Runt domains take the lead in hematopoiesis and osteogenesis

Expression of Runx1, -2 and -3 during tooth, palate and craniofacial bone development

We describe the expression of three Runt-related RUNX genes (previously termed AML, Cbfa, or Pebp2alpha) Runx1 and Runx3 during the development of teeth and other craniofacial tissues and compare them to Runx2 expression reported earlier. All three genes were expressed in mesenchymal condensates. Runx1 was expressed in several cartilage primordia earlier than Runx3, and Runx2 was intense in all mesenchymal condensations of bones and teeth. Only Runx1 was expressed in epithelia, and in tooth germs transcripts were detected in outer dental epithelium. Runx1 was also intensely expressed in the midline epithelium of palatal shelves. In early tooth morphogenesis Runx3 was coexpressed with Runx2 in a thin layer of mesenchymal cells underlying dental epithelium. Unlike Runx2, Runx3 was expressed in odontoblasts. However, Runx3 mutant mice did not show obvious tooth phenotype or deviations of Runx1 and Runx2 expression patterns in the tooth.

Identification of genes interacting with rnt-1 through large-scale RNAi screening in Caenorhabditis elegans

Although many critical roles of the RUNX family proteins have already been identified, little attention has been given to how these proteins interact with other factors. Elucidating RUNX protein interactions will help extend our understanding of their roles in normal development and tumorigenesis. In this study, we performed large-scale RNAi screening to identify genes that genetically interact with rnt-1, the sole homolog of RUNX protein in the nematode Caenorhabditis elegans. To this end, we took advantage of the fact that C. elegans can survive a severe loss of RNT-1 function with only mild phenotypes, and we looked for genes that caused a synthetic phenotype in the rnt-1 mutant background. We identified seven genes, three of which (cdk-8, cic-1, and sur-2) are involved in transcription, two of which (pgp-2 and cct-5) are involved in stress response, and two of which (D2045.7 and W09D10.4) are involved in signaling cascades, according to their functional gene ontology terms. We further confirmed that the CDK8-containing mediator complex genetically interacts with RNT-1 by showing that knockdown of each component of the CDK8 mediator complex caused a synthetic phenotype, that is, the exploded intestine through the vulva (Eiv) phenotype, in the rnt-1 mutant background. We also identified a putative target gene, acs-4, which is regulated by the RNT-1 and CDK8 mediator complex. Our results strengthen the notion that the CDK8 mediator complex may also act together with RUNX proteins in mammals.

Physical and functional interaction of Runt-related protein 1 with hypoxia-inducible factor-1alpha

Angiogenesis and hematopoiesis are closely linked and interactive with each other, but few studies were given to identify possible links between angiogenesis-promoting proteins and hematopoiesis-related transcription factors. Here we investigated the potential relationship of oxygen-sensitive alpha-subunit of angiogenesis-related hypoxia-inducible factor-1alpha (HIF-1alpha) with Runt-related protein 1 (Runx1, also known as acute myeloid leukemia-1, AML-1), an important hematopoietic transcription factor. The results demonstrated that Runx1 and HIF-1alpha proteins directly interacted with each other to a degree, in which Runt homology domain of Runx1 was mainly involved. Leukemia-related abnormal Runx1 fusion protein AML1-ETO, which fuses the N-terminal 177 amino acid residues of the Runx1 protein in frame to ETO (eight-twenty-one) protein, also interacted with HIF-1alpha protein with greater ability than Runx1 itself. More intriguingly, Runx1 overexpression inhibited DNA-binding and transcriptional activity of HIF-1 protein with reduced expression of HIF-1-targeted genes such as vascular endothelial growth factor, while silence of Runx1 expression by specific small interfering RNA significantly increased transcriptional activity of HIF-1 protein, suggesting that Runx1 inhibited transcription-dependent function of HIF-1. Vice versa, HIF-1alpha increased DNA-binding ability and transcriptional activity of Runx1 protein. All these data would shed new insight to understanding Runx1 and HIF-1alpha-related hematopoietic cell differentiation and angiogenesis.

