The Runx3 transcription factor regulates development and survival of TrkC dorsal root ganglia neurons

Shared and distinct roles mediated through C-terminal subdomains of acute myeloid leukemia/Runt-related transcription factor molecules in murine development

AML1/Runx1 is a frequent target of human leukemia-associated gene aberration and encodes a transcription factor with nonredundant biologic functions in initial development of definitive hematopoiesis, T-cell development, and steady-state platelet production. AML1/Runx1 and 2 closely related family genes, AML2/Runx3 and AML3/Runx2/Cbfa1, present in mammals, comprise the Runt-domain transcription factor family. Although they have similar structural and biochemical properties, gene-targeting experiments have identified distinct biologic roles. To directly determine the presence of functional overlap among runt-related transcription factor (Runx) family molecules, we replaced the C-terminal portion of acute myeloid leukemia 1 (AML1) with that derived from its family members, which are variable in contrast to conserved Runt domain, using the gene knock-in method. We found that C-terminal portions of either AML2 or AML3 could functionally replace that of AML1 for myeloid development in culture and within the entire mouse. However, while AML2 substituted for AML1 could effectively rescue lymphoid lineages, AML3 could not, resulting in a smaller thymus and lymphoid deficiency in peripheral blood. Substitution by the C-terminal portion of AML3 also led to high infantile mortality and growth retardation, suggesting that AML1 has as yet unidentified effects on these phenotypes. Thus, the C-terminal portions of Runx family members have both similar and distinct biologic functions.

Control of RUNX2 isoform expression: The role of promoters and enhancers

The three mammalian RUNX genes constitute the family of runt domain transcription factors that are involved in the regulation of a number of developmental processes such as haematopoiesis, osteogenesis and neuronal differentiation. All three genes show a complex temporo-spatial pattern of expression. Since the three proteins are probably mutually interchangeable with regard to function, most of the specificity of each family member seems to be based on a tightly controlled regulation of expression. While RUNX gene expression is driven by two promoters for each gene, the promoter sequence alone does not seem to suffice for a proper expressional control. This review focuses on the available evidence for the existence of such control mechanisms and studies aiming at discovering cis-acting regulatory sequences of the RUNX2 gene.

Runx3 is involved in hair shape determination

Transcriptional regulators of the Runx family play critical roles in normal organ development and, when mutated, lead to genetic diseases and cancer. Runx3 functions during cell lineage decisions in thymopoiesis and neurogenesis and mediates transforming growth factor-beta signaling in dendritic cells. Here, we study the function of Runx3 in the skin and its appendages, primarily the hair follicle, during mouse development. Runx3 is expressed predominantly in the dermal compartment of the hair follicles as they form and during the hair cycle, as well as in the nail and sweat gland skin appendages. Distinct expression is also detected periodically in isolated cells of the epidermis and in melanocytes, populating the hair bulb. Runx3-deficient mice display a perturbation of the normal hair coat, which we show to be due to hair type and hair shape changes. Thus, one of the functions of Runx3 in skin may be to regulate the formation of the epithelial derived structural hair by affecting dermal to epidermal interactions.

Overlapping expression of Runx1(Cbfa2) and Runx2(Cbfa1) transcription factors supports cooperative induction of skeletal development

Identifying the genetic pathways that regulate skeletal development is necessary to correct a variety of cartilage and bone abnormalities. The Runx family of transcription factors play a fundamental role in organ development and cell differentiation. Initial studies have shown that both Runx1 and Runx2 are expressed in pre-chondrogenic mesenchyme of the developing embryo at E12.5. Abrogation of the Runx2 gene completely inhibits bone formation yet the cartilage anlagen in these mice is fully formed. In the present study, we hypothesized that Runx1 may compensate for the lack of Runx2 in vivo to induce the early stages of skeletal formation and development. Histologic beta-gal stained sections using the Runx1(+/-)-Lac-Z mice demonstrate Runx1 promoter activity in pre-chondrocytic cell populations. In situ hybridization using Runx1 and Runx2 specific probes indicate that both factors are expressed in mesenchymal stem cell progenitors during early embryonic development. During later stages of mouse skeletal formation, Runx1 is excluded from the hypertrophic cartilage while Runx2 is present in these matured chondrocyte populations. Quantification of Runx expression by real time RT-PCR and Western blot analyses reveals that Runx1 and Runx2 are differentially modulated during embryogenesis suggesting a temporal role for each of these transcriptional regulators during skeletal formation. We provide evidence that haploinsufficiency results in normal appearing embryo skeletons of heterozygote Runx2 and Runx1 mutant mouse models; however, a delay in bone formation was identified in the calvarium. In summary, our results support a function for Runx1 and Runx2 during skeletal development with a possible role for Runx1 in mediating early events of endochondral and intramembranous bone formation, while Runx2 is a potent inducer of late stages of chondrocyte and osteoblast differentiation.

Runx2: A master organizer of gene transcription in developing and maturing osteoblasts

Runx2 is essential for osteoblast development and proper bone formation. A member of the Runt domain family of transcription factors, Runx2 binds specific DNA sequences to regulate transcription of numerous genes and thereby control osteoblast development from mesenchymal stem cells and maturation into osteocytes. Although necessary for gene transcription and osteoblast development, Runx2 is not sufficient for optimal gene expression or bone formation. Runx2 cooperates with numerous proteins, including transcription factors and cofactors, is posttranslationally modified, and associates with the nuclear matrix to integrate a variety of signals and organize crucial events during osteoblast development and maturation. Consistent with its role as a master organizer, alterations in Runx2 expression levels are associated with skeletal diseases. Runx2 haploinsufficiency causes cleidocranial dysplasia, while Runx2 overexpression is common in many bone-metastatic cancers. In this review, we summarize the molecular mechanisms by which Runx2 integrates signals through coregulatory interactions, and discuss how its role as a master organizer may shift depending on promoter structure, developmental cues, and cellular context.

A chemoattractant role for NT-3 in proprioceptive axon guidance

Neurotrophin-3 (NT-3) is required for proprioceptive neuron survival. Deletion of the proapoptotic gene Bax in NT-3 knockout mice rescues these neurons and allows for examination of their axon growth in the absence of NT-3 signaling. TrkC-positive peripheral and central axons from dorsal root ganglia follow proper trajectories and arrive in close proximity to their targets but fail to innervate them. Peripherally, muscle spindles are absent and TrkC-positive axons do not enter their target muscles. Centrally, proprioceptive axons branch in ectopic regions of the spinal cord, even crossing the midline. In vitro assays reveal chemoattractant effects of NT-3 on dorsal root ganglion axons. Our results show that survival factor NT-3 acts as a short-distance axon guidance molecule for muscle sensory afferents as they approach their proper targets.

An enigmatic fourth runt domain gene in the fugu genome: ancestral gene loss versus accelerated evolution

Background

The runt domain transcription factors are key regulators of developmental processes in bilaterians, involved both in cell proliferation and differentiation, and their disruption usually leads to disease. Three runt domain genes have been described in each vertebrate genome (the RUNX gene family), but only one in other chordates. Therefore, the common ancestor of vertebrates has been thought to have had a single runt domain gene.

