Identification of a conserved family of Meis1-related homeobox genes

Lead Optimization and Structure-Activity Relationship Studies on Myeloid Ecotropic Viral Integration Site 1 Inhibitor

The pivotal role of the myeloid ecotropic viral integration site 1 (MEIS1) transcriptional factor was reported in cardiac regeneration and hematopoietic stem-cell (HSC) regulation with our previous findings. MEIS1 as a promising target in the context of pharmacological inhibition, we identified a potent myeloid ecotropic viral integration site (MEIS) inhibitor, MEISi-1, to induce murine and human HSC expansion ex vivo and in vivo. In this work, we performed lead optimization on MEISi-1 by synthesizing 45 novel analogues. Structure-activity relationship studies revealed the significance of a para-methoxy group on ring A and a hydrophobic moiety at the meta position of ring B. Obtained biological data were supported by inhibitor docking and molecular dynamics simulation studies. Eleven compounds were depicted as potent inhibitors demonstrating a better inhibitory profile on MEIS1 and target genes Meis1, Hif-1α, and p21. Among those, 4h, 4f, and 4b were the most potent inhibitors. The predicted pharmacokinetics properties fulfill drug-likeness criteria. In addition, compounds exerted neither cytotoxicity on human dermal fibroblasts nor mutagenicity.

MEIS1 and its potential as a cancer therapeutic target (Review)

Meis homeobox 1 (Meis1) was initially discovered in 1995 as a factor involved in leukemia in an animal model. Subsequently, 2 years later, MEIS1, the human homolog, was cloned in the liver and cerebellum, and was found to be highly expressed in myeloid leukemia cells. The MEIS1 gene, located on chromosome 2p14, encodes a 390-amino acid protein with six domains. The expression of homeobox protein MEIS1 is affected by cell type, age and environmental conditions, as well as the pathological state. Certain types of modifications of MEIS1 and its protein interaction with homeobox or pre-B-cell leukemia homeobox proteins have been described. As a transcription factor, MEIS1 protein is involved in cell proliferation in leukemia and some solid tumors. The present review article discusses the molecular biology, modifications, protein-protein interactions, as well as the role of MEIS1 in cell proliferation of cancer cells and MEIS1 inhibitors. It is suggested by the available literature MEIS1 has potential to become a cancer therapeutic target.

CREB, NF-Y and MEIS1 conserved binding sites are essential to balance Myostatin promoter/enhancer activity during early myogenesis

Myostatin (MSTN) is a strong inhibitor of skeletal muscle growth in human and other vertebrates. Its transcription is controlled by a proximal promoter/enhancer (Mstn P/E) containing a TATA box besides CREB, NF-Y, MEIS1 and FXR transcription factor binding sites (TFBSs), which are conserved throughout evolution. The aim of this work was to investigate the role of these TFBSs on Mstn P/E activity and evaluate the potential of their putative ligands as Mstn trans regulators. Mstn P/E mutant constructs were used to establish the role of conserved TFBSs using dual-luciferase assays. Expression analyses were performed by RT-PCR and in situ hybridization in C2C12 myoblasts and E10.5 mouse embryos, respectively. Our results revealed that CREB, NF-Y and MEIS1 sites are required to balance Mstn P/E activity, keeping Mstn transcription within basal levels during myoblast proliferation. Furthermore, our data showed that NF-Y site is essential, although not sufficient, to mediate Mstn P/E transcriptional activity. In turn, CREB and MEIS1 binding sites seem to depend on the presence of NF-Y site to induce Mstn P/E. FXR appears not to confer any effect on Mstn P/E activity, except in the absence of all other conserved TFBS. Accordingly, expression studies pointed to CREB, NF-Y and MEIS1 but not to FXR factors as possible regulators of Mstn transcription in the myogenic context. Altogether, our findings indicated that CREB, NF-Y and MEIS1 conserved sites are essential to control basal Mstn transcription during early myogenesis, possibly by interacting with these or other related factors.

Complex integrated analysis of lncRNAs-miRNAs-mRNAs in oral squamous cell carcinoma

OBJECTIVES

This study aims to reveal regulatory network of lncRNAs-miRNAs-mRNAs in oral squamous cell carcinoma (OSCC) through gene expression data.

MATERIAL AND METHODS

Differentially expressed lncRNAs, miRNAs and mRNAs (cut-off: False discovery rate (FDR)<0.05 and |fold change|>1.5) were unveiled by package edgeR of R. Cox regression analysis was performed to screen prognostic factors in OSCC related with overall survival (OS) and relapse-free survival (RFS). Protein-protein interaction (PPI) network was constructed for differentially expressed mRNAs using BioGRID, HPRD and DIP. Key hub genes were identified from top 100 differentially expressed mRNAs ranked by betweenness centrality using recursive feature elimination. LncRNA-miRNA and miRNA-mRNA regulatory network were constructed and combined into ceRNAs regulatory network. Gene ontology biological terms and Kyoto Encyclopedia of Genes and Genomes pathways were identified using Fisher's exact test.

RESULTS

A total of 929 differentially expressed mRNAs, 23 differentially expressed lncRNAs and 29 differentially expressed miRNAs were identified. 59 mRNAs, 6 miRNAs (hsa-mir-133a-1, hsa-mir-1-2, hsa-mir-486, hsa-mir-135b, hsa-mir-196b, hsa-mir-193b) and 6 lncRNAs (C10orf91, C2orf48, SFTA1P, FLJ41941,PART1,TTTY14) were related with OS; and 52 mRNAs, 4 miRNAs (hsa-mir-133a-1, hsa-mir-135b, hsa-mir-196b, hsa-mir-193b) and 2 lncRNAs (PART1, TTTY14) were associated with RFS. A support vector machine (SVM) classifier containing 37 key hub genes was obtained. A ceRNA regulatory network containing 417 nodes and 696 edges was constructed. ECM-receptor interaction, cytokine-cytokine receptor interaction, focal adhesion, arachidonic acid metabolism, and p53 signaling pathway were significantly enriched in the network.

CONCLUSION

These findings uncover the pathogenesis of OSCC and might provide potential therapeutic targets.

