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.

A novel missense mutation of the CBFA1 gene in a family with cleidocranial dysplasia (CCD) and variable expressivity

The aim of this study was to analyze the CBFA1 gene in a phenotypically variable family with autosomal dominant cleidocranial dysplasia (CCD). Five members of a family with CCD were characterized clinically. X-rays and photographs of the two clinically affected family members were taken. The genotype of all five affected family members was determined with the use of single strand conformation polymorphism (SSCP) and direct sequencing. A point-mutation in exon 2 (R148G) was detected in a patient with the full-blown clinical phenotype. His son, demonstrating the same mutation, showed only the dental CCD characteristics. No mutation could be found in the three clinically healthy family members. To conclude, a missense mutation in the CBFA1 gene was detected in a family with variably expressed CCD syndrome. A detailed clinical examination is necessary to detect minimally affected gene mutation carriers.

A case of Japanese cleidocranial dysplasia with a CBFA1 frameshift mutation

Cleidocranial dysplasia (CCD), which is caused by mutations of the core binding factor alpha 1 (CBFA1)/runt-related gene 2 (Runx2), is an autosomal, dominantly inherited disorder of high penetrance affecting skeletal ossification and tooth development. Recently, we found a novel frameshift mutation 383-T-insertion (S128F) in exon 3 in the CBFA1 gene of a Japanese classic CCD patient. We describe our detailed investigation of the patient with CCD associated with the CBFA1 mutation. The patient showed the characteristic expression of CCD, such as dysplasia of the clavicles, patent fontanelles, short stature, impacted supernumerary teeth, and delayed eruption of the permanent teeth. In addition to these characteristics, orthopantomography delayed ossification of the mandibular symphysis and a three-dimensional computed tomograph (3D-CT) analysis showed hypoplasia of the zygomatic arch. Furthermore, the acellular cementum of an impacted supernumerary tooth was absent in this patient. Thus, the CBFA1 mutation was critical for the pathogenesis of CCD in this patient.

The developmental control of osteoblast-specific gene expression: role of specific transcription factors and the extracellular matrix environment

Bone formation is a carefully controlled developmental process involving morphogen-mediated patterning signals that define areas of initial mesenchyme condensation followed by induction of cell-specific differentiation programs to produce chondrocytes and osteoblasts. Positional information is conveyed via gradients of molecules, such as Sonic Hedgehog that are released from cells within a particular morphogenic field together with region-specific patterns of hox gene expression. These, in turn, regulate the localized production of bone morphogenetic proteins and related molecules which initiate chondrocyte- and osteoblast-specific differentiation programs. Differentiation requires the initial commitment of mesenchymal stem cells to a given lineage, followed by induction of tissue-specific patterns of gene expression. Considerable information about the control of osteoblast-specific gene expression has come from analysis of the promoter regions of genes encoding proteins like osteocalcin that are selectively expressed in bone. Both general and tissue-specific transcription factors control this promoter. Osf2/Cbfa1, the first osteoblast-specific transcription factor to be identified, is expressed early in the osteoblast lineage and interacts with specific DNA sequences in the osteocalcin promoter essential for its selective expression in osteoblasts. The OSF2/CBFA1 gene is necessary for the development of mineralized tissues, and its mutation causes the human disease, cleidocranial dysplasia. Committed osteoprogenitor cells already expressing Osf2/Cbfa1 must synthesize a collagenous ECM before they will differentiate. A cell:ECM interaction mediated by integrin-type cell-surface receptors is essential for the induction of osteocalcin and other osteoblast-related proteins. This interaction stimulates the binding of Osf2/Cbfa1 to the osteocalcin promoter through an as-yet-undefined mechanism.

The 'chef's hat' appearance of the femoral head in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is inherited as an autosomal dominant disorder characterised by failure of membranous ossification. The condition is due to a mutation of the cbfa1 gene on chromosome 6 which has a role in the development of osteoblasts from the mesenchymal cells. In their growing years, these patients have an unusual shape of the femoral head reminiscent of a 'chef's hat'. In order to confirm the consistency of this sign, we have reviewed the radiographs of 28 patients with CCD. All except three had this appearance. The sign was also seen in patients with coxa vara associated with a variety of other conditions. The chef's hat sign may occur secondary to the particular mechanical environment created by coxa vara as well as abnormal cellular function in patients with CCD. Although coxa vara has some influence on the shape of the femoral head, it is not entirely responsible for its morphology since it was present in only six of the 28 patients with CCD.

