Genetic mapping of cleidocranial dysplasia and evidence of a microdeletion in one family

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.

Cbfa1: a molecular switch in osteoblast biology

During the past 4 years, our molecular understanding of osteoblast biology has made rapid progress due to the characterization of the function of one molecule, Cbfa1. This member of the runt/Cbfa family of transcription factors was first identified as the nuclear protein binding to an osteoblast-specific cis-acting element activating the expression of Osteocalcin, the most osteoblast-specific gene. Cbfa1 was then shown to regulate the expression of all the major genes expressed by osteoblasts. Consistent with this ability, genetic experiments identified Cbfa1 as a key regulator of osteoblast differentiation in vivo. Indeed, analysis of Cbfa1-deficient mice revealed that osteoblast differentiation is arrested in absence of Cbfa1, demonstrating both that it is required for this process and that no parallel pathway can overcome its absence. The importance of Cbfa1 in controlling osteoblast differentiation was further emphasized by the identification of Cbfa1 haploinsufficiency as the cause of cleidocranial dysplasia in humans and mice, a syndrome characterized by generalized bone defects. Lastly, Cbfa1 was shown to have a role beyond development and differentiation, regulating the rate of bone matrix deposition by differentiated osteoblasts. Thus, Cbfa1 is a critical gene not only for osteoblast differentiation but also for osteoblast function. These aspects, as well as the more recent progresses in understanding Cbfa1 biology, are the focuses of this review.

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.

Regulation of osteoblast differentiation mediated by bone morphogenetic proteins, hedgehogs, and Cbfa1

Osteoblasts arise from common progenitors with chondrocytes, muscle and adipocytes, and various hormones and local factors regulate their differentiation. We review here regulation of osteoblast differentiation mediated by the local factors such as bone morphogenetic proteins (BMPs) and hedgehogs and the transcription factor, core-binding factor alpha-1 (Cbfa1). BMPs are the most potent regulators of osteoblast differentiation among the local factors. Sonic and Indian hedgehogs are involved in osteoblast differentiation by interacting with BMPs. Cbfa1, a member of the runt domain gene family, plays a major role in the processes of a determination of osteoblast cell lineage and maturation of osteoblasts. Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation, because Cbfa1-deficient mice completely lacked bone formation due to maturation arrest ofosteoblasts. Although the regulatory mechanism of Cbfa1 expression has not been fully clarified, BMPs are an important local factor that up-regulates Cbfa1 expression. Thus, the intimate interaction between local factors such as BMPs and hedgehogs and the transcription factor, Cbfa1, is important to osteoblast differentiation and bone formation.

Clavicular hypoplasia, zygomatic arch hypoplasia, and micrognathia: a newly defined syndrome

We report on a 6-year-old boy with a previously undefined syndrome of clavicular hypoplasia, frontonasal malformation, zygomatic arch hypoplasia, micrognathia, and normal intelligence. His condition differs from similar syndromes on the basis of unique facial findings such as microcornea, stellate irises, and a midline maxillary cleft. We present his case, a review of the literature, and propose the acronym CHZAM, for clavicular hypoplasia, zygomatic arch, and micrognathia, to represent this syndrome.

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.

Molecular basis of tissue-specific gene expression mediated by the runt domain transcription factor PEBP2/CBF

The Runt domain transcription factor PEBP2/CBF is a heterodimer of alpha and beta subunits. Of the three mammalian alpha subunit genes, which are homologues of the Drosophila gene runt, PEBP2alphaA/CBFA1 and PEBP2alphaB/AML1 are critical regulators of osteogenesis and haematopoiesis, respectively. A unique functional interaction between PEBP2alphaB/AML1 and Ets-1 was recently observed, and provides a clear example of context-dependent transcriptional regulation. An elaborate mechanism of cooperation between the two factors may represent a basis for tissue-specific gene expression, which is thought to be achieved by combinations of specific sets of transcription factors unique to each cell lineage or stage of differentiation. A possible molecular basis for the critical role(s) played by PEBP2/CBF in tissue determination is discussed in light of these observations.

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.