Expression patterns of homeobox genes in the mouse vomeronasal organ at postnatal stages

Homeodomain proteins are encoded by homeobox genes and regulate development and differentiation in many neuronal systems. The mouse vomeronasal organ (VNO) generates in situ mature chemosensory neurons from stem cells. The roles of homeodomain proteins in neuronal differentiation in the VNO are poorly understood. Here we have characterized the expression patterns of 28 homeobox genes in the VNO of C57BL/6 mice at postnatal stages using multicolor fluorescent in situ hybridization. We identified 11 homeobox genes (Dlx3, Dlx4, Emx2, Lhx2, Meis1, Pbx3, Pknox2, Pou6f1, Tshz2, Zhx1, Zhx3) that were expressed exclusively in neurons; 4 homeobox genes (Pax6, Six1, Tgif1, Zfhx3) that were expressed in all non-neuronal cell populations, with Pax6, Six1 and Tgif1 also expressed in some neuronal progenitors and precursors; 12 homeobox genes (Adnp, Cux1, Dlx5, Dlx6, Meis2, Pbx2, Pknox1, Pou2f1, Satb1, Tshz1, Tshz3, Zhx2) with expression in both neuronal and non-neuronal cell populations; and one homeobox gene (Hopx) that was exclusively expressed in the non-sensory epithelium. We studied further in detail the expression of Emx2, Lhx2, Meis1, and Meis2. We found that expression of Emx2 and Lhx2 initiated between neuronal progenitor and neuronal precursor stages. As far as the sensory neurons of the VNO are concerned, Meis1 and Meis2 were only expressed in the apical layer, together with Gnai2, but not in the basal layer.

MEIS1, a Promising Candidate Gene, Is Not Associated with the Core Symptoms of Antipsychotic-Induced Restless Legs Syndrome in Korean Schizophrenia Patients

OBJECTIVE

Restless legs syndrome (RLS) is a distressing sleep disorder to which individuals appear to be genetically predisposed. In the present study, we assumed that antipsychotic-induced RLS symptoms were attributable to differences in individual genetic susceptibility, and investigated whether MEIS1, a promising candidate gene, was associated with antipsychotic-induced RLS symptoms in schizophrenia patients.

METHODS

All subjects were diagnosed with schizophrenia by board-certified psychiatrists using the Korean version of the Structured Clinical Interview for DSM-IV. We assessed antipsychotic-induced RLS symptoms in 190 Korean schizophrenic patients using the diagnostic criteria of the International Restless Legs Syndrome Study Group. Genotyping was performed for the rs2300478 and rs6710341 polymorphisms of the MEIS1 gene.

RESULTS

We divided subjects into RLS symptom (n=96) and non-symptom (n=94) groups. There was no significant between-group difference in the genotype or allele frequencies of the two polymorphisms investigated, nor in the frequency of the rs2300478-rs6710341 haplotype.

CONCLUSION

Our data do not suggest that the rs2300478 and rs6710341 polymorphisms of the MEIS1 gene are associated with the core symptoms of antipsychotic-induced RLS in schizophrenia; different genetic mechanisms may underlie antipsychotic-induced vs. primary RLS.

Proteomic profiling of cardiac tissue by isolation of nuclei tagged in specific cell types (INTACT)

The proper dissection of the molecular mechanisms governing the specification and differentiation of specific cell types requires isolation of pure cell populations from heterogeneous tissues and whole organisms. Here, we describe a method for purification of nuclei from defined cell or tissue types in vertebrate embryos using INTACT (isolation of nuclei tagged in specific cell types). This method, previously developed in plants, flies and worms, utilizes in vivo tagging of the nuclear envelope with biotin and the subsequent affinity purification of the labeled nuclei. In this study we successfully purified nuclei of cardiac and skeletal muscle from Xenopus using this strategy. We went on to demonstrate the utility of this approach by coupling the INTACT approach with liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomic methodologies to profile proteins expressed in the nuclei of developing hearts. From these studies we have identified the Xenopus orthologs of 12 human proteins encoded by genes, which when mutated in human lead to congenital heart disease. Thus, by combining these technologies we are able to identify tissue-specific proteins that are expressed and required for normal vertebrate organ development.

Biochemistry of the tale transcription factors PREP, MEIS, and PBX in vertebrates

TALE (three amino acids loop extension) homeodomain transcription factors are required in various steps of embryo development, in many adult physiological functions, and are involved in important pathologies. This review focuses on the PREP, MEIS, and PBX sub-families of TALE factors and aims at giving information on their biochemical properties, i.e., structure, interactors, and interaction surfaces. Members of the three sets of protein form dimers in which the common partner is PBX but they can also directly interact with other proteins forming higher-order complexes, in particular HOX. Finally, recent advances in determining the genome-wide DNA-binding sites of PREP1, MEIS1, and PBX1, and their partial correspondence with the binding sites of some HOX proteins, are reviewed. These studies have generated a few general rules that can be applied to all members of the three gene families. PREP and MEIS recognize slightly different consensus sequences: PREP prefers to bind to promoters and to have PBX as a DNA-binding partner; MEIS prefers HOX as partner, and both PREP and MEIS drive PBX to their own binding sites. This outlines the clear individuality of the PREP and MEIS proteins, the former mostly devoted to basic cellular functions, the latter more to developmental functions.

The pancreatic and duodenal homeobox protein PDX-1 regulates the ductal specific keratin 19 through the degradation of MEIS1 and DNA binding

Background

Pancreas organogenesis is the result of well-orchestrated and balanced activities of transcription factors. The homeobox transcription factor PDX-1 plays a crucial role in the development and function of the pancreas, both in the maintenance of progenitor cells and in determination and maintenance of differentiated endocrine cells. However, the activity of homeobox transcription factors requires coordination with co-factors, such as PBX and MEIS proteins. PBX and MEIS proteins belong to the family of three amino acid loop extension (TALE) homeodomain proteins. In a previous study we found that PDX-1 negatively regulates the transcriptional activity of the ductal specific keratin 19 (Krt19). In this study, we investigate the role of different domains of PDX-1 and elucidate the functional interplay of PDX-1 and MEIS1 necessary for Krt19 regulation.