PEBP2alphaA/CBFA1 mutations in Japanese cleidocranial dysplasia patients

Cleidocranial dysplasia (CCD) is an autosomal dominant human bone disease whose genetic locus has been located on chromosome 6p21, where the PEBP2alphaA/CBFA1 gene essential for osteogenesis also maps. Previously, several heterozygous mutations in PEBP2alphaA/CBFA1 were found in CCD patients. In this study, we identified six different types of mutations in PEBP2alphaA/CBFA1 in Japanese CCD patients. Four cases were similar to those reported previously: two were nonsense mutations in the Runt domain, one was a hemizygous deletion, and the other was a missense mutation in the Runt domain which abolished the DNA-binding activity of Runx2/PEBP2alphaA/CBFA1. The remaining two mutations were novel: one had a heterozygous gt-to-tt mutation at the splice donor site (gt) between the exon3-intron junction, which resulted in abnormal exon3 skipping, and the other had a mutation in exon7, which led to the introduction of a translational stop codon in the middle of the transactivation domain. Thus, defects in either the DNA-binding domain or transactivation domain of Runx2/PEBP2alphaA/CBFA1 can cause CCD. The results not only provide a strong genetic evidence that mutations involving in PEBP2alphaA/CBFA1 contribute to CCD, but also provide a useful tool to study how Runx2/PEBP2alphaA/CBFA1 plays its pivotal role during osteoblastic differentiation.

Early prenatal ultrasound diagnosis of cleidocranial dysplasia

A woman was referred in the first trimester of her third pregnancy because of a family history of cleidocranial dysplasia. An ultrasound examination at 14 weeks 4 days revealed a fetus with appropriate biometric measurements. However, the clavicles were noted to be hypoplastic and the cranium appeared less well ossified than expected for gestational age, suggesting the diagnosis of cleidocranial dysplasia. On subsequent examination at 21 weeks, the findings were essentially unchanged. Induced vaginal delivery owing to decreased amniotic fluid volume occurred at 37 weeks, and a female weighing 3200 g was delivered. The infant had clinical and X-ray signs of cleidocranial dysplasia.

CBFA1 mutation analysis and functional correlation with phenotypic variability in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a dominantly inherited skeletal dysplasia caused by mutations in the osteoblast-specific transcription factor CBFA1. To correlate CBFA1 mutations in different functional domains with the CCD clinical spectrum, we studied 26 independent cases of CCD and a total of 16 new mutations were identified in 17 families. The majority of mutations were de novo missense mutations that affected conserved residues in the runt domain and completely abolished both DNA binding and transactivation of a reporter gene. These, and mutations which result in premature termination in the runt domain, produced a classic CCD phenotype by abolishing transactivation of the mutant protein with consequent haploinsufficiency. We further identified three putative hypomorphic mutations (R391X, T200A and 90insC) which result in a clinical spectrum including classic and mild CCD, as well as an isolated dental phenotype characterized by delayed eruption of permanent teeth. Functional studies show that two of the three mutations were hypomorphic in nature and two were associated with significant intrafamilial variable expressivity, including isolated dental anomalies without the skeletal features of CCD. Together these data show that variable loss of function due to alterations in the runt and PST domains of CBFA1 may give rise to clinical variability, including classic CCD, mild CCD and isolated primary dental anomalies.

Mutation analysis of core binding factor A1 in patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a dominantly inherited disorder characterized by patent fontanelles, wide cranial sutures, hypoplasia of clavicles, short stature, supernumerary teeth, and other skeletal anomalies. We recently demonstrated that mutations in the transcription factor CBFA1, on chromosome 6p21, are associated with CCD. We have now analyzed the CBFA1 gene in 42 unrelated patients with CCD. In 18 patients, mutations were detected in the coding region of the CBFA1 gene, including 8 frameshift, 2 nonsense, and 9 missense mutations, as well as 2 novel polymorphisms. A cluster of missense mutations at arginine 225 (R225) identifies this residue as crucial for CBFA1 function. In vitro green fluorescent protein fusion studies show that R225 mutations interfere with nuclear accumulation of CBFA1 protein. There is no phenotypic difference between patients with deletions or frameshifts and those with other intragenic mutations, suggesting that CCD is generally caused by haploinsufficiency. However, we were able to extend the CCD phenotypic spectrum. A missense mutation identified in one family with supernumerary teeth and a radiologically normal skeleton indicates that mutations in CBFA1 can be associated exclusively with a dental phenotype. In addition, one patient with severe CCD and a frameshift mutation in codon 402 had osteoporosis leading to recurrent bone fractures and scoliosis, providing first evidence that CBFA1 may help maintain adult bone, in addition to its function in bone development.

