Expression of AML1-d, a short human AML1 isoform, in embryonic stem cells suppresses in vivo tumor growth and differentiation

Augmented expression of RUNX1 deregulates the global gene expression of U87 glioblastoma multiforme cells and inhibits tumor growth in mice

Glioblastoma multiforme is the most common and aggressive primary brain tumor in adults. A mesenchymal phenotype was associated with tumor aggressiveness and poor prognosis in glioblastoma multiforme patients. Recently, the transcription factor RUNX1 was suggested as a driver of the glioblastoma multiforme mesenchymal gene expression signature; however, its independent role in this process is yet to be described. Here, we assessed the role of RUNX1 in U87 glioblastoma multiforme cells in correspondence to its mediated transcriptome and genome-wide occupancy pattern. Overexpression of RUNX1 led to diminished tumor growth in nude and severe combined immunodeficiency mouse xenograft tumor model. At the molecular level, RUNX1 occupied thousands of genomic regions and regulated the expression of hundreds of target genes, both directly and indirectly. RUNX1 occupied genomic regions that corresponded to genes that were shown to play a role in brain tumor progression and angiogenesis and upon overexpression led to a substantial down-regulation of their expression level. When overexpressed in U87 glioblastoma multiforme cells, RUNX1 down-regulated key pathways in glioblastoma multiforme progression including epithelial to mesenchymal transition, MTORC1 signaling, hypoxia-induced signaling, and TNFa signaling via NFkB. Moreover, master regulators of the glioblastoma multiforme mesenchymal phenotype including CEBPb, ZNF238, and FOSL2 were directly regulated by RUNX1. The data suggest a central role for RUNX1 as master regulator of gene expression in the U87 glioblastoma multiforme cell line and mark RUNX1 as a potential target for novel future therapies for glioblastoma multiforme.

Runx1 Structure and Function in Blood Cell Development

RUNX transcription factors belong to a highly conserved class of transcriptional regulators which play various roles in the development of the majority of metazoans. In this review we focus on the founding member of the family, RUNX1, and its role in the transcriptional control of blood cell development in mammals. We summarize data showing that RUNX1 functions both as activator and repressor within a chromatin environment, a feature that requires its interaction with multiple other transcription factors and co-factors. Furthermore, we outline how RUNX1 works together with other factors to reshape the epigenetic landscape and the three-dimensional structure of gene loci within the nucleus. Finally, we review how aberrant forms of RUNX1 deregulate blood cell development and cause hematopoietic malignancies.

Addiction of t(8;21) and inv(16) acute myeloid leukemia to native RUNX1

The t(8;21) and inv(16) chromosomal aberrations generate the oncoproteins AML1-ETO (A-E) and CBFβ-SMMHC (C-S). The role of these oncoproteins in acute myeloid leukemia (AML) etiology has been well studied. Conversely, the function of native RUNX1 in promoting A-E- and C-S-mediated leukemias has remained elusive. We show that wild-type RUNX1 is required for the survival of t(8;21)-Kasumi-1 and inv(16)-ME-1 leukemic cells. RUNX1 knockdown in Kasumi-1 cells (Kasumi-1(RX1-KD)) attenuates the cell-cycle mitotic checkpoint, leading to apoptosis, whereas knockdown of A-E in Kasumi-1(RX1-KD) rescues these cells. Mechanistically, a delicate RUNX1/A-E balance involving competition for common genomic sites that regulate RUNX1/A-E targets sustains the malignant cell phenotype. The broad medical significance of this leukemic cell addiction to native RUNX1 is underscored by clinical data showing that an active RUNX1 allele is usually preserved in both t(8;21) or inv(16) AML patients, whereas RUNX1 is frequently inactivated in other forms of leukemia. Thus, RUNX1 and its mitotic control targets are potential candidates for new therapeutic approaches.

