is a potent morphogen that regulates HOX gene expression in embryogenesis and organogenesis. HOX genes are also involved in hematopoiesis and leukemogenesis. Thirty-nine mammalian HOX genes have been identified and classified into 13 paralogous groups clustered on 4 chromosomes. They encode a complex network of transcription regulatory proteins whose precise targets remain poorly understood. The overall function of the network appears to be dictated by gene dosage. To investigate the mechanisms involved in HOX gene regulation in hematopoiesis and leukemogenesis by precise measurement of individual HOX genes, a small-array real-time HOX (SMART-HOX) quantitative polymerase chain reaction (PCR) platform was designed and validated. Application of SMART-HOX to 16 APL bone marrow samples revealed a global down-regulation of 26 HOX genes compared with normal controls. HOX gene expression was also altered during differentiation induced by ATRA in the PML-RARalpha(+) NB4 cell line. PML-RARalpha fusion proteins have been reported to act as part of a repressor complex during myeloid cell differentiation, and a model linking HOX gene expression to this PML-RARalpha repressor complex is now proposed.

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.

The HOX homeodomain proteins block CBP histone acetyltransferase activity

Despite the identification of PBC proteins as cofactors that provide DNA affinity and binding specificity for the HOX homeodomain proteins, HOX proteins do not demonstrate robust activity in transient-transcription assays and few authentic downstream targets have been identified for these putative transcription factors. During a search for additional cofactors, we established that each of the 14 HOX proteins tested, from 11 separate paralog groups, binds to CBP or p300. All six isolated homeodomain fragments tested bind to CBP, suggesting that the homeodomain is a common site of interaction. Surprisingly, CBP-p300 does not form DNA binding complexes with the HOX proteins but instead prevents their binding to DNA. The HOX proteins are not substrates for CBP histone acetyltransferase (HAT) but instead inhibit the activity of CBP in both in vitro and in vivo systems. These mutually inhibitory interactions are reflected by the inability of CBP to potentiate the low levels of gene activation induced by HOX proteins in a range of reporter assays. We propose two models for HOX protein function: (i) HOX proteins may function without CBP HAT to regulate transcription as cooperative DNA binding molecules with PBX, MEIS, or other cofactors, and (ii) the HOX proteins may inhibit CBP HAT activity and thus function as repressors of gene transcription.

CREB binding protein recruitment to the transcription complex requires growth factor-dependent phosphorylation of its GF box

Growth factors such as epidermal growth factor (EGF) and insulin regulate development and metabolism via genes containing both POU homeodomain (Pit-1) and phorbol ester (AP-1) response elements. Although CREB binding protein (CBP) functions as a coactivator on these elements, the mechanism of transactivation was previously unclear. We now demonstrate that CBP is recruited to these elements only after it is phosphorylated at serine 436 by growth factor-dependent signaling pathways. In contrast, p300, a protein closely related to CBP that lacks this phosphorylation site, binds only weakly to the transcription complex and in a growth factor-independent manner. A small region of CBP (amino acids 312-440), which we term GF box, contains a potent transactivation domain and mediates this effect. Direct phosphorylation represents a novel mechanism controlling coactivator recruitment to the transcription complex.

BP1, a new homeobox gene, is frequently expressed in acute leukemias

Aberrant expression of homeobox genes has been described in primary leukemia blasts. We recently cloned a new cDNA, BP1, which is a member of the homeobox gene family. BP1 expression was investigated in bone marrow samples from acute myeloid leukemia (AML), acute T cell lymphocytic leukemia (ALL) and pre-B cell ALL. Expression levels of two apparent isoforms of BP1, DLX7 and DLX4, were measured in the same samples. They are weakly if at all detectable in normal bone marrow, PHA-stimulated T cells or B cells. BP1 RNA was highly expressed in 63% of AML cases, including 81% of the pediatric and 47% of the adult cases, and in 32% of T-ALL cases, but was not found in any of the pre-B ALL cases. Coexpression of BP1, DLX7 and DLX4 occurred in a significant number of leukemias. Our data, including co-expression of BP1 with c-myb and GATA-1, markers of early progenitors, suggest that BP1 expression occurs in primitive cells in AML. Analysis of CD34+ and CD34- normal bone marrow cells revealed BP1 is expressed in CD34- cells and virtually extinguished in CD34+ cells. Ectopic expression of BP1 in the leukemia cell line K562 increased clonogenicity, consistent with a role for BP1 in leukemogenesis. The presence of BP1 RNA in leukemic blasts may therefore be a molecular marker for primitive cells and/or may indicate that BP1 is an important upstream factor in an oncogenic pathway.

