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

Heterozygous pathogenic variants involving CBFB cause a new skeletal disorder resembling cleidocranial dysplasia

BACKGROUND

Cleidocranial dysplasia (CCD) is a rare skeletal dysplasia with significant clinical variability. Patients with CCD typically present with delayed closure of fontanels and cranial sutures, dental anomalies, clavicular hypoplasia or aplasia and short stature. Runt-related transcription factor 2 (RUNX2) is currently the only known disease-causing gene for CCD, but several studies have suggested locus heterogeneity.

METHODS

The cohort consists of eight subjects from five unrelated families partially identified through GeneMatcher. Exome or genome sequencing was applied and in two subjects the effect of the variant was investigated at RNA level.

RESULTS

In each subject a heterozygous pathogenic variant in CBFB was detected, whereas no genomic alteration involving RUNX2 was found. Three CBFB variants (one splice site alteration, one nonsense variant, one 2 bp duplication) were shown to result in a premature stop codon. A large intragenic deletion was found to delete exon 4, without affecting CBFB expression. The effect of a second splice site variant could not be determined but most likely results in a shortened or absent protein. Affected individuals showed similarities with RUNX2-related CCD, including dental and clavicular abnormalities. Normal stature and neurocognitive problems were however distinguishing features. CBFB encodes the core-binding factor β subunit, which can interact with all RUNX proteins (RUNX1, RUNX2, RUNX3) to form heterodimeric transcription factors. This may explain the phenotypic differences between CBFB-related and RUNX2-related CCD.

CONCLUSION

We confirm the previously suggested locus heterogeneity for CCD by identifying five pathogenic variants in CBFB in a cohort of eight individuals with clinical and radiographic features reminiscent of CCD.

Appropriateness of standard cephalometric norms for the assessment of dentofacial characteristics in patients with cleidocranial dysplasia

OBJECTIVES

Cleidocranial dysplasia (CCD) is a rare skeletal syndrome affecting craniofacial and dental development. As a consequence, conventional cephalometric landmarks may not be valid for CCD patients, and the appropriateness of norms used for the general population should be critically discussed.

METHODS

Five patients 9- to 22-year-old (three females, two males) with CCD were included. Lateral-cephalograms, orthopantomographies, and intra-oral photos were retrospectively analysed. Lateral-cephalograms of 50 normal controls (ten for each CCD patient) matched for age and sex were selected from an online database. Cephalometric measurements of each CCD patients were compared with average values of matched controls using Wilcoxon signed-rank test for paired values (α = 0.05).

RESULTS

In CCD patients, a shortening of the cranial base was present (ΔSN = -17.1 mm, p = 0.043). Thus, the mandible (ΔSNPg = +9.5°, p = 0.043) and the maxilla (ΔSNA = +11.2°, p = 0.043) showed protrusion compared to the cranial base, despite a reduced maxillary (ΔCo-A = -15.1 mm, p = 0.043) and mandibular (ΔCo-Gn = -15.2 mm, p = 0.080) length. The mandibular divergence was reduced (ΔSN/GoGn = -6.4°, p = 0.043), a reduced overbite was present (ΔOverbite = -2.9 mm, p = 0.043), and the interincisal angle was increased (ΔInterincisalAngle = +13.7°, p = 0.043), mainly due to retro-inclination of lower incisors.

CONCLUSIONS

Standard cephalometric norms for the assessment of horizontal jaw position may not be applicable to CCD patients because of a reduced anterior cranial base length compared to normal subjects. Vertical relationships may not be affected, and mandibular hypodivergency was confirmed.

Genetic Pattern, Orthodontic and Surgical Management of Multiple Supplementary Impacted Teeth in a Rare, Cleidocranial Dysplasia Patient: A Case Report

Background: Cleidocranial dysplasia (CCD) is a rare, autosomal dominant skeletal dysplasia with a prevalence of one per million births. The main causes of CCD are mutations in the core-binding factor alpha-1 (CBFA1) or runt-related transcription factor-2 (RUNX2), located at the 6p21 chromosomal region. RUNX2 plays important roles in osteoblast differentiation, chondrocyte proliferation and differentiation, and tooth formation. The disease is characterized by clavicular aplasia or hypoplasia, Wormian bones, delayed closure of cranial suture, brachycephalic head, maxillary deficiency, retention of primary teeth, inclusion of permanent teeth, and multiple supernumerary teeth. Materials and Methods: A 22-year-old girl suffering from cleidocranial dysplasia with short stature, narrow shoulders, craniofacial manifestations (short face, broad forehead, etc.) and dental anomalies (different lower dental elements under eruption, supernumerary and impacted multiple teeth, etc.) was examined at our service (Complex Operative Unit of Odontostomatology of Policlinico of Bari). RX Orthopantomography (OPG) and cone beam computed tomography (CBCT) were requested to better assess the position of the supernumerary teeth and their relationships with others and to evaluate the bone tissue. Results: Under eruption was probably caused by dental interferences with supernumerary teeth; hence, extractions of supernumerary upper canines and lower premolars were performed under general anaesthesia. Surgery outcome was excellent with good tissue healing and improvements in the therapeutic possibilities with future orthodontics. Conclusions: The objective of this article is to give an update about radiological, clinical, and molecular features of CCD and to alert the health team about the importance of establishing an early diagnosis and an appropriate treatment in these patients to prevent impacted teeth complications and to offer them a better quality of life.

Cleidocranial Dysplasia Causing Respiratory Distress in Neonates: A Case Report and Literature Review

Cleidocranial dysplasia (CCD; OMIM 119600) is a rare autosomal dominant skeletal dysplasia, which is mainly characterized by persistently open or delayed closure of fontanelle, patent skull sutures, abnormal clavicles, pectus excavatum, short stature, supernumerary teeth, and sinus and middle ear infections. It is caused by Runt-related transcription factor 2 (RUNX2; OMIM 600211) mutations. Herein, we present a rare case of CCD with neonatal respiratory distress, who had abnormal midfacial features and wide fontanelle. Also, pectus excavatum was noted. He was transferred to our department, administered standard medical treatment, and discharged after 4 weeks. Therefore, we recommend the early suspicion and identification of this rare inherited disease to adequate treatment.