Cell-autonomous function of Runx1 transcriptionally regulates mouse megakaryocytic maturation

RUNX1 transcription factor (TF) is a key regulator of megakaryocytic development and when mutated is associated with familial platelet disorder and predisposition to acute myeloid leukemia (FPD-AML). We used mice lacking Runx1 specifically in megakaryocytes (MK) to characterized Runx1-mediated transcriptional program during advanced stages of MK differentiation. Gene expression and chromatin-immunoprecipitation-sequencing (ChIP-seq) of Runx1 and p300 identified functional Runx1 bound MK enhancers. Runx1/p300 co-bound regions showed significant enrichment in genes important for MK and platelet homeostasis. Runx1 occupied genomic regions were highly enriched in RUNX and ETS motifs and to a lesser extent in GATA motif. Megakaryocytic specificity of Runx1/P300 bound enhancers was validated by transfection mutagenesis and Runx1/P300 co-bound regions of two key megakaryocytic genes Nfe2 and Selp were tested by in vivo transgenesis. The data provides the first example of genome wide Runx1/p300 occupancy in maturating primary FL-MK, unravel the Runx1-regulated program controlling MK maturation in vivo and identify a subset of its bona fide regulated genes. It advances our understanding of the molecular events that upon RUNX1mutations in human lead to the predisposition to familial platelet disorders and FPD-AML.

Human embryonic stem cell (hESC) and human embryo research

Human embryonic stem cell research has emerged as an important platform for the understanding and treatment of pediatric diseases. From its inception, however, it has raised ethical concerns based not on the use of stem cells themselves but on objections to the source of the cells--specifically, the destruction of preimplantation human embryos. Despite differences in public opinion on this issue, a large majority of the public supports continued research using embryonic stem cells. Given the possible substantial benefit of stem cell research on child health and development, the American Academy of Pediatrics believes that funding and oversight for human embryo and embryonic stem cell research should continue.

The actin regulator N-WASp is required for muscle-cell fusion in mice

A fundamental aspect of skeletal myogenesis involves extensive rounds of cell fusion, in which individual myoblasts are incorporated into growing muscle fibers. Here we demonstrate that N-WASp, a ubiquitous nucleation-promoting factor of branched microfilament arrays, is an essential contributor to skeletal muscle-cell fusion in developing mouse embryos. Analysis both in vivo and in primary satellite-cell cultures, shows that disruption of N-WASp function does not interfere with the program of skeletal myogenic differentiation, and does not affect myoblast motility, morphogenesis and attachment capacity. N-WASp-deficient myoblasts, however, fail to fuse. Furthermore, our analysis suggests that myoblast fusion requires N-WASp activity in both partners of a fusing myoblast pair. These findings reveal a specific role for N-WASp during mammalian myogenesis. WASp-family elements appear therefore to act as universal mediators of the myogenic cell-cell fusion mechanism underlying formation of functional muscle fibers, in both vertebrate and invertebrate species.

Dynamic combinatorial interactions of RUNX1 and cooperating partners regulates megakaryocytic differentiation in cell line models

Specific interactions of transcription factors (TFs) with their targets are crucial for specifying gene expression programs during cell differentiation. How specificity is maintained despite limited selectivity of individual TF-DNA interactions is not fully understood. RUNX1 TF is among the most frequently mutated genes in human leukemia and an important regulator of megakaryopoiesis. We used megakaryocytic cell lines to characterize the network of RUNX1 targets and cooperating TFs in differentiating megakaryocytes and demonstrated how dynamic partnerships between RUNX1 and cooperating TFs facilitated regulatory plasticity and specificity during this process. After differentiation onset, RUNX1 directly activated a large number of genes through interaction with preexisting and de novo binding sites. Recruitment of RUNX1 to de novo occupied sites occurred at H3K4me1-marked preprogrammed enhancers. A significant number of these de novo bound sites lacked RUNX motif but were occupied by AP-1 TFs. Reciprocally, AP-1 TFs were up-regulated by RUNX1 after 12-O-tetradecanoylphorbol-13-acetate induction and recruited to RUNX1-occupied sites lacking AP-1 motifs. At other differentiation stages, additional combinatorial interactions occurred between RUNX1 and its coregulators, GATA1 and ETS. The findings suggest that in differentiating megakaryocytic cell lines, RUNX1 cooperates with GATA1, AP-1, and ETS to orchestrate cell-specific transcription programs through dynamic TF partnerships.

