Groucho-dependent and -independent repression activities of Runt domain proteins

Identification of the Transcriptional Regulatory Role of RUNX2 by Network Analysis in Lung Cancer Cells

The use of a new bioinformatics pipeline allowed the identification of deregulated transcription factors (TFs) coexpressed in lung cancer that could become biomarkers of tumor establishment and progression. A gene regulatory network (GRN) of lung cancer was created with the normalized gene expression levels of differentially expressed genes (DEGs) from the microarray dataset GSE19804. Moreover, coregulatory and transcriptional regulatory network (TRN) analyses were performed for the main regulators identified in the GRN analysis. The gene targets and binding motifs of all potentially implicated regulators were identified in the TRN and with multiple alignments of the TFs' target gene sequences. Six transcription factors (E2F3, FHL2, ETS1, KAT6B, TWIST1, and RUNX2) were identified in the GRN as essential regulators of gene expression in non-small-cell lung cancer (NSCLC) and related to the lung tumoral process. Our findings indicate that RUNX2 could be an important regulator of the lung cancer GRN through the formation of coregulatory complexes with other TFs related to the establishment and progression of lung cancer. Therefore, RUNX2 could become an essential biomarker for developing diagnostic tools and specific treatments against tumoral diseases in the lung after the experimental validation of its regulatory function.

Myeloid neoplasms and clonal hematopoiesis from the RUNX1 perspective

RUNX1 is a critical transcription factor for the emergence of definitive hematopoiesis and the precise regulation of adult hematopoiesis. Dysregulation of its regulatory network causes aberrant hematopoiesis. Recurrent genetic alterations in RUNX1, including chromosomal translocations and mutations, have been identified in both inherited and sporadic diseases. Recent genomic studies have revealed a vast mutational landscape surrounding genetic alterations in RUNX1. Accumulating pieces of evidence also indicate the leukemogenic role of wild-type RUNX1 in certain situations. Based on these efforts, part of the molecular mechanisms of disease development as a consequence of dysregulated RUNX1-regulatory networks have become increasingly evident. This review highlights the recent advances in the field of RUNX1 research and discusses the critical roles of RUNX1 in hematopoiesis and the pathobiological function of its alterations in the context of disease, particularly myeloid neoplasms, and clonal hematopoiesis.

Enhancer recruitment of transcription repressors RUNX1 and TLE3 by mis-expressed FOXC1 blocks differentiation in acute myeloid leukemia

Despite absent expression in normal hematopoiesis, the Forkhead factor FOXC1, a critical mesenchymal differentiation regulator, is highly expressed in ∼30% of HOXAhigh acute myeloid leukemia (AML) cases to confer blocked monocyte/macrophage differentiation. Through integrated proteomics and bioinformatics, we find that FOXC1 and RUNX1 interact through Forkhead and Runt domains, respectively, and co-occupy primed and active enhancers distributed close to differentiation genes. FOXC1 stabilizes association of RUNX1, HDAC1, and Groucho repressor TLE3 to limit enhancer activity: FOXC1 knockdown induces loss of repressor proteins, gain of CEBPA binding, enhancer acetylation, and upregulation of nearby genes, including KLF2. Furthermore, it triggers genome-wide redistribution of RUNX1, TLE3, and HDAC1 from enhancers to promoters, leading to repression of self-renewal genes, including MYC and MYB. Our studies highlight RUNX1 and CEBPA transcription factor swapping as a feature of leukemia cell differentiation and reveal that FOXC1 prevents this by stabilizing enhancer binding of a RUNX1/HDAC1/TLE3 transcription repressor complex to oncogenic effect.

Transcriptional Programming in Arteriosclerotic Disease: A Multifaceted Function of the Runx2 (Runt-Related Transcription Factor 2)

Despite successful therapeutic strategies in the prevention and treatment of arteriosclerosis, the cardiovascular complications remain a major clinical and societal issue worldwide. Increased vascular calcification promotes arterial stiffness and accelerates cardiovascular morbidity and mortality. Upregulation of the Runx2 (Runt-related transcription factor 2), an essential osteogenic transcription factor for bone formation, in the cardiovascular system has emerged as an important regulator for adverse cellular events that drive cardiovascular pathology. This review discusses the regulatory mechanisms that are critical for Runx2 expression and function and highlights the dynamic and complex cross talks of a wide variety of posttranslational modifications, including phosphorylation, acetylation, ubiquitination, and O-linked β-N-acetylglucosamine modification, in regulating Runx2 stability, cellular localization, and osteogenic transcriptional activity. How the activation of an array of signaling cascades by circulating and local microenvironmental factors upregulates Runx2 in vascular cells and promotes Runx2-mediated osteogenic transdifferentiation of vascular smooth muscle cells and expression of inflammatory cytokines that accelerate macrophage infiltration and vascular osteoclast formation is summarized. Furthermore, the increasing appreciation of a new role of Runx2 upregulation in promoting vascular smooth muscle cell phenotypic switch, and Runx2 modulated by O-linked β-N-acetylglucosamine modification and Runx2-dependent repression of smooth muscle cell-specific gene expression are discussed. Further exploring the regulation of this key osteogenic transcription factor and its new perspectives in the vasculature will provide novel insights into the transcriptional regulation of vascular smooth muscle cell phenotype switch, reprograming, and vascular inflammation that promote the pathogenesis of arteriosclerosis.

Evidence That Runt Acts as a Counter-Repressor of Groucho During Drosophila melanogaster Primary Sex Determination

Runx proteins are bifunctional transcription factors that both repress and activate transcription in animal cells. Typically, Runx proteins work in concert with other transcriptional regulators, including co-activators and co-repressors to mediate their biological effects. In Drosophila melanogaster the archetypal Runx protein, Runt, functions in numerous processes including segmentation, neurogenesis and sex determination. During primary sex determination Runt acts as one of four X-linked signal element (XSE) proteins that direct female-specific activation of the establishment promoter (Pe) of the master regulatory gene Sex-lethal (Sxl). Successful activation of SxlPe requires that the XSE proteins overcome the repressive effects of maternally deposited Groucho (Gro), a potent co-repressor of the Gro/TLE family. Runx proteins, including Runt, contain a C-terminal peptide, VWRPY, known to bind to Gro/TLE proteins to mediate transcriptional repression. We show that Runt’s VWRPY co-repressor-interaction domain is needed for Runt to activate SxlPe. Deletion of the Gro-interaction domain eliminates Runt-ability to activate SxlPe, whereas replacement with a higher affinity, VWRPW, sequence promotes Runt-mediated transcription. This suggests that Runt may activate SxlPe by antagonizing Gro function, a conclusion consistent with earlier findings that Runt is needed for Sxl expression only in embryonic regions with high Gro activity. Surprisingly we found that Runt is not required for the initial activation of SxlPe. Instead, Runt is needed to keep SxlPe active during the subsequent period of high-level Sxl transcription suggesting that Runt helps amplify the difference between female and male XSE signals by counter-repressing Gro in female, but not in male, embryos.