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.

Regulation of granulopoiesis by transcription factors and cytokine signals

The development of mature granulocytes from hematopoietic precursor cells is controlled by a myriad of transcription factors which regulate the expression of essential genes, including those encoding growth factors and their receptors, enzymes, adhesion molecules, and transcription factors themselves. In particular, C/EBPalpha, PU.1, CBF, and c-Myb have emerged as critical players during early granulopoiesis. These transcription factors interact with one another as well as other factors to regulate the expression of a variety of genes important in granulocytic lineage commitment. An important goal remains to understand in greater detail how these various factors act in concert with signals emanating from cytokine receptors to influence the various steps of maturation, from the pluripotent hematopoietic stem cell, to a committed myeloid progenitor, to myeloid precursors, and ultimately to mature granulocytes.

Sequence analysis and tissue specific expression of human HOXA7

A 3071-bp fragment containing the human HOXA7 gene was sequenced. It contained two exons, one intron, and two polyadenylation signals (AATAAA) at positions 1844 and 2923. The exon encoded 230 aa residues, while the hexapeptide, homeodomain, and C-terminal acidic domains were detected. When the total sequences were compared with those of murine Hoxa-7, the 5' untranslated region (UTR), exon I, intron, exon II, and 3' UTR exhibited 99, 92, 65, 85, and 72% homology, respectively. Through Northern analysis, about a 1.9-kb transcript was detected in the fetal kidney. Minor transcripts of 1.5 and 1.1-kb were also detected in the fetal liver as well as in the kidney. In the case of adult tissues, most of the tissues tested (lung, liver, skeletal muscle, kidney, pancreas, and placenta), except brain tissue, expressed a 5.3 kb transcript with various intensities. Our results here suggested that not only Abd-B type Hox genes, but the ones in paralogous groups I-VIII could generate multiple transcripts. The characterization of these tissue-specific and stage-specific alternative transcripts would help to understand the regulatory function of the HOXA7 gene during development, and possibly the pathology of human disease caused by Hox genes.

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.

CBP and histone deacetylase inhibition enhance the transactivation potential of the HOXB7 homeodomain-containing protein

Homeodomain-containing proteins are transcription factors regulating the coordinated expression of multiple target genes involved in development, differentiation and cellular transformation. In this study, we demonstrated that HOXB7, one member of this family, behaved as a transactivator in breast cancer cells. Deletion of either the HOXB7 N-terminal domain or the C-terminal acidic tail abolished this transcriptional effect, suggesting a combination of distinct functional transactivating domains. HOXB7 physically interacted both in vitro and in vivo with the coactivator CREB-binding protein (CBP). This interaction led to an enhanced transactivating potential and required the N-terminal of HOXB7 as well as two domains located at the C-terminal part of CBP. Moreover, trichostatin A, a deacetylase inhibitor, strongly enhanced the transcriptional properties of HOXB7. Our data therefore indicate that HOX proteins can directly interact with CBP and that acetylation/deacetylation may regulate their transcriptional properties.