A Serrate-expressing signaling center controls Drosophila hematopoiesis

The differentiation of Drosophila blood cells relies on a functional hierarchy between the GATA protein, Serpent (Srp), and multiple lineage-specific transcription factors, such as the AML1-like protein, Lozenge (Lz). Two major branches of Drosophila hematopoiesis give rise to plasmatocytes/macrophages and crystal cells. Serrate signaling through the Notch pathway is critical in the regulation of Lz expression and the specification of crystal cell precursors, thus providing a key distinction between the two lineages. The expression of Serrate marks a discrete cluster of cells in the lymph gland, a signaling center, with functional similarities to stromal signaling in mammalian hematopoiesis.

Transcriptional regulation of Xbr-1a/Xvent-2 homeobox gene: analysis of its promoter region

Xvent homeobox proteins are induced by BMP-4 signaling and have been known to mediate many BMP-4 activities as key downstream transcriptional factors. In order to investigate the regulatory mode of Xvent transcription, we isolated genomic DNA of the Xbr-1a/Xvent-2 containing the promoter region responsive to BMP-4 signaling. The cis-acting elements located within the Xbr-1a/Xvent-2 promoter and the regulation modes by BMP-4 signaling were analyzed by serial deletion and site-directed mutagenesis experiments. The upstream -235bp of the promoter retained the full transcriptional activity and BMP-4-response when compared with the longest promoter construct. Further analysis indicated that two separated 15bp regions contained a strong positive element and BMP-4-response element. Site-directed mutagenesis of those regions suggests that those two regions cooperate for the promoter activity and BMP-4-response. Moreover, we found that the transcription factors, Oaz and PEBP2alphaA, were able to elicit additive effects with BMP-4 signaling on Xbr-1a/Xvent-2 reporter activities. These results indicate that transcriptional regulation of the Xbr-1a/Xvent-2 gene occurs in a complex mode through the cooperation of various transcription factors.

Functional analysis of RUNX2 mutations in Japanese patients with cleidocranial dysplasia demonstrates novel genotype-phenotype correlations

Cleidocranial dysplasia (CCD) is an autosomal dominant heritable skeletal disease caused by heterozygous mutations in the osteoblast-specific transcription factor RUNX2. We have performed mutational analysis of RUNX2 on 24 unrelated patients with CCD. In 17 patients, 16 distinct mutations were detected in the coding region of RUNX2: 4 frameshift, 3 nonsense, 6 missense, and 2 splicing mutations, in addition to 1 polymorphism. The missense mutations were all clustered within the Runt domain, and their protein products were severely impaired in DNA binding and transactivation. In contrast, two RUNX2 mutants had the Runt domain intact and remained partially competent for transactivation. One criterion of CCD, short stature, was much milder in the patients with the intact Runt domain than in those without. Furthermore, a significant correlation was found between short stature and the number of supernumerary teeth. On the one hand, these genotype-phenotype correlations highlight a general, quantitative dependency, by skeleto-dental developments, on the gene dosage of RUNX2, which has hitherto been obscured by extreme clinical diversities of CCD; this gene-dosage effect is presumed to manifest on small reductions in the total RUNX2 activity, by approximately one-fourth of the normal level at minimum. On the other hand, the classic CCD phenotype, hypoplastic clavicles or open fontanelles, was invariably observed in all patients, including those with normal height. Thus, the cleidocranial bone formation, as mediated by intramembranous ossification, may require a higher level of RUNX2 than does skeletogenesis (mediated by endochondral ossification), as well as odontogenesis (involving still different complex processes). Overall, these results suggest that CCD could result from much smaller losses in the RUNX2 function than has been envisioned on the basis of the conventional haploinsufficiency model.