Soybean aphid biotype 1 genome: Insights into the invasive biology and adaptive evolution of a major agricultural pest

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) is a serious pest of the soybean plant, Glycine max, a major world-wide agricultural crop. We assembled a de novo genome sequence of Ap. glycines Biotype 1, from a culture established shortly after this species invaded North America. 20.4% of the Ap. glycines proteome is duplicated. These in-paralogs are enriched with Gene Ontology (GO) categories mostly related to apoptosis, a possible adaptation to plant chemistry and other environmental stressors. Approximately one-third of these genes show parallel duplication in other aphids. But Ap. gossypii, its closest related species, has the lowest number of these duplicated genes. An Illumina GoldenGate assay of 2380 SNPs was used to determine the world-wide population structure of Ap. Glycines. China and South Korean aphids are the closest to those in North America. China is the likely origin of other Asian aphid populations. The most distantly related aphids to those in North America are from Australia. The diversity of Ap. glycines in North America has decreased over time since its arrival. The genetic diversity of Ap. glycines North American population sampled shortly after its first detection in 2001 up to 2012 does not appear to correlate with geography. However, aphids collected on soybean Rag experimental varieties in Minnesota (MN), Iowa (IA), and Wisconsin (WI), closer to high density Rhamnus cathartica stands, appear to have higher capacity to colonize resistant soybean plants than aphids sampled in Ohio (OH), North Dakota (ND), and South Dakota (SD). Samples from the former states have SNP alleles with high F_ST values and frequencies, that overlap with genes involved in iron metabolism, a crucial metabolic pathway that may be affected by the Rag-associated soybean plant response. The Ap. glycines Biotype 1 genome will provide needed information for future analyses of mechanisms of aphid virulence and pesticide resistance as well as facilitate comparative analyses between aphids with differing natural history and host plant range.

Post-Translational Regulations of Transcriptional Activity of RUNX2

Runt-related transcription factor 2 (RUNX2) is a key transcription factor for bone formation and osteoblast differentiation. Various signaling pathways and mechanisms that regulate the expression and transcriptional activity of RUNX2 have been thoroughly investigated since the involvement of RUNX2 was first reported in bone formation. As the regulation of Runx2 expression by extracellular signals has recently been reviewed, this review focuses on the regulation of post-translational RUNX2 activity. Transcriptional activity of RUNX2 is regulated at the post-translational level by various enzymes including kinases, acetyl transferases, deacetylases, ubiquitin E3 ligases, and prolyl isomerases. We describe a sequential and linear causality between post-translational modifications of RUNX2 by these enzymes. RUNX2 is one of the most important osteogenic transcription factors; however, it is not a suitable drug target. Here, we suggest enzymes that directly regulate the stability and/or transcriptional activity of RUNX2 at a post-translational level as effective drug targets for treating bone diseases.

RUNX1 Dosage in Development and Cancer

The transcription factor RUNX1 first came to prominence due to its involvement in the t(8;21) translocation in acute myeloid leukemia (AML). Since this discovery, RUNX1 has been shown to play important roles not only in leukemia but also in the ontogeny of the normal hematopoietic system. Although it is currently still challenging to fully assess the different parameters regulating RUNX1 dosage, it has become clear that the dose of RUNX1 can greatly affect both leukemia and normal hematopoietic development. It is also becoming evident that varying levels of RUNX1 expression can be used as markers of tumor progression not only in the hematopoietic system, but also in non-hematopoietic cancers. Here, we provide an overview of the current knowledge of the effects of RUNX1 dosage in normal development of both hematopoietic and epithelial tissues and their associated cancers.

Regulation of Runx2 by post-translational modifications in osteoblast differentiation

Osteogenesis is the process of new bone formation where transcription factors play an important role in controlling cell proliferation and differentiation. Runt-related transcription factor 2 (Runx2), a key transcription factor, regulates the differentiation of mesenchymal stem cells into osteoblasts, which further mature into osteocytes. Runx2 acts as a modulator such that it can either stimulate or inhibit the osteoblast differentiation. A defect/alteration in the expression/activity of this gene may lead to skeletal dysplasia. Runx2 thus serves as the best therapeutic model gene for studying bone and bone-related diseases. In this review, we briefly outline the regulation of Runx2 and its activity at the post-translational levels by the virtue of phosphorylation, acetylation, and ubiquitination in controlling the bone homeostasis.

RUNX1-dependent mechanisms in biological control and dysregulation in cancer

The RUNX1 transcription factor has recently been shown to be obligatory for normal development. RUNX1 controls the expression of genes essential for proper development in many cell lineages and tissues including blood, bone, cartilage, hair follicles, and mammary glands. Compromised RUNX1 regulation is associated with many cancers. In this review, we highlight evidence for RUNX1 control in both invertebrate and mammalian development and recent novel findings of perturbed RUNX1 control in breast cancer that has implications for other solid tumors. As RUNX1 is essential for definitive hematopoiesis, RUNX1 mutations in hematopoietic lineage cells have been implicated in the etiology of several leukemias. Studies of solid tumors have revealed a context-dependent function for RUNX1 either as an oncogene or a tumor suppressor. These RUNX1 functions have been reported for breast, prostate, lung, and skin cancers that are related to cancer subtypes and different stages of tumor development. Growing evidence suggests that RUNX1 suppresses aggressiveness in most breast cancer subtypes particularly in the early stage of tumorigenesis. Several studies have identified RUNX1 suppression of the breast cancer epithelial-to-mesenchymal transition. Most recently, RUNX1 repression of cancer stem cells and tumorsphere formation was reported for breast cancer. It is anticipated that these new discoveries of the context-dependent diversity of RUNX1 functions will lead to innovative therapeutic strategies for the intervention of cancer and other abnormalities of normal tissues.

The carboxy-terminus, a key regulator of protein function

All proteins end with a carboxyl terminus that has unique biophysical properties and is often disordered. Although there are examples of important C-termini functions, a more global role for the C-terminus is not yet established. In this review, we summarize research on C-termini, a unique region in proteins that cells exploit. Alternative splicing and proteolysis increase the diversity of proteins and peptides in cells with unique C-termini. The C-termini of proteins contain minimotifs, short peptides with an encoded function generally characterized as binding, posttranslational modifications, and trafficking. Many of these activities are specific to minimotifs on the C-terminus. Approximately 13% of C-termini in the human proteome have a known minimotif, and the majority, if not all of the remaining termini have conserved motifs inferring a function that remains to be discovered. C-termini, their predictions, and their functions are collated in the C-terminome, Proteus, and Terminus Oriented Protein Function INferred Database (TopFIND) database/web systems. Many C-termini are well conserved, and some have a known role in health and disease. We envision that this summary of C-termini will guide future investigation of their biochemical and physiological significance.

RUNX family: Oncogenes or tumor suppressors (Review)

Runt-related transcription factor (RUNX) proteins belong to a transcription factors family known as master regulators of important embryonic developmental programs. In the last decade, the whole family has been implicated in the regulation of different oncogenic processes and signaling pathways associated with cancer. Furthermore, a suppressor tumor function has been also reported, suggesting the RUNX family serves key role in all different types of cancer. In this review, the known biological characteristics, specific regulatory abilities and experimental evidence of RUNX proteins will be analyzed to demonstrate their oncogenic potential and tumor suppressor abilities during oncogenic processes, suggesting their importance as biomarkers of cancer. Additionally, the importance of continuing with the molecular studies of RUNX proteins' and its dual functions in cancer will be underlined in order to apply it in the future development of specific diagnostic methods and therapies against different types of cancer.

Thermodynamic investigation of DNA-binding affinity of wild-type and mutant transcription factor RUNX1

Transcription factor RUNX1 and its binding partner CBFβ play a critical role in gene regulation for hematopoiesis. Mutations of RUNX1 cause ~10% of acute myeloid leukemia (AML) with a particularly poor prognosis. The current paradigm for the leukemogenesis is that insufficient activity of wild-type (WT) RUNX1 impairs hematopoietic differentiation. The majority of mutant RUNX1 proteins lose the DNA-binding affinity and inhibit WT RUNX1 by depletion of CBFβ. Here, isothermal titration calorimetry (ITC) was used to quantitatively study the interactions of WT and three clinical mutant RUNX1, CBFβ and DNA. Our data show that the binding of RUNX1 to DNA is enthalpy-driven, and the affinity decreases in the order of WT > S114L > R139Q >> K83E, which support previous observations and conclusion. To find potentially beneficial RUNX1 mutations that could increase the overall RUNX1 activity, K83R and H179K mutations of RUNX1 were designed, using structure-based computational modeling, and found to possess significantly higher DNA-binding affinities than does WT RUNX1. K83R and H179K mutant RUNX1 could therefore be protein-based RUNX1 activators.

