PEBP2alphaA/CBFA1 mutations in Japanese cleidocranial dysplasia patients

Functional consequences of C-terminal mutations in RUNX2

Cleidocranial dysplasia (CCD) is a genetic disorder caused by mutations in the RUNX2 gene, affecting bone and teeth development. Previous studies focused on mutations in the RUNX2 RHD domain, with limited investigation of mutations in the C-terminal domain. This study aimed to investigate the functional consequences of C-terminal mutations in RUNX2. Eight mutations were analyzed, and their effects on transactivation activity, protein expression, subcellular localization, and osteogenic potential were studied. Truncating mutations in the PST region and a missense mutation in the NMTS region resulted in increased transactivation activity, while missense mutations in the PST showed activity comparable to the control. Truncating mutations produced truncated proteins, while missense mutations produced normal-sized proteins. Mutant proteins were mislocalized, with six mutant proteins detected in both the nucleus and cytoplasm. CCD patient bone cells exhibited mislocalization of RUNX2, similar to the generated mutant. Mislocalization of RUNX2 and reduced expression of downstream genes were observed in MSCs from a CCD patient with the p.Ser247Valfs*3 mutation, leading to compromised osteogenic potential. This study provides insight into the functional consequences of C-terminal mutations in RUNX2, including reduced expression, mislocalization, and aberrant transactivation of downstream genes, contributing to the compromised osteogenic potential observed in CCD.

High mobility group AT-hook 2 regulates osteoblast differentiation and facial bone development

The mutation and deletion of high mobility group AT-hook 2 (Hmga2) gene exhibit skeletal malformation, but almost nothing is known about the mechanism. This study examined morphological anomaly of facial bone in Hmga2-/- mice and osteoblast differentiation of pre-osteoblast MC3T3-E1 cells with Hmga2 gene knockout (A2KO). Hmga2-/- mice showed the size reduction of anterior frontal part of facial bones. Hmga2 protein and mRNA were expressed in mesenchymal cells at ossification area of nasal bone. A2KO cells differentiation into osteoblasts after reaching the proliferation plateau was strongly suppressed by alizarin red and alkaline phosphatase staining analyses. Expression of osteoblast-related genes, especially Osterix, was down-regulated in A2KO cells. These results demonstrate a close association of Hmga2 with osteoblast differentiation of mesenchymal cells and bone growth. Although future studies are needed, the present study suggests an involvement of Hmga2 in osteoblast-genesis and bone growth.

Identification of a Novel Splice Site Mutation in RUNX2 Gene in a Family with Rare Autosomal Dominant Cleidocranial Dysplasia

Background:

Pathogenic variants of RUNX2, a gene that encodes an osteoblast-specific transcription factor, have been shown as the cause of CCD, which is a rare hereditary skeletal and dental disorder with dominant mode of inheritance and a broad range of clinical variability. Due to the relative lack of clinical complications resulting in CCD, the medical diagnosis of this disorder is challenging, which leaves it underdiagnosed.

Methods:

In this study, nine healthy and affected members of an Iranian family were investigated. PCR and sequencing of all exons and exon-intron boundaries of RUNX2 (NM_001024630) gene was performed on proband. Co-segregation analysis was conducted in the other family members for the identified variant. Additionally, a cohort of 100 Iranian ethnicity-matched healthy controls was screened by ARMS-PCR method.

Results:

The novel splice site variant (c.860-2A>G), which was identified in the intron 6 of RUNX2 gene, co-segregated with the disease in the family, and it was absent in healthy controls. Pathogenicity of this variant was determined by several software, including HSF, which predicts the formation or disruption of splice donor sites, splice acceptor sites, exonic splicing silencer sites, and exonic splicing enhancer sites. In silico analysis predicted this novel variant to be disease causing.

Conclusion:

The identified variant is predicted to have an effect on splicing, which leads to exon skipping and producing a truncated protein via introducing a premature stop codon.

Targeted reversion of induced pluripotent stem cells from patients with human cleidocranial dysplasia improves bone regeneration in a rat calvarial bone defect model

Background

Runt-related transcription factor 2 (RUNX2) haploinsufficiency causes cleidocranial dysplasia (CCD) which is characterized by supernumerary teeth, short stature, clavicular dysplasia, and osteoporosis. At present, as a therapeutic strategy for osteoporosis, mesenchymal stem cell (MSC) transplantation therapy is performed in addition to drug therapy. However, MSC-based therapy for osteoporosis in CCD patients is difficult due to a reduction in the ability of MSCs to differentiate into osteoblasts resulting from impaired RUNX2 function. Here, we investigated whether induced pluripotent stem cells (iPSCs) properly differentiate into osteoblasts after repairing the RUNX2 mutation in iPSCs derived from CCD patients to establish normal iPSCs, and whether engraftment of osteoblasts derived from properly reverted iPSCs results in better regeneration in immunodeficient rat calvarial bone defect models.