Identification of a Novel Mutation in the Runt-Related Transcription Factor 2 Gene in a Chinese Family With Cleidocranial Dysplasia

ABSTRACT

Cleidocranial dysplasia (CCD) is an uncommon autosomal dominant disease, characterized by hypoplasia of clavicles, delayed fontanelle closure, dental anomalies, and short stature. It has been reported in the literature that the pathogenic variants of Runt-related transcription factor 2 (RUNX2) gene are correlated with CCD patients.Here, we report a consanguineous Chinese family with 2 patients suffering from CCD, presenting similar skeleton and dentition malformation. Upon whole-exome sequencing, 52863 variants were detected in the propositus. Based on the genotype phenotype correlation, a frameshift deletion c.1554delG p.(Trp518Cysfs∗61), located in exon 8 of RUNX2, remained after filtration. For Sanger sequencing, all exons of the RUNX2 gene in members of this Chinese family were amplified by polymerase chain reaction. Mutation mentioned above was confirmed in the propositus and his mother, which has not been reported previously and cannot be found in the publicly available databases. The present study expands the pathogenic variant spectrum of RUNX2 gene and contributes to molecular diagnosis.

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

Background:

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

Methods:

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

Results:

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

Conclusion:

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

Cleidocranial dysplasia with growth hormone deficiency: a case report

Background

Cleidocranial dysplasia (CCD) is a rare skeletal disorder with autosomal dominant inheritance that is characterized by hypoplastic clavicles, delayed closure of the cranial sutures, dental abnormalities, and short stature, among other features. The responsible gene for CCD is RUNX2 located on the short arm of chromosome 6p21. In general, there are intrafamilial variations in height among CCD patients. Few studies have reported data on recombinant human growth hormone (rhGH) treatment for patients with CCD; thus, it remains to be elucidated whether rhGH treatment can improve short stature. Here, we report a case of a 6-year-old girl with CCD who has growth hormone deficiency (GHD) and a novel mutation of RUNX2.

Case presentation

At 5 years of age, this patient was diagnosed with GHD and rhGH treatment was initiated. Thereafter, she was diagnosed with CCD due to the presence of hypoplastic clavicles and an open fontanelle, which was also observed in her mother and brother. She responded well to rhGH treatment; her height improved from − 3.2 SD to − 2.4 SD after 13 months.

Conclusion

A detailed patient history and physical examination are necessary for the early diagnosis of CCD. Similarly, to ascertain the effect of rhGH treatment, careful evaluation of the patient’s final height post-therapy is needed.

Surgical Management and Evaluation of the Craniofacial Growth and Morphology in Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD, MIM 119600) is a rare autosomal dominant disorder affecting bone, cartilage, craniofacial growth, and tooth formation leading to supernumerary teeth. Few reports delineate the genotype-phenotype correlations related to the variations in craniofacial morphology and patterning of the dentition and the complexity of treating patient's malocclusion. Successful management of the craniofacial deformities in patients with CCD requires a multidisciplinary team of healthcare specialists. Approximately 70% of patients are due to point mutations in RUNX2 and <20% due to copy number variations with the remainder unidentified. There is no literature to date, describing the orthognathic management of CCD patients with deletion in one of the RUNX2 alleles. The purpose of this study was to evaluate the craniofacial morphology and dental patterning in a 14-year-old Caucasian female with CCD resulting from a novel microdeletion of RUNX2 in 1 allele. The CCD patient with RUNX2 haploinsufficiency due to microdeletion had decreased craniofacial bone and ankyloses in the permanent dentition. An altered extraction protocol of supernumerary teeth was followed in this patient. Craniofacial growth and morphologic analysis demonstrated atypical skull shape, persistent metopic suture, and decreased mandibular size.

Cleidocranial Dysplasia with 6p21.1-p12.3 Microdeletion: A Case Report and Literature Review

OBJECTIVE

The aim of this article is to publish a literature review and report on a new case of cleidocranial dysplasia syndrome with 6p21.1-p12.3 microdeletion.

DESIGN

A PubMed search using "cleidocranial dysplasia syndrome (CCD)" or "6p microdeletion" was performed. Articles with information relevant to our case were obtained for review. A new case of cleidocranial dysplasia syndrome is presented to describe and discuss clinical manifestations, pathogenesis, clinical progression of cleidocranial dysplasia syndrome, and management.

RESULTS

There were 22 articles with reports of cleidocranial dysplasia syndrome or 6p microdeletion. Cleidocranial dysplasia syndrome, a rare genetic disorder, documented to have an autosomal dominant inheritance pattern and caused by caused by mutations of the transcription factor RUNX2. RUNX2 has been mapped to chromosome 6p21. The anomalies in cleidocranial dysplasia syndrome can involve not only the clavicle and skull but the entire skeleton because the membranous as well as endochondral bone formation may be affected. Upon follow-up, our patient was found to have global developmental delay.

CONCLUSIONS

We report a near-term neonate with characteristic features of cleidocranial dysplasia and a 6p21.1-p12.3 microdeletion. Cleidocranial dysplasia syndrome is a rare autosomal dominant skeletal dysplasia. The mutation of the RUNX2 gene results in cleidocranial dysplasia syndrome.