HOXC5 and HOXC8 expression are selectively turned on in human cervical cancer cells compared to normal keratinocytes

A growing number of data have sustained the involvement of homeobox genes expression deregulation in cancer. In this study, we have performed an exhaustive survey of the expression of the 39 class I HOX genes expressed in normal and malignant human cervix keratinocytes. Using RT-PCR, we observed that the vast majority (34/39) of HOX genes are expressed in normal keratinocytes. Only HOXA2, HOXA7, HOXC5, HOXC8 and HOXD12 were found to be silent. Interestingly, this pattern is conserved in the transformed keratinocytes (SiHa cells) except for the appearance of HOXC5 and HOXC8 mRNA. The HOXC5 and HOXC8 expression was also observed in two other transformed keratinocytes cell lines of independent origins, Eil-8 and 18-11S3, and confirmed by in situ hybridization. Our data add weight to the body of evidence attributing to a specific adult tissue a particular combination of expressed HOX genes and suggest that HOXC5 and/or HOXC8 could be involved in the process leading to the transformation of cervical keratinocytes.

c-myc intron element-binding proteins are required for 1, 25-dihydroxyvitamin D3 regulation of c-myc during HL-60 cell differentiation and the involvement of HOXB4

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) suppresses c-myc expression during differentiation of HL-60 cells along the monocytic pathway by blocking transcriptional elongation at the first exon/intron border of the c-myc gene. In the present study, the physiological relevance of three putative regulatory protein binding sites found within a 280-base pair region in intron 1 of the c-myc gene was explored. HL-60 promyelocytic leukemia cells were transiently transfected with three different c-myc promoter constructs cloned upstream of a chloramphenicol acetyltransferase (CAT) reporter gene. With the wild-type c-myc promoter construct (pMPCAT), which contains MIE1, MIE2, and MIE3 binding sites, 1,25-(OH)2D3 was able to decrease CAT activity by 45.4 +/- 7.9% (mean +/- S.E., n = 8). The ability of 1, 25-(OH)2D3 to inhibit CAT activity was significantly decreased to 18. 5 +/- 4.3% (59.3% reversal, p < 0.02) when examined with a MIE1 deletion construct (pMPCAT-MIE1). Moreover, 1,25-(OH)2D3 was completely ineffective at suppressing CAT activity in cells transfected with pMPCAT-287, a construct without MIE1, MIE2, and MIE3 binding sites (-6.5 +/- 10.9%, p < 0.002). MIE1- and MIE2-binding proteins induced by 1,25-(OH)2D3 had similar gel shift mobilities, while MIE3-binding proteins migrated differently. Furthermore, chelerythrine chloride, a selective protein kinase C (PKC) inhibitor, and a PKCbeta antisense oligonucleotide completely blocked the binding of nuclear proteins induced by 1,25-(OH)2D3 to MIE1, MIE2, and MIE3. A 1,25-(OH)2D3-inducible MIE1-binding protein was identified to be HOXB4. HOXB4 levels were significantly increased in response to 1,25-(OH)2D3. Taken together, these results indicate that HOXB4 is one of the nuclear phosphoproteins involved in c-myc transcription elongation block during HL-60 cell differentiation by 1,25-(OH)2D3.

IkappaB-alpha enhances transactivation by the HOXB7 homeodomain-containing protein

Combinatorial interactions between distinct transcription factors generate specificity in the controlled expression of target genes. In this report, we demonstrated that the HOXB7 homeodomain-containing protein, which plays a key role in development and differentiation, physically interacted in vitro with IkappaB-alpha, an inhibitor of NF-kappaB activity. This interaction was mediated by the IkappaB-alpha ankyrin repeats and C-terminal domain as well as by the HOXB7 N-terminal domain. In transient transfection experiments, IkappaB-alpha markedly increased HOXB7-dependent transcription from a reporter plasmid containing a homeodomain consensus-binding sequence. This report therefore showed a novel function for IkappaB-alpha, namely a positive regulation of transcriptional activation by homeodomain-containing proteins.

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.

HOXA10 is expressed in response to sex steroids at the time of implantation in the human endometrium

Hox genes are well-known transcriptional regulators that play an essential role in directing embryonic development. Mice that are homozygous for a targeted disruption of the Hoxa10 gene exhibit uterine factor infertility. We have recently demonstrated that HOXA10 is expressed in the adult human uterus. To examine expression of HOXA10 during the menstrual cycle, Northern blot analysis and in situ hybridization were performed. Expression of HOXA10 dramatically increased during the midsecretory phase of the menstrual cycle, corresponding to the time of implantation and increase in circulating progesterone. Expression of HOXA10 in cultured endometrial cells was stimulated by estrogen or progesterone. Stimulation of HOXA10 by progesterone was concentration-dependent within the physiologic range, and the effect of estrogen was inhibited by cycloheximide. These results identify sex steroids as novel regulators of HOX gene expression. HOXA10 may have an important function in regulating endometrial development during the menstrual cycle and in establishing conditions necessary for implantation in the human.