Methods

Two cases of CCD patient-derived induced pluripotent stem cells (CCD-iPSCs) were generated using retroviral vectors (OCT3/4, SOX2, KLF4, and c-MYC) or a Sendai virus SeVdp vector (KOSM302L). Reverted iPSCs were established using programmable nucleases, clustered regularly interspaced short palindromic repeats (CRISPR)/Cas-derived RNA-guided endonucleases, to correct mutations in CCD-iPSCs. The mRNA expressions of osteoblast-specific markers were analyzed using quantitative reverse-transcriptase polymerase chain reaction. iPSCs-derived osteoblasts were transplanted into rat calvarial bone defects, and bone regeneration was evaluated using microcomputed tomography analysis and histological analysis.

Results

Mutation analysis showed that both contained nonsense mutations: one at the very beginning of exon 1 and the other at the initial position of the nuclear matrix-targeting signal. The osteoblasts derived from CCD-iPSCs (CCD-OBs) expressed low levels of several osteoblast differentiation markers, and transplantation of these osteoblasts into calvarial bone defects created in rats with severe combined immunodeficiency showed poor regeneration. However, reverted iPSCs improved the abnormal osteoblast differentiation which resulted in much better engraftment into the rat calvarial bone defect.

Conclusions

Taken together, these results demonstrate that patient-specific iPSC technology can not only provide a useful disease model to elucidate the role of RUNX2 in osteoblastic differentiation but also raises the tantalizing prospect that reverted iPSCs might provide a practical medical treatment for CCD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0754-4) contains supplementary material, which is available to authorized users.

Novel Mutation of the RUNX2 Gene in Patients with Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder linked to mutations in the Runt-related transcription factor 2, encoded by the RUNX2 gene, which is essential for osteoblast differentiation and skeletal development. Here, we describe a novel nonsense mutation (c.532C>T; p.Q178X) in RUNX2 identified in 3 affected members of a Polish family with CCD. The localization and transcriptional transactivation studies show that the mutated form of the protein has altered the subcellular localization and significantly decreased transactivation properties, respectively. Consequently, our data show that the c.532C>T mutation generates a defective RUNX2 protein and is genetically linked to the CCD phenotype.

Skp2 inhibits osteogenesis by promoting ubiquitin-proteasome degradation of Runx2

Osteogenic transcription factor Runx2 is essential for osteoblast differentiation. The activity of Runx2 is tightly regulated at transcriptional as well as post-translational level. However, regulation of Runx2 stability by ubiquitin mediated proteasomal degradation by E3 ubiquitin ligases is little-known. Here, for the first time we demonstrate that Skp2, an SCF family E3 ubiquitin ligase negatively targets Runx2 by promoting its polyubiquitination and proteasome dependent degradation. Co-immunoprecipitation studies revealed that Skp2 physically interacts with Runx2 both in a heterologous as well as physiologically relevant system. Functional consequences of Runx2-Skp2 physical interaction were then assessed by promoter reporter assay. We show that Skp2-mediated downregulation of Runx2 led to reduced Runx2 transactivation and osteoblast differentiation. On the contrary, inhibition of Skp2 restored Runx2 levels and promoted osteoblast differentiation. We further show that Skp2 and Runx2 proteins are co-expressed and show inverse relation in vivo such as in lactating, ovariectomized and estrogen-treated ovariectomized animals. Together, these data demonstrate that Skp2 targets Runx2 for ubiquitin mediated degradation and hence negatively regulate osteogenesis. Therefore, the present study provides a plausible therapeutic target for osteoporosis or cleidocranial dysplasia caused by the heterozygous mutation of Runx2 gene.

Characterisation of novel RUNX2 mutation with alanine tract expansion from Japanese cleidocranial dysplasia patient