Runx-dependent and silencer-independent repression of a maturation enhancer in the Cd4 gene

An intronic silencer, S4, in the Cd4 gene has been shown to be responsible for the helper-lineage-specific expression of CD4; S4 requires Runx complex binding to exert its silencer function against the enhancer-mediated Cd4 activation by modulating the epigenetic state of the Cd4 gene. Here we identify a late-acting maturation enhancer. Bcl11b plays essential roles for activation of both the early-acting proximal enhancer and maturation enhancer of Cd4. Notably, Runx complexes suppress these enhancers by distinct mechanisms. Whereas repression of the proximal enhancer depends on the S4 silencer, the maturation enhancer is repressed by Runx in the absence of S4. Moreover, ThPOK, known to antagonize S4-mediated Cd4 repression, assists Runx complexes to restrain maturation enhancer activation. Distinct modes of S4 silencer action upon distinct enhancers thus unravel a pathway that restricts CD4 expression to helper-lineage cells by silencer-independent and Runx-dependent repression of maturation enhancer activity in cytotoxic-lineage cells.

The commitment of helper and cytotoxic lineages for CD4 and CD8 T cells, respectively, is associated with the regulation of Cd4 gene expression. Here the authors show that an intronic silencer, S4, has differential effects and synergy with the RUNX complex to act on two enhancer elements of the CD4 gene to control T cell lineage commitment in the thymus.

Glucocorticoids Inhibit Oncogenic RUNX1-ETO in Acute Myeloid Leukemia with Chromosome Translocation t(8;21)

Acute myeloid leukemia (AML) is a major blood cancer with poor prognosis. New therapies are needed to target oncogene-driven leukemia stem cells, which account for relapse and resistance. Chromosome translocation t(8;21), which produces RUNX1-ETO (R-E) fusion oncoprotein, is found in ~13% AML. R-E dominance negatively inhibits global gene expression regulated by RUNX1, a master transcription factor for hematopoiesis, causing increased self-renewal and blocked cell differentiation of hematopoietic progenitor cells, and eventually leukemia initiation. Methods: Connectivity-Map followed by biological activity testing were used to identify candidate compounds that can inhibit R-E-mediated gene transcription. Molecular mechanistic studies were also performed. Results: Glucocorticoid drugs, such as betamethasone and dexamethasone, were found to exhibit potent and selective in vitro and in vivo activities against R-E leukemia, as well as strong synergy when combined with chemotherapeutics. Microarray analysis showed that treatment with glucocorticoids significantly inhibited R-E's activity and reactivated that of RUNX1. Such gene expression changes caused differentiation and apoptosis of R-E leukemia cells. Our studies also show a possible molecular mechanism for the targeted therapy. Upon treatment with a glucocorticoid drug, more glucocorticoid receptor (GR) was translocated into the nucleus and bound to DNA, including promoters of RUNX1 target genes. GR was found to associate with RUNX1, but not R-E. This interaction increased binding of RUNX1 to DNA and reduced that of R-E, shifting to a RUNX1 dominance. Conclusion: Glucocorticoid drugs represent a targeted therapy for AML with chromosome translocation t(8:21). Given their high activity, favorable human pharmacokinetics as well as synergy with chemotherapeutics, glucocorticoids could be clinically useful to treat R-E AML.

Osteogenic Programming of Human Mesenchymal Stem Cells with Highly Efficient Intracellular Delivery of RUNX2

Mesenchymal stem cells (MSCs) are being exploited in regenerative medicine due to their tri-lineage differentiation and immunomodulation activity. Currently, there are two major challenges when directing the differentiation of MSCs for therapeutic applications. First, chemical and growth factor strategies to direct osteogenesis in vivo lack specificity for targeted delivery with desired effects. Second, MSC differentiation by gene therapy is difficult as transfection with existing approaches is clinically impractical (viral transfection) or have low efficacy (lipid-mediated transfection). These challenges can be avoided by directly delivering nonvirally derived recombinant protein transcription factors with the glycosaminoglycan-binding enhanced transduction (GET) delivery system (P21 and 8R peptides). We used the osteogenic master regulator, RUNX2 as a programming factor due to its stage-specific role in osteochondral differentiation pathways. Herein, we engineered GET-fusion proteins and compared sequential osteogenic changes in MSCs, induced by exposure to GET fusion proteins or conventional stimulation methods (dexamethasone and Bone morphogenetic protein 2). By assessing loss of stem cell-surface markers, upregulation of osteogenic genes and matrix mineralization, we demonstrate that GET-RUNX2 efficiently transduces MSCs and triggers osteogenesis by enhancing target gene expression directly. The high transduction efficiency of GET system holds great promise for stem cell therapies by allowing reproducible transcriptional control in stem cells, potentially bypassing problems observed with high-concentration growth-factor or pleiotropic steroid therapies. Stem Cells Translational Medicine 2017;6:2146-2159.

Aberrant AML1 gene expression in the diagnosis of childhood leukemias not characterized by AML1-involved cytogenetic abnormalities

The AML1 ( acute myeloid leukemia 1) gene, a necessary prerequisite of embryonic hematopoiesis and a critical regulator of normal hematopoietic development, is one of the most frequently mutated genes in human leukemia, involving over 50 chromosome translocations and over 20 partner genes. In the few existing studies investigating AML1 gene expression in childhood leukemias, aberrant upregulation seems to specifically associate with AML1 translocations and amplifications. The aim of this study was to determine whether overexpression also extends to other leukemic subtypes than the ones karyotypically involving AML1. We use quantitative real-time polymerase chain reaction methodology to investigate gene expression in 100 children with acute leukemias and compare them to those of healthy controls. We show that in childhood acute lymphoblastic leukemia, AML1 gene overexpression is associated with a variety of leukemic subtypes, both immunophenotypically and cytogenetically. Statistically significantly higher transcripts of the gene were detected in the acute lymphoblastic leukemia group as compared to the acute myeloid leukemia group, where AML1 overexpression appeared to associate with cytogenetic abnormalities additional to those that engage the AML1 gene, or that are reported as showing a "normal" karyotype. Collectively, our study shows that AML1 gene overexpression characterizes a broader range of leukemic subtypes than previously thought, including various maturation stages of B-cell acute lymphoblastic leukemia and cytogenetic types additional to those involving the AML1 gene.

Precocious Phenotypic Transcription-Factor Expression During Early Development

A novel role for phenotypic transcription factors in very early differentiation was recently observed and merits further study to elucidate what role this precocious expression may have in development. The RUNX1 transcription factor exhibits selective and transient upregulation during early mesenchymal differentiation. In contrast to phenotype-associated transcriptional control of gene expression to establish and sustain hematopoietic/myeloid lineage identity, precocious expression of RUNX1 is functionally linked to control of an epithelial to mesenchymal transition that is obligatory for development. This early RUNX1 expression spike provides a paradigm for precocious expression of a phenotypic transcription factor that invites detailed mechanistic study to fully understand its biological importance. J. Cell. Biochem. 118: 953-958, 2017. © 2016 Wiley Periodicals, Inc.