Methodology/Principal Findings

Here, we demonstrate that PDX-1 exerts a dual manner of regulation of Krt19 transcriptional activity. Deletion studies highlight that the NH₂-terminus of PDX-1 is functionally relevant for the down-regulation of Krt19, as it is required for DNA binding of PDX-1 to the Krt19 promoter. Moreover, this effect occurs independently of PBX. Second, we provide insight on how PDX-1 regulates the Hox co-factor MEIS1 post-transcriptionally. We find specific binding of MEIS1 and MEIS2 to the Krt19 promoter using IP-EMSA, and siRNA mediated silencing of Meis1, but not Meis2, reduces transcriptional activation of Krt19 in primary pancreatic ductal cells. Over-expression of PDX-1 leads to a decreased level of MEIS1 protein, and this decrease is prevented by inhibition of the proteasome.

Conclusions/Significance

Taken together, our data provide evidence for a dual mechanism of how PDX-1 negatively regulates Krt19 ductal specific gene expression. These findings imply that transcription factors may efficiently regulate target gene expression through diverse, non-redundant mechanisms.

MEIS and PBX homeobox proteins in ovarian cancer

Three amino-acid loop extension (TALE) homeobox proteins MEIS and PBX are cofactors for HOX-class homeobox proteins, which control growth and differentiation during embryogenesis and homeostasis. We showed that MEIS and PBX expression are related to cisplatin resistance in ovarian cancer cell lines. Therefore, MEIS1, MEIS2 and PBX expression were investigated immunohistochemically in a tissue microarray (N=232) of ovarian cancers and ovarian surface epithelium (N=15). Results were related to clinicopathologic characteristics and survival. All cancers expressed MEIS1, MEIS2 and PBX in nucleus and cytoplasm. MEIS1 and 2 only stained nuclear in surface epithelium. Nuclear MEIS2 was negatively related to stage, grade and overall survival in univariate analyses. Additionally, MEIS and PBX RNA expression in ovarian surface epithelium and other normal tissues and ovarian cancer versus other tumour types using public array data sets were studied. In ovarian cancer, MEIS1 is highly expressed compared to other cancer types. In conclusion, MEIS and PBX are extensively expressed in ovarian carcinomas and may play a role in ovarian carcinogenesis.

Gene expression analysis of hematopoietic progenitor cells identifies Dlg7 as a potential stem cell gene

Inducible hematopoietic stem/progenitor cell lines represent a model for studying genes involved in self-renewal and differentiation. Here, gene expression was studied in the inducible human CD34+ acute myelogenous leukemia cell line KG1 using oligonucleotide arrays and suppression subtractive cloning. Using this approach, we identified Dlg7, the homolog of the Drosophila Dlg1 tumor suppressor gene, as downregulated at the early stages of KG1 differentiation. Similarly, Dlg7 was expressed in normal purified umbilical cord blood CD34+CD38- progenitors but not in the more committed CD34+CD38+ population. Dlg7 expression was not detected in differentiated cells obtained from hematopoietic colonies, nor was expression detected in purified T-cells, B-cells, and monocytes. When analyzed in different types of stem cells, Dlg7 expression was detected in purified human bone marrow-derived CD133+ progenitor cells, human mesenchymal stem cells, and mouse embryonic stem (ES) cells. Overexpression of Dlg7 in mouse ES cells increased their growth rate and reduced the number of EBs emerging upon differentiation. In addition, the EBs were significantly smaller, indicating an inhibition in differentiation. This inhibition was further supported by higher expression of Bmp4, Oct4, Rex1, and Nanog in EBs overexpressing Dlg7 and lower expression of Brachyury. Finally, the Dlg7 protein was detected in liver and colon carcinoma tumors but not in normal adjacent tissues, suggesting a role for the gene in carcinogenesis. In conclusion, our results suggest that Dlg7 has a role in stem cell survival, in maintaining stem cell properties, and in carcinogenesis. Disclosure of potential conflicts of interest is found at the end of this article.

Hypomorphic mutation of the TALE gene Prep1 (pKnox1) causes a major reduction of Pbx and Meis proteins and a pleiotropic embryonic phenotype

The interaction of Prep1 and Pbx homeodomain transcription factors regulates their activity, nuclear localization, and likely, function in development. To understand the in vivo role of Prep1, we have analyzed an embryonic lethal hypomorphic mutant mouse (Prep1(i/i)). Prep1(i/i) embryos die at embryonic day 17.5 (E17.5) to birth with an overall organ hypoplasia, severe anemia, impaired angiogenesis, and eye anomalies, particularly in the lens and retina. The anemia correlates with delayed differentiation of erythroid progenitors and may be, at least in part, responsible for intrauterine death. At E14.5, Prep1 is present in fetal liver (FL) cMyb-positive cells, whose deficiency causes a marked hematopoietic phenotype. Prep1 is also localized to FL endothelial progenitors, consistent with the observed angiogenic phenotype. Likewise, at the same gestational day, Prep1 is present in the eye cells that bear Pax6, implicated in eye development. The levels of cMyb and Pax6 in FL and in the retina, respectively, are significantly decreased in Prep1(i/i) embryos, consistent with the hematopoietic and eye phenotypes. Concomitantly, Prep1 deficiency results in the overall decrease of protein levels of its related family member Meis1 and its partners Pbx1 and Pbx2. As both Prep1 and Meis interact with Pbx, the overall Prep1/Meis-Pbx DNA-binding activity is strongly reduced in whole Prep1(i/i) embryos and their organs. Our data indicate that Prep1 is an essential gene that acts upstream of and within a Pbx-Meis network that regulates multiple aspects of embryonic development.

MEIS homeobox genes in neuroblastoma

The common pediatric tumor neuroblastoma originates from primitive neural crest-derived precursor cells of the peripheral nervous system. Neuroblastoma especially affects very young children, and can already be present at birth. Its early onset and cellular origin predict the involvement of developmental control genes in neuroblastoma etiology. These genes are indispensable for the tight regulation of normal embryonic development but as a consequence cause cancer and congenital diseases upon mutation or aberrant expression. To date however, the connotation of these genes in neuroblastoma pathogenesis is scant. This review recapitulates data on the MEIS homeobox control genes in cancer and focuses on neuroblastoma.