Cleidocranial dysplasia (CCD; MIM 119600) is an autosomal dominant skeletal dysplasia characterised by hypopalstic and/or aplastic clavicles, midface hypoplasia, absent or delayed closure of cranial sutures, moderately short stature, delayed eruption of permanent dentition and supernumerary teeth. The molecular pathogenesis can be explained in about two-thirds of CCD patients by haploinsufficiency of the RUNX2 gene. In our current study, we identified a novel and rare variant of the RUNX2 gene (c.181_189dupGCGGCGGCT) in a Japanese patient with phenotypic features of CCD. The insertion led an alanine tripeptide expansion (+3Ala) in the polyalanine tract. To date, a RUNX2 variant with alanine decapeptide expansion (+10Ala) is the only example of a causative variant of RUNX2 with polyalanine tract expansion to be reported, whilst RUNX2 (+1Ala) has been isolated from the healthy population. Thus, precise analyses of the RUNX2 (+3Ala) variant were needed to clarify whether the tripeptide expanded RUNX2 is a second disease-causing mutant with alanine tract expansion. We therefore investigated the biochemical properties of the mutant RUNX2 (+3Ala), which contains 20 alanine residues in the polyalanine tract. When transfected in COS7 cells, RUNX2 (+3Ala) formed intracellular ubiquitinated aggregates after 24h, and exerted a dominant negative effect in vitro. At 24h after gene transfection, whereas slight reduction was observed in RUNX2 (+10Ala), all of these mutants significantly activated osteoblast-specific element-2, a cis-acting sequence in the promoter of the RUNX2 target gene osteocalcin. The aggregation growth of RUNX2 (+3Ala) was clearly lower and slower than that of RUNX2 (+10Ala). Furthermore, we investigated several other RUNX2 variants with various alanine tract lengths, and found that the threshold for aggregation may be RUNX2 (+3Ala). We conclude that RUNX2 (+3Ala) is the cause of CCD in our current case, and that the accumulation of intracellular aggregates in vitro is related to the length of the alanine tract.

Nuclear matrix-targeting of the osteogenic factor Runx2 is essential for its recognition and activation of the alkaline phosphatase gene

BACKGROUND

A good understanding of the mechanism of gene regulation that is involved in bone mineralization is critical for the design of anabolic treatments for bone deficiency diseases. Alkaline phosphatase (ALP) expressed by osteoblasts plays an important role in promoting bone mineralization by hydrolyzing pyrophosphate. However, the mechanism by which the expression of ALP is regulated during osteoblast differentiation has not been thoroughly investigated.

METHODS

Chromatin immunoprecipitation. EMSA and mutagenesis were used to identify the Runx2 binding sites on ALP gene and to analyze the role of nuclear matrix-localization of Runx2 on the recognition and activation of ALP gene.

RESULTS

Using chromatin immunoprecipitation, we determined that both ectopic and endogenous Runx2 bound to ALP intron 1 in a region containing a cluster of five putative core-sites. The third one (11C3) among those fives was bound most strongly in vitro by Runx2 and acted as a Runx2-dependent transcriptional enhancer. Furthermore, a Runx2 mutant lacking the nuclear matrix-targeting sequence (Runx2deltaNMTS) bound to the ALP gene less efficiently than the wild-type protein and a Runx2 mutant that is deficient in its ability to bind to DNA (Runx2K120A) accumulated largely in the nuclear matrix.

CONCLUSIONS

Nuclear matrix-localization of Runx2 influences its ALP gene recognition.

GENERAL SIGNIFICANCE

Our results showed for the first time that ALP is a direct target gene of Runx2 and illustrated that the recognition/binding and activation of the ALP by this transcription factor are dependent on its nuclear matrix-targeting.

Glutamine repeat variants in human RUNX2 associated with decreased femoral neck BMD, broadband ultrasound attenuation and target gene transactivation

RUNX2 is an essential transcription factor required for skeletal development and cartilage formation. Haploinsufficiency of RUNX2 leads to cleidocranial displaysia (CCD) a skeletal disorder characterised by gross dysgenesis of bones particularly those derived from intramembranous bone formation. A notable feature of the RUNX2 protein is the polyglutamine and polyalanine (23Q/17A) domain coded by a repeat sequence. Since none of the known mutations causing CCD characterised to date map in the glutamine repeat region, we hypothesised that Q-repeat mutations may be related to a more subtle bone phenotype. We screened subjects derived from four normal populations for Q-repeat variants. A total of 22 subjects were identified who were heterozygous for a wild type allele and a Q-repeat variant allele: (15Q, 16Q, 18Q and 30Q). Although not every subject had data for all measures, Q-repeat variants had a significant deficit in BMD with an average decrease of 0.7SD measured over 12 BMD-related parameters (p = 0.005). Femoral neck BMD was measured in all subjects (-0.6SD, p = 0.0007). The transactivation function of RUNX2 was determined for 16Q and 30Q alleles using a reporter gene assay. 16Q and 30Q alleles displayed significantly lower transactivation function compared to wild type (23Q). Our analysis has identified novel Q-repeat mutations that occur at a collective frequency of about 0.4%. These mutations significantly alter BMD and display impaired transactivation function, introducing a new class of functionally relevant RUNX2 mutants.