Alleles of RUNX2/CBFA1 gene are associated with differences in bone mineral density and risk of fracture

The aim of this study was to determine if DNA polymorphism within runt-related gene 2 (RUNX2)/core binding factor A1 (CBFA1) is related to bone mineral density (BMD). RUNX2 contains a glutamine-alanine repeat where mutations causing cleidocranial dysplasia (CCD) have been observed. Two common variants were detected within the alanine repeat: an 18-bp deletion and a synonymous alanine codon polymorphism with alleles GCA and GCG (noted as A and G alleles, respectively). In addition, rare mutations that may be related to low BMD were observed within the glutamine repeat. In 495 randomly selected women of the Geelong Osteoporosis Study (GOS), the A allele was associated with higher BMD at all sites tested. The effect was maximal at the ultradistal (UD) radius (p = 0.001). In a separate fracture study, the A allele was significantly protective against Colles' fracture in elderly women but not spine and hip fracture. The A allele was associated with increased BMD and was protective against a common form of osteoporotic fracture, suggesting that RUNX2 variants may be related to genetic effects on BMD and osteoporosis.

Structural analyses of DNA recognition by the AML1/Runx-1 Runt domain and its allosteric control by CBFbeta

The core binding factor (CBF) heterodimeric transcription factors comprised of AML/CBFA/PEBP2alpha/Runx and CBFbeta/PEBP2beta subunits are essential for differentiation of hematopoietic and bone cells, and their mutation is intimately related to the development of acute leukemias and cleidocranial dysplasia. Here, we present the crystal structures of the AML1/Runx-1/CBFalpha(Runt domain)-CBFbeta(core domain)-C/EBPbeta(bZip)-DNA, AML1/Runx-1/CBFalpha(Runt domain)-C/EBPbeta(bZip)-DNA, and AML1/Runx-1/CBFalpha(Runt domain)-DNA complexes. The hydrogen bonding network formed among CBFalpha(Runt domain) and CBFbeta, and CBFalpha(Runt domain) and DNA revealed the allosteric regulation mechanism of CBFalpha(Runt domain)-DNA binding by CBFbeta. The point mutations of CBFalpha related to the aforementioned diseases were also mapped and their effect on DNA binding is discussed.

Dimerization with PEBP2beta protects RUNX1/AML1 from ubiquitin-proteasome-mediated degradation

The RUNX family genes are the mammalian homologs of the Drosophila genes runt and lozenge, and members of this family function as master regulators of definitive hematopoiesis and osteogenesis. The RUNX genes encode the alpha subunit of the transcription factor PEBP2/CBF. The beta subunit consists of the non-RUNX protein PEBP2beta. We found that RUNX1/AML1, which is essential for hematopoiesis, is continuously subjected to proteolytic degradation mediated by the ubiquitin-proteasome pathway. When PEBP2beta is present, however, the ubiquitylation of RUNX1 is abrogated and this causes a dramatic inhibition of RUNX1 proteolysis. Heterodimerization between PEBP2beta and RUNX1 thus appears to be an essential step in the generation of transcriptionally competent RUNX1. Consistent with this notion, RUNX1 was barely detected in PEBP2beta(-/-) mouse. CBF(PEBP2)beta- SMMHC, the chimeric protein associated with inv(16) acute myeloid leukemia, was found to protect RUNX1 from proteolytic degradation more efficiently than PEBP2beta. These results reveal a hitherto unknown and major role of PEBP2beta, namely that it regulates RUNX1 by controlling its turnover. This has allowed us to gain new insights into the mechanism of leukemogenesis by CBFbeta-SMMHC.