Craniofacial features of cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal-dominant malformation syndrome affecting bones and teeth. The most common skeletal and dental abnormalities in affected individuals are hypoplastic/aplastic clavicles, open fontanelles, short stature, retention of primary teeth, delayed eruption of permanent teeth, supernumerary teeth, and multiple impacted teeth. Treatment of CCD requires a multidisciplinary approach that may include dental corrections, orthognathic surgery and cranioplasty along with management of any complications of CCD. Early diagnosis of this condition enables application of the treatment strategy that provides the best quality of life to such patients. Notably, Runx2 gene mutations have been identified in CCD patients. Therefore, further elucidation of the molecular mechanism of supernumerary teeth formation related to Runx2 mutations may improve understanding of dental development in CCD. The insights into CCD pathogenesis may assist in the development of new treatments for CCD.

Maxillary sinuses and midface in patients with cleidocranial dysostosis

The cleidocranial dysplasia is general skeletal disorder with an autosomal dominant inheritance. It is manifested by many craniofacial abnormalities, of which the maxillary hypoplasia is the most evident. The aim of the study was to use CBCT to evaluate the volume of the maxillary sinuses and the dimensions of maxillae in patients with CCD and compare them with healthy individuals. Files of four children with cleidocranial dysplasia were investigated. Volume of every maxillary sinus as well as two dimensional measurements of distances between particular points of interest located on surface of maxilla were calculated from reconstructed CBCT examination. Data were compared with a control group. Statistical analysis was performed. Linear and volumetric data obtained using CBCT was collected and compared with a control group. All affected children had both maxillary sinus volume and maxillary dimensions smaller than control values. The maxillary sinuses were underdeveloped up to half of normal values. The largest differences were recorded in vertical linear dimensions of the maxillae. Horizontal dimensions were also lower. There are morphological modifications of bone tissue which accompany CCD. It seems that these changes occur on the midfacial region and to a greater extent concern the maxillary sinus volume.

Transcriptome analysis of periodontitis-associated fibroblasts by CAGE sequencing identified DLX5 and RUNX2 long variant as novel regulators involved in periodontitis

Periodontitis is affecting over half of the adult population, and represents a major public health problem. Previously, we isolated a subset of gingival fibroblasts (GFs) from periodontitis patients, designated as periodontitis-associated fibroblasts (PAFs), which were highly capable of collagen degradation. To elucidate their molecular profiles, GFs isolated form healthy and periodontitis-affected gingival tissues were analyzed by CAGE-seq and integrated with the FANTOM5 atlas. GFs from healthy gingival tissues displayed distinctive patterns of CAGE profiles as compared to fibroblasts from other organ sites and characterized by specific expression of developmentally important transcription factors such as BARX1, PAX9, LHX8, and DLX5. In addition, a novel long non-coding RNA associated with LHX8 was described. Furthermore, we identified DLX5 regulating expression of the long variant of RUNX2 transcript, which was specifically active in GFs but not in their periodontitis-affected counterparts. Knockdown of these factors in GFs resulted in altered expression of extracellular matrix (ECM) components. These results indicate activation of DLX5 and RUNX2 via its distal promoter represents a unique feature of GFs, and is important for ECM regulation. Down-regulation of these transcription factors in PAFs could be associated with their property to degrade collagen, which may impact on the process of periodontitis.

RUNX2 mutation impairs bone remodelling of dental follicle cells and periodontal ligament cells in patients with cleidocranial dysplasia

RUNX2 is an important osteo-specific factor with crucial functions in bone formation and remodelling as well as resorption of teeth. Heterozygous mutation of RUNX2 can cause cleidocranial dysplasia (CCD), a systemic disease with extensive skeletal dysplasia and abnormality of tooth growth. In our study, dental follicle cells (DFCs) and periodontal ligament cells (PDLCs) were isolated, cultured and identified from one patient with CCD and compared with normal controls. This CCD patient was confirmed to have a heterozygous frameshift mutation of RUNX2 (c.514delT, p.Ser172fs) in the previous study. The results showed that the proliferation abilities of DFCs and PDLCs were both disturbed by the RUNX2 mutation in the CCD patient compared with the normal control. A co-culture system of these cells with human peripheral blood mononuclear cells was then used to investigate the effect of RUNX2 mutation on osteoclastogenesis. We found that the RUNX2 mutation in CCD reduced the expression of osteoclast-related genes, such as RUNX2, CTR, CTSK, RANKL and OPG The ability of osteoclastogenesis in DFCs and PDLCs detected by tartrate-resistant acid phosphatase staining in the co-culture system was also reduced by the RUNX2 mutation compared with the normal control. These outcomes indicate that the RUNX2 mutation disturbs the modulatory effects of DFCs and PDLCs on the differentiation of osteoclasts and osteoblasts, thereby interfering with bone remodelling. These effects may contribute in part to the pathological manifestations of retention of primary teeth and delayed eruption of permanent teeth in patients with CCD.

Increased trabecular bone and improved biomechanics in an osteocalcin-null rat model created by CRISPR/Cas9 technology

Osteocalcin, also known as bone γ-carboxyglutamate protein (Bglap), is expressed by osteoblasts and is commonly used as a clinical marker of bone turnover. A mouse model of osteocalcin deficiency has implicated osteocalcin as a mediator of changes to the skeleton, endocrine system, reproductive organs and central nervous system. However, differences between mouse and human osteocalcin at both the genome and protein levels have challenged the validity of extrapolating findings from the osteocalcin-deficient mouse model to human disease. The rat osteocalcin (Bglap) gene locus shares greater synteny with that of humans. To further examine the role of osteocalcin in disease, we created a rat model with complete loss of osteocalcin using the CRISPR/Cas9 system. Rat osteocalcin was modified by injection of CRISPR/Cas9 mRNA into the pronuclei of fertilized single cell Sprague-Dawley embryos, and animals were bred to homozygosity and compound heterozygosity for the mutant alleles. Dual-energy X-ray absorptiometry (DXA), glucose tolerance testing (GTT), insulin tolerance testing (ITT), microcomputed tomography (µCT), and a three-point break biomechanical assay were performed on the excised femurs at 5 months of age. Complete loss of osteocalcin resulted in bones with significantly increased trabecular thickness, density and volume. Cortical bone volume and density were not increased in null animals. The bones had improved functional quality as evidenced by an increase in failure load during the biomechanical stress assay. Differences in glucose homeostasis were observed between groups, but there were no differences in body weight or composition. This rat model of complete loss of osteocalcin provides a platform for further understanding the role of osteocalcin in disease, and it is a novel model of increased bone formation with potential utility in osteoporosis and osteoarthritis research.