Nonredundant roles for Runx1 alternative promoters reflect their activity at discrete stages of developmental hematopoiesis

The transcription factor Runx1 is a pivotal regulator of definitive hematopoiesis in mouse ontogeny. Vertebrate Runx1 is transcribed from 2 promoters, the distal P1 and proximal P2, which provide a paradigm of the complex transcriptional and translational control of Runx1 function. However, very little is known about the biologic relevance of alternative Runx1 promoter usage in definitive hematopoietic cell emergence. Here we report that both promoters are active at the very onset of definitive hematopoiesis, with a skewing toward the P2. Moreover, functional and morphologic analysis of a novel P1-null and an attenuated P2 mouse model revealed that although both promoters play important nonredundant roles in the emergence of definitive hematopoietic cells, the proximal P2 was most critically required for this. The nature of the observed phenotypes is indicative of a differential contribution of the P1 and P2 promoters to the control of overall Runx1 levels, where and when this is most critically required. In addition, the dynamic expression of P1-Runx1 and P2-Runx1 points at a requirement for Runx1 early in development, when the P2 is still the prevalent promoter in the emerging hemogenic endothelium and/or first committed hematopoietic cells.

A regulatory interplay between miR-27a and Runx1 during megakaryopoiesis

The transcription factor Runx1 is a key regulator of definitive hematopoiesis in the embryo and the adult. Lineage-specific expression of Runx1 involves transcription and post-transcription control through usage of alternative promoters and diverse 3'UTR isoforms, respectively. We identified and mapped microRNA (miR) binding sites on Runx1 3'UTR and show that miR-27a, miR-9, miR-18a, miR-30c, and miR-199a* bind and post-transcriptionally attenuate expression of Runx1. miR-27a impacts on both the shortest (0.15 kb) and longest (3.8 kb) 3'UTRs and, along with additional miRs, might contribute to translation attenuation of Runx1 mRNA in the myeloid cell line 416B. Whereas levels of Runx1 mRNA in 416B and the B cell line 70Z were similar, the protein levels were not. Large amounts of Runx1 protein were found in 70Z cells, whereas only minute amounts of Runx1 protein were made in 416B cells and overexpression of Runx1 in 416B induced terminal differentiation associated with megakaryocytic markers. Induction of megakaryocytic differentiation in K562 cells by 12-o-tetradecanoylphorbol-13-acetate markedly increased miR-27a expression, concomitantly with binding of Runx1 to miR-27a regulatory region. The data indicate that miR-27a plays a regulatory role in megakaryocytic differentiation by attenuating Runx1 expression, and that, during megakaryopoiesis, Runx1 and miR-27a are engaged in a feedback loop involving positive regulation of miR-27a expression by Runx1.

Developmentally regulated promoter-switch transcriptionally controls Runx1 function during embryonic hematopoiesis

Background

Alternative promoters usage is an important paradigm in transcriptional control of mammalian gene expression. However, despite the growing interest in alternative promoters and their role in genome diversification, very little is known about how and on what occasions those promoters are differentially regulated. Runx1 transcription factor is a key regulator of early hematopoiesis and a frequent target of chromosomal translocations in acute leukemias. Mice deficient in Runx1 lack definitive hematopoiesis and die in mid-gestation. Expression of Runx1 is regulated by two functionally distinct promoters designated P1 and P2. Differential usage of these two promoters creates diversity in distribution and protein-coding potential of the mRNA transcripts. While the alternative usage of P1 and P2 likely plays an important role in Runx1 biology, very little is known about the function of the P1/P2 switch in mediating tissue and stage specific expression of Runx1 during development.

