Animal model: skeletal anomalies in mice with cleidocranial dysplasia

Drugs targeting SIRT1, a new generation of therapeutics for osteoporosis and other bone related disorders?

With an aging population and limited treatment options, osteoporosis currently represents a significant public health challenge. Recent animal studies indicate that longevity-associated SIRT1 may serve as an attractive pharmacological target for the treatment of osteoporosis and other bone related disorders. Pre-clinical studies demonstrate that mice treated with SIRT1 agonists show protection against age-related, post-menopausal, and disuse models of osteoporosis. Conversely, SIRT1 knockout models display low bone mass phenotypes associated with increased bone resorption and decreased bone formation. This review summarizes recent animal and human experimental data showing that pharmacological activation of SIRT1 may act in a manner that current treatments do not, namely by treating the imbalance in bone remodeling that is the root cause of osteoporosis and other bone disorders.

Prolyl 3-hydroxylase 1 null mice display abnormalities in fibrillar collagen-rich tissues such as tendons, skin, and bones

Osteogenesis imperfecta (OI) is a skeletal disorder primarily caused by mutations in the type I collagen genes. However, recent investigations have revealed that mutations in the genes encoding for cartilage-associated protein (CRTAP) or prolyl 3-hydroxylase 1 (P3H1) can cause a severe, recessive form of OI. These reports show minimal 3-hydroxylation of key proline residues in type I collagen as a result of CRTAP or P3H1 deficiency and demonstrate the importance of P3H1 and CRTAP to bone structure and development. P3H1 and CRTAP have previously been shown to form a stable complex with cyclophilin B, and P3H1 was shown to catalyze the 3-hydroxylation of specific proline residues in procollagen I in vitro. Here we describe a mouse model in which the P3H1 gene has been inactivated. Our data demonstrate abnormalities in collagen fibril ultrastructure in tendons from P3H1 null mice by electron microscopy. Differences are also seen in skin architecture, as well as in developing limbs by histology. Additionally bone mass and strength were significantly lower in the P3H1 mice as compared with wild-type littermates. Altogether these investigations demonstrate disturbances of collagen fiber architecture in tissues rich in fibrillar collagen, including bone, tendon, and skin. This model system presents a good opportunity to study the underlying mechanisms of recessive OI and to better understand its effects in humans.

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.

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.

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.

The osteoblast: a sophisticated fibroblast under central surveillance

The study of the biology of osteoblasts, or bone-forming cells, illustrates how mammalian genetics has profoundly modified our understanding of cell differentiation and physiologic processes. Indeed, genetic-based studies over the past 5 years have revealed how osteoblast differentiation is controlled through growth and transcription factors. Likewise, the recent identification, using mutant mouse models, of a central component in the regulation of bone formation expands our understanding of the control of bone remodeling. This regulatory loop, which involves the hormone leptin, may help to explain the protective effect of obesity on bone mass in humans. In addition, it provides a novel physiologic concept that may shed light on the etiology of osteoporosis and help to identify new therapeutic targets.

Transcriptional autoregulation of the bone related CBFA1/RUNX2 gene

The runt related transcription factor CBFA1 (AML3/PEBP2alphaA/RUNX2) regulates expression of several bone- and cartilage-related genes and is required for bone formation in vivo. The gene regulatory mechanisms that control activation and repression of CBFA1 gene transcription during osteoblast differentiation and skeletal development are essential for proper execution of the osteogenic program. We have therefore defined functional contributions of 5' regulatory sequences conserved in rat, mouse and human CBFA1 genes to transcription. Deletion analysis reveals that 0.6 kB of the bone-related rat or mouse CBFA1 promoter (P1, MASNS protein isoform) is sufficient to confer transcriptional activation, and that there are multiple promoter domains which positively and negatively regulate transcription. Progressive deletion of promoter segments between nt -351 and -92 causes a striking 30- to 100-fold combined decrease in promoter activity. Additionally, 5' UTR sequences repress reporter gene transcription 2- to 3-fold. Our data demonstrate that CBFA1 is a principal DNA binding protein interacting with the 5' region of the CBFA1 gene in osseous cells, that there are at least three CBFA1 recognition motifs in the rat CBFA1 promoter, and that there are three tandemly repeated CBFA1 sites within the 5' UTR. We find that forced expression of CBFA1 protein downregulates CBFA1 promoter activity and that a single CBFA1 site is sufficient for transcriptional autosuppression. Thus, our data indicate that the CBFA1 gene is autoregulated in part by negative feedback on its own promoter to stringently control CBFA1 gene expression and function during bone formation.