Perspectives on the morphology and biology of CD34-negative stem cells

The CD34 antigen is the classical indicator molecule of pluripotent hematopoietic stem cells. But there is more and more evidence that progenitors of a yet uncommitted stem cell population do not express this surrogate marker. The bone marrow and other sites of hematopoiesis consist also of fibroblast-like stromal cells, quiescent hematopoietic stem cells, and mesenchymal stem cells. Depending on their stage of differentiation, CD34- stem cells cannot only generate hematopoietic progenitors, but also more specified mesenchymal precursors, such as osteoblasts, chondrocytes, myocytes, adipocyts, and others. The stromal cell compartment produces not only matrix proteins, such as collagens, fibronectin and others, but also the essential growth factors, which initiate and support the differentiation of primary quiescent, but eventually activated CD34- stem cells into CD34+ hematopoietic progenitors. In vivo studies have shown that long-term hematopoietic and mesenchymal reconstitution can be achieved with CD34- stem cell lines, isolated from various sources, although the frequency of CD34- stem cells seams to be quite low among the progenitor population. Some authors deny the reconstitution ability of CD34- cells. The majority of CD34- stem cells are quiescent fibroblast-like cells, which can be identified in the bone marrow biopsy as "bone lining cells". Some of those bone lining cells show protein synthesis and contain secretory vesicles. Recent studies have demonstrated that there is a surprising plasticity of the earliest stem cell population, consisting of cells with stromal cell function as well as hematopoietic and mesenchymal progenitors. The new insights into the biology of totipotent stem cells give us novel perspectives for cell- and gene therapy of various malignant and nonmalignant diseases and the possibility to replace defective organ functions with autologous CD34- stem cells.

High incidence of biallelic point mutations in the Runt domain of the AML1/PEBP2αB gene in Mo acute myeloid leukemia and in myeloid malignancies with acquired trisomy 21

The AML1 gene, situated in 21q22, is often rearranged in acute leukemias through t(8;21) translocation, t(12;21) translocation, or less often t(3;21) translocation. Recently, point mutations in the Runt domain of the AML1 gene have also been reported in leukemia patients. Observations for mutations of the Runt domain of the AML1 gene in bone marrow cells were made in 300 patients, including 131 with acute myeloid leukemia (AML), 94 with myelodysplastic syndrome (MDS), 28 with blast crisis chronic myeloid leukemia (CML), 3 with atypical CML, 41 with acute lymphoblastic leukemia (ALL), and 3 with essential thrombocythemia (ET). Forty-one of the patients had chromosome 21 abnormalities, including t(8;21) in 6 of the patients with AML, t(12;21) in 8 patients with ALL, acquired trisomy 21 in 17 patients, tetrasomy 21 in 7 patients, and constitutional trisomy 21 (Down syndrome) in 3 patients. A point mutation was found in 14 cases (4.7%), including 9 (22%) of the 41 patients with AML of the Mo type (MoAML) (none of them had detectable chromosome 21 rearrangement) and 5 (38%) of the 13 myeloid malignancies with acquired trisomy 21 (1 M1AML, 2 M2AML, 1 ET, and 1 atypical CML). In at least 8 of 9 mutated cases of MoAML, both AML alleles were mutated: 3 patients had different stop codon mutations of the 2 AML1 alleles, and 5 patients had the same missense or stop codon mutation in both AML1 alleles, which resulted in at least 3 of the patients having duplication of the mutated allele and deletion of the normal residual allele, as shown by FISH analysis and by comparing microsatellite analyses of several chromosome 21 markers on diagnosis and remission samples. In the remaining mutated cases, with acquired trisomy 21, a missense mutation of AML1, which involved 2 of the 3 copies of the AML1 gene, was found. Four of the 7 mutated cases could be reanalyzed in complete remission, and no AML1 mutation was found, showing that mutations were acquired in the leukemic clone. In conclusion, these findings confirm the possibility of mutations of the Runt domain of the AML1 gene in leukemias, mainly in MoAML and in myeloid malignancies with acquired trisomy 21. AML1 mutations, in MoAML, involved both alleles and probably lead to nonfunctional AML1 protein. As AML1 protein regulates the expression of the myeloperoxidase gene, the relationship between AML1 mutations and Mo phenotype in AML will have to be further explored.