Results

We employed mice bearing a hypomorphic Runx1 allele, with a largely diminished P2 activity, to investigate the biological role of alternative P1/P2 usage. Mice homozygous for the hypomorphic allele developed to term, but died within a few days after birth. During embryogenesis the P1/P2 activity is spatially and temporally modulated. P2 activity is required in early hematopoiesis and when attenuated, development of liver hematopoietic progenitor cells (HPC) was impaired. Early thymus development and thymopoiesis were also abrogated as reflected by thymic hypocellularity and loss of corticomedullary demarcation. Differentiation of CD4/CD8 thymocytes was impaired and their apoptosis was enhanced due to altered expression of T-cell receptors.

Conclusion

The data delineate the activity of P1 and P2 in embryogenesis and describe previously unknown functions of Runx1. The findings show unequivocally that the role of P1/P2 during development is non redundant and underscore the significance of alternative promoter usage in Runx1 biology.

Specification and connectivity of neuronal subtypes in the sensory lineage

During the development of the nervous system, many different types of neuron are produced. As well as forming the correct type of neuron, each must also establish precise connections. Recent findings show that, because of shared gene programmes, neuronal identity is intimately linked to and coordinated with axonal behaviour. Peripheral sensory neurons provide an excellent system in which to study these interactions. This review examines how neuronal diversity is created in the PNS and describes proteins that help to direct the diversity of neuronal subtypes, cell survival, axonal growth and the establishment of central patterns of modality-specific connections.

Transcription factors T-bet and Runx3 cooperate to activate Ifng and silence Il4 in T helper type 1 cells

Cell differentiation involves activation and silencing of lineage-specific genes. Here we show that the transcription factor Runx3 is induced in T helper type 1 (T(H)1) cells in a T-bet-dependent manner, and that both transcription factors T-bet and Runx3 are required for maximal production of interferon-gamma (IFN-gamma) and silencing of the gene encoding interleukin 4 (Il4) in T(H)1 cells. T-bet does not repress Il4 in Runx3-deficient T(H)2 cells, but coexpression of Runx3 and T-bet induces potent repression in those cells. Both T-bet and Runx3 bind to the Ifng promoter and the Il4 silencer, and deletion of the silencer decreases the sensitivity of Il4 to repression by either factor. Our data indicate that cytokine gene expression in T(H)1 cells may be controlled by a feed-forward regulatory circuit in which T-bet induces Runx3 and then 'partners' with Runx3 to direct lineage-specific gene activation and silencing.

Dynamic expression of Runx1 in skin affects hair structure

The three mammalian Runx transcription factors, some of which are known to be involved in human genetic diseases and cancer, are pivotal players in embryo development and function as key regulators of cell fate determination and organogenesis. Here, we report the expression of Runx1 during the development of hair and other skin appendages in the mouse and describe the effect of Runx1 on the structural hair output. In hair follicles, where the three Runx proteins are expressed, Runx1 expression is most prominent in both mesenchymal and epithelial compartments. The epithelial expression includes the hair keratin forming layers of the hair shaft and the bulge, where interestingly, Runx1 is co-expressed with keratin 15, a putative hair follicle stem cell marker. In the hair mesenchyme, during early stages of hair morphogenesis, Runx1 is expressed in a discrete dermal sub-epithelial layer, while at later stages it is found in a hair cycle dependent pattern in the dermal papilla. To elucidate the function of Runx1 in the hair follicle we have generated a Runx1 epidermal conditional knockout and found that the mutant mice display a remarkable structural deformation of the zigzag hair type. The data delineate Runx1 as a novel specific marker of several hair follicle cell types and sheds light on its role in hair morphogenesis and differentiation.

Normal and transforming functions of RUNX1: a perspective

Converging studies from many investigators indicate that RUNX1 has a critical role in the correct maintenance of essential cellular functions during embryonic development and after birth. The discovery that this gene is also frequently mutated in human leukemia has increased the interest in the role that RUNX1 plays in both normal and transforming pathways. Here, we provide an overview of the many roles of RUNX1 in hematopoietic self-renewal and differentiation and summarize the information that is currently available on the many mechanisms of RUNX1 deregulation in human leukemia.

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.

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.