Results

Analysis of the genome draft of the fugu pufferfish (Takifugu rubripes) reveals the existence of a fourth runt domain gene, FrRUNT, in addition to the orthologs of human RUNX1, RUNX2 and RUNX3. The tiny FrRUNT packs six exons and two putative promoters in just 3 kb of genomic sequence. The first exon is located within an intron of FrSUPT3H, the ortholog of human SUPT3H, and the first exon of FrSUPT3H resides within the first intron of FrRUNT. The two gene structures are therefore "interlocked". In the human genome, SUPT3H is instead interlocked with RUNX2. FrRUNT has no detectable ortholog in the genomes of mammals, birds or amphibians. We consider alternative explanations for an apparent contradiction between the phylogenetic data and the comparison of the genomic neighborhoods of human and fugu runt domain genes. We hypothesize that an ancient RUNT locus was lost in the tetrapod lineage, together with FrFSTL6, a member of a novel family of follistatin-like genes.

Conclusions

Our results suggest that the runt domain family may have started expanding in chordates much earlier than previously thought, and exemplify the importance of detailed analysis of whole-genome draft sequence to provide new insights into gene evolution.

Loss of Runx3 function in leukocytes is associated with spontaneously developed colitis and gastric mucosal hyperplasia

RUNX transcription factors are key regulators of lineage-specific gene expression and might be involved in autoimmune diseases. Runx3 plays a role during the development of sensory neurons and T cells and regulates transforming growth factor beta (TGF-beta) signaling in dendritic cells. Here, we report that at 4 weeks of age, Runx3 knockout (KO) mice spontaneously develop inflammatory bowel disease (IBD) characterized by leukocyte infiltration, mucosal hyperplasia, formation of lymphoid clusters, and increased production of IgA. Additionally, at a considerably older age (8 months), the KO mice also develop progressive hyperplasia of the gastric mucosa associated with disturbed epithelial differentiation and cellular hyaline degeneration. Analysis of cytokines in the colonic mucosa of Runx3 KO mice revealed a mixed T helper 1/T helper 2 response. By using immunohistochemistry and RNA in situ hybridization, Runx3 expression in the gastrointestinal tract is detected in lymphoid and myeloid populations but not in the epithelium. The data indicate that loss of leukocytic cell-autonomous function of Runx3 results in IBD and gastric lesion in the KO mice. IBD in humans is viewed as a complex genetic disorder. Several susceptibility loci were identified on different human chromosomes including the chromosomal region 1p36 where RUNX3 resides. It is thus tempting to speculate that mutations in RUNX3 may constitute an IBD risk factor in humans.

Conservation and expression of an alternative 3' exon of Runx2 encoding a novel proline-rich C-terminal domain

The Runx2 (Cbfa1, Aml3, PEBP2alphaA) gene plays an essential role in bone development and is one of a three-member family of closely related genes that encode the alpha-chain DNA binding components of the heterodimeric core binding factor complex. While all three mammalian Runx genes share a complex dual promoter structure (P1, P2) and display alternative splicing, a distinctive feature of Runx2 is the potential to encode larger isoforms in which the C-terminal domain encoded by the standard 3' terminal exon (exon 6) is replaced by an extended 200-201 amino acid C-terminal sequence including an extensive proline-rich domain and a C-terminal amphipathic helix. We report that the novel exon that gives rise to these variants (exon 6.1) is located over 100 kb downstream of exon 6 in the mouse, rat and human genomes. Exon 6.1 spans a CpG-rich island, and human/rodent conservation is evident through the coding sequence and the 3' untranslated region (UTR). Reverse transcriptase polymerase chain reaction (RT-PCR) and blot hybridisation analyses reveal that exon 6.1 is utilised at low levels in all mouse tissues and cell lines that express Runx2, regardless of which promoter is active, giving Runx2 the potential to encode more than 12 distinct isoforms. RT-PCR analysis of human RUNX2 exon 6.1 expression shows that utilisation of this exon is also conserved. In vitro transcription/translation of cDNAs encoding several exon 6.1 isoforms reveals that the novel Runx proteins are able to bind specifically to canonical Runx DNA target sequences. Antibodies raised to the unique C-terminal domain were shown to be reactive by immunoprecipitation and immunoblot assay, and were used in confocal immunofluorescence microscopy to reveal low level cytoplasmic staining in osteosarcoma and lymphoma cells that express high levels of Runx2 mRNA. However, reactive protein could not be detected in immunoblots of extracts from either cell type, suggesting that these proteins are unstable in lymphoid and osteosarcoma cells. In conclusion, the conservation and widespread utilisation of Runx2 exon 6.1 suggest that its encoded isoforms play an as yet undetermined role in mammalian development.

RNT-1, the C. elegans homologue of mammalian RUNX transcription factors, regulates body size and male tail development

The rnt-1 gene is the only Caenorhabditis elegans homologue of the mammalian RUNX genes. Several lines of molecular biological evidence have demonstrated that the RUNX proteins interact and cooperate with Smads, which are transforming growth factor-beta (TGF-beta) signal mediators. However, the involvement of RUNX in TGF-beta signaling has not yet been supported by any genetic evidence. The Sma/Mab TGF-beta signaling pathway in C. elegans is known to regulate body length and male tail development. The rnt-1(ok351) mutants show the characteristic phenotypes observed in mutants of the Sma/Mab pathway, namely, they have a small body size and ray defects. Moreover, RNT-1 can physically interact with SMA-4 which is one of the Smads in C. elegans, and double mutant animals containing both the rnt-1(ok351) mutation and a mutation in a known Sma/Mab pathway gene displayed synergism in the aberrant phenotypes. In addition, lon-1(e185) mutants was epistatic to rnt-1(ok351) mutants in terms of long phenotype, suggesting that lon-1 is indeed downstream target of rnt-1. Our data reveal that RNT-1 functionally cooperates with the SMA-4 proteins to regulate body size and male tail development in C. elegans.

Early B cell factor cooperates with Runx1 and mediates epigenetic changes associated with mb-1 transcription

Cd79a (called mb-1 here) encodes the Ig-alpha signaling component of the B cell receptor. The early B cell-specific mb-1 promoter was hypermethylated at CpG dinucleotides in hematopoietic stem cells but became progressively unmethylated as B cell development proceeded. The transcription factor Pax5 activated endogenous mb-1 transcription in a plasmacytoma cell line, but could not when the promoter was methylated. In this context, early B cell factor (EBF), a transcription factor required for B lymphopoiesis, potentiated activation of mb-1 by Pax5. EBF and the basic helix-loop-helix transcription factor E47 each contributed to epigenetic modifications of the mb-1 promoter, including CpG demethylation and nucleosomal remodeling. EBF function was enhanced by interaction with the transcription factor Runx1. These data suggest a molecular basis for the hierarchical dependence of Pax5 function on EBF and E2A in B lymphocyte development.