Summary: A complete null of osteocalcin, generated by the CRISPR/Cas9 system, results in an increase in trabecular bone, increased bone strength and altered glucose homeostasis in Sprague-Dawley rats.

Genetic background of supernumerary teeth

Supernumerary teeth (ST) are odontostomatologic anomaly characterized by as the existence excessive number of teeth in relation to the normal dental formula. This condition is commonly seen with several congenital genetic disorders such as Gardner's syndrome, cleidocranial dysostosis and cleft lip and palate. Less common syndromes that are associated with ST are; Fabry Disease, Ellis-van Creveld syndrome, Nance-Horan syndrome, Rubinstein-Taybi Syndrome and Trico–Rhino–Phalangeal syndrome. ST can be an important component of a distinctive disorder and an important clue for early diagnosis. Certainly early detecting the abnormalities gives us to make correct management of the patient and also it is important for making well-informed decisions about long-term medical care and treatment. In this review, the genetic syndromes that are related with ST were discussed.

Molecular characterization of cbfβ gene and identification of new transcription variants: implications for function

The CBFβ gene encodes a transcription factor that, in combination with CBFα (also called Runx, runt-related transcription factor) regulates expression of several target genes. CBFβ interacts with all Runx family members, such as RUNX2, a regulator of bone-related gene transcription that contains a conserved DNA-binding domain. CBFβ stimulates DNA binding of the Runt domain, and is essential for most of the known functions of RUNX2. A comparative analysis of the zebrafish cbfβ gene and protein, and of its orthologous identified homologous proteins in different species indicates a highly conserved function. We cloned eleven zebrafish cbfβ gene transcripts, one resulting in the known Cbfβ protein (with 187 aa), and three additional variants resulting from skipping exon 5a (resulting in a protein with 174 aa) or exon 5b (resulting in a protein with 201 aa), both observed for the first time in zebrafish, and a completely novel isoform containing both exon 5a and 5b (resulting in a protein with 188 aa). Functional analysis of these isoforms provides insight into their role in regulating gene transcription. From the other variants two are premature termination Cbfβ forms, while the others show in-frame exon-skipping causing changes in the Cbfβ domain that may affect its function.

Cleidocranial dysplasia: a review of the dental, historical, and practical implications with an overview of the South African experience

Cleidocranial dysplasia (CCD) is an uncommon but well-known genetic skeletal condition. Several hundred affected persons are members of a large extended family in the Cape Town Mixed Ancestry community of South Africa. The clinical manifestations are often innocuous, but hyperdontia and other developmental abnormalities of the teeth are a major feature and may require special dental management. Over the past 40 years, the authors have encountered more than 100 affected persons in Cape Town. Emphasis has been on dental management, but medical, genetic, and social problems have also been addressed. In this article, we have reviewed the manifestations of the disorder in the light of our own experience, and performed a literature search with emphasis on the various approaches to dental management and treatment options in CCD. Advances in the understanding of the biomolecular pathogenesis of CCD are outlined and the international and local history of the disorder is documented.

Prevailing importance of the hedgehog signaling pathway and the potential for treatment advancement in sarcoma

The hedgehog signaling pathway is important in embryogenesis and post natal development. Constitutive activation of the pathway due to mutation of pathway components occurs in ~25% of medulloblastomas and also in basal cell carcinomas. In many other malignancies the therapeutic role for hedgehog inhibition though intriguing, based on preclinical data, is far from assured. Hedgehog inhibition is not an established part of the treatment paradigm of sarcoma but the scientific rationale for a possible benefit is compelling. In chondrosarcoma there is evidence of hedgehog pathway activation and an ontologic comparison between growth plate chondrocyte differentiation and different chondrosarcoma subtypes. Immunostaining epiphyseal growth plate for Indian hedgehog is particularly positive in the zone of pre-hypertrophic chondrocytes which correlates ontologically with conventional chondrosarcoma. In Ewing sarcoma/PNET tumors the Gli1 transcription factor is a direct target of the EWS-FLI1 oncoprotein present in 85% of cases. In many cases of rhabdomyosarcomas there is increased expression of Gli1 (Ragazzini et al., 2004). Additionally, a third of embryonal rhabdomyosarcomas have loss of Chr.9q22 that encompasses the patched locus (Bridge et al., 2000). The potential to treat osteosarcoma by inhibition of Gli2 and the role of the pathway in ovarian fibromas and other connective tissue tumors is also discussed (Nagao et al., 2011; Hirotsu et al., 2010). Emergence of acquired secondary resistance to targeted therapeutics is an important issue that is also relevant to hedgehog inhibition. In this context secondary resistance of medulloblastomas to treatment with a smoothened antagonist in two tumor mouse models is examined.

RUNX2 analysis of Danish cleidocranial dysplasia families

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

A complex chromosome rearrangement, der(6)ins(6)(p21.1q25.3q27)inv(6)(p25.3q27), in a child with cleidocranial dysplasia

Complex chromosome rearrangements (CCRs) are structural abnormalities involving >2 chromosomes or >3 breakpoints. It has been suggested that the probability of imbalance increases as the number of breakpoints increase. Here we report a 7-month-old, Hispanic girl presenting with cleidocranial dysplasia (CCD) who was found to have a complex chromosome rearrangement of chromosome 6. Fluorescence in situ hybridization studies with bacterial artificial chromosome (BAC) clones showed that the rearrangement involved insertion of 6q into 6p disrupting the "Runt related transcription factor 2 (RUNX2)" gene at chromosome 6p21.1. In addition, a pericentric inversion of chromosome 6 was identified. Despite the complex nature of the rearrangement, no cryptic deletions or duplications could be detected by array comparative genomic hybridization.