Different modes of enhancer-specific regulation by Runt and Even-skipped during Drosophila segmentation

Expression of the Drosophila slp1 gene depends on nonadditive interactions between two cis-regulatory enhancers. These enhancers are repressed by preventing either Pol II recruitment or release of promoter-proximal paused Pol II in a manner that is both enhancer and transcription factor specific and can account for their nonadditive interaction.

The initial metameric expression of the Drosophila sloppy paired 1 (slp1) gene is controlled by two distinct cis-regulatory DNA elements that interact in a nonadditive manner to integrate inputs from transcription factors encoded by the pair-rule segmentation genes. We performed chromatin immunoprecipitation on reporter genes containing these elements in different embryonic genotypes to investigate the mechanism of their regulation. The distal early stripe element (DESE) mediates both activation and repression by Runt. We find that the differential response of DESE to Runt is due to an inhibitory effect of Fushi tarazu (Ftz) on P-TEFb recruitment and the regulation of RNA polymerase II (Pol II) pausing. The proximal early stripe element (PESE) is also repressed by Runt, but in this case, Runt prevents PESE-dependent Pol II recruitment and preinitiation complex (PIC) assembly. PESE is also repressed by Even-skipped (Eve), but, of interest, this repression involves regulation of P-TEFb recruitment and promoter-proximal Pol II pausing. These results demonstrate that the mode of slp1 repression by Runt is enhancer specific, whereas the mode of repression of the slp1 PESE enhancer is transcription factor specific. We propose a model based on these differential regulatory interactions that accounts for the nonadditive interactions between the PESE and DESE enhancers during Drosophila segmentation.

Mechanism of ETV6-RUNX1 Leukemia

The t(12;21)(p13;q22) translocation is the most frequently occurring single genetic abnormality in pediatric leukemia. This translocation results in the fusion of the ETV6 and RUNX1 genes. Since its discovery in the 1990s, the function of the ETV6-RUNX1 fusion gene has attracted intense interest. In this chapter, we will summarize current knowledge on the clinical significance of ETV6-RUNX1, the experimental models used to unravel its function in leukemogenesis, the identification of co-operating mutations and the mechanisms responsible for their acquisition, the function of the encoded transcription factor and finally, the future therapeutic approaches available to mitigate the associated disease.

The Groucho/Transducin-like enhancer of split protein family in animal development

Corepressors are proteins that cannot bind DNA directly but repress transcription by interacting with partner proteins. The Groucho/Transducin-Like Enhancer of Split (TLE) are a conserved family of corepressor proteins present in animals ranging from invertebrates such as Drosophila to vertebrates such as mice and humans. Groucho/TLE proteins perform important functions throughout the life span of animals, interacting with several pathways and regulating fundamental processes such as metabolism. However, these proteins have especially crucial functions in animal development, where they are required in multiple tissues in a temporally regulated manner. In this review, we summarize the functions of the Groucho/TLE proteins during animal development, emphasizing on specific tissues where they play essential roles.

Origins of context-dependent gene repression by capicua

Receptor Tyrosine Kinase (RTK) signaling pathways induce multiple biological responses, often by regulating the expression of downstream genes. The HMG-box protein Capicua (Cic) is a transcriptional repressor that is downregulated in response to RTK signaling, thereby enabling RTK-dependent induction of Cic targets. In both Drosophila and mammals, Cic is expressed as two isoforms, long (Cic-L) and short (Cic-S), whose functional significance and mechanism of action are not well understood. Here we show that Drosophila Cic relies on the Groucho (Gro) corepressor during its function in the early embryo, but not during other stages of development. This Gro-dependent mechanism requires a short peptide motif, unique to Cic-S and designated N2, which is distinct from other previously defined Gro-interacting motifs and functions as an autonomous, transferable repressor element. Unexpectedly, our data indicate that the N2 motif is an evolutionary innovation that originated within dipteran insects, as the Cic-S isoform evolved from an ancestral Cic-L-type form. Accordingly, the Cic-L isoform lacking the N2 motif is completely inactive in early Drosophila embryos, indicating that the N2 motif endowed Cic-S with a novel Gro-dependent activity that is obligatory at this stage. We suggest that Cic-S and Gro coregulatory functions have facilitated the evolution of the complex transcriptional network regulated by Torso RTK signaling in modern fly embryos. Notably, our results also imply that mammalian Cic proteins are unlikely to act via Gro and that their Cic-S isoform must have evolved independently of fly Cic-S. Thus, Cic proteins employ distinct repressor mechanisms that are associated with discrete structural changes in the evolutionary history of this protein family.

Understanding the evolution of developmental regulatory mechanisms is a central challenge of biology. Here we uncover a newly evolved mechanism of transcriptional repression by Capicua (Cic), a conserved sensor of Receptor Tyrosine Kinase (RTK) signaling. In Drosophila, Cic patterns the central regions of the embryo by repressing genes induced by Torso RTK signaling at the poles. We show that Cic performs this function by recruiting the Groucho (Gro) corepressor and that this mechanism is an evolutionary innovation of dipteran insects. Indeed, we find that recruitment of Gro depends on a short motif of Cic (N2) specific to dipterans. Strikingly, moreover, the form of Cic that existed before the origin of dipterans is completely inactive in fly embryos, whereas the equivalent form carrying N2 displays significant function. This suggests that evolution of the N2 motif caused a fundamental change in Cic repressor activity, which we propose has enabled the complex roles of Cic, Gro and Torso signaling in fly embryonic patterning. In contrast, Cic functions independently of Gro in other Drosophila tissues and probably also in mammals, where Cic lacks the N2 sequence. Thus, our results illustrate the structural and evolutionary origins of essential functional variations within a highly conserved family of developmental regulators.

LMO2 induces T-cell leukemia with epigenetic deregulation of CD4

In this study, we present a remarkable clonal cell line, 32080, derived from a CD2-Lmo2- transgenic T-cell leukemia with differentiation arrest at the transition from the intermediate single positive to double positive stages of T-cell development. We observed that 32080 cells had a striking variegated pattern in CD4 expression. There was cell-to-cell variability, with some cells expressing no CD4 and others expressing high CD4. The two populations were isogenic and yet differed in their rates of apoptosis and sensitivity to glucocorticoid. We sorted the 32080 line for CD4-positive or CD4-negative cells and observed them in culture. After 1 week, both sorted populations showed variegated CD4 expression, like the parental line, showing that the two populations could interconvert. We determined that cell replication was necessary to transit from CD4(+) to CD4(-) and CD4(-) to CD4(+). Lmo2 knockdown decreased CD4 expression, while inhibition of intracellular NOTCH1 or histone deacetylase activity induced CD4 expression. Enforced expression of RUNX1 repressed CD4 expression. We analyzed the CD4 locus by Histone 3 chromatin immunoprecipitation and found silencing marks in the CD4(-) cells and activating marks in the CD4(+) population. The 32080 cell line is a striking model of intermediate single positive to double positive T-cell plasticity and invokes a novel mechanism for LMO2's oncogenic functions.