Groucho homologue Grg5 interacts with the transcription factor Runx2-Cbfa1 and modulates its activity during postnatal growth in mice

Runx2-Cbfa1, a Runt transcription factor, plays important roles during skeletal development. It is required for differentiation and function of osteoblasts. In its absence, chondrocyte hypertrophy is severely impaired and there is no vascularization of cartilage templates during skeletal development. These tissue-specific functions of Runx2 are likely to be dependent on its interaction with other proteins. We have therefore searched for proteins that may modulate the activity of Runx2. The yeast two-hybrid system was used to identify a groucho homologue, Grg5, as a Runx2-interacting protein. Grg5 enhances Runx2 activity in a cell culture-based assay and by analyses of postnatal growth in mice we demonstrate that Grg5 and Runx2 interact genetically. We also show that Runx2 haploinsufficiency in the absence of Grg5 results in a more severe delay in ossification of cranial sutures and fontanels than occurs with Runx2 haploinsufficiency on a wild-type background. Finally, we find shortening of the proliferative and hypertrophic zones, and expansion of the resting zone in the growth plates of Runx2(+/-) Grg5(-/-) mice that are associated with reduced Ihh expression and Indian hedgehog (Ihh) signaling. We therefore conclude that Grg5 enhances Runx2 activity in vivo.

AML1/Runx1 is important for the development of hindbrain cholinergic branchiovisceral motor neurons and selected cranial sensory neurons

The mechanisms that regulate the acquisition of distinctive neuronal traits in the developing nervous system are poorly defined. It is shown here that the mammalian runt-related gene Runx1 is expressed in selected populations of postmitotic neurons of the embryonic central and peripheral nervous systems. These include cholinergic branchial and visceral motor neurons in the hindbrain, restricted populations of somatic motor neurons of the median and lateral motor columns in the spinal cord, as well as nociceptive and mechanoreceptor neurons in trigeminal and vestibulocochlear ganglia. In mouse embryos lacking Runx1 activity, hindbrain branchiovisceral motor neuron precursors of the cholinergic lineage are correctly specified but then fail to progress to a more differentiated state and undergo increased cell death, resulting in a neuronal loss in the mantle layer. In contrast, the development of cholinergic somatic motor neurons is unaffected. Runx1 inactivation also leads to a loss of selected sensory neurons in trigeminal and vestibulocochlear ganglia. These findings uncover previously unrecognized roles for Runx1 in the regulation of mammalian neuronal subtype development.

Transforming Growth Factor-β Stimulates p300-dependent RUNX3 Acetylation, Which Inhibits Ubiquitination-mediated Degradation

The Runt domain transcription factors (RUNXs) play essential roles in normal development and neoplasias. Genetic analyses of animals and humans have revealed the involvement of RUNX1 in hematopoiesis and leukemia, RUNX2 in osteogenesis and cleidocranial dysplasia, and RUNX3 in the development of T-cells and dorsal root ganglion neurons and in the genesis of gastric cancer. Here we report that RUNX3 is a target of the acetyltransferase activity of p300. The p300-dependent acetylation of three lysine residues protects RUNX3 from ubiquitin ligase Smurf-mediated degradation. The extent of the acetylation is up-regulated by the transforming growth factor-beta signaling pathway and down-regulated by histone deacetylase activities. Our findings demonstrate that the level of RUNX3 protein is controlled by the competitive acetylation and deacetylation of the three lysine residues, revealing a new mechanism for the posttranslational regulation of RUNX3 expression.

Runx2 control of organization, assembly and activity of the regulatory machinery for skeletal gene expression

We present an overview of Runx involvement in regulatory mechanisms that are requisite for fidelity of bone cell growth and differentiation, as well as for skeletal homeostasis and the structural and functional integrity of skeletal tissue. Runx-mediated control is addressed from the perspective of support for biological parameters of skeletal gene expression. We review recent findings that are consistent with an active role for Runx proteins as scaffolds for integration, organization and combinatorial assembly of nucleic acids and regulatory factors within the three-dimensional context of nuclear architecture.

Role of RUNX family members in transcriptional repression and gene silencing

RUNX family members are DNA-binding transcription factors that regulate the expression of genes involved in cellular differentiation and cell cycle progression. The RUNX family includes three mammalian RUNX proteins (RUNX1, -2, -3) and two homologues in Drosophila. Experiments in Drosophila and mouse indicate that the RUNX proteins are required for gene silencing of engrailed and CD4, respectively. RUNX-mediated repression involves recruitment of corepressors such as mSin3A and Groucho as well as histone deacetylases. Furthermore, RUNX1 and RUNX3 associate with SUV39H1, a histone methyltransferase involved in gene silencing. RUNX1 is frequently targeted in human leukemia by chromosomal translocations that fuse the DNA-binding domain of RUNX1 to other transcription factors and corepressor molecules. The resulting leukemogenic fusion proteins are transcriptional repressors that form stable complexes with corepressors, histone deacetylases and histone methyltransferases. Thus, transcriptional repression and gene silencing through RUNX1 contribute to the mechanisms of leukemogenesis of the fusion proteins. Therapies directed at the associated cofactors may be beneficial for treatment of these leukemias.

The Runx genes: lineage-specific oncogenes and tumor suppressors

The Runx genes present a challenge to the simple binary classification of cancer genes as oncogenes or tumor suppressors. There is evidence that loss of function of two of the three mammalian Runx genes promotes cancer, but in a highly lineage-restricted manner. In human leukemias, the RUNX1 gene is involved in various chromosomal translocation events that create oncogenic fusion proteins, at least some of which appear to function as dominant-negative inhibitors of the normal gene product. Paradoxically, evidence is mounting that structurally intact Runx genes are also oncogenic when overexpressed. All the three murine genes act as targets for transcriptional activation by retroviral insertional mutagenesis, and the oncogenic potential of Runx2 has been confirmed in transgenic mice. Moreover, the RUNX1 gene is often amplified or overexpressed in cases of acute leukemia. The state of progress in elucidating the oncogenic roles of the Runx genes is the subject of this review, and we draw together recent observations in a tentative model for the effects of Runx deregulation on hematopoietic cell differentiation. We suggest that lineage-specific factors determine the sensitivity to the oncogenic effects of loss or overexpression of Runx factors.

Tumor suppressor activity of RUNX3

Recent analyses have revealed that RUNX family members play important roles in both normal developmental processes and carcinogenesis. Of the three known RUNX family members, RUNX3 has been shown to be involved in neurogenesis of the dorsal root ganglia, T-cell differentiation and tumorigenesis of gastric epithelium. Deletion of the Runx3 locus in mice resulted in hyperplasia of the gastric epithelium due to the stimulation of proliferation and suppression of apoptosis that was accompanied by a reduced sensitivity to TGF-beta1. In primary human gastric cancer specimens, RUNX3 is frequently inactivated by allele loss or gene silencing due to promoter hypermethylation. The tumorigenicity of human gastric cancer cell lines in nude mice decreased as the level of RUNX3 expression increased, which indicates that RUNX3 is a bona fide tumor suppressor of gastric cancers.

Growth regulation of gastric epithelial cells by Runx3

RUNX3: is expressed by gastric epithelial cells throughout development. Mice whose Runx3 gene has been knocked out died soon after birth. In the knockout mouse, gastric epithelia exhibited hyperplasia and epithelial apoptosis was suppressed. Analysis using a primary culture system for the epithelial cells suggested that this is caused by the reduced sensitivity of Runx3-/- gastric epithelial cells to the growth-inhibiting and apoptosis-inducing activities of TGF-beta. In human and mouse gastric cancer cell lines, RUNX3/Runx3 was silenced due to hypermethylation of CpG islands in the promoter region. Exogenous expression of RUNX3 in the cells that do not express the endogenous gene caused an inhibition of growth both in vivo and in vitro. These observations indicate that Runx3 is a major growth regulator of gastric epithelial cells, and that it is deeply involved in gastric tumorigenesis in both humans and mice.

