Cleidocranial dysplasia in mice

Chapter 2. Evolution of vertebrate cartilage development

Major advances in the molecular genetics, paleobiology, and the evolutionary developmental biology of vertebrate skeletogenesis have improved our understanding of the early evolution and development of the vertebrate skeleton. These studies have involved genetic analysis of model organisms, human genetics, comparative developmental studies of basal vertebrates and nonvertebrate chordates, and both cladistic and histological analyses of fossil vertebrates. Integration of these studies has led to renaissance in the area of skeletal development and evolution. Among the major findings that have emerged is the discovery of an unexpectedly deep origin of the gene network that regulates chondrogenesis. In this chapter, we discuss recent progress in each these areas and identify a number of questions that need to be addressed in order to fill key gaps in our knowledge of early skeletal evolution.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

High-dose estrogen-induced osteogenesis is decreased in aged RUNX2(+/-) mice

Runx2 is a transcription factor that is not only critical in embryonic skeletal development but also important in regulating osteoblast function in the adult. Heterozygosity of RUNX2 (RUNX2(+/-)) leads to haploinsufficiency and manifests as a condition with distinctive skeletal features in humans and mice. Aged but not young RUNX2(+/-) adult mice may also display reduced intramembranous bone formation. To clarify the role of Runx2 in intramembranous bone formation in adult mice a histomorphometric study was performed to compare the osteogenic response to high-dose estrogen in RUNX2(+/-) and wild-type mice. Young (10 weeks) and aged (26 weeks) RUNX2(+/-) and wild-type littermate mice were treated with vehicle or high-dose estrogen (0.5 mg/animal/week) by subcutaneous injection for 4 weeks. Mice were divided into 8 groups according to age, genotype and treatment with 6 animals per group. Following sacrifice, longitudinal tibial sections were prepared and examined by static and dynamic histomorphometry. Estrogen treatment induced formation of new cancellous bone in both wild-type and RUNX2(+/-) mice. This occurred to the same extent in young mice of both genotypes. However, in the aged RUNX2(+/-) mice this response as assessed by bone volume (BV/TV%) was decreased by over 70% (p<0.001) when compared to aged wild-type mice. Furthermore, significant reductions in cancellous double-labelled surfaces (dls/TV, 1.7+/-0.2 vs 1.0+/-0.4 mm(2)/mm(3), p<0.05) and mineral apposition rate (1.8+/-0.1 vs 1.4+/-0.1 microm/day, p<0.01) were observed in aged RUNX2(+/-) mice compared to wild-types. Aged RUNX2(+/-) mice display an abrogated osteogenic response to high-dose estrogen. This may have occurred through combined reductions in recruitment of osteoprogenitor cells, osteoblast activity and mineralization. Since the characteristic histological changes in the marrow cavity which precede the formation of cancellous bone following estrogen treatment was seen in the aged RUNX2(+/-) mice we suggest that they may eventually be capable of a full osteogenic response but haploinsufficiency leads to delayed bone formation.

Type X collagen gene regulation by Runx2 contributes directly to its hypertrophic chondrocyte-specific expression in vivo

The alpha1(X) collagen gene (Col10a1) is the only known hypertrophic chondrocyte-specific molecular marker. Until recently, few transcriptional factors specifying its tissue-specific expression have been identified. We show here that a 4-kb murine Col10a1 promoter can drive beta-galactosidase expression in lower hypertrophic chondrocytes in transgenic mice. Comparative genomic analysis revealed multiple Runx2 (Runt domain transcription factor) binding sites within the proximal human, mouse, and chick Col10a1 promoters. In vitro transfection studies and chromatin immunoprecipitation analysis using hypertrophic MCT cells showed that Runx2 contributes to the transactivation of this promoter via its conserved Runx2 binding sites. When the 4-kb Col10a1 promoter transgene was bred onto a Runx2(+/-) background, the reporter was expressed at lower levels. Moreover, decreased Col10a1 expression and altered chondrocyte hypertrophy was also observed in Runx2 heterozygote mice, whereas Col10a1 was barely detectable in Runx2-null mice. Together, these data suggest that Col10a1 is a direct transcriptional target of Runx2 during chondrogenesis.

Revisiting the role of retinoid signaling in skeletal development

Several years ago, it was discovered that an imbalance of vitamin A during embryonic development has dramatic teratogenic effects. These effects have since been attributed to vitamin A's most active metabolite, retinoic acid (RA), which itself profoundly influences the development of multiple organs including the skeleton. After decades of study, researchers are still uncovering the molecular basis whereby retinoids regulate skeletal development. Retinoid signaling involves several components, from the enzymes that control the synthesis and degradation of RA, to the cytoplasmic RA-binding proteins, and the nuclear receptors that modulate gene transcription. As new functions for each component continue to be discovered, their developmental roles appear increasingly complex. Interestingly, each component has been implicated in skeletal development. Moreover, retinoid signaling comes into play at distinct stages throughout the developmental sequence of skeletogenesis, highlighting a fundamental role for this pathway in forming the adult skeleton. Consistent with these roles, manipulation of the retinoid signaling pathway significantly affects the expression of the skeletogenic master regulatory factors, Sox9 and Cbfa1. In addition to the fact that we now have a greater understanding of the retinoid signaling pathway on a molecular level, much more information is now available to begin placing retinoid signaling within the context of other factors that regulate skeletogenesis. Here we review these recent advances and describe our current understanding of how retinoid signaling functions to coordinate skeletal development. We also discuss future directions and clinical implications in this field.