Runx2 promotes both osteoblastogenesis and novel osteoclastogenic signals in ST2 mesenchymal progenitor cells

We profiled the global gene expression of a bone marrow-derived mesenchymal pluripotent cell line in response to Runx2 expression. Besides osteoblast differentiation, Runx2 promoted the osteoclastogenesis of co-cultured splenocytes. This was attributable to the upregulation of many novel osteoclastogenic genes and the downregulation of anti-osteoclastogenic genes.

INTRODUCTION

In addition to being a master regulator for osteoblast differentiation, Runx2 controls osteoblast-driven osteoclastogenesis. Previous studies profiling gene expression during osteoblast differentiation had limited focus on Runx2 or paid little attention to its role in mediating osteoblast-driven osteoclastogenesis.

METHODS

ST2/Rx2(dox), a bone marrow-derived mesenchymal pluripotent cell line that expresses Runx2 in response to Doxycycline (Dox), was used to profile Runx2-induced gene expression changes. Runx2-induced osteoblast differentiation was assessed based on alkaline phosphatase staining and expression of classical marker genes. Osteoclastogenic potential was evaluated by TRAP staining of osteoclasts that differentiated from primary murine splenocytes co-cultured with the ST2/Rx2(dox) cells. The BeadChip™ platform (Illumina) was used to interrogate genome-wide expression changes in ST2/Rx2(dox) cultures after treatment with Dox or vehicle for 24 or 48 h. Expression of selected genes was also measured by RT-qPCR.

RESULTS

Dox-mediated Runx2 induction in ST2 cells stimulated their own differentiation along the osteoblast lineage and the differentiation of co-cultured splenocytes into osteoclasts. The latter was attributable to the stimulation of osteoclastogenic genes such as Sema7a, Ltc4s, Efnb1, Apcdd1, and Tnc as well as the inhibition of anti-osteoclastogenic genes such as Tnfrsf11b (OPG), Sema3a, Slco2b1, Ogn, Clec2d (Ocil), Il1rn, and Rspo2.

CONCLUSION

Direct control of osteoblast differentiation and concomitant indirect control of osteoclast differentiation, both through the activity of Runx2 in pre-osteoblasts, constitute a novel mechanism of coordination with a potential crucial role in coupling bone formation and resorption.

RUNX2 analysis of Danish cleidocranial dysplasia families

Cleidocranial dysplasia (CCD) is an autosomal dominant inherited disease caused by mutations in the Runt gene RUNX2. Screening of 19 Danish CCD families revealed 16 pathogenic mutations (84%) representing 8 missense mutations, 2 nonsense mutations, 4 frame-shift mutations and 2 large deletions in the RUNX2 locus. Eight mutations were novel, two were found twice, and polymorphisms were found in the promoter region and in the conserved polyglutamine/polyalanine repeat. A large duplication downstream of RUNX2 found in one patient suggests a possible regulatory RUNX2 element. The CCD phenotypes and genotypes adhere to the large phenotypic variability reported in previous CCD studies. Identification of large chromosome aberrations in or near the RUNX2 locus in 3 of the 19 cases suggests copy number analyses to be included in future RUNX2 mutation analyses.

Runx2 overexpression in bone marrow stromal cells accelerates bone formation in critical-sized femoral defects

The repair of large nonunions in long bones remains a significant clinical problem due to high failure rates and limited tissue availability for auto- and allografts. Many cell-based strategies for healing bone defects deliver bone marrow stromal cells (BMSCs) to the defect site to take advantage of the inherent osteogenic capacity of this cell type. However, many factors, including donor age and ex vivo expansion of the cells, cause BMSCs to lose their differentiation ability. To overcome these limitations, we have genetically engineered BMSCs to constitutively overexpress the osteoblast-specific transcription factor Runx2. In the present study, we examined Runx2-modified BMSCs, delivered via polycaprolactone scaffolds loaded with type I collagen meshes, in critical-sized segmental defects in rats compared to unmodified cells, cell-free scaffolds, and empty defects. Runx2 expression in BMSCs accelerated healing of critical-sized defects compared to unmodified BMSCs and defects receiving cell-free treatments. These findings provide an accelerated method for healing large bone defects, which may reduce recovery time and the need for external fixation of critical-sized defects.