Characterization of a human homologue of the Saccharomyces cerevisiae transcription factor spt3 (SUPT3H)

Spt3 is a Saccharomyces cerevisiae transcription factor that is required in vivo for the transcription of a number of RNA polymerase II-transcribed genes. We report the cloning of the gene encoding the human homologue of Spt3, SUPT3H, and its initial functional analysis. The human and yeast Spt3 homologues share an overall identity of 30% that defines three conserved regions, suggesting possible functional domains. To determine whether SUPT3H is a true functional Spt3 homologue, we tested for complementation of an spt3Delta mutation in yeast. While expression of the full-length SUPT3H is unable to complement an spt3Delta mutation, expression of a human-yeast chimeric gene that contains 42% human sequences can partially complement an spt3Delta mutation. These data suggest that the transcriptional control carried out by Spt3 has been conserved from yeast to human.

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.

Genomic organization, expression of the human CBFA1 gene, and evidence for an alternative splicing event affecting protein function

The Cbfa1 gene, which encodes the transcription factor Osf2/Cbfa1 required for osteoblast differentiation in mouse and human, is mutated in cleidocranial dysplasia, a skeletal dysplasia. We describe here the isolation of the full-length human OSF2/CBFA1 cDNAs, the genomic organization of the entire CBFA1 gene, its expression, and the existence of an alternative splicing event. Nucleotide sequence analysis of the human and mouse OSF2/CBFA1 cDNAs showed a 98% homology in the coding sequence and 96% in the 5' untranslated (UTR) sequence. Analysis of CBFA1 genomic clones revealed that the 5' UTR sequence of the human OSF2/CBFA1 cDNA lies 75 kb upstream of the originally described 5' end of the gene. The existence of two OSF2/CBFA1 cDNAs is due to an alternative splicing event around exon 8 that affects the transcriptional activity of the protein. Northern blot analysis demonstrates that the expression of the human OSF2/CBFA1 gene is restricted to osteoblastic cells.

Mouse clavicular development: analysis of wild-type and cleidocranial dysplasia mutant mice

Cleidocranial dysplasia (CCD) is an autosomal dominant disease characterized by hypoplasia or aplasia of clavicles, open fontanelles, and other skeletal anomalies. A mouse mutant, shown by clinical and radiographic analysis to be strikingly similar to the human disorder and designated Ccd, was used as a model for the human disorder. Since malformation of the clavicle is the hallmark of CCD, we studied clavicular development in wild-type and Ccd mice. Histology and in situ hybridization experiments were performed to compare the temporal and spatial expression of several genes in wild-type and Ccd mutant mouse embryos. Bone and cartilage specific markers--type I, II, and X collagens, Sox9, aggrecan, and osteopontin were used as probes. The analyses covered the development of the clavicle from the initial mesenchymal condensation at embryonic day 13 (E13) to the late mineralization stage at embryonic day 15.5. At day 13.5, cells in the center of the condensation differentiate into characteristic precursor cells that were not observed in other bone anlagen. In the medial part of the anlage these cells express markers of the early cartilage lineage (type II collagen and Sox9), whereas cells of the lateral part express markers of the osteoblast lineage (type I collagen). With further development the medial cells differentiate into chondrocytes and start to express chondrocyte-specific markers such as aggrecan. Cells of the lateral part differentiate into osteoblasts as indicated by the production of bone matrix and the expression of osteopontin. At day 14.5 a regular growth plate has developed between the two parts where type X collagen expression can be demonstrated in hypertrophic chondrocytes. The data indicate that the medial part of the clavicle develops by endochondral bone formation while the lateral part ossifies as a membranous bone. The clavicle of Ccd mice showed a smaller band of mesenchymal cell condensation than in wild-type mice. Cells of the condensation failed to express type I and type II collagen at E13.5. In the lateral part of the clavicle type I collagen expression was not detected until E14.5 and osteopontin expression only appeared at E15.5. At E15.5, a small ossification center appears in the lateral part which is, in contrast to the wild-type clavicular bone, solid and without primary spongiosa as well as bone marrow. In the medial portion, type II collagen expression and endochondral ossification never occurs in Ccd mice; this portion of the clavicle is therefore missing in Ccd.