High bone resorption in adult aging transgenic mice overexpressing cbfa1/runx2 in cells of the osteoblastic lineage

The runt family transcription factor core-binding factor alpha1 (Cbfa1) is essential for bone formation during development. Surprisingly, transgenic mice overexpressing Cbfa1 under the control of the 2.3-kb collagen type I promoter developed severe osteopenia that increased progressively with age and presented multiple fractures. Analysis of skeletally mature transgenic mice showed that osteoblast maturation was affected and that specifically in cortical bone, bone resorption as well as bone formation was increased, inducing high bone turnover rates and a decreased degree of mineralization. To understand the origin of the increased bone resorption, we developed bone marrow stromal cell cultures and reciprocal coculture of primary osteoblasts and spleen cells from wild-type or transgenic mice. We showed that transgenic cells of the osteoblastic lineage induced an increased number of tartrate-resistant acid phosphatase-positive multinucleated cells, suggesting that primary osteoblasts as well as bone marrow stromal cells from transgenic mice have stronger osteoclastogenic properties than cells derived from wild-type animals. We investigated the candidate genes whose altered expression could trigger this increase in bone resorption, and we found that the expression of receptor activator of NF-kappaB ligand (RANKL) and collagenase 3, two factors involved in bone formation-resorption coupling, was markedly increased in transgenic cells. Our data thus suggest that overexpression of Cbfa1 in cells of the osteoblastic lineage does not necessarily induce a substantial increase in bone formation in the adult skeleton but has a positive effect on osteoclast differentiation in vitro and can also dramatically enhance bone resorption in vivo, possibly through increased RANKL expression.

Role of Runx genes in chondrocyte differentiation

Runx2/Cbfa1 plays a central role in skeletal development as demonstrated by the absence of osteoblasts/bone in mice with inactivated Runx2/Cbfa1 alleles. To further investigate the role of Runx2 in cartilage differentiation and to assess the potential of Runx2 to induce bone formation, we cloned chicken Runx2 and overexpressed it in chick embryos using a retroviral system. Infected chick wings showed multiple phenotypes consisting of (1) joint fusions, (2) expansion of carpal elements, and (3) shortening of skeletal elements. In contrast, bone formation was not affected. To investigate the function of Runx2/Cbfa1 during cartilage development, we have generated transgenic mice that express a dominant negative form of Runx2 in cartilage. The selective inactivation of Runx2 in chondrocytes results in a severe shortening of the limbs due to a disturbance in chondrocyte differentiation, vascular invasion, osteoclast differentiation, and periosteal bone formation. Analysis of the growth plates in transgenic mice and in chick limbs shows that Runx2 is a positive regulator of chondrocyte differentiation and vascular invasion. The results further indicate that Runx2 promotes chondrogenesis either by maintaining or by initiating early chondrocyte differentiation. Furthermore, Runx2 is essential but not sufficient to induce osteoblast differentiation. To analyze the role of runx genes in skeletal development, we performed in situ hybridization with Runx2- and Runx3-specific probes. Both genes were coexpressed in cartilaginous condensations, indicating a cooperative role in the regulation of early chondrocyte differentiation and thus explaining the expansion/maintenance of cartilage in the carpus and joints of infected chick limbs.

Identification of novel CBFA1/RUNX2 mutations causing cleidocranial dysplasia

Core binding factor A1 (CBFA1/RUNX2) is a runt-like transcription factor essential for osteoblast differentiation. Haplotype insufficiency causes cleidocranial dysplasia (CCD), a syndrome featuring supernumerary tooth buds, delayed tooth eruption, patent fontanels, Wormian bones, short stature, dysplasia of the clavicles, growth retardation and hypoplasia of the distal phalanges. We identified novel CBFAI/RUNX2 mutations after PCR and direct sequencing of patient leukocyte DNA. In family 1 mother and son are affected by CCD. Both carry the missense mutation R190W (CGG > TGG). This nucleotide change introduced a BsmI restriction site, which was used to independently confirm the mutation. It was absent in healthy members of the family. Family 2, in which father and daughter are affected by CCD, shows a deletion of nucleotide C821. This deletion causes a frameshift mutation with premature stop after the insertion of 18 aberrant amino acids. Healthy family members did not have this mutation. The clavicular dysplasia was more pronounced with the R19OW mutation, while the bone density was markedly reduced in individuals with either mutation, suggesting a previously underemphasized increased risk for osteoporosis in CCD.

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.

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.

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.

Regulation of chondrocyte differentiation by Cbfa1

Cbfa1, a developmentally expressed transcription factor of the runt family, was recently shown to be essential for osteoblast differentiation. We have investigated the role of Cbfa1 in endochondral bone formation using Cbfa1-deficient mice. Histology and in situ hybridization with probes for indian hedgehog (Ihh), collagen type X and osteopontin performed at E13.5, E14.5 and E17.5 demonstrated a lack of hypertrophic chondrocytes in the anlagen of the humerus and the phalanges and a delayed onset of hypertrophy in radius/ulna in Cbfa1-/- mice. Detailed analysis of Cbfa1 expression using whole mount in situ hybridization and a lacZ reporter gene reveled strong expression not only in osteoblasts but also in pre-hypertrophic and hypertrophic chondrocytes. Our studies identify Cbfa1 as a major positive regulator of chondrocyte differentiation.

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.

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.

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.