Apoptosis in murine calvarial bone and suture development

To study the possible role of apoptosis in calvarial bone and suture development, terminal deoxynucleotidyl transferase-mediated nick-end labeling (TUNEL) was performed on whole mount and sectioned calvariae from mice aged between E14 and P6. We also analyzed by in situ hybridization the expression of Msx2, Bmp4 and Bmp7 genes, which are known to act in conserved signaling pathways leading to apoptosis. We found TUNEL-positive cells from E16 onwards in the calvarial bones, intervening sutures and fontanelles. TUNEL-positive osteoblasts and preosteoblasts were identified at or close to the osteogenic fronts, areas of intense osteogenic activity, with TUNEL-positive mesenchymal cells located in the midsutural mesenchyme. TUNEL-positive osteoclasts and osteocytes were also observed in a sporadic fashion, as well as TUNEL-positive dural cells. Msx2 was expressed in the sutural mesenchyme and the dura mater. Bmp4 was expressed in the developing bone, underlying dura mater, the osteogenic fronts, and also weakly in the sutural mesenchyme. Bmp7 was detected at the same locations as Bmp4 but with noticeably stronger intensity in the meninges and overlying epidermis. We propose that this apoptosis is part of normal suture development, and is integral to the balance between bone formation and resorption, so that abnormal apoptosis may lead to premature (Craniosynostosis) or delayed (Cleidocranial dysplasia) suture closure.

Anomalies of craniofacial skeleton and teeth in cleidocranial dysplasia

Mutations involving the transcription factor CBFA1 cause cleidocranial dysplasia (CCD) in man. Recently, a mouse model of CCD has been generated (Cbfal +/-) [Komori et al., 1997], and disturbances of osteoclast differentiation have been documented. It has been shown that these animals exhibit hypoplastic clavicles and nasal bones, and retarded ossification of parietal, interparietal, and supraoccipital bones. Humans with CCD show all these features, including severely retarded ossification of the cranial base, strongly suggesting that both intramembranous ossification and endochondral ossification are affected. In addition, CCD patients have multiple supernumerary teeth and delayed tooth eruption. The present report presents 3D reconstructions of computerised tomography (CT) scans of the craniofacial region of a CCD boy examined at both 1 and 7 years of age. The anomalies in craniofacial skeleton and teeth are analysed and compared to the findings of our previous clinical studies and to the findings in the animal model. Based on the available information, we suggest that osteoblast, osteoclast, and dentinoclast differentiation may be disturbed in CCD.

Cleidocranial dysplasia: clinical and molecular genetics

Cleidocranial dysplasia (CCD) (MIM 119600) is an autosomal dominant skeletal dysplasia characterised by abnormal clavicles, patent sutures and fontanelles, supernumerary teeth, short stature, and a variety of other skeletal changes. The disease gene has been mapped to chromosome 6p21 within a region containing CBFA1, a member of the runt family of transcription factors. Mutations in the CBFA1 gene that presumably lead to synthesis of an inactive gene product were identified in patients with CCD. The function of CBFA1 during skeletal development was further elucidated by the generation of mutated mice in which the Cbfa1 gene locus was targeted. Loss of one Cbfa1 allele (+/-) leads to a phenotype very similar to human CCD, featuring hypoplasia of the clavicles and patent fontanelles. Loss of both alleles (-/-) leads to a complete absence of bone owing to a lack of osteoblast differentiation. These studies show that haploinsufficiency of CBFA1 causes the CCD phenotype. CBFA1 controls differentiation of precursor cells into osteoblasts and is thus essential for membranous as well as endochondral bone formation.

Ischio-vertebral dysplasia: a distinct entity

BACKGROUND

Kyphoscoliosis is a complication of some bone dysplasias, including cleidocranial dysplasia.

OBJECTIVES

We report a distinct disorder with defective ossification of the ischial rami, severe kyphoscoliosis and normal clavicles. Early recognition of this syndrome allows prevention of complications.

MATERIALS AND METHODS

All patient cases (aged 1 day to 33 years) were selected according to the above criteria, with special attention to radiological findings, family history and follow-up (5-30 years).

RESULTS

In all eight patients, we observed the following: (a) Severe thoracic scoliosis of early onset and rapid progression, leading to rotatory dislocation. Spinal cord compression occurred in four cases with respiratory problems related to chest deformity. (b) Bilateral and symmetrical incomplete ossification of the ischial rami. (c) Peculiar facies with retrognathia. (d) Normal clavicles. Three patients were from the same family (grandmother, mother and daughter).