NCAM(CD56) and RUNX1(AML1) are up-regulated in human ischemic cardiomyopathy and a rat model of chronic cardiac ischemia

Chronic myocardial ischemia is the leading cause of impaired myocardial contractility and heart failure. To identify differentially expressed genes in human ischemic cardiomyopathy (ICM), we constructed a subtracted cDNA library using specimens of ICM compared to normal human heart. Among 100 randomly sequenced clones, seven sequences represented recently identified candidate genes for differential expression in cardiac hypertrophy. A further clone without a known hypertrophy-association coded for the adhesion molecule NCAM(CD56). RNase protection assay, immunohistochemistry, and Western blotting revealed strong overexpression of NCAM(CD56) in all hearts with ICM (n = 14) compared to normal hearts (n = 8), whereas in congestive cardiomyopathy (CCM) (n = 8), hypertrophic obstructive cardiomyopathy (n = 2), myocarditis (n = 4), and sarcoidosis (n = 2), at most slight overexpression of NCAM(CD56) was observed. NCAM(CD56) overexpression abnormally involved the whole cell membrane and the cytoplasma of cardiomyocytes only inside and adjacent to ischemia-induced cardiac scars. Normal or hypertrophic fibers at a distance from ischemic scars were devoid of NCAM overexpression. Identical alterations were observed in an experimental rat ICM model, but not in normal nor in spontaneously hypertensive rat hearts. In search of NCAM(CD56)-related transcription factors we found RUNX1(AML1) up-regulation in ICM and detected RUNX1(AML1) binding within the NCAM(CD56) promoter by electromobility shift assay. We concluded that strong overexpression of NCAM(CD56) and RUNX1(AML1) is a constant and characteristic feature of cardiomyocytes within or adjacent to scars in ICM.

Overexpression of transcription factor BACH1 in fetal Down syndrome brain

There is a series of about 12 transcription factors expressed on chromosome 21. These transcription factors (TFs) are major candidates for playing a pathogenetic role for the abnormal wiring of the brain in fetal Down Syndrome (DS) as approximately 5,000 TFs are developmentally involved in the complex architecture of the human brain. TF derangement in DS has been already reported and we decided to contribute to the problem by studying four TFs encoded on chromosome 21 in fetal DS brain. We used fetal cortex of 8 DS fetuses and 6 controls (females) from the 18-19th week of gestation. Brain homogenates were subject to immunoblotting using goat-anti-BACH1, rabbit anti-heme oxygenase 1 (HO1), rabbit anti-ERG, rabbit anti-RUNX1 and goat anti-SIM2 l. Antibodies against beta-actin were used to normalise cell loss and antibodies against neuron-specific enolase were used to compensate neuronal loss. BACH1 was significantly overexpressed in fetal DS (p < 0.008) as compared to controls whereas RUNX1 and ERG proteins were comparable between groups, and SIM2 l was not detectable in any specimen. BACH1 was even significantly increased in the DS panel when normalised versus the housekeeping protein beta-actin (p < 0.01) or the neuron specific enolase (p < 0.01). HO-1 was found comparable between groups. BACH1, a member of the family of BTB-basic leucine zipper transcription factors, regulates gene expression through the NF-E2 site. More specifically, BACH1 suppresses expression of HO1. Increased BACH1, however, did not lead to decreased HO1, which would have explained oxidative stress observed in fetal DS.

Aberrant protein expression of transcription factors BACH1 and ERG, both encoded on chromosome 21, in brains of patients with Down syndrome and Alzheimer’s disease