Novel RUNX2 Mutations in Chinese Individuals with Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD) is an inherited autosomal-dominant skeletal disease caused by heterozygous mutations in the osteoblast-specific transcription factor, RUNX2. We performed mutation analysis of RUNX2 on four unrelated Chinese individuals with CCD. Three novel distinct mutations were detected in the coding region of RUNX2: two missense and one frameshift. These mutations were exclusively clustered within the Runt domain. One missense mutation converts threonine to isoleucine at codon 200 (T200I). The other one substitutes leucine for arginine at codon 225 (R225L), which affects many family members. The frame-shift mutation (214fs) in exon3 leads to the introduction of a translational stop codon at codon 221, resulting in a truncated RUNX2 protein. The reporter gene assays revealed that all the mutants exhibited significantly reduced transactivation activities on the osteocalcin promoter. Our results provide new genetic evidence that mutations involved in RUNX2 contribute to CCD. Abbreviations: AML3, gene encoding acute myeloid leukemia protein 3; bp, base pair; CBFA1, gene encoding core-binding factor 1; CBFβ, gene encoding core-binding factor β; CCD, cleidocranial dysplasia; NLS, nuclear localization signal; OSE2, osteoblast-specific cis-acting element 2; PEBP2A, gene encoding polyoma enhancer binding protein 2A; PST, proline/serine/ threonine-rich domain; Q/A, glutamine-alanine repeat domain; Runt, Runt Homology Domain; RUNX2, the mammalian runt-related genes 2; RUNX2, Runt-related protein 2.

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.

Cleidocranial dysplasia with severe parietal bone dysplasia: C-terminal RUNX2 mutations

BACKGROUND

Cleidocranial dysplasia (CCD) is an autosomal-dominant skeletal dysplasia syndrome that is characterized by widely patent calvarial sutures, clavicular hypoplasia, supernumerary teeth, and short stature. CCD is caused by mutations in the transcription factor RUNX2, which is known to function as a major regulator of bone differentiation. Despite the characterization of 67 unique mutations in 97 individual cases, and the availability of animal models, no obvious genotype-phenotype correlation has emerged.

METHODS

We describe 3 new cases that were ascertained on the basis of a severe calvarial phenotype, that were associated with 3 novel mutations in the C-terminal region of RUNX2 distal to the DNA-binding runt domain. In addition, a review of all previously described cases was undertaken in an effort to standardize mutation nomenclature, characterize the position of known mutations relative to the runt domain, and explore the hypothesis that C-terminal mutations that preserve the runt domain may lead to more-severe craniofacial phenotypes.

RESULTS

Upon mutational analysis of RUNX2, we identified either frameshift or splice-site mutations that affect the C-terminal region of the resultant protein distal to the runt domain.

CONCLUSIONS

In the context of previously described mutations, these cases suggest that C-terminal mutations that preserve the DNA-binding runt domain while disrupting the SMAD 1,2,3,5 binding domain and the nuclear matrix targeting signal may be responsible for the severe phenotype observed.

A novel RUNX2 missense mutation predicted to disrupt DNA binding causes cleidocranial dysplasia in a large Chinese family with hyperplastic nails

Background

Cleidocranial dysplasia (CCD) is a dominantly inherited disease characterized by hypoplastic or absent clavicles, large fontanels, dental dysplasia, and delayed skeletal development. The purpose of this study is to investigate the genetic basis of Chinese family with CCD.

Methods

Here, a large Chinese family with CCD and hyperplastic nails was recruited. The clinical features displayed a significant intrafamilial variation. We sequenced the coding region of the RUNX2 gene for the mutation and phenotype analysis.

Results

The family carries a c.T407C (p.L136P) mutation in the DNA- and CBFβ-binding Runt domain of RUNX2. Based on the crystal structure, we predict this novel missense mutation is likely to disrupt DNA binding by RUNX2, and at least locally affect the Runt domain structure.

Conclusion

A novel missense mutation was identified in a large Chinese family with CCD with hyperplastic nails. This report further extends the mutation spectrum and clinical features of CCD. The identification of this mutation will facilitate prenatal diagnosis and preimplantation genetic diagnosis.

Networks and hubs for the transcriptional control of osteoblastogenesis

We present an overview of the concepts of tissue-specific transcriptional control mechanisms essential for development of the bone cell phenotype. BMP2 induced transcription factors constitute a network of activities and molecular switches for bone development and osteoblast differentiation. Among these regulators are Runx2 (Cbfa1/AML3), the principal osteogenic master gene for bone formation, as well as homeodomain proteins and osterix. Runx2 has multiple regulatory activities, including activation or repression of gene expression, and integration of biological signals from developmental cues, such as BMP/TGFbeta, Wnt and Src signaling pathways. Runx2 provides a new paradigm for transcriptional control by functioning as a principal scaffolding protein in nuclear microenvironments to control gene expression in response to physiologic signals (growth factors, cytokines and hormones). The protein serves as a hub for the coordination of activities essential for the expansion and differentiation of osteogenic lineage cells through the formation of co-regulatory protein complexes organized in subnuclear domains. Mechanisms by which Runx2 supports commitment to osteogenesis and determines cell fate involve its retention on mitotic chromosomes. Disruption of a unique protein module, the subnuclear targeting signal of Runx2, has profound effects on osteoblast differentiation and metastasis of cancer cells in the bone microenvironment. Runx2 target genes include regulators of cell growth control, components of the bone extracellular matrix, angiogenesis, and signaling proteins for development of the osteoblast phenotype and bone turnover. The specificity of Runx2 regulatory activities provides a basis for novel therapeutic strategies to correct bone disorders.