The osteoblast-specific transcription factor Cbfa1 contributes to the expression of osteoprotegerin, a potent inhibitor of osteoclast differentiation and function

Bone formation and resorption are tightly coupled under normal conditions, and the interaction of osteoclast precursors with cells of the osteoblast lineage is a prerequisite for osteoclast formation. Cbfa1 is an osteoblast-specific transcription factor that is essential for osteoblast differentiation and bone formation. At present, it is not known whether Cbfa1 regulates any of the osteoblast-derived factors involved in the bone resorption pathway. Osteoprotegerin (OPG) is an osteoblast-secreted glycoprotein that functions as a potent inhibitor of osteoclast differentiation and bone resorption. Cloning and computer analysis of a 5.9-kilobase human OPG promoter sequence revealed the presence of 12 putative Cbfa1 binding elements (osteoblast-specific element 2 (OSE(2))), suggesting a possible regulation of OPG by Cbfa1. We cloned the promoter upstream of the beta-galactosidase reporter gene (pOPG5. 9betagal) and evaluated whether Cbfa1 could regulate its expression in transient transfection assays. The 5.9-kilobase promoter directed increased levels of reporter gene expression, reminiscent of OPG protein levels in osteoblastic cell lines (BALC and U2OS) as compared with the nonosteoblastic cell line COS1. Cotransfection of a Cbfa1 expression construct along with pOPG5.9betagal reporter construct led to 39-, 7-, and 16-fold increases in beta-galactosidase activity in COS1, BALC, and U2OS cells, respectively. Removal of all the putative OSE(2) elements led to an almost complete loss of transactivation. Mutational analysis demonstrated that the proximal OSE(2) element contributes to a majority of the effects of Cbfa1, and Cbfa1 bound to the proximal element in a sequence-specific manner. Further, overexpression of Cbfa1 led to a 54% increase in OPG protein levels in U2OS cells. These results indicate that Cbfa1 regulates the expression of OPG, thereby further contributing to a molecular link between bone formation and resorption.

Two domains unique to osteoblast-specific transcription factor Osf2/Cbfa1 contribute to its transactivation function and its inability to heterodimerize with Cbfbeta

Osf2/Cbfa1, hereafter called Osf2, is a member of the Runt-related family of transcription factors that plays a critical role during osteoblast differentiation. Like all Runt-related proteins, it contains a runt domain, which is the DNA-binding domain, and a C-terminal proline-serine-threonine-rich (PST) domain thought to be the transcription activation domain. Additionally, Osf2 has two amino-terminal domains distinct from any other Runt-related protein. To understand the mechanisms of osteoblast gene regulation by Osf2, we performed an extensive structure-function analysis. After defining a short Myc-related nuclear localization signal, a deletion analysis revealed the existence of three transcription activation domains and one repression domain. AD1 (for activation domain 1) comprises the first 19 amino acids of the molecule, which form the first domain unique to Osf2, AD2 is formed by the glutamine-alanine (QA) domain, the second domain unique to Osf2, and AD3 is located in the N-terminal half of the PST domain and also contains sequences unique to Osf2. The transcription repression domain comprises the C-terminal 154 amino acids of Osf2. DNA-binding, domain-swapping, and protein interaction experiments demonstrated that full-length Osf2 does not interact with Cbfbeta, a known partner of Runt-related proteins, whereas a deletion mutant of Osf2 containing only the runt and PST domains does. The QA domain appears to be responsible for preventing this heterodimerization. Thus, our results uncover the unique functional organization of Osf2 by identifying functional domains not shared with other Runt-related proteins that largely control its transactivation and heterodimerization abilities.