Missense mutations abolishing DNA binding of the osteoblast-specific transcription factor OSF2/CBFA1 in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder characterized by hypoplastic or absent clavicles, large fontanelles, dental anomalies and delayed skeletal development. The phenotype is suggestive of a generalized defect in ossification and is one of the most common skeletal dysplasias not associated with disproportionate stature. To date, no genetic determinants of ossification have been identified. CCD has been mapped to chromosome 6p21, where CBFA1, a gene encoding OSF2/CBFA1, a transcriptional activator of osteoblast differentiation, has been localized. Here, we describe two de novo missense mutations, Met175Arg and Ser191Asn, in the OSF2/CBFA1 gene in two patients with CCD. These two mutations result in substitution of highly conserved amino acids in the DNA-binding domain. DNA-binding studies with the mutant polypeptides show that these amino acid substitutions abolish the DNA-binding ability of OSF2/CBFA1 to its known target sequence. Concurrent studies show that heterozygous nonsense mutations in OSF2/CBFA1 also result in CCD, while mice homozygous for the osf2/cbfa1 mull allele exhibit a more severe lethal phenotype. Thus, these results together suggest that CCD is produced by haploinsufficiency of OSF2/CBFA1 and provide direct genetic evidence that the phenotype is secondary to an alteration of osteoblast differentiation.

Cbfa1, a candidate gene for cleidocranial dysplasia syndrome, is essential for osteoblast differentiation and bone development

We have generated Cbfa1-deficient mice. Homozygous mutants die of respiratory failure shortly after birth. Analysis of their skeletons revealed an absence of osteoblasts and bone. Heterozygous mice showed specific skeletal abnormalities that are characteristic of the human heritable skeletal disorder, cleidocranial dysplasia (CCD). These defects are also observed in a mouse Ccd mutant for this disease. The Cbfa1 gene was shown to be deleted in the Ccd mutation. Analysis of embryonic Cbfa1 expression using a lacZ reporter gene revealed strong expression at sites of bone formation prior to the earliest stages of ossification. Thus, the Cbfa1 gene is essential for osteoblast differentiation and bone formation, and the Cbfa1 heterozygous mouse is a paradigm for a human skeletal disorder.

Mutations involving the transcription factor CBFA1 cause cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal-dominant condition characterized by hypoplasia/aplasia of clavicles, patent fontanelles, supernumerary teeth, short stature, and other changes in skeletal patterning and growth. In some families, the phenotype segregates with deletions resulting in heterozygous loss of CBFA1, a member of the runt family of transcription factors. In other families, insertion, deletion, and missense mutations lead to translational stop codons in the DNA binding domain or in the C-terminal transactivating region. In-frame expansion of a polyalanine stretch segregates in an affected family with brachydactyly and minor clinical findings of CCD. We conclude that CBFA1 mutations cause CCD and that heterozygous loss of function is sufficient to produce the disorder.

Osf2/Cbfa1: a transcriptional activator of osteoblast differentiation

The osteoblast is the bone-forming cell. The molecular basis of osteoblast-specific gene expression and differentiation is unknown. We previously identified an osteoblast-specific cis-acting element, termed OSE2, in the Osteocalcin promoter. We have now cloned the cDNA encoding Osf2/Cbfa1, the protein that binds to OSE2. Osf2/Cbfa1 expression is initiated in the mesenchymal condensations of the developing skeleton, is strictly restricted to cells of the osteoblast lineage thereafter, and is regulated by BMP7 and vitamin D3. Osf2/Cbfa1 binds to and regulates the expression of multiple genes expressed in osteoblasts. Finally, forced expression of Osf2/Cbfa1 in nonosteoblastic cells induces the expression of the principal osteoblast-specific genes. This study identifies Osf2/Cbfa1 as an osteoblast-specific transcription factor and as a regulator of osteoblast differentiation.