Oncogenic potential of the RUNX gene family: 'overview'

Runt-related (RUNX) gene family is composed of three members, RUNX1/AML1, RUNX2 and RUNX3, and encodes the DNA-binding (alpha) subunits of the Runt domain transcription factor polyomavirus enhancer-binding protein 2 (PEBP2)/core-binding factor (CBF), which is a heterodimeric transcription factor. RUNX1 is most frequently involved in human acute leukemia. RUNX2 shows oncogenic potential in mouse experimental system. RUNX3 is a strong candidate as a gastric cancer tumor suppressor. The beta subunit gene of PEBP2/CBF is also frequently involved in chromosome rearrangements associated with human leukemia. In this Overview, I will summarize how this growing field has been formed and what are the challenging new frontiers for better understanding of the oncogenic potential of this gene family.

Mechanism of leukemogenesis by the inv(16) chimeric gene CBFB/PEBP2B-MHY11

Inv(16)(p13q22) is associated with acute myeloid leukemia subtype M4Eo that is characterized by the presence of myelomonocytic blasts and atypical eosinophils. This chromosomal rearrangement results in the fusion of CBFB and MYH11 genes. CBF beta normally interacts with RUNX1 to form a transcriptionally active nuclear complex. The MYH11 gene encodes the smooth muscle myosin heavy chain. The CBF beta-SMMHC fusion protein is capable of binding to RUNX1 and form dimers and multimers through its myosin tail. Previous results from transgenic mouse models show that Cbfb-MYH11 is able to inhibit dominantly Runx1 function in hematopoiesis, and is a key player in the pathogenesis of leukemia. In recent years, molecular and cellular biological studies have led to the proposal of several models to explain the function of CBF beta-SMMHC. In this review, we will first focus our attention on the molecular mechanisms proposed in the recent publications. We will next examine recent gene expression profiling studies on inv(16) leukemia cells. Finally, we will describe a recent study from one of our labs on the identification of cooperating genes for leukemogenesis with CBFB-MYH11.

Coordinate regulation of cell growth and differentiation by TGF-beta superfamily and Runx proteins

Runx proteins regulate various biological processes, including growth and differentiation of hematopoietic cells, lymphocytes, osteoblasts, and gastric epithelial cells. Some of the biological activities of Runx proteins are reminiscent of those of transforming growth factor (TGF)-beta superfamily cytokines. Consistent with this notion, receptor-regulated Smads (R-Smads), signal mediators of the TGF-beta superfamily cytokines, and Runx proteins have been shown to physically interact with each other. R-Smads activated by TGF-beta and Runx proteins cooperatively induce synthesis of IgA in B lymphocytes, and those activated by bone morphogenetic proteins and Runx2 induce osteoblastic differentiation. Moreover, the R-Smad-Runx signaling pathways are regulated by an E3 ubiquitin ligase Smurf1, as well as a signal transducer of interferons, STAT1. Since Runxl and Runx3 are involved in the development of some cancers including acute leukemia and gastric cancer, it will be of interest to examine in detail whether TGF-beta-specific R-Smads and Runx proteins coordinately regulate growth and differentiation of hematopoietic cells and gastric epithelial cells.

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Structure and regulated expression of mammalian RUNX genes

The RUNX are key regulators of lineage-specific gene expression in major developmental pathways. The expression of RUNX genes is tightly regulated, leading to a highly specific spatio/temporal expression pattern and to distinct phenotypes of gene knockouts. This review highlights the extensive structural similarities between the three mammalian RUNX genes and delineates how regulation of their expression at the levels of transcription and translation are orchestrated into the unique RUNX expression pattern.

Identification of novel genes of the bone-specific transcription factor Runx2

INTRODUCTION

The transcription factor Runx2 is a key regulator of osteoblast development and plays a role in chondrocyte maturation. The identification of transcriptional target genes of Runx2 may yield insight into how osteoblastic differentiation is achieved on a molecular level.

MATERIALS AND METHODS

Using a differential hybridization technique (selective amplification through biotin and restriction-mediated enrichment [SABRE]) and cDNA microarray analysis, 15 differentially expressed genes were identified using mRNA from C3H 10Tl/2 cells with constitutive and inducible overexpression of Runx2.

RESULTS AND CONCLUSIONS

Among the 15 genes identified, 4 encode the extracellular matrix proteins Ecml, Mgp, Fbn5, and Osf-2, three represent the transcription factors Esxl, Osrl, and Sox9, whereas others were Ptn, Npdc-1, Higl, and Tem l. The gene for Pttg1ip was upregulated in Runx2-expressing cells. Pttg1ip is widely expressed during development, but at highest levels in limbs and gonads. The Pttg1ip promoter binds Runx2 in a sequence specific manner, and Runx2 is able to transactivate the Pttg lip promoter in MC3T3-El cells. Therefore, Pttg1ip is likely tobe a novel direct transcriptional target gene of Runx2. In conclusion, the genes identified in this study are important candidates for mediating Runx2 induced cellular differentiation.

Runx2 mediates FGF signaling from epithelium to mesenchyme during tooth morphogenesis

Runx2 (Cbfa1) is a runt domain transcription factor that is essential for bone development and tooth morphogenesis. Teeth form as ectodermal appendages and their development is regulated by interactions between the epithelium and mesenchyme. We have shown previously that Runx2 is expressed in the dental mesenchyme and regulated by FGF signals from the epithelium, and that tooth development arrests at late bud stage in Runx2 knockout mice [Development 126 (1999) 2911]. In the present study, we have continued to clarify the role of Runx2 in tooth development and searched for downstream targets of Runx2 by extensive in situ hybridization analysis. The expression of Fgf3 was downregulated in the mesenchyme of Runx2 mutant teeth. FGF-soaked beads failed to induce Fgf3 expression in Runx2 mutant dental mesenchyme whereas in wild-type mesenchyme they induced Fgf3 in all explants indicating a requirement of Runx2 for transduction of FGF signals. Fgf3 was absent also in cultured Runx2-/- calvarial cells and it was induced by overexpression of Runx2. Furthermore, Runx2 was downregulated in Msx1 mutant tooth germs, indicating that it functions in the dental mesenchyme between Msx1 and Fgf3. Shh expression was absent from the epithelial enamel knot in lower molars of Runx2 mutant and reduced in upper molars. However, other enamel knot marker genes were expressed normally in mutant upper molars, while reduced or missing in lower molars. These differences between mutant upper and lower molars may be explained by the substitution of Runx2 function by Runx3, another member of the runt gene family that was upregulated in upper but not lower molars of Runx2 mutants. Shh expression in mutant enamel knots was not rescued by FGFs in vitro, indicating that in addition to Fgf3, Runx2 regulates other mesenchymal genes required for early tooth morphogenesis. Also, exogenous FGF and SHH did not rescue the morphogenesis of Runx2 mutant molars. We conclude that Runx2 mediates the functions of epithelial FGF signals regulating Fgf3 expression in the dental mesenchyme and that Fgf3 may be a direct target gene of Runx2.