Ultraconserved elements in insect genomes: a highly conserved intronic sequence implicated in the control of homothorax mRNA splicing

Recently, we identified a large number of ultraconserved (uc) sequences in noncoding regions of human, mouse, and rat genomes that appear to be essential for vertebrate and amniote ontogeny. Here, we used similar methods to identify ultraconserved genomic regions between the insect species Drosophila melanogaster and Drosophila pseudoobscura, as well as the more distantly related Anopheles gambiae. As with vertebrates, ultraconserved sequences in insects appear to occur primarily in intergenic and intronic sequences, and at intron-exon junctions. The sequences are significantly associated with genes encoding developmental regulators and transcription factors, but are less frequent and are smaller in size than in vertebrates. The longest identical, nongapped orthologous match between the three genomes was found within the homothorax (hth) gene. This sequence spans an internal exon-intron junction, with the majority located within the intron, and is predicted to form a highly stable stem-loop RNA structure. Real-time quantitative PCR analysis of different hth splice isoforms and Northern blotting showed that the conserved element is associated with a high incidence of intron retention in hth pre-mRNA, suggesting that the conserved intronic element is critically important in the post-transcriptional regulation of hth expression in Diptera.

MEIS C Termini Harbor Transcriptional Activation Domains That Respond to Cell Signaling

MEIS proteins form heteromeric DNA-binding complexes with PBX monomers and PBX.HOX heterodimers. We have shown previously that transcriptional activation by PBX.HOX is augmented by either protein kinase A (PKA) or the histone deacetylase inhibitor trichostatin A (TSA). To examine the contribution of MEIS proteins to this response, we used the chromatin immunoprecipitation assay to show that MEIS1 in addition to PBX1, HOXA1, and HOXB1 was recruited to a known PBX.HOX target, the Hoxb1 autoregulatory element following Hoxb1 transcriptional activation in P19 cells. Subsequent to TSA treatment, MEIS1 recruitment lagged behind that of HOX and PBX partners. MEIS1A also enhanced the transcriptional activation of a reporter construct bearing the Hoxb1 autoregulatory element after treatment with TSA. The MEIS1 homeodomain and protein-protein interaction with PBX contributed to this activity. We further mapped TSA-responsive and CREB-binding protein-dependent PKA-responsive transactivation domains to the MEIS1A and MEIS1B C termini. Fine mutation of the 56-residue MEIS1A C terminus revealed four discrete regions required for transcriptional activation function. All of the mutations impairing the response to TSA likewise reduced activation by PKA, implying a common mechanistic basis. C-terminal deletion of MEIS1 impaired transactivation without disrupting DNA binding or complex formation with HOX and PBX. Despite sequence similarity to MEIS and a shared ability to form heteromeric complexes with PBX and HOX partners, the PREP1 C terminus does not respond to TSA or PKA. Thus, MEIS C termini possess transcriptional regulatory domains that respond to cell signaling and confer functional differences between MEIS and PREP proteins.

Meis1-mediated apoptosis is caspase dependent and can be suppressed by coexpression of HoxA9 in murine and human cell lines

Coexpression of the homeodomain protein Meis1 and either HoxA7 or HoxA9 is characteristic of many acute myelogenous leukemias. Although Meis1 can be overexpressed in bone marrow long-term repopulating cells, it is incapable of mediating their transformation. Although overexpressing HoxA9 alone transforms murine bone marrow cells, concurrent Meis1 overexpression greatly accelerates oncogenesis. Meis1-HoxA9 cooperation suppresses several myeloid differentiation pathways. We now report that Meis1 overexpression strongly induces apoptosis in a variety of cell types in vitro through a caspase-dependent process. Meis1 requires a functional homeodomain and Pbx-interaction motif to induce apoptosis. Coexpressing HoxA9 with Meis1 suppresses this apoptosis and provides protection from several apoptosis inducers. Pbx1, another Meis1 cofactor, also induces apoptosis; however, coexpressing HoxA9 is incapable of rescuing Pbx-mediated apoptosis. This resistance to apoptotic stimuli, coupled with the previously reported ability to suppress multiple myeloid differentiation pathways, would provide a strong selective advantage to Meis1-HoxA9 coexpressing cells in vivo, leading to leukemogenesis.

Overexpression of PREP-1 in F9 teratocarcinoma cells leads to a functionally relevant increase of PBX-2 by preventing its degradation

To bind DNA and to be retained in the nucleus, PBX proteins must form heterodimeric complexes with members of the MEINOX family. Therefore the balance between PBX and MEINOX must be an important regulatory feature. We show that overexpression of PREP-1 influences the level of PBX-2 protein maintaining the PREP-1-PBX balance. This effect has important functional consequences. F9 teratocarcinoma cells stably transfected with PREP-1 had an increased DNA binding activity to a PREP-PBX-responsive element. Because PREP-1 binds DNA efficiently only when dimerized to PBX, the increased DNA binding activity suggests that the level of PBX might also have increased. Indeed PREP-1-overexpressing cells had a higher level of PBX-2 and PBX-1b proteins. PBX-2 increase did not depend on increased mRNA level or a higher rate of translation but rather because of a protein stabilization process. Indeed, PBX-2 level drastically decreased after 3 h of cycloheximide treatment in control but not in PREP-1-overexpressing cells and the proteasome inhibitor MG132 prevented PBX-2 decay in control cells. Hence, dimerization with PREP-1 appears to decrease proteasomal degradation of PBX-2. Retinoic acid induces differentiation of F9 teratocarcinoma cells with a cascade synthesis of HOX proteins. In PREP-1-overexpressing cells, HOXb1 induction was more sustained (3 days versus 1 day) and the induced level of MEIS-1b, another TALE (three amino acid loop extension) protein involved in embryonal development, was higher. Thus an increase in PREP-1 leads to changes in the fate-determining HOXb1 and has therefore important functional consequences.

Engrailed cooperates with extradenticle and homothorax to repress target genes in Drosophila

Engrailed is a key transcriptional regulator in the nervous system and in the maintenance of developmental boundaries in Drosophila, and its vertebrate homologs regulate brain and limb development. Here, we show that the functions of both of the Hox cofactors Extradenticle and Homothorax play essential roles in repression by Engrailed. Mutations that remove either of them abrogate the ability of Engrailed to repress its target genes in embryos, both cofactors interact directly with Engrailed, and both stimulate repression by Engrailed in cultured cells. We suggest a model in which Engrailed, Extradenticle and Homothorax function as a complex to repress Engrailed target genes. These studies expand the functional requirements for extradenticle and homothorax beyond the Hox proteins to a larger family of non-Hox homeodomain proteins.