The Role of RUNX2 in Osteosarcoma Oncogenesis

Osteosarcoma is an aggressive but ill-understood cancer of bone that predominantly affects adolescents. Its rarity and biological heterogeneity have limited studies of its molecular basis. In recent years, an important role has emerged for the RUNX2 "platform protein" in osteosarcoma oncogenesis. RUNX proteins are DNA-binding transcription factors that regulate the expression of multiple genes involved in cellular differentiation and cell-cycle progression. RUNX2 is genetically essential for developing bone and osteoblast maturation. Studies of osteosarcoma tumours have revealed that the RUNX2 DNA copy number together with RNA and protein levels are highly elevated in osteosarcoma tumors. The protein is also important for metastatic bone disease of prostate and breast cancers, while RUNX2 may have both tumor suppressive and oncogenic roles in bone morphogenesis. This paper provides a synopsis of the current understanding of the functions of RUNX2 and its potential role in osteosarcoma and suggests directions for future study.

Runx2 deficiency in mice causes decreased thyroglobulin expression and hypothyroidism

We recently reported on the overexpression of Runx2 (Cbfa1/AML3), an osteoblast-specific transcription factor, in human papillary thyroid cancer tissues. We report here that normal thyrocytes also express Runx2 and that Runx2(+/-) mice are in a hypothyroid state. To clarify the mechanism, we studied the effects of small interfering RNA-mediated silencing of Runx2 on thyroid-specific gene expression in FRTL-5 cells. Lowering the levels of Runx2 had no effect on the amount of Na(+)/I(-) symporter mRNA but markedly decreased the amount of thyroglobulin (Tg) mRNA. A Runx2 binding consensus sequence is present on the Tg gene promoter, and gel-shift assay revealed that Runx2 binds to this region. Reporter assay showed that deletion of the region or introduction of a mutation into the binding site significantly impairs promoter function. These results indicate that Runx2 deficiency in mice causes decreased Tg expression and a novel type of hypothyroidism.

Severe cleidocranial dysplasia and hypophosphatasia in a child with microdeletion of the C-terminal region of RUNX2

Cleidocranial dysplasia (CCD) is a rare autosomal dominant skeletal dysplasia due to mutations causing haploinsufficiency of RUNX2, an osteoblast transcription factor specific for bone and cartilage. The classic form of CCD is characterized by delayed closure of the fontanels, hypoplastic or aplastic clavicles and dental anomalies. Clinical reports suggest that a subset of patients with CCD have skeletal changes which mimic hypophosphatasia (HPP). Mutations in RUNX2 are detected in approximately 65% of cases of CCD, and microdeletions occur in 13%. We present clinical and radiological features in a 6-year-old child with severe CCD manifested by absence of the clavicles marked calvarial hypomineralization, osteoporosis and progressive kyphoscoliosis. HPP features included Bowdler spurs, severe osteopenia, and low alkaline phosphatase. Following negative mutation analysis of RUNX2, comparative genomic hybridization (CGH) microarray was performed. The result revealed a microdeletion in RUNX2, disrupting the C-terminal part of the gene.

The role of periodontal ligament cells in delayed tooth eruption in patients with cleidocranial dysostosis

OBJECTIVE

The clinical appearance of patients with cleidocranial dysplasia (CCD), which is caused by mutations in the RUNX2 gene, is characterized by anomalies of the clavicles, thorax, spine, pelvis and extremities and by disturbances of the skull and tooth development. Of orthodontic relevance are multiple supernumerary teeth associated with delayed tooth eruption. The present investigation is based on the hypothesis that an altered phenotypic expression of periodontal ligament (PDL) cells from CCD patients and a reduced ability of those cells to support the differentiation of bone-resorbing osteoclasts might contribute to delayed tooth eruption.

MATERIALS AND METHODS

To test this hypothesis, PDL cells from healthy donors and from two patients with clinically and molecular biologically diagnosed CCD were characterized for the basal and induced mRNA expression of osteoblast marker genes. The physiological relevance of the findings for the differentiation of osteoclasts was examined in an osteoclast assay, as well as in a co-culture model of PDL cells and osteoclast precursors.

RESULTS

Both CCD patients displayed missense mutations of the RUNX2 gene. The in vitro experiments revealed an unaltered expression of RUNX2 mRNA, however especially in CCD patient 2 there was a reduced basal expression of mRNA for the key regulatory gene for bone remodeling RANKL. Furthermore, compared to the control cells from healthy donors, these factors were less inducible by stimulation of the cultures with 1alpha,25(OH)(2)D(3). In the osteoclast assays as well as in the co-culture experiments, PDL cells from the CCD patients showed a reduced capacity to induce the differentiation of active osteoclasts.

CONCLUSIONS

These data indicate that PDL cells from CCD patients express a less distinctive osteoblastic phenotype resulting in an impaired ability to support osteoclastogenesis which might, in part, account for the delayed tooth eruption that can be observed clinically.

Cleidocranial dysplasia: report of 3 cases and literature review

BACKGROUND

Cleidocranial dysplasia (CCD) is an autosomal-dominant skeletal dysplasia syndrome that is characterized by widely patent calvarial sutures, clavicular hypoplasia, supernumerary teeth, and short stature. It is caused by mutations of the transcription factor RUNX2, which is known as a major regulator of bone differentiation.

OBJECTIVE

To report on 3 Chinese pediatric cases of CCD with an emphasis on the clinical presentation and diagnostic modalities.