The essential roles of core binding factors CfRunt and CfCBFβ in hemocyte production of scallop Chlamys farreri

Core binding factor (CBF) is a family of heterodimeric transcription factors composed of a DNA-binding CBFα subunit and a non-DNA-binding CBFβ subunit, which plays critical roles in regulating hematopoiesis, osteogenesis and neurogenesis. In the present study, two genes encoding Runt (designed as CfRunt) and CBFβ (designed as CfCBFβ) were cloned and characterized from scallop Chlamys farreri. The full-length cDNA of CfRunt and CfCBFβ consists of 2128 bp and 1729 bp encoding a predicted polypeptide of 530 and 183 amino acids with a conserved Runt domain and CBFβ domain, respectively. Electrophoretic mobility shift assay demonstrated that the recombinant CfRunt protein (rCfRunt) exhibited solid ability to bind specific DNA, whereas rCfCBFβ could remarkably increase the DNA-binding affinity of rCfRunt. The mRNA transcripts of CfRunt and CfCBFβ could be detected in all tested tissues, especially in hemocytes, heart, hepatopancreas or muscle. After bacterial challenge, the circulating total hemocyte count (THC) of scallop reduced to the lowest level at 6h (P<0.05), and then it recovered gradually to the control level at 48-96 h, while the mRNA expressions of CfRunt and CfCBFβ were significant up-regulated between 6 and 48 h (P<0.05). After CfRunt gene was silenced by RNA interference, the hemocyte renewal rate and circulating THC both decreased significantly (P<0.05). However, following the RNA interference of CfRunt, the mRNA expression of CfRunt was significantly induced (P<0.05) and the attenuated hemocyte renewal rate and circulating THC could be repaired partially by LPS stimulation in the CfRunt-silenced scallops. The results collectively indicated that CfRunt and CfCBFβ, as conserved transcription factors, played essential roles in regulating hemocyte production of scallop.

Control of Drosophila embryo patterning by transcriptional co-regulators

A combination of broadly expressed transcriptional activators and spatially restricted repressors are used to pattern embryos into cells of different fate. Transcriptional co-regulators are essential mediators of transcription factor function, and contribute to selective transcriptional responses in embryo development. A two step mechanism of transcriptional regulation is discussed, where remodeling of chromatin is initially required, followed by stimulation of recruitment or release of RNA polymerase from the promoter. Transcriptional co-regulators are essential for both of these steps. In particular, most co-activators are associated with histone acetylation and co-repressors with histone deacetylation. In the early Drosophila embryo, genome-wide studies have shown that the CBP co-activator has a preference for associating with some transcription factors and regulatory regions. The Groucho, CtBP, Ebi, Atrophin and Brakeless co-repressors are selectively used to limit zygotic gene expression. New findings are summarized which show that different co-repressors are often utilized by a single repressor, that the context in which a co-repressor is recruited to DNA can affect its activity, and that co-regulators may switch from co-repressors to co-activators and vice versa. The possibility that co-regulator activity is regulated and plays an instructive role in development is discussed as well. This review highlights how findings in Drosophila embryos have contributed to the understanding of transcriptional regulation in eukaryotes as well as to mechanisms of animal embryo patterning.

Runx transcription factors repress human and murine c-Myc expression in a DNA-binding and C-terminally dependent manner

The transcription factors Runx1 and c-Myc have individually been shown to regulate important gene targets as well as to collaborate in oncogenesis. However, it is unknown whether there is a regulatory relationship between the two genes. In this study, we investigated the transcriptional regulation of endogenous c-Myc by Runx1 in the human T cell line Jurkat and murine primary hematopoietic cells. Endogenous Runx1 binds to multiple sites in the c-Myc locus upstream of the c-Myc transcriptional start site. Cells transduced with a C-terminally truncated Runx1 (Runx1.d190), which lacks important cofactor interaction sites and can block C-terminal-dependent functions of all Runx transcription factors, showed increased transcription of c-Myc. In order to monitor c-Myc expression in response to early and transiently-acting Runx1.d190, we generated a cell membrane-permeable TAT-Runx1.d190 fusion protein. Murine splenocytes treated with TAT-Runx1.d190 showed an increase in the transcription of c-Myc within 2 hours, peaking at 4 hours post-treatment and declining thereafter. This effect is dependent on the ability of Runx1.d190 to bind to DNA. The increase in c-Myc transcripts is correlated with increased c-Myc protein levels. Collectively, these data show that Runx1 directly regulates c-Myc transcription in a C-terminal- and DNA-binding-dependent manner.

GRG5/AES interacts with T-cell factor 4 (TCF4) and downregulates Wnt signaling in human cells and zebrafish embryos

Transcriptional control by TCF/LEF proteins is crucial in key developmental processes such as embryo polarity, tissue architecture and cell fate determination. TCFs associate with β-catenin to activate transcription in the presence of Wnt signaling, but in its absence act as repressors together with Groucho-family proteins (GRGs). TCF4 is critical in vertebrate intestinal epithelium, where TCF4-β-catenin complexes are necessary for the maintenance of a proliferative compartment, and their abnormal formation initiates tumorigenesis. However, the extent of TCF4-GRG complexes' roles in development and the mechanisms by which they repress transcription are not completely understood. Here we characterize the interaction between TCF4 and GRG5/AES, a Groucho family member whose functional relationship with TCFs has been controversial. We map the core GRG interaction region in TCF4 to a 111-amino acid fragment and show that, in contrast to other GRGs, GRG5/AES-binding specifically depends on a 4-amino acid motif (LVPQ) present only in TCF3 and some TCF4 isoforms. We further demonstrate that GRG5/AES represses Wnt-mediated transcription both in human cells and zebrafish embryos. Importantly, we provide the first evidence of an inherent repressive function of GRG5/AES in dorsal-ventral patterning during early zebrafish embryogenesis. These results improve our understanding of TCF-GRG interactions, have significant implications for models of transcriptional repression by TCF-GRG complexes, and lay the groundwork for in depth direct assessment of the potential role of Groucho-family proteins in both normal and abnormal development.

B-lineage transcription factors and cooperating gene lesions required for leukemia development

Differentiation of hematopoietic stem cells into B lymphocytes requires the concerted action of specific transcription factors, such as RUNX1, IKZF1, E2A, EBF1 and PAX5. As key determinants of normal B-cell development, B-lineage transcription factors are frequently deregulated in hematological malignancies, such as B-cell precursor acute lymphoblastic leukemia (BCP-ALL), and affected by either chromosomal translocations, gene deletions or point mutations. However, genetic aberrations in this developmental pathway are generally insufficient to induce BCP-ALL, and often complemented by genetic defects in cytokine receptors and tyrosine kinases (IL-7Rα, CRLF2, JAK2 and c-ABL1), transcriptional cofactors (TBL1XR1, CBP and BTG1), as well as the regulatory pathways that mediate cell-cycle control (pRB and INK4A/B). Here we provide a detailed overview of the genetic pathways that interact with these B-lineage specification factors, and describe how mutations affecting these master regulators together with cooperating lesions drive leukemia development.

Expression of the pair-rule gene homologs runt, Pax3/7, even-skipped-1 and even-skipped-2 during larval and juvenile development of the polychaete annelid Capitella teleta does not support a role in segmentation

Background

Annelids and arthropods each possess a segmented body. Whether this similarity represents an evolutionary convergence or inheritance from a common segmented ancestor is the subject of ongoing investigation.

Methods

To investigate whether annelids and arthropods share molecular components that control segmentation, we isolated orthologs of the Drosophila melanogaster pair-rule genes, runt, paired (Pax3/7) and eve, from the polychaete annelid Capitella teleta and used whole mount in situ hybridization to characterize their expression patterns.