Prep2: cloning and expression of a new prep family member

We describe Prep2, a new murine homeobox-containing gene closely related to Prep1. The PREP2 protein belongs to the three amino acid loop extension (TALE) superclass of homeodomain-containing proteins and encodes a polypeptide of 462 residues. As for PREP1, PREP2 binds an appropriate site on DNA as a heterodimer with PBX1A. Northern analysis, immunoblotting, immunohistochemistry, and in situ hybridization show widespread Prep2 expression during organogenesis and in the adult. The data suggest that Prep2 functions to varying degrees in a broad array of tissues and developmental processes.

Meis1a suppresses differentiation by G-CSF and promotes proliferation by SCF: potential mechanisms of cooperativity with Hoxa9 in myeloid leukemia

Hoxa9 and Meis1a are homeodomain transcription factors that heterodimerize on DNA and are down-regulated during normal myeloid differentiation. Hoxa9 and Meis1a cooperate to induce acute myeloid leukemia (AML) in mice, and are coexpressed in human AML. Despite their cooperativity in leukemogenesis, we demonstrated previously that retroviral expression of Hoxa9 alone--in the absence of coexpressed retroviral Meis1 or of expression of endogenous Meis genes--blocks neutrophil and macrophage differentiation of primary myeloid progenitors cultured in granulocyte-macrophage colony-stimulating factor (GM-CSF). Expression of Meis1 alone did not immortalize any factor-dependent marrow progenitor. Because HoxA9-immortalized progenitors still execute granulocytic differentiation in response to granulocyte CSF (G-CSF) and monocyte differentiation in response to macrophage CSF (M-CSF), we tested the possibility that Meis1a cooperates with Hoxa9 by blocking viable differentiation pathways unaffected by Hoxa9 alone. Here we report that Meis1a suppresses G-CSF-induced granulocytic differentiation of Hoxa9-immortalized progenitors, permitting indefinite self-renewal in G-CSF. Meis1a also reprograms Hoxa9-immortalized progenitors to proliferate, rather than die, in response to stem cell factor (SCF) alone. We propose that Meis1a and Hoxa9 are part of a molecular switch that regulates progenitor abundance by suppressing differentiation and maintaining self-renewal in response to different subsets of cytokines during myelopoiesis. The independent differentiation pathways targeted by Hoxa9 and Meis1a prompt a "cooperative differentiation arrest" hypothesis for a subset of leukemia, in which cooperating transcription factor oncoproteins block complementary subsets of differentiation pathways, establishing a more complete differentiation block in vivo.

Novel alternative PBX3 isoforms in leukemia cells with distinct interaction specificities

PBX3 is a member of the PBX family of TALE homeobox genes. The prototypic member, PBX1, was first identified in chromosomal translocations in B-lineage leukemia and is required for normal hematopoiesis. PBX2 and PBX3 were later identified as members of this highly conserved family by their strong homology to PBX1. While the expression pattern of PBX1 is restricted, PBX2 and PBX3 are ubiquitously expressed. Little is known about the functional role of PBX3. Our studies identified two PBX3 transcripts alternative to the canonical forms, PBX3A and PBX3B, resulting from a novel splice in PBX3. These new isoforms, named PBX3C and PBX3D, have been detected in all tissues and cell lines tested. Intriguingly, expression of PBX3D is favored in normal cells, whereas PBX3C expression is favored in leukemia cells. Functional studies showed that PBX3C and PBX3D proteins were unable to interact with the PBX-interacting factor PREP1 and weakly interacted with MEIS proteins. We propose that PBX3C and PBX3D may affect PBX3-mediated transcriptional regulation by acting in opposition to the known PBX proteins through alternative PBX3 complex formation. The identification and characterization of these novel PBX3 isoforms provide a foundation for a better understanding of the biological role of PBX3.

From hematopoiesis to neuropoiesis: evidence of overlapping genetic programs

It is reasonable to propose that gene expression profiles of purified stem cells could give clues for the molecular mechanisms of stem cell behavior. We took advantage of cDNA subtraction to identify a set of genes selectively expressed in mouse adult hematopoietic stem cells (HSC) as opposed to bone marrow (BM). Analysis of HSC-enriched genes revealed several key regulatory gene candidates, including two novel seven transmembrane (7TM) receptors. Furthermore, by using cDNA microarray techniques we found a large set of HSC-enriched genes that are expressed in mouse neurospheres (a population greatly enriched for neural progenitor cells), but not present in terminally differentiated neural cells. In situ hybridization demonstrated that many of them, including one HSC-enriched 7TM receptor, were selectively expressed in the germinal zones of fetal and adult brain, the regions harboring mouse neural stem cells. We propose that at least some of the transcripts that are selectively and commonly expressed in two or more types of stem cells define a functionally conserved group of genes evolved to participate in basic stem cell functions, including stem cell self-renewal.

Cloning and developmental expression of a zebrafish meis2 homeobox gene

We show here that a zebrafish meis2 gene homolog has a dynamic expression pattern in the developing mesoderm and central nervous system. Meis family homeodomain proteins are known to act as cofactors with other homeodomain proteins. We find expression of meis2.1 in the developing zebrafish hindbrain and somites, correlating with reported sites of zebrafish hox gene expression, as well as in presumptive cerebellum, midbrain, retina and ventral forebrain. The expression pattern shares some, but not all, features with that of murine Meis2.