HOXA10 is expressed in response to sex steroids at the time of implantation in the human endometrium

Hox genes are well-known transcriptional regulators that play an essential role in directing embryonic development. Mice that are homozygous for a targeted disruption of the Hoxa10 gene exhibit uterine factor infertility. We have recently demonstrated that HOXA10 is expressed in the adult human uterus. To examine expression of HOXA10 during the menstrual cycle, Northern blot analysis and in situ hybridization were performed. Expression of HOXA10 dramatically increased during the midsecretory phase of the menstrual cycle, corresponding to the time of implantation and increase in circulating progesterone. Expression of HOXA10 in cultured endometrial cells was stimulated by estrogen or progesterone. Stimulation of HOXA10 by progesterone was concentration-dependent within the physiologic range, and the effect of estrogen was inhibited by cycloheximide. These results identify sex steroids as novel regulators of HOX gene expression. HOXA10 may have an important function in regulating endometrial development during the menstrual cycle and in establishing conditions necessary for implantation in the human.

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.

HOXC4, HOXC5, and HOXC6 Expression in Non-Hodgkin's Lymphoma: Preferential Expression of the HOXC5 Gene in Primary Cutaneous Anaplastic T-Cell and Oro-Gastrointestinal Tract Mucosa-Associated B-Cell Lymphomas

Most of the 39 members of the homeobox (HOX) gene family are believed to control blood cell development. HOXC4 and HOXC6 gene expression levels increase with differentiation of lymphoid cells. In contrast, HOXC5 is not expressed in the lymphoid lineage, but was found in lymphoid cell lines, representing the neoplastic equivalents of various differentiation stages of T and B lymphocytes. In the present study, we investigated the HOXC4, HOXC5, and HOXC6 gene expression pattern in 89 non-Hodgkin's lymphomas (NHLs) of different histologic subtypes and originating from different sites. Using RNA in situ hybridization and semiquantitative reverse transcription-polymerase chain reaction, we found expression of HOXC4 in 83 of 88 and HOXC6 in 77 of 88 NHLs and leukemias investigated. In contrast, HOXC5 expression was found in only 26 of 87 NHLs and appeared to be preferentially expressed by two specific subsets of lymphomas, ie, primary cutaneous anaplastic T-cell lymphomas (9 of 9) and extranodal marginal zone B-cell lymphomas (maltomas; 7 of 9). These results indicate that, in contrast to HOXC4 and HOXC6, HOXC5 shows a type- and site-restricted expression pattern in both T- and B-cell NHLs.

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.

AbdB-like Hox proteins stabilize DNA binding by the Meis1 homeodomain proteins

Recent studies show that Hox homeodomain proteins from paralog groups 1 to 10 gain DNA binding specificity and affinity through cooperative binding with the divergent homeodomain protein Pbx1. However, the AbdB-like Hox proteins from paralogs 11, 12, and 13 do not interact with Pbx1a, raising the possibility of different protein partners. The Meis1 homeobox gene has 44% identity to Pbx within the homeodomain and was identified as a common site of viral integration in myeloid leukemias arising in BXH-2 mice. These integrations result in constitutive activation of Meis1. Furthermore, the Hoxa-9 gene is frequently activated by viral integration in the same BXH-2 leukemias, suggesting a biological synergy between these two distinct classes of homeodomain proteins in causing malignant transformation. We now show that the Hoxa-9 protein physically interacts with Meis1 proteins by forming heterodimeric binding complexes on a DNA target containing a Meis1 site (TGACAG) and an AbdB-like Hox site (TTTTACGAC). Hox proteins from the other AbdB-like paralogs, Hoxa-10, Hoxa-11, Hoxd-12, and Hoxb-13, also form DNA binding complexes with Meis1b, while Hox proteins from other paralogs do not appear to interact with Meis1 proteins. DNA binding complexes formed by Meis1 with Hox proteins dissociate much more slowly than DNA complexes with Meis1 alone, suggesting that Hox proteins stabilize the interactions of Meis1 proteins with their DNA targets.