Four novelRUNX2 mutations including a splice donor site result in the cleidocranial dysplasia phenotype

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder caused by haploinsufficiency of the RUNX2 gene. In this study, we analyzed by direct sequencing RUNX2 mutations from eleven CCD patients. Four of seven mutations were novel: two nonsense mutations resulted in a translational stop at codon 50 (Q50X) and 112 (E112X); a missense mutation converted arginine to glycine at codon 131 (R131G); and an exon 1 splice donor site mutation (donor splice site GT/AT, IVS1 + 1G > A) at exon 1-intron junction resulted in the deletion of QA stretch contained in exon 1 of RUNX2. We focused on the functional analysis of the IVS1 + 1G > A mutation. A full-length cDNA of this mutation was cloned (RUNX2Deltae1) and expressed in Chinese hamster ovary (CHO) and HeLa cells. Functional analysis of RUNX2Deltae1 was performed with respect to protein stability, nuclear localization, DNA binding, and transactivation activity of a downstream RUNX2 target gene. Protein stability of RUNX2Deltae1 is similar to wild-type RUNX2 as determined by Western blot analysis. Subcellular localization of RUNX2Deltae1, assessed by in situ immunofluorescent staining, was observed with partial retention in both the nucleus and cytoplasm. This finding is in contrast to RUNX2 wild-type, which is detected exclusively in the nucleus. DNA binding activity was also compromised by the RUNX2Deltae1 in gel shift assay. Finally, RUNX2Deltae1 blocked transactivation of the osteocalcin gene determined by transient transfection assay. Our findings demonstrate for the first time that the CCD phenotype can be caused by a splice site mutation, which results in the deletion of N-terminus amino acids containing the QA stretch in RUNX2 that contains a previously unidentified second nuclear localization signal (NLS). We postulate that the QA sequence unique to RUNX2 contributes to a competent structure of RUNX2 that is required for nuclear localization, DNA binding, and transactivation function.

Neonatal lethal osteochondrodysplasia with low serum levels of alkaline phosphatase and osteocalcin

Neonatal lethal skeletal dysplasias are rare and typically involve thoracic malformations and severe limb shortening. We report on a newborn boy manifesting an osteochondrodysplasia associated with fatal respiratory insufficiency who had normal lung volumes and extremity lengths. His disorder featured aberrant skeletal patterning and defective ossification including a severely osteopenic skull, apparent absence of clavicles, and clefting of the mandible and vertebrae. Serum alkaline phosphatase and osteocalcin levels were markedly low. Biochemical studies suggested parathyroid insufficiency probably from critical illness. Histopathology at autopsy excluded impaired mineralization of skeletal matrix, but endochondral bone formation appeared disorganized with growth plate clustering of chondrocytes in hypertrophic zones and in zones of provisional calcification. Parathyroid glands were not found. Despite features of two distinctive heritable entities, hypophosphatasia and cleidocranial dysplasia, the cumulative findings did not match either condition, and no mutations were found in either the tissue nonspecific ALP isoenzyme or core-binding factor genes, respectively, or in the genes encoding osteocalcin or the osteoblast transcription factor osterix. This patient could represent the extreme of cleidocranial dysplasia (a disorder not always associated with structural mutation in core-binding factor A1), but more likely he defines a unique osteochondrodysplasia disrupting both intramembranous and endochondral bone formation.

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

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

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

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

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

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

The role of a Runt domain transcription factor AML1/RUNX1 in leukemogenesis and its clinical implications

A Runt domain transcription factor AML1/RUNX1 is essential for generation and differentiation of definitive hematopoietic stem cells. AML1 is the most frequent target of chromosomal translocations in acute leukemias. Several chimeric proteins such as AML1-MTG8 and TEL-AML1 have transdominant properties for wild-type AML1 and acts as transcriptional repressors. The transcriptional repression in AML1 fusion proteins is mediated by recruitment of nuclear corepressor complex that maintains local histone deacetylation. Inhibition of the expression of AML1-responsive genes leads to a block in hematopoietic cell differentiation and consequent leukemic transformation. On the other hand, mutations in the Runt domain of the AML1 are identified in both sporadic acute myeloblastic leukemia (AML) without AML1 translocation and familial platelet disorder with predisposition to AML. These observations indicate that a decrease in AML1 dosage resulting from chromosomal translocations or mutations contributes to leukemogenesis. Furthermore, dysregulated chromatin remodeling and transcriptional control appears to be a common pathway in AML1-associated leukemias that could be an important target for the development of new therapeutic agents.