High incidence of biallelic point mutations in the Runt domain of the AML1/PEBP2αB gene in Mo acute myeloid leukemia and in myeloid malignancies with acquired trisomy 21

The AML1 gene, situated in 21q22, is often rearranged in acute leukemias through t(8;21) translocation, t(12;21) translocation, or less often t(3;21) translocation. Recently, point mutations in the Runt domain of the AML1 gene have also been reported in leukemia patients. Observations for mutations of the Runt domain of the AML1 gene in bone marrow cells were made in 300 patients, including 131 with acute myeloid leukemia (AML), 94 with myelodysplastic syndrome (MDS), 28 with blast crisis chronic myeloid leukemia (CML), 3 with atypical CML, 41 with acute lymphoblastic leukemia (ALL), and 3 with essential thrombocythemia (ET). Forty-one of the patients had chromosome 21 abnormalities, including t(8;21) in 6 of the patients with AML, t(12;21) in 8 patients with ALL, acquired trisomy 21 in 17 patients, tetrasomy 21 in 7 patients, and constitutional trisomy 21 (Down syndrome) in 3 patients. A point mutation was found in 14 cases (4.7%), including 9 (22%) of the 41 patients with AML of the Mo type (MoAML) (none of them had detectable chromosome 21 rearrangement) and 5 (38%) of the 13 myeloid malignancies with acquired trisomy 21 (1 M1AML, 2 M2AML, 1 ET, and 1 atypical CML). In at least 8 of 9 mutated cases of MoAML, both AML alleles were mutated: 3 patients had different stop codon mutations of the 2 AML1 alleles, and 5 patients had the same missense or stop codon mutation in both AML1 alleles, which resulted in at least 3 of the patients having duplication of the mutated allele and deletion of the normal residual allele, as shown by FISH analysis and by comparing microsatellite analyses of several chromosome 21 markers on diagnosis and remission samples. In the remaining mutated cases, with acquired trisomy 21, a missense mutation of AML1, which involved 2 of the 3 copies of the AML1 gene, was found. Four of the 7 mutated cases could be reanalyzed in complete remission, and no AML1 mutation was found, showing that mutations were acquired in the leukemic clone. In conclusion, these findings confirm the possibility of mutations of the Runt domain of the AML1 gene in leukemias, mainly in MoAML and in myeloid malignancies with acquired trisomy 21. AML1 mutations, in MoAML, involved both alleles and probably lead to nonfunctional AML1 protein. As AML1 protein regulates the expression of the myeloperoxidase gene, the relationship between AML1 mutations and Mo phenotype in AML will have to be further explored.

Mechanisms of transcriptional regulation by Runt domain proteins

Runt domain proteins have vital roles in regulating transcription in developmental pathways extending from sex determination and segmentation in fruit fly embryos to the development of blood and bone in mammals. Many of the insights into the mechanisms by which these proteins act to regulate transcription originate either from studies on the Drosophila runt gene, the founding member of this family, or from work on the mammalian PEBP2/CBF transcription factor. Genetic experiments in the Drosophila system reveal that runt functions both to activate and to repress transcription of different downstream target genes and indicate that different mechanisms are used in the regulation of different specific downstream target genes. These studies have also identified other nuclear factors that work with Runt in some of these pathways. Studies in mammalian systems have provided additional evidence for the complexity of transcriptional regulation by Runt domain proteins and have identified other transcription factors that cooperate with Runt domain proteins to regulate the activity of different specific cis-regulatory enhancers. The emerging view from studies in both systems is that these proteins act as context-dependent regulators of transcription, activating or repressing gene expression dependent upon the constititution of a particular promoter/enhancer in a particular cell type. These results have yielded new insights into the molecular mechanisms that control animal development and provide a framework for investigating fundamental issues in eukaryotic transcriptional regulation.