Results

When segments first appear, expression of the single C. teleta runt ortholog is only detected in the brain. Later, Ct-runt is expressed in the ventral nerve cord, foregut and hindgut. Analysis of Pax genes in the C. teleta genome reveals the presence of a single Pax3/7 ortholog. Ct-Pax3/7 is initially detected in the mid-body prior to segmentation, but is restricted to two longitudinal bands in the ventral ectoderm. Each of the two C. teleta eve orthologs has a unique and complex expression pattern, although there is partial overlap in several tissues. Prior to and during segment formation, Ct-eve1 and Ct-eve2 are both expressed in the bilaterial pair of mesoteloblasts, while Ct-eve1 is expressed in the descendant mesodermal band cells. At later stages, Ct-eve2 is expressed in the central and peripheral nervous system, and in mesoderm along the dorsal midline. In late stage larvae and adults, Ct-eve1 and Ct-eve2 are expressed in the posterior growth zone.

Conclusions

C. teleta eve, Pax3/7 and runt homologs all have distinct expression patterns and share expression domains with homologs from other bilaterians. None of the pair-rule orthologs examined in C. teleta exhibit segmental or pair-rule stripes of expression in the ectoderm or mesoderm, consistent with an independent origin of segmentation between annelids and arthropods.

Groucho: a corepressor with instructive roles in development

Drosophila Groucho (Gro) is the founding member of a family of metazoan corepressors. Gro mediates repression through interactions with a myriad of DNA-binding repressor proteins to direct the silencing of genes involved in many developmental processes, including neurogenesis and patterning of the main body axis, as well as receptor tyrosine kinase/Ras/MAPK, Notch, Wingless (Wg)/Wnt, and Decapentaplegic (Dpp) signaling. Gro mediates repression by multiple molecular mechanisms, depending on the regulatory context. Because Gro is a broadly expressed nuclear factor, whereas its repressor partners display restricted temporal and spatial distribution, it was presumed that this corepressor played permissive rather than instructive roles in development. However, a wide range of studies demonstrates that this is not the case. Gro can sense and integrate many cellular inputs to modulate the expression of variety of genes, making it a versatile corepressor with crucial instructive roles in development and signaling.

RUNX1 and RUNX1-ETO: roles in hematopoiesis and leukemogenesis

RUNX1 is a transcription factor that regulates critical processes in many aspects of hematopoiesis. RUNX1 is also integral in defining the definitive hematopoietic stem cell. In addition, many hematological diseases like myelodysplastic syndrome and myeloproliferative neoplasms have been associated with mutations in RUNX1. Located on chromosomal 21, the RUNX1 gene is involved in many forms of chromosomal translocations in leukemia. t(8;21) is one of the most common chromosomal translocations found in acute myeloid leukemia (AML), where it results in a fusion protein between RUNX1 and ETO. The RUNX1-ETO fusion protein is found in approximately 12% of all AML patients. In this review, we detail the structural features, functions, and models used to study both RUNX1 and RUNX1-ETO in hematopoiesis over the past two decades.

How to make stripes: deciphering the transition from non-periodic to periodic patterns in Drosophila segmentation

The generation of metameric body plans is a key process in development. In Drosophila segmentation, periodicity is established rapidly through the complex transcriptional regulation of the pair-rule genes. The 'primary' pair-rule genes generate their 7-stripe expression through stripe-specific cis-regulatory elements controlled by the preceding non-periodic maternal and gap gene patterns, whereas 'secondary' pair-rule genes are thought to rely on 7-stripe elements that read off the already periodic primary pair-rule patterns. Using a combination of computational and experimental approaches, we have conducted a comprehensive systems-level examination of the regulatory architecture underlying pair-rule stripe formation. We find that runt (run), fushi tarazu (ftz) and odd skipped (odd) establish most of their pattern through stripe-specific elements, arguing for a reclassification of ftz and odd as primary pair-rule genes. In the case of run, we observe long-range cis-regulation across multiple intervening genes. The 7-stripe elements of run, ftz and odd are active concurrently with the stripe-specific elements, indicating that maternal/gap-mediated control and pair-rule gene cross-regulation are closely integrated. Stripe-specific elements fall into three distinct classes based on their principal repressive gap factor input; stripe positions along the gap gradients correlate with the strength of predicted input. The prevalence of cis-elements that generate two stripes and their genomic organization suggest that single-stripe elements arose by splitting and subfunctionalization of ancestral dual-stripe elements. Overall, our study provides a greatly improved understanding of how periodic patterns are established in the Drosophila embryo.

Repression of the RHOH gene by JunD

RhoH is a member of the Rho family of small GTP-binding proteins that lacks GTPase activity. Since RhoH is constantly bound by GTP, it is thought to be constitutively active and controlled predominantly by changes in quantitative expression. RhoH is produced specifically in haematopoietic cells and aberrant expression has been linked to various forms of leukaemia. Transcription of the RHOH gene is the first level at which the quantitative levels of the RhoH protein are regulated. Previous studies have demonstrated that RHOH gene transcription is initiated by three distinct promoter regions designated P1, P2 and P3 that define the 5' end of exons 1, 2 and 4 respectively. In the present study we report that the P3 promoter is largely responsible for RHOH gene transcription in the B-lymphocytic cell line Raji. The P3 promoter contains a minimal promoter region and a repressor region extending from -236 to +67 and +68 to +245 respectively, relative to the 5' end of exon 4. Chromatin immunoprecipitation demonstrated that two AP1 (activator protein 1) sites in the minimal promoter region bind JunD. When JUND is overexpressed, the endogenous RHOH gene is repressed; however, when JUND is inhibited, expression of endogenous RHOH is induced both in the Raji cell line and AML (acute myeloid leukaemia) cells. In the HCL (hairy cell leukaemia) cell line JOK-1, induction of RHOH increases expression of the α isoform of protein kinase C. This downstream target of RHOH is also induced in AML cells by JUND inhibition. Collectively, these data indicate that JunD is an inhibitor of RHOH gene expression.

RUNX2 mutations in Taiwanese patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant human skeletal disorder comprising hypoplastic clavicles, wide cranial sutures, supernumerary teeth, short stature, and other skeletal abnormalities. It is known that mutations in the human RUNX2 gene mapped at 6p21 are responsible for CCD. We analyzed the mutation patterns of the RUNX2 gene by direct sequencing in six Taiwanese index cases with typical CCD. One of the patients was a familial case and the others were sporadic cases. Sequencing identified four mutations. Three were caused by single nucleotide substitutions, which created a nonsense (p.R391X), two were missense mutations (p.R190W, p.R225Q), and the forth was a novel mutation (c.1119delC), a one-base deletion. Real time quantitative PCR adapted to determine copy numbers of the promoter, all exons and the 3’UTR region of the RUNX2 gene detected the deletion of a single allele in a sporadic case. The results extend the spectrum of RUNX2 mutations in CCD patients and indicate that complete deletions of the RUNX2 gene should be considered in those CCD patients lacking a point mutation detected by direct sequencing.

Hairless is a cofactor for Runt-dependent transcriptional regulation

Runt is a vital transcriptional regulator in the developmental pathway responsible for segmentation in the Drosophila embryo. Runt activates or represses transcription in a manner that is dependent on both cellular context and the specific downstream target. Here we identify Hairless (H) as a Runt-interacting molecule that functions during segmentation. We find that H is important for maintenance of engrailed (en) repression as was previously demonstrated for Groucho (Gro), Rpd3, and CtBP. H also contributes to the Runt-dependent repression of sloppy-paired-1 (slp1), a role that is not shared with these other corepressors. We further find distinct roles for these different corepressors in the regulation of other Runt targets in the early Drosophila embryo. These findings, coupled with observations on the distinct functional requirements for Runt in regulating these several different targets, indicate that Runt-dependent regulation in the Drosophila blastoderm embryo relies on unique, target-gene-specific molecular interactions.