Xmeis1, a protooncogene involved in specifying neural crest cell fate in Xenopus embryos

Meis1 (Myeloid Ecotropic viral Integration Site 1) is a homeobox gene that was originally isolated as a common site of viral integration in myeloid tumors of the BXH-2 recombinant inbred mice strain. We previously isolated a Xenopus homolog of Meis1 (Xmeis1). Here we show that Xmeis1 may play a significant role in neural crest development. In developing Xenopus embryos, Xmeis1 displays a broad expression pattern, but strong expression is observed in tissue of neural cell fate, such as midbrain, hindbrain, the dorsal portion of the neural tube, and neural crest derived branchial arches. In animal cap explants, overexpression of Xmeis1b, an alternatively spliced form of Xmeis1, induces expression of neural crest marker genes in the absence of mesoderm. Moreover, Xmeis1b induces XGli-3 and XZic3, pre-pattern genes involved at the earliest stages of neural crest development, and like these two genes, can induce ectopic pigmented cell masses when overexpressed in developing embryos. Misexpression of Xmeis1b also induces ectopic expression of neural crest markers along the antero-posterior axis of the neural tube in developing Xenopus embryos. In contrast, Xmeis1a, another splice variant, is much less effective at inducing these effects. These data suggest that Xmeis1b is involved in neural crest cell fate specification during embryogenesis, and can functionally intersect with the Gli/Zic signal transduction pathway.

The homeobox gene MEIS1 is amplified in IMR-32 and highly expressed in other neuroblastoma cell lines

Neuroblastoma is a childhood tumour of the sympathetic nervous system that demonstrates striking clinical heterogeneity. In order to determine which genes are abnormally expressed in neuroblastoma, we screened regions of amplification from the short arm of chromosome 2 in the neuroblastoma cell line IMR-32 and found that the homeobox gene, myeloid ecotropic integration site 1 (MEIS1), is highly amplified. MEIS1 normally maps to chromosome band 2p14. High expression of MEIS1 without amplification was also found in other neuroblastoma cell lines, with and without MYCN amplification, and in medulloblastoma and crythroleukaemia cell lines. MEIS1 is highly expressed in cerebellum and ubiquitously expressed in normal immunohaematopoietic tissues and is thought to be important in cell proliferation and differentiation. While several lines of evidence point towards a role for homeobox genes in the development of other malignancies, this is the first report showing the amplification of a homeobox gene in neuroblastoma.

Direct interaction of two homeoproteins, homothorax and extradenticle, is essential for EXD nuclear localization and function

The Drosophila Homothorax (HTH) and Extradenticle (EXD) are two homeoproteins required in a number of developmental processes. EXD can function as a cofactor to Hox proteins. Its nuclear localization is dependent on HTH. In this study we present evidence of in vivo physical interaction between HTH and EXD, mediated primarily through an evolutionarily conserved MH domain in HTH. This interaction is essential for the mutual stabilization of both proteins, for EXD nuclear localization, and for the cooperative DNA binding of the EXD-HTH heterodimer. Some in vivo functions require both EXD and HTH in the nucleus, suggesting that the EXD-HTH complex may function as a transcriptional regulator.

Developmental patterning genes and their conserved functions: from model organisms to humans

Molecular and genetic evidence accumulated during the past 20 years in the field of developmental biology indicates that different animals possess many common genetic systems for embryonic patterning. In this review we describe the conserved functions of such developmental patterning genes and their relevance for human pathological conditions. Special attention is given to the Hox genetic system, involved in establishing cell identities along the anterior-posterior axis of all higher metazoans. We also describe other conserved genetic systems, such as the involvement of Pax6 genes in eye development and the role of Nkx2.5-type proteins in heart development. Finally, we outline some fascinating problems at the forefront of the studies of developmental patterning genes and show how knowledge obtained from model genetic organisms such as Drosophila helps to explain normal human morphogenesis and the genetic basis of some birth defects.

Regulation of Hox target genes by a DNA bound Homothorax/Hox/Extradenticle complex

To regulate their target genes, the Hox proteins of Drosophila often bind to DNA as heterodimers with the homeodomain protein Extradenticle (EXD). For EXD to bind DNA, it must be in the nucleus, and its nuclear localization requires a third homeodomain protein, Homothorax (HTH). Here we show that a conserved N-terminal domain of HTH directly binds to EXD in vitro, and is sufficient to induce the nuclear localization of EXD in vivo. However, mutating a key DNA binding residue in the HTH homeodomain abolishes many of its in vivo functions. HTH binds to DNA as part of a HTH/Hox/EXD trimeric complex, and we show that this complex is essential for the activation of a natural Hox target enhancer. Using a dominant negative form of HTH we provide evidence that similar complexes are important for several Hox- and exd-mediated functions in vivo. These data suggest that Hox proteins often function as part of a multiprotein complex, composed of HTH, Hox, and EXD proteins, bound to DNA.

HOXA9 forms triple complexes with PBX2 and MEIS1 in myeloid cells

Aberrant activation of the HOX, MEIS, and PBX homeodomain protein families is associated with leukemias, and retrovirally driven coexpression of HOXA9 and MEIS1 is sufficient to induce myeloid leukemia in mice. Previous studies have demonstrated that HOX-9 and HOX-10 paralog proteins are unique among HOX homeodomain proteins in their capacity to form in vitro cooperative DNA binding complexes with either the PBX or MEIS protein. Furthermore, PBX and MEIS proteins have been shown to form in vivo heterodimeric DNA binding complexes with each other. We now show that in vitro DNA site selection for MEIS1 in the presence of HOXA9 and PBX yields a consensus PBX-HOXA9 site. MEIS1 enhances in vitro HOXA9-PBX protein complex formation in the absence of DNA and forms a trimeric electrophoretic mobility shift assay (EMSA) complex with these proteins on an oligonucleotide containing a PBX-HOXA9 site. Myeloid cell nuclear extracts produce EMSA complexes which appear to contain HOXA9, PBX2, and MEIS1, while immunoprecipitation of HOXA9 from these extracts results in coprecipitation of PBX2 and MEIS1. In myeloid cells, HOXA9, MEIS1, and PBX2 are all strongly expressed in the nucleus, where a portion of their signals are colocalized within nuclear speckles. However, cotransfection of HOXA9 and PBX2 with or without MEIS1 minimally influences transcription of a reporter gene containing multiple PBX-HOXA9 binding sites. Taken together, these data suggest that in myeloid leukemia cells MEIS1 forms trimeric complexes with PBX and HOXA9, which in turn can bind to consensus PBX-HOXA9 DNA targets.