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.

The Abd-B-like Hox homeodomain proteins can be subdivided by the ability to form complexes with Pbx1a on a novel DNA target

Previous studies showed that the Hox homeodomain proteins from paralog groups 1-8 display cooperative DNA binding with the non-Hox homeodomain protein Pbx, mediated by a canonical YPWM. Although the Abd-B-like Hox proteins in paralogs 9-13 lack this sequence, Hoxb-9 and Hoxa-10 were reported to bind with Pbx1a to DNA. We show that these interactions require a tryptophan 6 amino acids N-terminal to the homeodomain. Binding site selection for Hoxb-9 with Pbx1a yielded ATGATTTACGAC, containing a novel TTAC Hox-binding site adjacent to a Pbx site. In the presence of Pbx1a, Hoxb-9 and Hoxa-10 bound to targets containing either TTAC or TTAT. These data extend previous findings that interactions with Pbx define a Hox protein binding code for different DNA sequences across paralog groups 1 through 10. Members of the 11, 12, and 13 paralogs do not cooperatively bind DNA with Pbx1a, despite the presence of tryptophan residues N-terminal to the homeodomain in Hoxd-12 and Hoxd-13. Hoxa-11, Hoxd-12, or Hoxd-13, in the presence of Pbx1a, selected a TTAC Hox site but lacking a Pbx1a site. These data suggest that Abd-B-like Hox proteins bind to a novel TTAC site and can be divided by their cooperative binding to DNA with Pbx1a.

Mice Bearing a Targeted Interruption of the Homeobox Gene HOXA9 Have Defects in Myeloid, Erythroid, and Lymphoid Hematopoiesis

Several homeobox genes of the HOXA and HOXB clusters are expressed in primitive blood cells, suggesting a role for HOX genes in normal hematopoiesis. The HOXA9 gene is expressed in CD34+ marrow cells and in developing lymphocytes. We examined blood-forming organs of mice homozygous for an interrupted HOXA9 allele to determine if loss of HOX gene function is deleterious to hematopoiesis. HOXA9−/− mice have approximately 30% to 40% reductions in total leukocytes and lymphocytes (P < .001) and a blunted granulocytic response to granulocyte colony-stimulating factor (G-CSF ). Homozygous mice have significantly smaller spleens and thymuses. Myeloid/erythroid and pre-B progenitors in the marrow are significantly reduced, but no significant decreases are noted in mixed colonies, day 12 colony-forming units-spleen (CFU-S), or long-term culture–initiating cells (LTC-IC), suggesting little or no perturbation in earlier progenitors. Heterozygous animals display no hematopoietic defects. The abnormalities in leukocyte production are transplantable, indicating that the defect resides in the hematopoietic cells. These studies demonstrate a physiologic role for a HOX gene in blood cell differentiation, with the greatest apparent influence of HOXA9 at the level of the committed progenitor.

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.

Hox homeodomain proteins exhibit selective complex stabilities with Pbx and DNA

Eight of the nine homeobox genes of the Hoxb locus encode proteins which contain a conserved hexapeptide motif upstream from the homeodomain. All eight proteins (Hoxb-1-Hoxb-8) bind to a target oligonucleotide in the presence of Pbx1a under conditions where minimal or no binding is detected for the Hox or Pbx1a proteins alone. The stabilities of the Hox-Pbx1a-DNA complexes vary >100-fold, with the proteins from the middle of the locus (Hoxb-5 and Hoxb-6) forming very stable complexes, while Hoxb-4, Hoxb-7 and Hoxb-8 form complexes of intermediate stability and proteins at the 3'-side of the locus (Hoxb-1-Hoxb-3) form complexes which are very unstable. Although Hox-b proteins containing longer linker sequences between the hexapeptide and homeodomains formed unstable complexes, shortening the linker did not confer complex stability. Homeodomain swapping experiments revealed that this motif does not independently determine complex stability. Naturally occurring variations within the hexapeptides of specific Hox proteins also do not explain complex stability differences. However, two core amino acids (tryptophan and methionine) which are absolutely conserved within the hexapeptide domains appear to be required for complex formation. Removal of N- and C-terminal flanking regions did not influence complex stability and the members of paralog group 4 (Hoxa-4, b-4, c-4 and d-4), which share highly conserved hexapeptides, linkers and homeodomains but different flanking regions, form complexes of similar stability. These data suggest that the structural features of Hox proteins which determine Hox-Pbx1a-DNA complex stability reside within the precise structural relationships between the homeodomain, hexapeptide and linker regions.