CONCLUSION

Ischio-vertebral dysplasia seems to represent a true entity, with radiological and genetic findings that make it distinct from cleidocranial dysostosis. The association of kyphoscoliosis and these pelvic abnormalities is specific for this condition. Neurological and respiratory complications can be avoided if the condition is recognised early and early treatment is instituted.

Discovery: Osf2/Cbfa1, a master gene of bone formation

This report reviews the current research that has impacted on our understanding of osteogenesis. Recent studies indicate that the transcription factor Osf2 (osteoblast specific transcription factor 2)/Cbfa1 (core binding factor activity 1) serves as a Master Gene regulating osteoblast-specific gene expression. The gene is expressed in cells of the osteoblast lineage only, and this expression is regulated by calciotropic agents. Moreover, when expressed in non-skeletal cells, the cells assume many of the characteristics of an osteoblast. In knockout experiments designed to assess the importance of the gene in osteogenesis, no evidence of bone formation could be observed in animals that are homozygous for the deletion. Studies of the heterozygote indicate that osteoblast function is compromised: there is a severe reduction in the number of bone cells, the tissue is deficient in bone proteins, and the activity of the enzyme alkaline phosphatase is low. It was noted that the heterozygote displays abnormalities that are remarkably similar to those exhibited by cleidocranial dysplastics. Indeed, Osf2 mapped close to a chromosomal locus on chromosome 6p21, long suspected of being involved with the disease. A search conducted for Osf2 mutations in kindreds with cleidocranial dysplasia revealed deletions, insertions, and missense mutations; these mutations are found to segregate with patients who are defined clinically as cleidocranial dysplastic. Aside from providing a new insight into a disease state that has so far avoided molecular analysis, results of the studies emphasize that the loss of a Master Gene drastically alters the development and maintenance of the appendicular skeleton and the craniofacial complex.

Genetic craniofacial aberrations

Many craniofacial and dental anomalies have a genetic background. Much research related to the molecular pathology of genetic conditions is being carried out, and new information related to mapping of disease genes, gene identification, and mutations in these genes is accumulating with incredible speed. It is important to be well informed of the molecular background of the conditions that we treat at anomaly clinics. This article reviews the most recent molecular findings related to Turner syndrome, Beckwith-Wiedemann syndrome, Marfan syndrome, Treacher Collins syndrome, cleidocranial dysplasia, and cleft lip and palate.

Delayed dental maturation in cleidocranial dysplasia

Cleidocranial dysplasia (CCD), a rare, inherited, generalized, skeletal and dental dysplasia, exhibiting an autosomal dominant mode of transmission, may be associated with delays during tooth maturation. To test whether permanent tooth formation is delayed in patients with CCD and if the presence of supernumerary teeth adversely influences maturation of the dentition, a group of CCD patients (eight females, three males) was compared to an equal number of control subjects matched for age and gender. Dental maturity was assessed using panoramic radiographs and the Dental Maturity Ratio, (DMR = mean dental age divided by the chronological age) was calculated. The mean DMR in CCD patients (0.87 +/- 0.14) was lower than in the control group (1.06 +/- 0.14), p < 0.01. Among patients with CCD, patients with supernumerary teeth, had a lower DMR (0.82 +/- 0.13 vs. 0.91 +/- 0.16), but the difference did not reach statistical significance. After adjusting for the presence of supernumerary teeth the diagnosis of CCD was still found to be associated with lower DMR than controls, p = 0.0569. We conclude that CCD patients have delayed tooth development of approximately 2.1 years and that among these patients, those with supernumerary teeth were further delayed by 1.5 years.

Transcriptional regulation of osteoblast differentiation during development

The osteoblast is the bone-forming cell. The molecular basis of osteoblast-specific gene expression and differentiation begin to be understood. Following the characterization of OSE2, an osteoblast-specific cis-acting element present in the Osteocalcin promoter Osf2/Cbfa1, the protein that binds to OSE2, was identified. Osf2/Cbfa1 is a member of the runt family of transcription factors. Its expression is initiated in the mesenchymal condensations of the developing skeleton and is strictly restricted to cells of the osteoblast lineage thereafter. Osf2/Cbfa1 binds to and regulates the expression of multiple genes expressed in osteoblasts, and forced expression of Osf2/Cbfa1 in nonosteoblastic cells induces the expression of the principal osteoblast-specific genes. Osf2/Cbfa1 gene inactivation in mice leads to failure of mesenchymal progenitor cells to differentiate into osteoblasts. Lastly, heterozygous mutations in the Osf2/Cbfa1 gene cause Cleidocranial dysplasia in human and mice, a condition marked by generalized bone defects. These result demonstrate that Osf2/Cbfa1 is an osteoblast-specific transcriptional activator of osteoblast differentiation whose function is non redundant with the function of other gene products during development.