Transcriptional and epigenetic regulation of CD4/CD8 lineage choice

The helper versus cytotoxic-lineage choice of CD4(+)CD8(+) DP thymocytes correlates with MHC restriction of their T cell receptors and the termination of either CD8 or CD4 coreceptor expression. It has been hypothesized that transcription factors regulating the expression of the Cd4/Cd8 coreceptor genes must play a role in regulating the lineage decision of DP thymocytes. Indeed, progress made during the past decade led to the identification of several transcription factors that regulate CD4/CD8 expression that are as well important regulators of helper/cytotoxic cell fate choice. These studies provided insight into the molecular link between the regulation of coreceptor expression and lineage decision. However, studies initiated by the identification of ThPOK, a central transcription factor for helper T cell development, have offered another perspective on the cross-regulation between these two processes. Here, we review advances in our understanding of regulatory circuits composed of transcription factors and their link to epigenetic mechanisms, which play essential roles in specifying and sealing cell lineage identity during the CD4/CD8 commitment process of DP thymocytes.

Transcriptional repressors, corepressors and chromatin modifying enzymes in T cell development

Gene expression is regulated by the combined action of transcriptional activators and transcriptional repressors. Transcriptional repressors function by recruiting corepressor complexes containing histone-modifying enzymes to specific sites within DNA. Chromatin modifying complexes are subsequently recruited, either directly by transcriptional repressors, or indirectly via corepressor complexes and/or histone modifications, to remodel chromatin into either a transcription-friendly 'open' form or an inhibitory 'closed' form. Transcriptional repressors, corepressors and chromatin modifying complexes play critical roles throughout T cell development. Here, we highlight those genes that function to repress transcription and that have been shown to be required for T cell development.

Requirement of male-specific dosage compensation in Drosophila females--implications of early X chromosome gene expression

Dosage compensation equates between the sexes the gene dose of sex chromosomes that carry substantially different gene content. In Drosophila, the single male X chromosome is hypertranscribed by approximately two-fold to effect this correction. The key genes are male lethal and appear not to be required in females, or affect their viability. Here, we show these male lethals do in fact have a role in females, and they participate in the very process which will eventually shut down their function—female determination. We find the male dosage compensation complex is required for upregulating transcription of the sex determination master switch, Sex-lethal, an X-linked gene which is specifically activated in females in response to their two X chromosomes. The levels of some X-linked genes are also affected, and some of these genes are used in the process of counting the number of X chromosomes early in development. Our data suggest that before the female state is set, the ground state is male and female X chromosome expression is elevated. Females thus utilize the male dosage compensation process to amplify the signal which determines their fate.

When substantially different, sex chromosomes present the challenge of not only gene dose inequity between the sexes, in the heterogametic sex where one chromosome (frequently the Y) carries few genes, but also an inequity relative to the autosomes which are diploid. Dosage compensation refers to the process which equates gene dose between the sexes. Recent results, however, indicate that the mammalian X chromosome avoids monosomy and has a level of expression that is two-fold relative to the autosomes. Hyperactive X chromosome expression in Caenorhabditis elegans has also been suggested, and dosage compensation in the hermaphrodite appears to lower expression of the X chromosomes to match autosome levels. We find that, before the female state is set in Drosophila, the X chromosomes may also express their genes at the two-fold male level and that this level of expression is used to female advantage to consolidate their sex determination. Together, the results suggest that elevated X chromosome expression may be the norm, and that the various dosage compensation processes different organisms utilize reflect a mechanism to counteract an initial hyperactive X chromosome state.

Distinct contributions of conserved modules to Runt transcription factor activity

An investigation of the in vivo roles of conserved regions of the Drosophila Runt protein outside of the DNA-binding Runt domain reveals distinct requirements in different regulatory activities. The conserved VWRPY-containing C-terminus required for repression of only a subset of targets is also found to participate in activation of other targets.

Runx proteins play vital roles in regulating transcription in numerous developmental pathways throughout the animal kingdom. Two Runx protein hallmarks are the DNA-binding Runt domain and a C-terminal VWRPY motif that mediates interaction with TLE/Gro corepressor proteins. A phylogenetic analysis of Runt, the founding Runx family member, identifies four distinct regions C-terminal to the Runt domain that are conserved in Drosophila and other insects. We used a series of previously described ectopic expression assays to investigate the functions of these different conserved regions in regulating gene expression during embryogenesis and in controlling axonal projections in the developing eye. The results indicate each conserved region is required for a different subset of activities and identify distinct regions that participate in the transcriptional activation and repression of the segmentation gene sloppy-paired-1 (slp1). Interestingly, the C-terminal VWRPY-containing region is not required for repression but instead plays a role in slp1 activation. Genetic experiments indicating that Groucho (Gro) does not participate in slp1 regulation further suggest that Runt's conserved C-terminus interacts with other factors to promote transcriptional activation. These results provide a foundation for further studies on the molecular interactions that contribute to the context-dependent properties of Runx proteins as developmental regulators.

The Runx3 transcription factor augments Th1 and down-modulates Th2 phenotypes by interacting with and attenuating GATA3

Recently, it was reported that the expression of Runt-related transcription factor 3 (Runx3) is up-regulated in CD4(+) helper T cells during Th1 cell differentiation, and that Runx3 functions in a positive feed-forward manner with the T-box family transcription factor, T-bet, which is a master regulator of Th1 cell differentiation. The relative expression levels of IFN-gamma and IL-4 are also regulated by the Th2-associated transcription factor, GATA3. Here, we demonstrate that Runx3 was induced in Th2 as well as Th1 cells and that Runx3 interacted with GATA3 and attenuated GATA3 transcriptional activity. Ectopic expression of Runx3 in vitro in cultured cells or transgenic expression of Runx3 in mice accelerated CD4(+) cells to a Th1-biased population or down-modulated Th2 responses, in part by neutralizing GATA3. Our results suggest that the balance of Runx3 and GATA3 is one factor that influences the manifestation of CD4(+) cells as the Th1 or Th2 phenotypes.

Domain analyses of the Runx1 transcription factor responsible for modulating T-cell receptor-beta/CD4 and interleukin-4/interferon-gamma expression in CD4(+) peripheral T lymphocytes

The Runx1 transcription factor is one of the master regulators of T-lymphocyte differentiation. There have been several reports trying to assign a domain within the Runx1 protein that is responsible for gene expression in thymocytes. The Runx1 domains involved in regulating the expression of several genes in peripheral CD4(+) T cells were analysed. It was observed that Runx1 over-expression enhanced the surface expression of CD4 and CD69 molecules via its activation domain and VWRPY domain, and decreased that of T-cell receptor-beta via its activation domain. Runx1 over-expression enhanced interferon-gamma expression via its activation and VWRPY domains, and abolished interleukin-4 expression through its activation domain. Transduction of Runx1 did not down-regulate CD4 expression until 72 hr of culture, but the repression of CD4 expression became evident after 96 hr. The main region responsible for repressing CD4 expression was the inhibitory domain of Runx1. Taken together, these results lead to a proposal that the regions in Runx1 responsible for modulating gene expression are distinct in thymocytes and in peripheral CD4(+) T cells.

RUNX2 mutations in cleidocranial dysplasia patients

OBJECTIVE

Mutations in the RUNX2 gene, a master regulator of bone formation, have been identified in cleidocranial dysplasia (CCD) patients. CCD is a rare autosomal-dominant disease characterized by the delayed closure of cranial sutures, defects in clavicle formation, and supernumerary teeth. The purposes of this study were to identify genetic causes of two CCD nuclear families and to report their clinical phenotypes.

MATERIALS AND METHODS

We identified two CCD nuclear families and performed mutational analyses to clarify the underlying molecular genetic etiology.