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.

Identification of homeobox genes expressed during the process of rat liver regeneration after partial hepatectomy

Homeobox (HBox) genes are well-known to be involved in development and differentiation. To ascertain a role of HBox genes in the process of liver regeneration, we identified HBox genes expressed at various times after partial hepatectomy in rats (at 0 hr, 1 hr, 2 days, and 4 days) by using reverse transcription-polymerase chain reaction (RT-PCR), cloning, and sequencing techniques. By the competitive RT-PCR method using generic primers, expression levels of HBox genes in regenerating livers were estimated at as low as only 0.4-2% of that in 14-day embryonic liver; however, we identified multiple HBox genes at different stages. Comparing sets of HBox genes identified at different stages, we could find two candidates of stage specifically expressed HBox genes (one rat caudal-related gene, RCdx-3, stimulated at 1 hr, and one rat Hox gene, RHoxB5, repressed after hepatectomy) and continuous expression of five Hox genes (RHoxA1, A4, A5, B2, and B3) before and after hepatectomy. These HBox genes are considered to correlate with the process of liver regeneration.

Nuclear factor binding sites in human beta globin IVS2

The second intron of the human beta globin gene (beta IVS2) has been previously identified as a region required for proper expression of beta globin. To further characterize this region, we have footprinted the entire beta IVS2 and have analyzed regions of interest by electrophoretic mobility shift assay. Through these studies we have identified four utilized binding sites for the erythroid regulatory factor GATA-1, two sites bound by general transcription factor Oct-1, two sites bound by the nuclear matrix attachment DNA binding protein special A-T-rich binding protein 1, and a site bound by a potential homeobox protein. Additionally, we have found several factors displaying temporal or tissue specificity by electrophoretic mobility shift assay, which may be potentially involved in the regulation of beta globin expression. These proteins are not supershifted by antibodies to factors important in erythroid regulation such as GATA-1, NFE-2, or YY1, or by antibodies against more general transcription factors.

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.

Potential roles for two human homeodomain containing proteins in the proliferation and differentiation of human hematopoietic progenitors

Two human homeobox genes, HB9 and HLX, are expressed in hematopoietic progenitors and activated lymphocytes. They are implicated in the proliferation of hematopoietic progenitors in response to growth factors and the differentiation of hematopoietic progenitors to mature cell lineages. RNAs from bone marrow cells of patients with acute myeloid or lymphocytic leukemia have high levels of these two genes while similar RNAs from patients with chronic lymphocytic or myeloid leukemias have nearly normal levels. While the significance of these two genes in leukemogenesis is unknown, they are likely to regulate gene transcription during hematopoiesis and their dysregulation may have dire consequences for hematopoietic cells.

Expression of a novel human homeobox-containing gene that maps to chromosome 7q36.1 in hematopoietic cells

A homeobox is a DNA sequence of 180 base pairs that encodes a DNA-binding domain known as a homeodomain. The polymerase chain reaction (PCR) has been used to prepare probes of homeobox-containing genes. We cloned and sequenced the amplified products of PCR that was performed with human genomic DNA and two primers that correspond to well-conserved regions in homeoboxes. Fifteen kinds of homeobox gene were identified and 13 of them were assigned to HOX genes that have already been reported. Two others represented novel homeobox genes and one of them, GBX1, was mapped to chromosome 7q36.1 by fluorescence in situ hybridization. Northern hybridization of mRNA for various kinds of hematopoietic cell showed that the newly identified GBX1 gene is expressed in K562 cells and Daudi cells.