Targeted disruption of Cbfa1 results in a complete lack of bone formation owing to maturational arrest of osteoblasts

A transcription factor, Cbfa1, which belongs to the runt-domain gene family, is expressed restrictively in fetal development. To elucidate the function of Cbfa1, we generated mice with a mutated Cbfa1 locus. Mice with a homozygous mutation in Cbfa1 died just after birth without breathing. Examination of their skeletal systems showed a complete lack of ossification. Although immature osteoblasts, which expressed alkaline phophatase weakly but not Osteopontin and Osteocalcin, and a few immature osteoclasts appeared at the perichondrial region, neither vascular nor mesenchymal cell invasion was observed in the cartilage. Therefore, our data suggest that both intramembranous and endochondral ossification were completely blocked, owing to the maturational arrest of osteoblasts in the mutant mice, and demonstrate that Cbfa1 plays an essential role in osteogenesis.

Sibs with cleidocranial dysplasia born to normal parents: germ line mosaicism?

Cleidocranial dysplasia (CCD) is classically an autosomal dominant disorder. However, the possibility of an autosomal recessive form of CCD has been suggested based on a report of 2 consanguineous families, one with a single affected child, the second with affected sibs, born to normal parents. We present a family with sibs with CCD born to normal parents, and suggest germ line mosaicism as the more likely mechanism for this occurrence.

Mapping of the gene for cleidocranial dysplasia in the historical Cape Town (Arnold) kindred and evidence for locus homogeneity

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder, features of which include a patient anterior fontanelle, a bulging calvarium, hypoplasia or aplasia of the clavicles, a wide public symphysis, dental anomalies, vertebral malformation, and short stature. The Cape Town kindred which is under our genetic management was originally described more than four decades ago and now consists of more than 1000 people. Following reports of rearrangements on chromosomes 6 and 8 in people with CCD, we have carried out linkage analyses between highly information microsatellite dinucleotide repeat markers in the rearranged regions and the disorder in a branch of this South African CCD kindred, consisting of 38 subjects, 18 of whom are affected. Maximum lod scores (at theta = 0.00) of 7.14 (for marker D6S459), 4.32 (TCTE), 4.99 (D6S452), 5.97 (D6S269), and 3.95 (D6S465) confirm linkage of the disorder to the short arm of chromosome 6. Our data indicate that the CCD gene is located within a minimal region of approximately 10 cM flanked by the marker D6S451 distally and D6S466 proximally. This information is vital towards isolating and characterising the gene for CCD, and is being used to construct a physical map of 6p21.1-6p21.3. More importantly, mapping of the locus in the South African kindred of mixed ancestry, in which the "founder" of the disorder was of Chinese origin, suggests that a single locus is responsible for classic CCD.

Genetic mapping of the cleidocranial dysplasia (CCD) locus on chromosome band 6p21 to include a microdeletion

Cleidocranial dysplasia (CCD) is a generalized skeletal dysplasia with autosomal dominant inheritance. Recently, the CCD disease locus was localized to 23 [Mundlos et al., 1995] and 17 cM regions [Feldman et al., 1995], of chromosome band 6p21 by linkage studies of seven affected families. Of note, the 23 cM region contained a microdeletion detected in one family at D6S459, an interval that was excluded in the 17 cM overlapping region. Here, linkage of CCD to 6p21 was independently confirmed with a maximal two-point LOD score of Z = 5.12 with marker D6S452 at theta = 0.00. Recombinant events in two affected individuals defined a CCD region of 7 cM from D6S465 to D6S282, which overlapped with the CCD region containing the microdeletion but did not overlap with the 17 cM critical region from D6S282 to D6S291. These results suggest the refined localization of the CCD region to 6 cM spanning markers D6S438 to D6S282, thereby reviving the possibility that the CCD gene lies within the microdeletion at D6S459.

Finding genes involved in human developmental disorders

Recent advances in the human genome initiative have accelerated positional cloning efforts toward identification of a number of genes responsible for human developmental anomalies, particularly those involving the skeletal system. Genotype/phenotype comparison and functional analysis of these genes will further elucidate pathways of normal and abnormal human development of the skeletal and other organ systems.