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.

Integration of Runx and Smad regulatory signals at transcriptionally active subnuclear sites

Runx factors control lineage commitment and are transcriptional effectors of Smad signaling. Genetic defects in these pathways interfere with normal development. The in situ localization of Runx and Smad proteins must impact the mechanisms by which these proteins function together in gene regulation. We show that the integration of Runx and Smad signals is mediated by in situ interactions at specific foci within the nucleus. Activated Smads are directed to these subnuclear foci only in the presence of Runx proteins. Smad-Runx complexes are associated in situ with the nuclear matrix, and this association requires the intranuclear targeting signal of Runx factors. The convergence of Smad and Runx proteins at these sites supports transcription as reflected by BrUTP labeling and functional cooperativity between the proteins. Thus, Runx-mediated intranuclear targeting of Smads is critical for the integration of two distinct pathways essential for fetal development.

New mutations in the CBFA1 gene in two Mexican patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal disorder exhibiting a wide clinical spectrum ranging from minimal anomalies to classic CCD. Mutations scattered throughout the entire CBFA1 gene have been related to this disorder. However, it seems that most of them affect the highly conserved Runt domain, abolishing the DNA-binding ability of this transcription factor. Moreover, no systematic effect has been found to relate the type of mutation to the severity of the clinical features. In this paper, we studied two unrelated patients with classic CCD. DNA analysis revealed two novel mutations and three undescribed polymorphisms. One of the substitutions was a missense mutation in the Q/A domain leading to the replacement of a polar residue by a nonpolar one (158 A --> T [Q53L]). The second was an uncommon heterozygous stop codon mutation (1565 G --> C [X522S]) which theoretically results in a longer protein with 23 additional amino acids. This is the first report of this type of mutation in CBFA1. We discuss the possible consequences of these mutant sequences, although no phenotype-genotype correlation could be established. Our findings expand the existing number of allelic variants in this pathology.

Mutations in the RUNX2 gene in patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a autosomal dominant disorder characterized by skeletal anomalies such as patent fontanels, late closure of cranial sutures with Wormian bones, late erupting secondary dentition, rudimentary clavicles, and short stature. The locus for this disease was mapped to chromosome 6p21. RUNX2 is a member of the runt family of transcription factors and its expression is restricted to developing osteoblasts and a subset of chondrocytes. Mutations in the RUNX2 gene have been shown to cause CCD. Chromosomal translocations, deletions, insertions, nonsense and splice-site mutations, as well as missense mutations of the RUNX2 gene have been described in CCD patients. Although there is a wide spectrum in phenotypic variability ranging from primary dental anomalies to all CCD features plus osteoporosis, no clear phenotype-genotype correlation has been established. However analysis of the three-dimensional structure of the DNA binding runt domain of the RUNX proteins and its interaction with DNA, as well as the cofactor CBFB, start to provide an insight into how missense mutations affect RUNX2 function.

A case of a Japanese patient with cleidocranial dysplasia possessing a mutation of CBFA1 gene

Cleidocranial dysplasia (CCD) is an autosomal dominant human bone disease characterized by hypoplastic or aplastic clavicles, wide cranial sutures, supernumerary teeth, short stature, and other skeletal disorders. Recently, various mutations of the core binding factor (CBFA1) gene have been detected in CCD patients. The CBFA1 gene is a member of the runt family of transcription factors. We experienced one Japanese case of CCD with open sutures, hypoplasia of clavicles and brachydactyly, combined with atlant-axis dislocation. We performed the sequence analysis of the CBFA1 gene and detected a missense mutation of R225W in exon 3.

A specific targeting signal directs Runx2/Cbfa1 to subnuclear domains and contributes to transactivation of the osteocalcin gene

Key components of DNA replication and the basal transcriptional machinery as well as several tissue-specific transcription factors are compartmentalized in specialized nuclear domains. In the present study, we show that determinants of subnuclear targeting of the bone-related Runx2/Cbfa1 protein reside in the C-terminus. With a panel of C-terminal mutations, we further demonstrate that targeting of Runx2 to discrete subnuclear foci is mediated by a 38 amino acid sequence (aa 397-434). This nuclear matrix-targeting signal (NMTS) directs the heterologous Gal4 protein to nuclear-matrix-associated Runx2 foci and enhances transactivation of a luciferase gene controlled by Gal4 binding sites. Importantly, we show that targeting of Runx2 to the NM-associated foci contributes to transactivation of the osteoblast-specific osteocalcin gene in osseous cells. Taken together, these findings identify a critical component of the mechanisms mediating Runx2 targeting to subnuclear foci and provide functional linkage between subnuclear organization of Runx2 and bone-specific transcriptional control.