RESULTS

Mutational analysis revealed a novel nonsense mutation (c.273T>A, p.L93X) in family 1 and a de novo missense one (c.673C>T, p.R225W) in family 2. Individuals with a nonsense mutation showed maxillary hypoplasia, delayed eruption, multiple supernumerary teeth, and normal stature. In contrast, an individual with a de novo missense mutation in the Runt domain showed only one supernumerary tooth and short stature.

CONCLUSIONS

Mutational and phenotypic analyses showed that the severity of mutations on the skeletal system may not necessarily correlate with that of the disruption of tooth development.

Transcriptional co-repressors of Runx2

Runx2 is an essential transcription factor for skeletal mineralization because it stimulates osteoblast differentiation of mesenchymal stem cells, promotes chondrocyte hypertrophy, and contributes to endothelial cell migration and vascular invasion of developing bones. Runx2 is also expressed during mouse embryo development in nascent mammary gland epithelium. Recent evidence implicates deregulation of Runx2 as a contributing factor in breast cancer-induced osteolysis and invasion, as well as in ectopic vascular calcification. Like other Runt domain proteins, Runx2 is a context-dependent transcriptional activator and repressor of genes that regulate cellular proliferation and differentiation. Proteins that temporally and spatially associate with Runx2 dictate these opposing transcriptional activities. Recent studies have identified several co-repressor proteins that bind to Runx2 to regulate gene expression. These co-factors include histone deacetylases (HDACs), transducin-like enhancer of split (TLE) proteins, mSin3a, and yes-associated protein (YAP). These proteins do not bind DNA themselves and appear to act by preventing Runx2 from binding DNA, altering chromatin structure, and/or by possibly blocking co-activator complexes. The nuclear localization of several of these factors is regulated by extracellular signaling events. Understanding the mechanisms whereby co-repressor proteins affect Runx2 activity during normal cellular development and tumor progression will identify new therapeutic targets for skeletal disorders such as osteoporosis and for bone metastatic cancers.

The homeodomain protein Cux1 interacts with Grg4 to repress p27 kip1 expression during kidney development

The homeodomain protein Cux1 is highly expressed in the nephrogenic zone of the developing kidney where it functions to regulate cell proliferation. Here we show that Cux1 directly interacts with the co-repressor Grg4 (Groucho 4), a known effector of Notch signaling. Promoter reporter based luciferase assays revealed enhanced repression of p27(kip1) promoter activity by Cux1 in the presence of Grg4. Chromatin immunoprecipitation (ChIP) assays demonstrated the direct interaction of Cux1 with p27(kip1) in newborn kidney tissue in vivo. ChIP assays also identified interactions of Cux1, Grg4, HDAC1, and HDAC3 with p27(kip1) at two separate sites in the p27(kip1) promoter. DNAse1 footprinting experiments revealed that Cux1 binds to the p27(kip1) promoter on the sequence containing two Sp1 sites and a CCAAT box approximately 500 bp from the transcriptional start site, and to an AT rich sequence approximately 1.5 kb from the transcriptional start site. Taken together, these results identify Grg4 as an interacting partner for Cux1 and suggest a mechanism of p27(kip1) repression by Cux1 during kidney development.

Mitogen activated protein kinase-dependent inhibition of osteocalcin gene expression by transforming growth factor-beta1

TGF-beta (transforming growth factor-beta) plays a key role in osteoblast differentiation and bone development. While the ability of TGF-beta to inhibit the expression of osteoblast differentiation genes has been well documented, the mechanism of this inhibition is not yet completely characterized. Runx2, a transcription factor necessary for expression of osteoblast differentiation genes is a central target of inhibition by TGF-beta. In this study, we found that TGF-beta1 inhibits expression of osteoblast differentiation genes without altering expression of Runx2. Transient transfection experiments determined that TGF-beta1 inhibited osteocalcin promoter activity and this effect is mediated through Runx2. We further identified that there was no change in protein expression, cellular localization, or DNA binding affinity of Runx2 after TGF-beta1-treatment of osteoblasts, suggesting that Runx2 undergoes post-translational modifications following TGF-beta1 treatment. Co-immunoprecipitation experiments identified increased phosphorylation of Runx2 when differentiating osteoblasts were treated with TGF-beta1. Mitogen activated protein kinase (MAPK) inhibitors relieved the TGF-beta1-inhibitory effect of Runx2-mediated osteocalcin expression. Thus, our results suggest that TGF-beta1-inhibition of osteoblast differentiation is dependent on the MAPK pathway and this effect is most likely mediated by post-translational modification of Runx2 such as phosphorylation rather than other regulatory mechanisms.

Co-stimulation of the bone-related Runx2 P1 promoter in mesenchymal cells by SP1 and ETS transcription factors at polymorphic purine-rich DNA sequences (Y-repeats)

Transcriptional control of Runx2 gene expression through two alternative promoters (P1 and P2) is critical for the execution of its function as an osteogenic cell fate determining factor. In all vertebrates examined to date, the bone related P1 promoter contains a purine-rich region (-303 to -128 bp in the rat) that separates two regulatory domains. The length of this region differs dramatically between species even within the same order. Using deletion analysis, we show that part of this purine-rich region (-200 to -128) containing a duplicated element (Y-repeat) positively regulates Runx2 P1 transcription. Electrophoretic mobility assays and chromatin immunoprecipitations reveal that Y-repeat binds at least two different classes of transcription factors related to GC box binding proteins (e.g. SP1 and SP7/Osterix) and ETS-like factors (e.g. ETS1 and ELK1). Forced expression of SP1 increases Runx2 P1 promoter activity through the Y-repeats, and small interfering RNA depletion of SP1 decreases Runx2 expression. Similarly, exogenous expression of wild type ELK1, but not a defective mutant that cannot be phosphorylated, enhances Runx2 gene expression. SP1 is most abundant in proliferating cells, and ELK1 is most abundant in postconfluent cells; during MC3T3-E1 osteoblast differentiation, both proteins are transiently co-expressed when Runx2 expression is enhanced. Taken together, our data suggest that basal Runx2 gene transcription is regulated by dynamic interactions between SP1 and ETS-like factors during progression of osteogenesis.

Proleukemic RUNX1 and CBFbeta mutations in the pathogenesis of acute leukemia

The existence of non-random mutations in critical regulators of cell growth and differentiation is a recurring theme in cancer pathogenesis and provides the basis for our modern, molecular approach to the study and treatment of malignant diseases. Nowhere is this more true than in the study of leukemogenesis, where research has converged upon a critical group of genes involved in hematopoietic stem and progenitor cell self-renewal and fate specification. Prominent among these is the heterodimeric transcriptional regulator, RUNX1/CBFbeta. RUNX1 is a site-specific DNA-binding protein whose consensus response element is found in the promoters of many hematopoietically relevant genes. CBFbeta interacts with RUNX1, stabilizing its interaction with DNA to promote the actions of RUNX1/CBFbeta in transcriptional control. Both the RUNX1 and the CBFbeta genes participate in proleukemic chromosomal alterations. Together they contribute to approximately one-third of acute myelogenous leukemia (AML) and one-quarter of acute lymphoblastic leukemia (ALL) cases, making RUNX1 and CBFbeta the most frequently affected genes known in the pathogenesis of acute leukemia. Investigating the mechanisms by which RUNX1, CBFbeta, and their proleukemic fusion proteins influence leukemogenesis has contributed greatly to our understanding of both normal and malignant hematopoiesis. Here we present an overview of the structural features of RUNX1/CBFbeta and their derivatives, their roles in transcriptional control, and their contributions to normal and malignant hematopoiesis.