CASE SUMMARY

3 Chinese children were admitted to our hospital because of short stature. All the children had hypoplastic clavicles, absent calvarium, open fontanel, and messy palmprint. Screening laboratory test results for bone mineral density, free thyroxine, and thyroid stimulating hormone were all normal. One child had mental retardation. Two were hereditary, and 1 was sporadic.

CONCLUSION

These results showed that CCD should be suspected in patients with short stature and poor clavicles, calvarium, or teeth. Timely recognition and hereditary tendency counseling is required and useful.

Orthodontic Treatment in a Patient with Cleidocranial Dysostosis

Cleidocranial dysostosis is a rare congenital skeletal disorder, associated with clavicular hypoplasia or aplasia, delayed closure of cranial fontanels, brachycephalic skull, delayed exfoliation of primary dentition, eruption of permanent teeth, and multiple supernumerary and morphologic abnormalities of the maxilla and mandible. The disorder is caused by mutation in the CBFA1 gene, on the short arm of chromosome 6p21. The prevalence of cleidocranial dysostosis is estimated one per million, without sex or ethnic group predilection. The purpose of this paper is to describe the orthodontic treatment in a patient with cleidocranial dysostosis. Therapy may include removal of supernumerary teeth, surgical exposure of impacted teeth, and orthodontic treatment.

Intragenic microdeletion of RUNX2 is a novel mechanism for cleidocranial dysplasia

Cleidocranial dysplasia (CCD; MIM 119600) is a rare autosomal dominant disorder characterized by facial, dental, and skeletal malformations. To date, rearrangement and mutations involving RUNX2, which encodes a transcription factor required for osteoblast differentiation on 6p21, has been the only known molecular etiology for CCD. However, only 70% patients were found to have point mutations, 13% large/contiguous deletion but the rest of 17% remains unknown. We ascertained a family consisted of eight affected individuals with CCD phenotypes. Direct sequencing analysis revealed no mutations in the RUNX2. Real time quantitative PCR were performed which revealed an exon 2 to exon 6 intragenic deletion in RUNX2. Our patients not only demonstrated a unique gene change as a novel mechanism for CCD, but also highlight the importance of considering "deletion" and "duplication" in suspected familial cases before extensive effort of gene hunting be carried.

De novo three-way chromosome translocation 46,XY,t(4;6;21)(p16;p21.1;q21) in a male with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal dysplasia associated with cranial, clavicular, and dental anomalies. It is caused by mutations in the RUNX2 gene, which encodes an osteoblast-specific transcription factor and maps to chromosome 6p21. We report clinical and molecular cytogenetic studies in a patient with clinical features of CCD including wormian bones, delayed fontanel closure, hypoplastic clavicles and pubic rami, and supernumerary dentition. Additional abnormalities of bone growth and connective tissue, including easy bruisability, scarring, bleeding, joint hypermobility, and developmental delay were also observed. Molecular cytogenetic studies identified a de novo apparently balanced three-way translocation 46,XY,t(4;6;21)(p16;p21.1;q21). Further mapping revealed the breakpoint on 6p21 to be ∼50 kb upstream of exon 1 of the RUNX2 gene, with RUNX2 being intact on the derivative chromosome 6. We hypothesize that the proband's CCD has arisen from disruption of the developmentally regulated gene RUNX2 at the 6p21 breakpoint, due to a position effect mutation which may have altered the expression of the gene. Further studies might unravel a new regulatory element for RUNX2.

[Immunohistochemical study on collagen I content in the gingiva in cleidocranial dysplasia]

BACKGROUND

Patients with cleidocranial dysplasia (CCD) present a thickend and fibrotic gingiva.

PURPOSE

To the best of our knowledge it was analysed for the first time, whether this is correlated with an increased rate of collagen I in oral mucosa.

PATIENTS AND METHODS

27 soft tissue biopsies of six CCD-patients and 17 tissue samples of 12 healthy persons were labled with a monoclonal antibody against collagen I and the bound antibodies were detected with alkaline phosphatase-anti-alkaline phophatase-kit. The histological slices were analysed by a digital image recognition software under a fully automated microscope and the rate of collagen I was converted into amounts of grey tones.

RESULTS

The amount of grey tones reached from 11.909 to 15.319 in the CCD-group, and from 2752 to 12.556 in the control group. The U-Test of Mann, Whitney and Wilcoxon for two independent samples generated a rank sum of 91,50 for CCD-patients, and of 79,50 for the control group. The Z-value was 3,246, the p-value 0,005. "Fisher's exact test" identified a p-value of 0,0003.

CONCLUSIONS

The rate of collagen I in the oral mucosa seems to be increased significantly in CCD. This could explain the typical thick and fibrotic consistency of the gingiva and could be one reason for the delayed or missing dentition.

[Cleidocranial dysplasia. Description and analysis of a patient cohort]

BACKGROUND

Cleidocranial dysplasia (CCD) is a rare dysplasia of bony and dental tissue. Characteristic are typical craniofacial and dental findings including morphological anomalies. CCD is possibly the only general syndrome that can be diagnosed based on the dental findings alone. CCD correlates with mutations in the RUNX2 gene.

PURPOSE

The present interdisciplinary study correlates phenotypic findings with genetic variations in the corresponding gene.

PATIENTS AND METHODS

The coding sequence of the RUNX2 gene from 31 CCD patients from 20 families was analyzed using molecular genetic methods including polymerase chain reaction and direct sequencing. The craniofacial and dental findings of each patient were evaluated according to a standardized scoring scheme and tested with homogeneity analysis for general phenotypic findings.

RESULTS

Several mutations of the RUNX2 gene were identified. Depending on the mutation type, they showed different distribution patterns within the gene coinciding with the functional domains of the gene product. With homogeneity analysis of the phenotype cardinal (especially dental findings) and minor findings (pneumatization disturbances, Wormian bones) were identified. In combination with the genetic data, the statistical analysis showed that loss-of-function mutations of the RUNX2 gene result in a milder markedness of the CCD phenotype than gain-of-function or decrease-of-function mutations.