Transcription mapping in a medulloblastoma breakpoint interval and Smith-Magenis syndrome candidate region: identification of 53 transcriptional units and new candidate genes

The chromosomal band 17p11.2 is associated with a number of neurological disorders and malignant diseases. This region is also characterized by the presence of complex repeat elements that are probably responsible for the frequent occurrence of interstitial deletions, duplications, and isochromosome formation. In the course of the molecular analysis of this interval, an integrated map with YACs, PACs, and cosmids covering approximately 6 Mb was established. Focusing on the 1.4-Mb interval containing the Smith-Magenis syndrome critical region and the breakpoint region for medulloblastomas, we constructed a detailed transcript map between the marker PS2 and the proximal CMT1A repeat. FISH analysis of the PACs allowed determination of the position of the transcripts with respect to the SMS critical region and the presumptive chromosomal breakpoint in medulloblastomas. One PAC (G21100) provided evidence for the presence of a novel complex repeat unit, indicating that there are at least three independent repeat elements within 2 Mb. Five genes were mapped to clone G21100 and are likely to form part of this novel complex sequence repeat. In summary, 53 new transcripts were isolated by using cDNA selection and exon trapping. This included 8 known but previously unmapped genes and 45 novel transcripts. The expression profile of 21 transcripts was determined by RT-PCR. Based on their homologies to known genes or proteins, some of the novel genes are considered candidate genes either for malignant diseases or for the Smith-Magenis syndrome.

Functional and cooperative interactions between the homeodomain PDX1, Pbx, and Prep1 factors on the somatostatin promoter

Expression of the somatostatin gene in endocrine pancreatic cells is controlled by several regulatory cis-elements located in the promoter region. Among these, the adjacent UE-A and TSEI elements, located from -113 to -85 relative to the transcription initiation site, function in combination and act as a pancreas-specific mini-enhancer. The TSEI element is recognized by the pancreatic homeodomain factor PDX1. In the present study, we show that the UE-A element binds a heterodimeric complex composed of a Pbx factor and the Prep1 protein, both belonging to the atypical three-amino acid loop extension homeodomain family. Recombinant Pbx1 and Prep1 proteins bind cooperatively to the UE-A site, whereas neither protein can bind this site alone. Transient transfection experiments reveal that both Pbx1 and Prep1 are required to generate a strong transcriptional activation from the UE-A element when this element is inserted close to the TATA box. In contrast, in the context of the intact somatostatin promoter or mini-enhancer, Pbx1 and Prep1 alone have no effect, but they produce a drastic activation when the pancreatic homeodomain factor PDX1 is also coexpressed. Thus, the activity of the somatostatin mini-enhancer is mediated by a cooperative interaction between the Pbx-Prep1 heterodimeric complex and the pancreatic factor PDX1.

A Meis family protein caudalizes neural cell fates in Xenopus

A homologue of the Drosophila homothorax (hth) gene, Xenopus Meis3 (XMeis3), was cloned from Xenopus laevis. XMeis3 is expressed in a single stripe of cells in the early neural plate stage. By late neurula, the gene is expressed predominantly in rhombomeres two, three and four, and in the anterior spinal cord. Ectopic expression of RNA encoding XMeis3 protein causes anterior neural truncations with a concomitant expansion of hindbrain and spinal cord. Ectopic XMeis3 expression inhibits anterior neural induction in neuralized animal cap ectoderm explants without perturbing induction of pan-neural markers. In naive animal cap ectoderm, ectopic XMeis3 expression activates transcription of the posteriorly expressed neural markers, but not pan-neural markers. These results suggest that caudalizing proteins, such as XMeis3, can alter A-P patterning in the nervous system in the absence of neural induction. Regionally expressed proteins like XMeis3 could be required to overcome anterior signals and to specify posterior cell fates along the A-P axis.

An endocrine-exocrine switch in the activity of the pancreatic homeodomain protein PDX1 through formation of a trimeric complex with PBX1b and MRG1 (MEIS2)

HOX proteins and some orphan homeodomain proteins form complexes with either PBX or MEIS subclasses of homeodomain proteins. This interaction can increase the binding specificity and transcriptional effectiveness of the HOX partner. Here we show that specific members of both PBX and MEIS subclasses form a multimeric complex with the pancreatic homeodomain protein PDX1 and switch the nature of its transcriptional activity. The two activities of PDX1 are exhibited through the 10-bp B element of the transcriptional enhancer of the pancreatic elastase I gene (ELA1). In pancreatic acinar cells the activity of the B element requires other elements of the ELA1 enhancer; in beta-cells the B element can activate a promoter in the absence of other enhancer elements. In acinar cell lines the activity is mediated by a complex comprising PDX1, PBX1b, and MRG1 (MEIS2). In contrast, beta-cell lines are devoid of PBX1b and MRG1, so that a trimeric complex does not form, and the beta-cell-type activity is mediated by PDX1 without PBX1b and MRG1. The presence of specific nuclear isoforms of PBX and MEIS is precisely regulated in a cell-type-specific manner. The beta-cell-type activity can be detected in acinar cells if the B element is altered to retain binding of PDX1 but prevent binding of the PDX1-PBX1b-MRG1 complex. These observations suggest that association with PBX and MEIS partners controls the nature of the transcriptional activity of the organ-specific PDX1 transcription factor in exocrine versus endocrine cells.

Hox proteins meet more partners

The Hox genes are clustered sets of homeobox-containing genes that play a central role in animal development. Recent genetic and molecular data suggest that Hox proteins interact with pre-existing homeodomain protein complexes. These complexes may help to regulate Hox activity and Hox specificity, and help cells to interpret signaling cascades during development.

Hoxa9 transforms primary bone marrow cells through specific collaboration with Meis1a but not Pbx1b

Hoxa9, Meis1 and Pbx1 encode homeodomaincontaining proteins implicated in leukemic transformation in both mice and humans. Hoxa9, Meis1 and Pbx1 proteins have been shown to physically interact with each other, as Hoxa9 cooperatively binds consensus DNA sequences with Meis1 and with Pbx1, while Meis1 and Pbx1 form heterodimers in both the presence and absence of DNA. In this study, we sought to determine if Hoxa9 could transform hemopoietic cells in collaboration with either Pbx1 or Meis1. Primary bone marrow cells, retrovirally engineered to overexpress Hoxa9 and Meis1a simultaneously, induced growth factor-dependent oligoclonal acute myeloid leukemia in <3 months when transplanted into syngenic mice. In contrast, overexpression of Hoxa9, Meis1a or Pbx1b alone, or the combination of Hoxa9 and Pbx1b failed to transform these cells acutely within 6 months post-transplantation. Similar results were obtained when FDC-P1 cells, engineered to overexpress these genes, were transplanted to syngenic recipients. Thus, these studies demonstrate a selective collaboration between a member of the Hox family and one of its DNA-binding partners in transformation of hemopoietic cells.