Core-binding factors in hematopoiesis and immune function

Core binding factors are heterodimeric transcription factors containing a DNA binding Runx1, Runx2, or Runx3 subunit, along with a non DNA binding CBF beta subunit. All four subunits are required at one or more stages of hematopoiesis. This review describes the role of Runx1 and CBF beta in the initiation of hematopoiesis in the embryo, and in the emergence of hematopoietic stem cells. We also discuss the later stages of hematopoiesis for which members of the core binding factor family are required, as well as the recently described roles for these proteins in autoimmunity.

Runx2 and Runx3 are essential for chondrocyte maturation, and Runx2 regulates limb growth through induction of Indian hedgehog

The differentiation of mesenchymal cells into chondrocytes and chondrocyte proliferation and maturation are fundamental steps in skeletal development. Runx2 is essential for osteoblast differentiation and is involved in chondrocyte maturation. Although chondrocyte maturation is delayed in Runx2-deficient (Runx2(-/-)) mice, terminal differentiation of chondrocytes does occur, indicating that additional factors are involved in chondrocyte maturation. We investigated the involvement of Runx3 in chondrocyte differentiation by generating Runx2-and-Runx3-deficient (Runx2(-/-)3(-/-)) mice. We found that chondrocyte differentiation was inhibited depending on the dosages of Runx2 and Runx3, and Runx2(-/-)3(-/-) mice showed a complete absence of chondrocyte maturation. Further, the length of the limbs was reduced depending on the dosages of Runx2 and Runx3, due to reduced and disorganized chondrocyte proliferation and reduced cell size in the diaphyses. Runx2(-/-)3(-/-) mice did not express Ihh, which regulates chondrocyte proliferation and maturation. Adenoviral introduction of Runx2 in Runx2(-/-) chondrocyte cultures strongly induced Ihh expression. Moreover, Runx2 directly bound to the promoter region of the Ihh gene and strongly induced expression of the reporter gene driven by the Ihh promoter. These findings demonstrate that Runx2 and Runx3 are essential for chondrocyte maturation and that Runx2 regulates limb growth by organizing chondrocyte maturation and proliferation through the induction of Ihh expression.

AML1/Runx1 recruits calcineurin to regulate granulocyte macrophage colony-stimulating factor by Ets1 activation

Acute myeloid leukemia 1 (AML1), also denoted Runx1, is a transcription factor essential for hematopoiesis, and the AML1 gene is the most common target of chromosomal translocations in human leukemias. AML1 binds to sequences present in the regulatory regions of a number of hematopoiesis-specific genes, including certain cytokines such as granulocyte macrophage colony-stimulating factor (GM-CSF) up-regulated after T cell receptor stimulation. Here we show that both subunits of the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin (CN), which is activated upon T cell receptor stimulation, interact directly with the N-terminal runt homology domain-containing part of AML1. The regulatory CN subunit binds AML1 with a higher affinity and in addition also interacts with the isolated runt homology domain. The related Runx2 transcription factor, which is essential for bone formation, also interacts with CN. A constitutively active derivative of CN is shown to activate synergistically the GM-CSF promoter/enhancer together with AML1 or Runx2. We also provide evidence that relief of the negative effect of the AML1 sites is important for Ca(2+) activation of the GM-CSF promoter/enhancer and that AML1 overexpression increases this Ca(2+) activation. Both subunits of CN interact with AML1 in coimmunoprecipitation analyses, and confocal microscopy analysis of cells expressing fluorescence-tagged protein derivatives shows that CN can be recruited to the nucleus by AML1 in vivo. Mutant analysis of the GM-CSF promoter shows that the Ets1 binding site of the promoter is essential for the synergy between AML1 and CN in Jurkat T cells. Analysis of the effects of inhibitors of the protein kinase glycogen synthase kinase-3beta and in vitro phosphorylation/dephosphorylation analysis of Ets1 suggest that glycogen synthase kinase-3beta-phosphorylated Ets1 is a target of AML1-recruited CN phosphatase at the GM-CSF promoter.

RUNX3 expression in primary and metastatic pancreatic cancer

AIM

Runx transcription factors are important regulators of lineage specific gene expression, cell proliferation, and differentiation. Runx3 expression is lost in a high proportion of gastric cancers, suggesting a tumour suppressive role in this malignancy. This study investigates the expression and localisation of Runx3 in pancreatic tissues.

METHODS

Quantitative polymerase chain reaction was used to measure Runx3 mRNA. Immunohistochemistry was carried out to localise Runx3 in normal pancreatic tissues, and in primary and metastatic pancreatic ductal adenocarcinoma (PDAC). Basal and transforming growth factor beta1 (TGFbeta1) induced Runx3 expression was analysed in cultured pancreatic cancer cell lines.

RESULTS

Runx3 expression was low to absent in normal pancreatic tissues, but increased in a third of cancer tissues. Runx3 was present only in islets in normal pancreas, whereas in pancreatic cancers, Runx3 was detected in the cancer cells of seven of 24 samples analysed. In addition, it was expressed by lymphocytes in six of the 16 cases with lymphocyte infiltration. In pancreatic cancer cell lines, Runx3 mRNA was present in Colo-357 and T3M4 cells, but was low to absent in the other cell lines tested. TGFbeta1 repressed Runx3 mRNA expressed in Colo-357 cells, and had no effect on Runx3 expression in the other pancreatic cancer cell lines.

CONCLUSION

Runx3 expression is restricted to islets in the normal pancreas. In contrast, a considerable proportion of pancreatic tumours express Runx3, and its expression is localised in the tumour cells and in the infiltrating lymphocytes. Thus, Runx3 might play a role in the pathogenesis of PDAC.

Runx3 regulates mouse TGF-beta-mediated dendritic cell function and its absence results in airway inflammation

Runx3 transcription factor regulates cell lineage decisions in thymopoiesis and neurogenesis. Here we report that Runx3 knockout (KO) mice develop spontaneous eosinophilic lung inflammation associated with airway remodeling and mucus hypersecretion. Runx3 is specifically expressed in mature dendritic cells (DC) and mediates their response to TGF-beta. In the absence of Runx3, DC become insensitive to TGF-beta-induced maturation inhibition, and TGF-beta-dependent Langerhans cell development is impaired. Maturation of Runx3 KO DC is accelerated and accompanied by increased efficacy to stimulate T cells and aberrant expression of beta2-integrins. Lung alveoli of Runx3 KO mice accumulate DC characteristic of allergic airway inflammation. Taken together, abnormalities in DC function and subset distribution may constitute the primary immune system defect, which leads to the eosinophilic lung inflammation in Runx3 KO mice. These data may help elucidate the molecular mechanisms underlying the pathogenesis of allergic airway inflammation in humans.