Down syndrome (DS; trisomy 21) is a genetic disorder associated with early mental retardation and patients inevitably develop Alzheimer's disease (AD)-like neuropathological changes. The molecular defects underlying the DS-phenotype may be due to overexpression of genes encoded on chromosome 21. This so-called gene dosage hypothesis is still controversial and demands systematic work on protein expression. A series of transcription factors (TF) are encoded on chromosome 21 and are considered to play a pathogenetic role in DS. We therefore decided to study brain expression of TF encoded on chromosome 21 in patients with DS and AD compared to controls: Frontal cortex of 6 male DS patients, 6 male patients with AD and 6 male controls were used for the experiments. Immunoblotting was used to determine protein levels of TF BACH1, ERG, SIM2 and RUNX1. SIM2 and RUNX1 were comparable between groups, while BACH1 was significantly reduced in DS, and ERG was increased in DS and AD as compared to controls. These findings may indicate that DS pathogenesis cannot be simply explained by the gene dosage effect hypothesis and that results of ERG expression in DS were paralleling those in AD probably reflecting a common pathogenetic mechanism possibly explaining why all DS patients develop AD like neuropathology from the fourth decade. We conclude that TF derangement is not only due to the process of neurodegeneration and propose that TFs BACH1 and ERG play a role for the development of AD-like neuropathology in DS and pathogenesis of AD per se and the manifold increase of ERG in both disorders may form a pivotal pathogenetic link.

Complex regulation of acetylcholinesterase gene expression in human brain tumors

To study the regulation of acetylcholinesterase (AChE) gene expression in human brain tumors, 3' splice variants of AChE mRNA and potentially relevant transcription factor mRNAs were labeled in primary astrocytomas and melanomas. AChE-S and AChE-R mRNA, as well as Runx1/AML1 mRNA accumulated in astrocytomas in correlation with tumor aggressiveness, but neither HNF3beta nor c-fos mRNA was observed in melanoma and astrocytomas. Immunohistochemistry demonstrated nuclear Runx1/AML1 and cellular AChE-S and AChE-R in melanomas, however, only AChE-S, and not the secreted AChE-R variant, was retained in astrocyte tumor cells. Runx1/AML1 revealed weak linkage with ACHE promoter sequences, yet enhanced ACHE gene expression in co-transfected COS1 cells. The p300 co-activator and the ACHE promoter's distal enhancer facilitated this effect, which was independent of much of the Runx1/AML1 trans-activation domain. Surprisingly, GASP, a fusion product of green fluorescence protein (GFP) and ASP(67), a peptide composed of the 67 C-terminal amino acid residues of AChE-S, localized to COS1 cell nuclei. However, GARP, the corresponding fusion product of GFP with a peptide having the 51 C-terminal residues of AChE-E or GFP alone, remained cytoplasmic. Runx1/AML1 exhibited improved nuclear retention in GASP-expressing COS1 cells, suggesting modulated nuclear localization processes. Together, these findings reveal brain tumor-specific regulation of both expression and cellular retention of variant ACHE gene products.

Spatial and temporal expression pattern of Runx3 (Aml2) and Runx1 (Aml1) indicates non-redundant functions during mouse embryogenesis

The human RUNX3/AML2 gene belongs to the 'runt domain' family of transcription factors that act as gene expression regulators in major developmental pathways. Here, we describe the expression pattern of Runx3 during mouse embryogenesis compared to the expression pattern of Runx1. E10.5 and E14.5-E16.5 embryos were analyzed using both immunohistochemistry and beta-galactosidase activity of targeted Runx3 and Runx1 loci. We found that Runx3 expression overlapped with that of Runx1 in the hematopoietic system, whereas in sensory ganglia, epidermal appendages, and developing skeletal elements, their expression was confined to different compartments. These data provide new insights into the function of Runx3 and Runx1 in organogenesis and support the possibility that cross-regulation between them plays a role in embryogenesis.