Members of the meis1 and pbx homeodomain protein families cooperatively bind a cAMP-responsive sequence (CRS1) from bovine CYP17

The mammalian Pbx homeodomain proteins provide specificity and increased DNA binding affinity to other homeodomain proteins. A cAMP-responsive sequence (CRS1) from bovine CYP17 has previously been shown to be a binding site for Pbx1. A member of a second mammalian homeodomain family, Meis1, is now also demonstrated to be a CRS1-binding protein upon purification using CRS1 affinity chromatography. CRS1 binding complexes from Y1 adrenal cell nuclear extract contain both Pbx1 and Meis1. This is the first transcriptional regulatory element reported as a binding site for members of the Meis1 homeodomain family. Pbx1 and Meis1 bind cooperatively to CRS1, whereas neither protein can bind this element alone. Mutagenesis of the CRS1 element indicates a binding site for Meis1 adjacent to the Pbx site. All previously identified Pbx binding partners have Pbx interacting motifs that contain a tryptophan residue amino-terminal to the homeodomain that is required for cooperative binding to DNA with Pbx. Members of the Meis1 family contain one tryptophan residue amino-terminal to the homeodomain, but site-directed mutagenesis indicates that this residue is not required for cooperative CRS1 binding with Pbx. Thus, the Pbx-Meis1 interaction is unique among Pbx complexes. Meis1 also cooperatively binds CRS1 with the Pbx homologs extradenticle from Drosophila melanogaster and ceh-20 from Caenorhabditis elegans, indicating that this interaction is evolutionarily conserved. Thus, CYP17 CRS1 is a transcriptional regulatory element containing both Pbx and Meis1 binding sites, which permit these two homeodomain proteins to bind and potentially regulate cAMP-dependent transcription through this sequence.

Dorsotonals/homothorax, the Drosophila homologue of meis1, interacts with extradenticle in patterning of the embryonic PNS

The homeotic genes of the bithorax complex are required, among other things, for establishing the patterns of sensory organs in the embryonic peripheral nervous system (PNS). However, the molecular mechanisms by which these genes affect pattern formation in the PNS are not understood and other genes that function in this pathway are not characterized. Here we report the phenotypic and molecular analysis of one such gene, homothorax (hth; also named dorsotonals). Mutations in the hth gene seem to alter the identity of the abdominal chordotonal neurons, which depend on Abd-A for their normal development. However, these mutations do not alter the expression of the abd-A gene, suggesting that hth may be involved in modulating abd-A activity. We have generated multiple mutations in the hth locus and cloned the hth gene. hth encodes a homeodomain-containing protein that is most similar to the murine proto-oncogene meis1. The hth gene is expressed throughout embryonic development in a spatially restricted pattern, which is modulated in abdominal segments by abd-A and Ubx. The spatial distribution of the HTH protein during embryonic development is very similar to the distribution of the Extradenticle (EXD) protein, a known modulator of homeotic gene activity. Here we show that the PNS phenotype of exd mutant embryos is virtually indistinguishable from that of hth mutant embryos and does not simply follow the homeotic transformations observed in the epidermis. We also show that the HTH protein is present in extremely low levels in embryos lacking exd activity as compared to wild-type embryos. In contrast, the EXD protein is present in fairly normal levels in hth mutant embryos, but fails to accumulate in nuclei and remains cytoplasmic. Ectopic expression of hth can drive ectopic nuclear localization of EXD. Based on our observations we propose that the genetic interactions between hth and exd serve as a novel mechanism for regulating homeotic protein activity in embryonic PNS development.

The Homothorax homeoprotein activates the nuclear localization of another homeoprotein, extradenticle, and suppresses eye development in Drosophila

The Extradenticle (Exd) protein in Drosophila acts as a cofactor to homeotic proteins. Its nuclear localization is regulated. We report the cloning of the Drosophila homothorax (hth) gene, a homolog of the mouse Meis1 proto-oncogene that has a homeobox related to that of exd. Comparison with Meis1 finds two regions of high homology: a novel MH domain and the homeodomain. In imaginal discs, hth expression coincides with nuclear Exd. hth and exd also have virtually identical, mutant clonal phenotypes in adults. These results suggest that hth and exd function in the same pathway. We show that hth acts upstream of exd and is required and sufficient for Exd protein nuclear localization. We also show that hth and exd are both negative regulators of eye development; their mutant clones caused ectopic eye formation. Targeted expression of hth, but not of exd, in the eye disc abolished eye development completely. We suggest that hth acts with exd to delimit the eye field and prevent inappropriate eye development.

Analysis of TALE superclass homeobox genes (MEIS, PBC, KNOX, Iroquois, TGIF) reveals a novel domain conserved between plants and animals

A new Caenorhabditis elegans homeobox gene, ceh-25, is described that belongs to the TALE superclass of atypical homeodomains, which are characterized by three extra residues between helix 1 and helix 2. ORF and PCR analysis revealed a novel type of alternative splicing within the homeobox. The alternative splicing occurs such that two different homeodomains can be generated, which differ in their first 25 amino acids. ceh-25 is an orthologue of the vertebrate Meis genes and it shares a new conserved domain of 130 amino acids with them. A thorough analysis of all TALE homeobox genes was performed and a new classification is presented. Four TALE classes are identified in animals: PBC, MEIS, TGIF and IRO (Iroquois); two types in fungi: the mating type genes (M-ATYP) and the CUP genes; and two types in plants: KNOX and BEL. The IRO class has a new conserved motif downstream of the homeodomain. For the KNOX class, a conserved domain, the KNOX domain, was defined upstream of the homeodomain. Comparison of the KNOX domain and the MEIS domain shows significant sequence similarity revealing the existence of an archetypal group of homeobox genes that encode two associated conserved domains. Thus TALE homeobox genes were already present in the common ancestor of plants, fungi and animals and represent a branch distinct from the typical homeobox 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.