CONCLUSIONS

We found that type and location of a specific mutation within the RUNX2 gene might have an impact on the expressivity of CCD. Due to the limited sampling size this hypothesis must be verified by investigations in larger patient groups.

Bone morphogenetic protein 2 activates Smad6 gene transcription through bone-specific transcription factor Runx2

BMP-2 plays an essential role in osteoblast and chondrocyte differentiation, but its signaling mechanism has not been fully defined. In the present studies, we investigated the mechanism through which BMP-2 activates the Smad6 gene. A -2006/+45 Smad6 promoter-luciferase construct was generated along with deletions and Runx2 binding site mutations to examine the role of Smad1 and Runx2 signaling following BMP-2 stimulation in osteoblasts. Transfection of Runx2 or treatment with BMP-2-stimulated promoter activity of the -2006/+45 and -1191/+45 reporters but not the -829/+45 and -374/+45 reporters. No Smad1/5 binding site is present in the -1191/-829 region of the Smad6 promoter. Mutation of the OSE2-a site (-1036/-1031) completely abolished the stimulatory effect of Runx2 as well as BMP-2 on the -2006/+45 and -1191/+45 Smad6 reporters. Gel shift and chromatin immunoprecipitation (ChIP) assays showed that Runx2 binds the OSE2-a element. ChIP assays demonstrated that Smad1 also interacts with the OSE2-a site at the Smad6 promoter through Runx2. The protein degradation of Runx2 is mediated by the E3 ubiquitin ligase Smurf1. In the present studies, we found that Smurf1 binds the OSE2-a site through Runx2 and inhibits Smad6 gene transcription. Treatment with BMP-2 and transfection of Smad1 abolished Smurf1 binding to the OSE2 site. These results show that Smad1 binding excludes Smurf1 interaction with the OSE2 site and promotes Smad6 gene transcription.

Runx2 and dental development

The Runx2 gene is a master transcription factor of bone and plays a role in all stages of bone formation. It is essential for the initial commitment of mesenchymal cells to the osteoblastic lineage and also controls the proliferation, differentiation, and maintenance of these cells. Control is complex, with involvement of a multitude of factors, thereby regulating the expression and activity of this gene both temporally and spatially. The use of multiple promoters and alternative splicing of exons further extends its diversity of actions. RUNX2 is also essential for the later stages of tooth formation, is intimately involved in the development of calcified tooth tissue, and exerts an influence on proliferation of the dental lamina. Furthermore, RUNX2 regulates the alveolar remodelling process essential for tooth eruption and may play a role in the maintenance of the periodontal ligament. In this article, the structure of Runx2 is described. The control and function of the gene and its product are discussed, with special reference to developing tooth tissues, in an attempt to elucidate the role of this gene in the development of the teeth and supporting structures.

Cyclin D1-cdk4 induce runx2 ubiquitination and degradation

Runx2 is a Runt domain transcription factor involved in the activation of genes encoding osteoblast and chondrocyte-specific proteins. Runx2 activity is regulated by transcriptional and post-transcriptional mechanisms. The functional significance of the post-translational modification of Runx2 has not been fully defined. We show that cyclin D1-Cdk4 induce Runx2 degradation in an ubiquitination-proteasome-dependent manner. Mutagenesis of Runx2 serine-472, a consensus Cdk site, to alanine increases the half-life of Runx2 and causes loss of sensitivity to cyclin D1-induced Runx2 degradation. The targeted Runx2 degradation by cyclin D1 identifies a novel mechanism through which Runx2 activity is regulated coordinately with the cell cycle machinery in bone cells.

Cleidocranial dysplasia in a mother and her daughter within the scope of neurosurgery

Cleidocranial dysplasia is an autosomal-dominant disorder characterized by late closure or nonclosure of the anterior fontanelle, late ossification of cranial sutures, defective clavicle, and delayed eruption of permanent teeth. In this article, two cases of cleidocranial dysplasia involving a mother and her daughter are reported, and a case management policy is suggested. The 1-year-old daughter was macrocephalic and brachycephalic, and had midface hypoplasia and hypertelorism. Plain radiographs revealed aplasia of the clavicles. Three-dimensional computerized tomography scanning demonstrated a large anterior fontanelle, a patent posterior fontanelle, and bone defects at the pterion and asterion, together with nonfused metopic and sagittal sutures. The mother was 22 years of age. She had an open anterior fontanelle, aplastic clavicles, and unerupted permanent teeth. Although it is a rare disorder, cleidocranial dysplasia should be recognized by neurosurgeons. A protective helmet can be provided in early childhood, and craniofacial remodeling can be undertaken at a later age, when the final size and shape of the skull become apparent.

Smad6 interacts with Runx2 and mediates Smad ubiquitin regulatory factor 1-induced Runx2 degradation

Runx2 is a bone-specific transcription factor that plays a critical role in bone development, postnatal bone formation, and chondrocyte maturation. The protein levels of Runx2 are regulated by the ubiquitin-proteasome pathway. In previous studies we discovered that E3 ubiquitin ligase Smad ubiquitin regulatory factor 1 (Smurf1) induces Runx2 degradation in a ubiquitin-proteasome-dependent manner, and Smurf1 plays an important role in osteoblast function and bone formation. In the present studies we investigated the molecular mechanism of Smurf1-induced Runx2 degradation. Smurf1 interacts with the PY motif of substrate proteins, and a PY motif has been identified in the C terminus of the Runx2 protein. To determine whether Smurf1 induces Runx2 degradation through the interaction with the PY motif of Runx2, we created a mutant Runx2 with a PY motif deletion and found that Smurf1 retained some of its ability to induce the degradation of the mutant Runx2, suggesting that Smurf1 could induce Runx2 degradation through an indirect mechanism. Smurf1 has been shown to interact with Smads 1, 5, 6, and 7, and Smads 1 and 5 also interact with Runx2. In the present studies we found that Smads 1 and 5 had no effect on Smurf1-induced Runx2 degradation. Although Smads 6 and 7 bind Smurf1, it is not known if Smads 6 or 7 interacts with Runx2 and mediate Runx2 degradation. We performed immunoprecipitation assays and found that Smad6 but not Smad7 interacts with Runx2. Smad6 enhances Smurf1-induced Runx2 degradation in an ubiquitin-proteasome-dependent manner. These results demonstrate that in addition to its interaction with the PY motif of Runx2, Smurf1 induces Runx2 degradation in a Smad6-dependent manner. Smurf1-induced Runx2 degradation serves as a negative regulatory mechanism for the BMP-Smad-Runx2 signaling pathway.