Morpholino antisense oligonucleotide-mediated gene knockdown during thymocyte development reveals role for Runx3 transcription factor in CD4 silencing during development of CD4-/CD8+ thymocytes

During thymic T cell development, immature CD4(+)/CD8(+) thymocytes develop into either CD4(+)/CD8(-) helper or CD4(-)/CD8(+) CTLs. The molecular mechanisms governing the complex selection and differentiation steps during thymic T cell development are not well understood. Here we developed a novel approach to investigate gene function during thymocyte development. We transfected ex vivo isolated immature thymocytes with gene-specific morpholino antisense oligonucleotides and induced differentiation in cell or organ cultures. A morpholino oligonucleotide specific for CD8alpha strongly reduces CD8 expression. To our knowledge, this is the first demonstrated gene knockdown by morpholino oligonucleotides in primary lymphocytes. Using this approach, we show here that the transcription factor Runx3 is involved in silencing of CD4 expression during CD8 T cell differentiation. Runx3 protein expression appears late in thymocyte differentiation and is confined to mature CD8 single-positive thymocytes, whereas Runx3 mRNA is transcribed in mature CD4 and CD8 thymocytes. Therefore, Runx3 protein expression is regulated at a post-transcriptional level. The knockdown of Runx3 protein expression through morpholino oligonucleotides inhibited the development of CD4(-)/CD8(+) T cells. Instead, mature cells with a CD4(+)/CD8(+) phenotype accumulated. Potential Runx binding sites were identified in the CD4 gene silencer element, which are bound by Runx protein in EMSAs. Mutagenesis of potential Runx binding sites in the CD4 gene silencer abolished silencing activity in a reporter gene assay, indicating that Runx3 is involved in CD4 gene silencing. The experimental approach developed here should be valuable for the functional analysis of other candidate genes in T cell differentiation.

RUNX/AML and C/EBP factors regulate CD11a integrin expression in myeloid cells through overlapping regulatory elements

The CD11a/CD18 (leukocyte function-associated antigen 1 [LFA-1]) integrin mediates critical leukocyte adhesive interactions during immune and inflammatory responses. The CD11a promoter directs CD11a/CD18 integrin expression, and its activity in lymphoid cells depends on a functional RUNX1/AML-1-binding site (AML-110) within the MS7 sequence. We now report that MS7 contains a C/EBP-binding site (C/EBP-100), which overlaps with AML-110 and is bound by C/EBP factors in myeloid cells. C/EBP and RUNX/AML factors compete for binding to their respective cognate elements and bind to the CD11a promoter MS7 sequence in a cell lineage- and differentiation-dependent manner. In myeloid cells MS7 is primarily recognized by C/EBP factors in proliferating cells whereas RUNX/AML factors (especially RUNX3/AML-2) bind to MS7 in differentiated cells. RUNX3/AML-2 binding to the CD11a promoter correlates with increased RUNX3/AML-2 protein levels and enhanced CD11a/CD18 cell surface expression. The relevance of the AML-110 element is underscored by the ability of AML-1/ETO to inhibit CD11a promoter activity, thus explaining the low CD11a/CD18 expression in t(8;21)-containing myeloid leukemia cells. Therefore, the expression of the CD11a/CD18 integrin in myeloid cells is determined through the differential occupancy of the CD11a proximal promoter by transcription factors implicated in the pathogenesis of myeloid leukemia.

Runx3 is required for hematopoietic development in zebrafish

We cloned zebrafish runx3/aml2/cbfa3 and examined its expression and function during embryogenesis. In the developing embryo, runx3 is dynamically expressed in hematopoietic, neuronal, and cartilaginous tissues. Hematopoietic expression of runx3 commences late in embryogenesis in the ventral tail intermediate cell mass and later colocalizes with spi1 and lyz in circulating blood cells. In the cloche mutant, hematopoietic expression was absent, suggesting that Runx3 functions downstream of cloche in a hematopoietic pathway. Neuronal tissues expressing runx3 include the trigeminal ganglia and Rohon-Beard neurons. Runx3 appears to contribute to normal development of primitive and definitive hematopoietic cells. When Runx3 function was compromised using morpholino oligonucleotides, a reduction in the number of mature blood cells was observed. Furthermore, Runx3 depletion decreased runx1 expression in the ventral wall of the dorsal aorta and reduced the number of spi1- and lyz-containing blood cells. Conversely, ubiquitous overexpression of runx3 led to an increase in primitive blood cell numbers, together with an increase in runx1-expressing cells in the ventral wall of the dorsal aorta. We propose a role for Runx3 in the regulation of blood cell numbers.

Mutagenesis of the Runt domain defines two energetic hot spots for heterodimerization with the core binding factor beta subunit

Core-binding factors (CBFs) are a small family of heterodimeric transcription factors that play critical roles in several developmental pathways and in human disease. Mutations in CBF genes are found in leukemias, bone disorders, and gastric cancers. CBFs consist of a DNA-binding CBF alpha subunit (Runx1, Runx2, or Runx3) and a non-DNA-binding CBF beta subunit. CBF alpha binds DNA in a sequence-specific manner, whereas CBF beta enhances DNA binding by CBF alpha. Both DNA binding and heterodimerization with CBF beta are mediated by a single domain in the CBF alpha subunits known as the "Runt domain." We analyzed the energetic contribution of amino acids in the Runx1 Runt domain to heterodimerization with CBF beta. We identified two energetic "hot spots" that were also found in a similar analysis of CBF beta (Tang, Y.-Y., Shi, J., Zhang, L., Davis, A., Bravo, J., Warren, A. J., Speck, N. A., and Bushweller, J. H. (2000) J. Biol. Chem. 275, 39579-39588). The importance of the hot spot residues for Runx1 function was demonstrated in in vivo transient transfection assays. These data refine the structural analyses and further our understanding of the Runx1-CBF beta interface.

Energetic contribution of residues in the Runx1 Runt domain to DNA binding

Core-binding factors (CBFs) are a small family of heterodimeric transcription factors that play critical roles in hematopoiesis and in the development of bone, stomach epithelium, and proprioceptive neurons. Mutations in CBF genes are found in leukemias, bone disorders, and gastric cancer. CBFs consist of a DNA-binding CBF alpha subunit and a non-DNA-binding CBF beta subunit. DNA binding and heterodimerization with CBF beta are mediated by the Runt domain in CBF alpha. Here we report an alanine-scanning mutagenesis study of the Runt domain that targeted amino acids identified by structural studies to reside at the DNA or CBF beta interface, as well as amino acids mutated in human disease. We determined the energy contributed by each of the DNA-contacting residues in the Runt domain to DNA binding both in the absence and presence of CBF beta. We propose mechanisms by which mutations in the Runt domain found in hematopoietic and bone disorders affect its affinity for DNA.

An important role for RUNX3 in human L1 transcription and retrotransposition

LINE-1s (long interspersed nuclear elements-1) are abundant non-LTR retrotransposons that comprise 17% of the human genome. The 5' untranslated region (5'UTR) of human L1 (L1Hs) houses a poorly understood internal promoter. Here we report that mutations at a putative runt-domain transcription factor (RUNX) site (+83 to +101) in the 5'UTR decreased L1Hs transcription and retrotransposition in cell culture-based assays. Exogenous expression of RUNX3, but not the other two RUNX family members, RUNX1 and RUNX2, increased L1Hs transcription and retrotransposition, which were otherwise decreased by siRNAs targeting RUNX3 and a dominant negative RUNX. Further more, the specific interaction between RUNX3 and its binding site was demonstrated by an electrophoretic mobility shift assay (EMSA) using an anti-RUNX3 antibody. Interestingly, RUNX3 may also regulate the antisense promoter activity of L1Hs 5'UTR via another putative RUNX site (+526 to +508), as revealed by site-directed mutations and exogenous expression of RUNX factors. Our results indicate an important role for RUNX3 in L1Hs retrotransposition as well as transcription from its 5'UTR in both sense and antisense directions, and they should contribute to our understanding of the mechanism underlying L1Hs retrotransposition and its impact on the expression of adjacent cellular genes.