New developments in bone formation

Two independent strategies have established that the transcription factor, Cbfa1, is a key regulator of both osteoblast differentiation and osteoblast-specific gene expression. Gene targeting experiments in mice have also shown that haploinsufficiency of Cbfa1 expression causes symptoms reminiscent of the Cleidocranial dysplasia syndrome (CCD), a heritable disorder of the skeleton. Direct analysis of the Cbfa1 gene in CCD families has revealed a direct correlation between mutations in this gene and disease phenotype.

Cleidocranial dysplasia with neonatal death due to central nervous system injury in utero: case report and literature review

Cleidocranial dysplasia (CCD), an uncommon disorder involving membranous bones, is rarely lethal in early life. The calvaria is defective and wormian bones are present. Abnormalities of the clavicles vary in severity from a minor unilateral defect to bilateral absence. This report concerns pre- and postmortem anatomical and radiological findings in a 15-day-old female neonate with CCD. Her postnatal course was characterized by seizures and recognition of hydrocephalus during the first day of life. The calvaria was hypoplastic with numerous wormian bones. A pseudofracture of the right clavicle was present. Hydrocephalus was present in the brachycephalic brain which had a severely thinned cerebral cortex. Hemosiderin in the ventricular lining and marked subependymal gliosis were interpreted as evidence of old intraventricular hemorrhage that had occurred in utero. A CCD-related condition, Yunis-Varon syndrome (YVS), is noted for early lethality and for developmental and secondary abnormalities of the central nervous system. The present case only partially matches the phenotype of YVS and might represent a part of a spectrum of phenotypic variants ranging from viable CCD to lethal YVS.

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.

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.

Increased expression of TGF-beta 2 in osteoblasts results in an osteoporosis-like phenotype

The development of the skeleton requires the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The activities of these two cell types are likely to be regulated by TGF-beta, which is abundant in bone matrix. We have used transgenic mice to evaluate the role of TGF-beta 2 in bone development and turnover. Osteoblast-specific overexpression of TGF-beta 2 from the osteocalcin promoter resulted in progressive bone loss associated with increases in osteoblastic matrix deposition and osteoclastic bone resorption. This phenotype closely resembles the bone abnormalities seen in human hyperparathyroidism and osteoporosis. Furthermore, a high level of TGF-beta 2 overexpression resulted in defective bone mineralization and severe hypoplasia of the clavicles, a hallmark of the developmental disease cleidocranial dysplasia. Our results suggest that TGF-beta 2 functions as a local positive regulator of bone remodeling and that alterations in TGF-beta 2 synthesis by bone cells, or in their responsiveness to TGF-beta 2, may contribute to the pathogenesis of metabolic bone disease.

Apparent cleidocranial dysplasia associated with abnormalities of 8q22 in three individuals

Cleidocranial dysplasia is an autosomal dominant, generalised skeletal disorder characterised by variable clavicular hypoplasia, frontal bossing, multiple Wormian bones, and delayed eruption of the teeth. The gene locus for this syndrome has not yet been assigned. Three individuals with manifestations of cleidocranial dysplasia associated with rearrangements of chromosome 8q22 are described. The evidence presented suggests that the gene for cleidocranial dysplasia may be located on chromosome 8q in humans in a region showing homology to mouse chromosome 3.

Malformation syndromes: a review of mouse/human homology

The purpose of this paper is to review the known and possible homologies between mouse and human multiple congenital anomaly syndromes. By identifying single gene defects causing similar developmental abnormalities in mouse and man, comparative gene mapping can be carried out, and if the loci in mouse and man are situated in homologous chromosome segments, further molecular studies can be performed to show that the loci are identical. This paper puts forward tentative homologies in the hope that some will be investigated and shown to be true homologies at the molecular level, thus providing mouse models for complex developmental syndromes. The mouse malformation syndromes are reviewed according to their major gene effects. X linked syndromes are reviewed separately because of the greater ease of establishing homology for these conditions.