Differential expression patterns of Runx2 isoforms in cranial suture morphogenesis

Runx2 (previously known as Cbfal/Pebp2alphaA/AML3), a key transcription factor in osteoblast differentiation, has at least two different isoforms using alternative promoters, which suggests that the isoforms might be expressed differentially. Haploinsufficiency of the Runx2 gene is associated with cleidocranial dysplasia (CCD), the main phenotype of which is inadequate development of calvaria. In spite of the biological relevance, Runx2 gene expression patterns in developing calvaria has not been explored previously, and toward this aim we developed three probes: pRunx2, which comprises the common coding sequence of Runx2 and hybridizes with all isoforms; pPebp2alphaA, which specifically hybridizes with the isoform transcribed with the proximal promoter; and pOsf2, which hybridizes with the isoform transcribed with the distal promoter. These probes were hybridized with tissue sections of mouse calvaria taken at various time points in development. Runx2 expression was localized to the critical area of cranial suture closure, being found in parietal bones, osteogenic fronts, and sutural mesenchyme. Pebp2alphaA and Osf2 showed tissue-specific expression patterns. The sites of Pebp2alphaA expression were almost identical to that of pRunx2 hybridization but expression was most intense in the sutural mesenchyme, where undifferentiated mesenchymal cells reside. The Osf2 isoform was strongly expressed in the osteogenic fronts, as well as in developing parietal bones, where osteopontin (OP) and osteocalcin (OC) also were expressed. However, in contrast to Pebp2alphaA, Osf2 expression did not occur in sutural mesenchyme. Pebp2alphaA also was expressed prominently in primordial cartilage that is found under the sutural mesenchyme and is not destined to be mineralized. Thus, Osf2 isoforms contribute to events later in osteoblast differentiation whereas the Pebp2alphaA isoform participates in a wide variety of cellular activities ranging from early stages of osteoblast differentiation to the final differentiation of osteoblasts.

Functional mutagenesis of AML1/RUNX1 and PEBP2 beta/CBF beta define distinct, non-overlapping sites for DNA recognition and heterodimerization by the Runt domain

The Runt domain family of transcription factors play key roles in transcriptional regulation of definitive hematopoiesis and osteogenesis. This transcription factor family is characterized by a DNA-binding alpha-subunit harboring the Runt domain and a secondary subunit, beta, which binds to the Runt domain and enhances its interaction with DNA. Missense mutations in the Runt domain from either the blood or bone-related gene product are associated with the onset of acute human leukemia as well as a disease of skeletal patterning known as cleidocranial dysplasia. NMR "footprinting" analysis of Runt domain/beta/DNA ternary complexes in solution previously identified the likely residues that form the heterodimerization and DNA-binding surfaces of the Runt domain. Functional mutagenesis at 37 positions in the Runt domain or beta confirms the original identification of these interaction surfaces and reveals that the heterodimerization and DNA-binding surfaces of the Runt domain occur at distinct, non-overlapping sites within the domain. The analysis of an additional 21 disease-related missense mutations identified from patients with either blood or bone disease demonstrates that the primary defect in these patients is a failure in DNA-recognition by the Runt domain. The molecular basis for the DNA-binding defect is analyzed in the context of the three-dimensional structure of the Runt domain in binary and ternary protein/DNA complexes.

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.

A RUNX2/PEBP2alpha A/CBFA1 mutation displaying impaired transactivation and Smad interaction in cleidocranial dysplasia

Cleidocranial dysplasia (CCD), an autosomal-dominant human bone disease, is thought to be caused by heterozygous mutations in runt-related gene 2 (RUNX2)/polyomavirus enhancer binding protein 2alphaA (PEBP2alphaA)/core-binding factor A1 (CBFA1). To understand the mechanism underlying the pathogenesis of CCD, we studied a novel mutant of RUNX2, CCDalphaA376, originally identified in a CCD patient. The nonsense mutation, which resulted in a truncated RUNX2 protein, severely impaired RUNX2 transactivation activity. We show that signal transducers of transforming growth factor beta superfamily receptors, Smads, interact with RUNX2 in vivo and in vitro and enhance the transactivation ability of this factor. The truncated RUNX2 protein failed to interact with and respond to Smads and was unable to induce the osteoblast-like phenotype in C2C12 myoblasts on stimulation by bone morphogenetic protein. Therefore, the pathogenesis of CCD may be related to the impaired Smad signaling of transforming growth factor beta/bone morphogenetic protein pathways that target the activity of RUNX2 during bone formation.