Runx1/AML1 hematopoietic transcription factor contributes to skeletal development in vivo

The requirement of Runx2 (Cbfal/AML3), a runt homology domain transcription factor essential for bone formation and osteoblast differentiation, is well established. Although Runx2 is expressed in the developing embryo prior to ossification, yet in the absence of Runx2 initial formation of the skeleton is normal, suggesting a potential redundancy in function of Runx family members. Here we addressed expression of the hematopoietic family member Runx1 (AML1/Cbfa2) in relation to skeletal development using a LacZ knock-in mouse model (Runx1(lz/+)). The resulting fusion protein reflects Runx1 promoter activity in its native context. Our studies show that Runx1 is expressed by prechondrocytic tissue forming the cartilaginous anlagen in the embryo, resting zone chondrocytes, suture lines of the calvarium, and in periosteal and perichondral membranes of all bone. Runx1 continues to be expressed in these tissues in adult mice, but is absent in mature cartilage or mineralized bone. However, hyaline cartilage outside the bone environment (trachea, xiphoid tissues), and epithelium of many soft tissues (trachea, thyroid, lung, skin) also express Runx1. The robust expression of Runx1 in vivo in chondroblasts at sites of cartilage growth and in osteoblasts at sites of new bone formation, suggests that Runx1 expression may be related to osteochondroprogenitor cell differentiation. This observation is further supported by high expression of Runx1 in ex vivo cultures of marrow stromal cells and calvarial derived osteoblasts from Runx1(lz/+) mice. These data indicate that Runx1 may contribute to the early stages of skeletogenesis and continues to function in the progenitor cells of tissues that support bone formation in the adult.

Runx3 and Runx1 are required for CD8 T cell development during thymopoiesis

The RUNX transcription factors are important regulators of lineage-specific gene expression. RUNX are bifunctional, acting both as activators and repressors of tissue-specific target genes. Recently, we have demonstrated that Runx3 is a neurogenic transcription factor, which regulates development and survival of proprioceptive neurons in dorsal root ganglia. Here we report that Runx3 and Runx1 are highly expressed in thymic medulla and cortex, respectively, and function in development of CD8 T cells during thymopoiesis. Runx3-deficient (Runx3 KO) mice display abnormalities in CD4 expression during lineage decisions and impairment of CD8 T cell maturation in the thymus. A large proportion of Runx3 KO peripheral CD8 T cells also expressed CD4, and in contrast to wild-type, their proliferation ability was largely reduced. In addition, the in vitro cytotoxic activity of alloimmunized peritoneal exudate lymphocytes was significantly lower in Runx3 KO compared with WT mice. In a compound mutant mouse, null for Runx3 and heterozygous for Runx1 (Runx3-/-;Runx1+/-), all peripheral CD8 T cells also expressed CD4, resulting in a complete lack of single-positive CD8+ T cells in the spleen. The results provide information on the role of Runx3 and Runx1 in thymopoiesis and suggest that both act as transcriptional repressors of CD4 expression during T cell lineage decisions.

Runx3 knockouts and stomach cancer

Gene targeting often results in knockout mice that show several phenotypes, some of which may not directly relate to the intrinsic function of the disrupted gene. Hence, to study the biological function of genes using knockout mice, one must identify the defects that are directly due to the loss of the targeted gene. Runx3 is a transcription factor that regulates lineage-specific gene expression in developmental processes. Recently, two groups produced Runx3 knockout mice. Two comparable defects were identified in both knockout strains, one involved neurogenesis and the other thymopoiesis. In addition, a stomach defect pertaining to gastric cancer was observed in one of the mutant strains, but not in the other. Here, we assess the differences between the two Runx3 mutant strains and discuss further studies that could reconcile these discrepancies. This article highlights the difficulties of inferring gene function through the interpretation of knockout phenotypes.

Peripheral NT3 signaling is required for ETS protein expression and central patterning of proprioceptive sensory afferents

To study the role of NT3 in directing axonal projections of proprioceptive dorsal root ganglion (DRG) neurons, NT3(-/-) mice were crossed with mice carrying a targeted deletion of the proapoptotic gene Bax. In Bax(-/-)/NT3(-/-) mice, NT3-dependent neurons survived and expressed the proprioceptive neuronal marker parvalbumin. Initial extension and collateralization of proprioceptive axons into the spinal cord occurred normally, but proprioceptive axons extended only as far as the intermediate spinal cord. This projection defect is similar to the defect in mice lacking the ETS transcription factor ER81. Few if any DRG neurons from Bax(-/-)/NT3(-/-) mice expressed ER81 protein. Expression of a NT3 transgene in muscle restored DRG ER81 expression in NT3(-/-) mice. Finally, addition of NT3 to DRG explant cultures resulted in induction of ER81 protein. Our data indicate that NT3 mediates the formation of proprioceptive afferent-motor neuron connections via regulation of ER81.

In vivo analysis of a developmental circuit for direct transcriptional activation and repression in the same cell by a Runx protein

Runx proteins have been implicated in acute myeloid leukemia, cleidocranial dysplasia, and stomach cancer. These proteins control key developmental processes in which they function as both transcriptional activators and repressors. How these opposing regulatory modes can be accomplished in the in vivo context of a cell has not been clear. In this study we use the developing cone cell in the Drosophila visual system to elucidate the mechanism of positive and negative regulation by the Runx protein Lozenge (Lz). We describe a regulatory circuit in which Lz causes transcriptional activation of the homeodomain protein Cut, which can then stabilize a Lz repressor complex in the same cell. Whether a gene is activated or repressed is determined by whether the Lz activator or the repressor complex binds to its upstream sequence. This study provides a mechanistic basis for the dual function of Runx proteins that is likely to be conserved in mammalian systems.

Phylogenesis and regulated expression of the RUNT domain transcription factors RUNX1 and RUNX3

The RUNX transcription factors are key regulators of lineage specific gene expression in developmental pathways. The mammalian RUNX genes arose early in evolution and maintained extensive structural similarities. Sequence analysis suggested that RUNX3 is the most ancient of the three mammalian genes, consistent with its role in neurogenesis of the monosynaptic reflex arc, the simplest neuronal response circuit, found in Cnidarians, the most primitive animals. All RUNX proteins bind to the same DNA motif and act as activators or repressors of transcription through recruitment of common transcriptional modulators. Nevertheless, analysis of Runx1 and Runx3 expression during embryogenesis revealed that their function is not redundant. In adults both Runx1 and Runx3 are highly expressed in the hematopoietic system. At early embryonic stages we found strong Runx3 expression in dorsal root ganglia neurons, confined to TrkC sensory neurons. In the absence of Runx3, knockout mice develop severe ataxia due to the early death of the TrkC neurons. Other phenotypic defects of Runx3 KO mice including abnormalities in thymopoiesis are also being investigated.