The RUNX3 gene--sequence, structure and regulated expression

The RUNX3 gene belongs to the runt domain family of transcription factors that act as master regulators of gene expression in major developmental pathways. In mammals the family includes three genes, RUNX1, RUNX2 and RUNX3. Here, we describe a comparative analysis of the human chromosome 1p36.1 encoded RUNX3 and mouse chromosome 4 encoded Runx3 genomic regions. The analysis revealed high similarities between the two genes in the overall size and organization and showed that RUNX3/Runx3 is the smallest in the family, but nevertheless exhibits all the structural elements characterizing the RUNX family. It also revealed that RUNX3/Runx3 bears a high content of the ancient mammalian repeat MIR. Together, these data delineate RUNX3/Runx3 as the evolutionary founder of the mammalian RUNX family. Detailed sequence analysis placed the two genes at a GC-rich H3 isochore with a sharp transition of GC content between the gene sequence and the downstream intergenic region. Two large conserved CpG islands were found within both genes, one around exon 2 and the other at the beginning of exon 6. RUNX1, RUNX2 and RUNX3 gene products bind to the same DNA motif, hence their temporal and spatial expression during development should be tightly regulated. Structure/function analysis showed that two promoter regions, designated P1 and P2, regulate RUNX3 expression in a cell type-specific manner. Transfection experiments demonstrated that both promoters were highly active in the GM1500 B-cell line, which endogenously expresses RUNX3, but were inactive in the K562 myeloid cell line, which does not express RUNX3.

Architecture and anatomy of the genomic locus encoding the human leukemia-associated transcription factor RUNX1/AML1

The RUNX1 gene on human chromosome 21q22.12 belongs to the 'runt domain' gene family of transcription factors (also known as AML/CBFA/PEBP2alpha). RUNX1 is a key regulator of hematopoiesis and a frequent target of leukemia associated chromosomal translocations. Here we present a detailed analysis of the RUNX1 locus based on its complete genomic sequence. RUNX1 spans 260 kb and its expression is regulated through two distinct promoter regions, that are 160 kb apart. A very large CpG island complex marks the proximal promoter (promoter-2), and an additional CpG island is located at the 3' end of the gene. Hitherto, 12 different alternatively spliced RUNX1 cDNAs have been identified. Genomic sequence analysis of intron/exon boundaries of these cDNAs has shown that all consist of properly spliced authentic coding regions. This indicates that the large repertoire of RUNX1 proteins, ranging in size between 20-52 kDa, are generated through usage of alternatively spliced exons some of which contain in frame stop codons. The gene's introns are largely depleted of repetitive sequences, especially of the LINE1 family. The RUNX1 locus marks the transition from a ~1 Mb of gene-poor region containing only pseudogenes, to a gene-rich region containing several functional genes. A search for RUNX1 sequences that may be involved in the high frequency of chromosomal translocations revealed that a 555 bp long segment originating in chromosome 11 FLI1 gene was transposed into RUNX1 intron 4.1. This intron harbors the t(8;21) and t(3;21) chromosomal breakpoints involved in acute myeloid leukemia. Interestingly, the FLI1 homologous sequence contains a breakpoint of the t(11;22) translocation associated with Ewing's tumors, and may have a similar function in RUNX1.

Transcription-coupled translation control of AML1/RUNX1 is mediated by cap- and internal ribosome entry site-dependent mechanisms

AML1/RUNX1 belongs to the runt domain transcription factors that are important regulators of hematopoiesis and osteogenesis. Expression of AML1 is regulated at the level of transcription by two promoters, distal (D) and proximal (P), that give rise to mRNAs bearing two distinct 5' untranslated regions (5'UTRs) (D-UTR and P-UTR). Here we show that these 5'UTRs act as translation regulators in vivo. AML1 mRNAs bearing the uncommonly long (1,631-bp) P-UTR are poorly translated, whereas those with the shorter (452-bp) D-UTR are readily translated. The low translational efficiency of the P-UTR is attributed to its length and the cis-acting elements along it. Transfections and in vitro assays with bicistronic constructs demonstrate that the D-UTR mediates cap-dependent translation whereas the P-UTR mediates cap-independent translation and contains a functional internal ribosome entry site (IRES). The IRES-containing bicistronic constructs are more active in hematopoietic cell lines that normally express the P-UTR-containing mRNAs. Furthermore, we show that the IRES-dependent translation increases during megakaryocytic differentiation but not during erythroid differentiation, of K562 cells. These results strongly suggest that the function of the P-UTR IRES-dependent translation in vivo is to tightly regulate the translation of AML1 mRNAs. The data show that AML1 expression is regulated through usage of alternative promoters coupled with IRES-mediated translation control. This IRES-mediated translation regulation adds an important new dimension to the fine-tuned control of AML1 expression.