Cbfa1 in bone development

A factor fundamental to bone formation has been identified. Gene targeting shows that core-binding factor alpha 1 (Cbfa1) plays an essential role in bone formation and osteoblast differentiation. Thus, it is now possible to begin examining the molecular mechanism of bone formation--especially osteoblast differentiation.

Cleidocranial dysostosis: report of a case

This case of cleidocranial dysostosis showed nearly 40 accessory teeth and the unerupted teeth on radiological examinations. Other diagnostic procedures found hypoplasia in maxillary and zygomatic bones, deep palate, open fontanel and open sutures, the collapse of sagittal suture, and aplasia of the clavicle. There were some abnormalities of the fingers. Under general anesthesia eleven supernumerary teeth in the mandible and eight in the maxilla were surgically extracted. Upper and lower removable acrylic appliances were fabricated.

[Syndromes. 1. Cleidocranial dysplasia]

Dysostosis cleido-cranialis is mainly characterized by a pathognomonic deformity of the skull, a hypoplastic midface, lack of eruption of permanent teeth, supernumerary teeth, defects in the clavicles, short stature and multiple other skeletal deformities. The inheritance is autosomal dominant, but there is a considerable number of new mutations.

Exclusion of BMP6 as a candidate gene for cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant, generalized skeletal dysplasia in humans that has been mapped to the short arm of chromosome 6. We report linkage of a CCD mutation to 6p21 in a large family and exclude the bone morphogenetic protein 6 gene (BMP6) as a candidate for the disease by cytogenetic localization and genetic recombination. CCD was linked with a maximal two-point LOD score of 7.22 with marker D6S452 at theta = 0. One relative with a recombination between D6S451 and D6S459 and another individual with a recombination between D6S465 and CCD places the mutation within a 7 cM region between D6S451 and D6S465 at 6p21. A phage P1 genomic clone spanning most of the BMP6 gene hybridized to chromosome 6 in band region p23-p24 using FISH analysis, placing this gene cytogenetically more distal than the region of linkage for CCD. We derived a new polymorphic marker from this same P1 clone and found recombinations between the marker and CCD in this family. The results confirm the map position of CCD on 6p21, further refine the CCD genetic interval by identifying a recombination between D6S451 and D6S459, and exclude BMP6 as a candidate gene.

Prenatal sonographic diagnosis of cleidocranial dysostosis

Cleidocranial dysostosis is an autosomal dominant disorder characterized by absence or hypoplasia of the clavicles, skull abnormalities, and abnormal dentition. The prenatal diagnosis of this condition has been reported once previously in a known high-risk pregnancy. In this report we describe the prenatal findings of cleidocranial dysostosis at 19 weeks' gestation in a woman affected with this disorder but undiagnosed before the fetal scan. This report is unique in the sense that an autosomal dominant condition diagnosed in the fetus led to a similar diagnosis in the mother.

The cDNA cloning of the transcripts of human PEBP2alphaA/CBFA1 mapped to 6p12.3-p21.1, the locus for cleidocranial dysplasia

PEBP2/CBF is a heterodimeric transcription factor composed of alpha and beta subunits. There are at least three closely related genes, PEBP2alphaA/Cbfa1, AML1/PEBP2alphaB/Cbfa2 and PEBP2alphaC/Cbfa3, encoding the alpha subunit and one beta subunit encoding gene. Structural alterations of AML1 and the beta subunit gene by chromosome translocations are frequently associated with several types of human leukemia. Structural changes of any of these gene products would have potential to affect the function of others. In this study, we isolated the human PEBP2alphaA cDNA by which we mapped the gene to 6p12.3-p21.1. Human chromosome 6p21 is the locus for cleidocranial dysplasia, an autosomal dominant bone disease. Recent gene disruption study revealed that PEBP2alphaA/Cbfa1 plays an essential role in osteogenesis (Komori et al., Cell, 1997, in press). Therefore, a close relationship between human PEBP2alphaA/CBFA1 and this bone disease is strongly implicated.