Intranuclear organization of RUNX transcriptional regulatory machinery in biological control of skeletogenesis and cancer

RUNX (AML/CBFA/PEBP2) transcription factors serve as paradigms for obligatory relationships between nuclear structure and physiological control of phenotypic gene expression. The RUNX proteins contribute to tissue restricted transcription by sequence-specific binding to promoter elements of target genes and serving as scaffolds for the assembly of coregulatory complexes that mediate biochemical and architectural control of activity. We will present an overview of approaches we are pursuing to address: (1) the involvement of RUNX proteins in governing competency for protein/DNA and protein/protein interactions at promoter regulatory sequences; (2) the recruitment of RUNX factors to subnuclear sites where the machinery for expression or repression of target genes is organized; and (3) the trafficking and integration of regulatory signals that control RUNX-mediated transcription.

The Runx genes as dominant oncogenes

We have shown previously that Runx2 is a frequent target (approximately equal to 30%) for proviral insertion in murine leukemia virus (MLV) induced T cell tumors in CD2-MYC transgenic mice. Further investigation of a large panel of these tumors revealed that a small number also contain insertions at either Runx3 or Runx1. None of the tumors contained insertions at more than one family member, but in each case proviral insertion was associated with a high level of expression from the upstream (P1) promoter of the respective target gene. Moreover, we confirmed that transcriptional activation of Runx1 does not affect the integrity of the coding sequence, as previously observed for Runx2. These observations suggest that the three Runx genes act as functionally redundant oncogenes in T-cell lymphoma development. To explore the oncogenic potential of Runx2 further we created transgenic mice that over-express this gene in the T cell compartment. These CD2-Runx2 animals show a preneoplastic enlargement of the CD8 immature single positive (ISP) thymocyte pool and develop lymphomas at a low incidence. Although the CD8 ISP population is greatly increased, unlike their wild type counterparts these cells are largely non-cycling. Co-expression of c-MYC in this lineage accentuates the CD8 ISP skew and induces rapid tumor development, confirming the potent synergy that exists between these two oncogenes. Experiments designed to understand the nature of the observed synergy are ongoing and are based on the hypothesis that Runx2 may exert a survival effect in c-MYC expressing tumors in vivo while c-MYC may rescue cells from the antiproliferative effects of Runx2. The oncogenic potential of Runx1 is also being assessed using primary murine embryonic fibroblasts (MEFs). These studies have revealed that while Runx1 exerts a growth suppressive effect in wild type cells a growth promoting effect is seen in the absence of p53, suggesting that the Runx genes may harbor latent oncogene-like properties.

Structural and functional characterization of Runx1, CBF beta, and CBF beta-SMMHC

Core binding factors (CBFs) are heterodimeric transcription factors consisting of a DNA-binding CBF alpha subunit and non-DNA-binding CBF beta subunit. DNA binding and heterodimerization is mediated by a single domain in the CBF alpha subunit called the Runt domain, while sequences flanking the Runt domain confer other biochemical activities such as transactivation. On the other hand, the heterodimerization domain in CBF beta is the only functional domain that has been identified in this subunit. The biophysical properties of the Runt domain and the CBF beta heterodimerization domain, and the structures of the isolated domains as well as of the Runt domain-DNA, Runt domain-CBF beta, and ternary Runt domain-CBF beta-DNA complexes, have been characterized over the past several years, and are summarized in this review.

Functional analysis of RUNX2 mutations in cleidocranial dysplasia: novel insights into genotype-phenotype correlations

Cleidocranial dysplasia (CCD) is an inherited autosomal-dominant skeletal disease caused by heterozygous mutations in the osteoblast-specific transcription factor, RUNX2. We have performed mutational analysis of RUNX2 on 24 unrelated patients with CCD. In 17 patients, 16 distinct mutations were detected in the coding region of RUNX2: 4 frameshift, 3 nonsense, 6 missense, and 2 splicing mutations alongside one polymorphism. The missense mutations were all clustered within the Runt domain and their protein products showed neither DNA binding nor transactivation. On the other hand, some mutant RUNX2 had the Runt domain intact and remained partially competent for transactivation. Coincidentally, one important phenotype of CCD, the short stature, was significantly milder in the patients with the intact Runt domain than those without. Furthermore, a remarkable correlation was found between the short stature and the number of supernumerary teeth. On the other hand, the classic CCD phenotype, hypoplastic clavicles or open fontanelles, was invariably observed regardless of the degree of short stature or supernumerary teeth. Overall, these results suggest that CCD could result from a much smaller loss in the RUNX2 function than envisioned on the basis of the conventional haploinsufficiency model. This makes an interesting contrast to the case of familial and sporadic leukemias mediated by RUNX1 mutations, in which mutants acting in a dominant negative manner have been suggested to confer a higher propensity to develop leukemia.

Cbf beta is involved in maturation of all lineages of hematopoietic cells during embryogenesis except erythroid

The transcription factor Cbf beta forms a heterodimeric complex with members of the Runx family of proteins. Together, Cbf beta and Runx1 play a critical role in the establishment of definitive hematopoiesis in mouse embryos. Previously, we used a Cbfb-GFP "knock-in" mouse model to demonstrate that Cbf beta is expressed in hematopoietic stem cells of the mouse fetal liver and aorta-gonad-mesonephros (Blood 100 (2002), 2449). We also examined the expression pattern of Cbf beta in different lineages of adult hematopoietic cells and found that it is expressed uniformly in all lineages except B lymphocytes and erythroid cells. Cbf beta expression decreases during maturation of B cells in the adult bone marrow, and is not expressed in nucleated erythroid precursors. Here, we examine the expression of Cbf beta in various hematopoietic lineages, including myeloid, lymphoid, and erythroid during late stages of embryonic development, and compare it to the pattern observed in adults. We find that there are subtle differences in expression of Cbf beta-GFP in embryonic hematopoietic cells compared to their adult counterparts, but that the overall pattern is the same. Our data complement recently published data on hematopoetic defects observed in transgenic Cbfb-null mouse embryos partially rescued by ectopic expression of Cbfb (Nature Genet. 32 (2002), 633; Nature Genet. 32 (2002), 645). and supports the emerging view that Cbf beta and Runx proteins are required for normal maturation of hematopoietic cells as well as establishment of definitive hematopoiesis.

RUNX transcription factors as key targets of TGF-beta superfamily signaling

The Runt domain transcription factors, RUNX1, RUNX2 and RUNX3, are integral components of signaling cascades mediated by both TGF-beta and bone morphogenetic proteins (BMPs) in several important biological systems. RUNX2 functions synergistically with Smad1 and Smad5 to regulate bone-specific genes when BMP induces osteogenesis. RUNX3, which has been mapped to locus 1p36, is a major tumor suppressor of gastric cancer and appears to be an important component of the TGF-beta-induced tumor suppressor pathway. A possible relationship between the TGF-beta-induced tumor-suppressor pathway and a postulated tumor suppressor gene on 1p36 must be examined.