Cleido cranial dysplasia: report of a family

A family case of Cleidocranial Dysplasia is presented. A mother and two adolescent girls were examined. In all three cases, a radiological series was performed over the entire body. Generalized dysplasia in bones, prolonged retention of primary teeth, and delayed eruption of permanent, as well as supernumerary teeth was diagnosed. The citogenetic study with GTG band showed normal 46, XX. Bilateral audiometry in the mother demonstrated a mild to moderate hypoacustic condition. Radiological findings are presented and the importance of early diagnosis is discussed.

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

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

Prenatal diagnosis of a cleidocranial dysplasia-like phenotype associated with a de novo balanced t(2q;6q)(q36;q16) translocation

Cleidocranial dysplasia (CCD) is a congenital disorder of bone development characterized by persistently open or delayed closure of cranial sutures and wormian bones, hypoplastic and/or aplastic clavicles, wide pubic symphysis, dental anomalies and short stature. The condition is inherited as an autosomal-dominant trait and the human CBFA1 gene has been identified as the CCD gene. We describe a prenatal form of the skeletal disorder that included clavicular hypoplasia, absence of ossification of the cranial parietal bones and very poor ossification of the frontal and pubic bones. Growth restriction affecting only the long bones was also noted. The fetal karyotype revealed an apparently de novo balanced t(2q;6q)(q36;q16) translocation. This particular form of skeletal disorder associated with the absence of family history and an apparently de novo balanced translocation led the parents to opt for termination of the pregnancy.

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.

Tooth eruption and cementum formation in the Runx2/Cbfa1 heterozygous mouse

Cleidocranial dysplasia (CCD) is an autosomal dominant human disorder that affects development of bones and teeth. The dental disorders in CCD patients include formation of supernumerary teeth, delayed tooth eruption, and lack of formation of cellular cementum in permanent teeth. This disorder involves a mutation in the osteoblast-specific transcription factor Runx2/Cbfa1, leading to haploinsufficiency of the Runx2/Cbfa1 protein. Here, we examined if Runx2/Cbfa1 heterozygous mice (with one functional allele for Runx2/Cbfa1) exhibit similar changes in tooth eruption, and dental cementum formation as in CCD patients. Heads of Runx2/Cbfa1 heterogeneous and wildtype mice aged days 16-35 postnatally were serially sectioned and stained with hematoxylin-eosin or for tartrate resistant acid phosphatase (TRAP) to identify osteoclasts. The results showed that the eruption pattern of the first and second molars in maxilla and mandible in Runx2/Cbfa1 +/- mice was the same as in wildtype animals. No clear difference in distribution or in the (estimated) number of osteoclasts was found. Cellular cement at the apical portions of the molar roots was present in both groups. The data suggests that in the mouse one allele for Runx2/Cbfa1 is sufficient for an undisturbed tooth eruption and an apparently normal formation of the periodontium.

A novel dysmorphic syndrome with open calvarial sutures and sutural cataracts maps to chromosome 14q13-q21

We describe a new dysmorphic syndrome in an inbred Saudi Arabian family with 21 members. Five males and one female have similar craniofacial features including wide open calvarial sutures with large and late-closing anterior fontanels, frontal bossing, hyperpigmentation with capillary hemangioma of the forehead, significant hypertelorism, and a broad and prominent nose. In addition, these individuals have Y-shaped sutural cataracts diagnosed by 1-2 years of age. No chromosomal or biochemical abnormalities were identified. A genome-wide scan was performed, and two-point LOD score analysis, assuming autosomal recessive inheritance, detected linkage to chromosome 14q13-q21. The highest LOD scores were obtained for marker GATA136A04 (LOD=4.58 at theta=0.00) and for the adjacent telomeric marker D14S1048 (LOD=4.32 at theta=0.00). Multipoint linkage analysis resulted in a maximum LOD score of 5.44 between markers D14S1048 and GATA136A04. Model independent analysis by SIBPAL confirmed linkage to the same chromosomal region. Haplotype analysis indicated that all affected individuals were homozygous for the interval on chromosome 14q13-q21 with two recombinants for D14S1014 (centromeric) and one recombinant for D14S301 (telomeric). These recombinations limit the disease locus to a region of approximately 7.26 Mb. Candidate genes localized to this region were identified, and analysis of PAX9 did not identify mutations in these patients. The unique clinical phenotype and the mapping data suggest that this family represents a novel autosomal recessive syndrome.

Genetically altered mouse models: the good, the bad, and the ugly

Targeted gene disruption in mice is a powerful tool for generating murine models for human development and disease. While the human genome program has helped to generate numerous candidate genes, few genes have been characterized for their precise in vivo functions. Gene targeting has had an enormous impact on our ability to delineate the functional roles of these genes. Many gene knockout mouse models faithfully mimic the phenotypes of the human diseases. Because some models display an unexpected or no phenotype, controversy has arisen about the value of gene-targeting strategies. We argue in favor of gene-targeting strategies, provided they are used with caution, particularly in interpreting phenotypes in craniofacial and oral biology, where many genes have pleiotropic roles. The potential pitfalls are outweighed by the unique opportunities for developing and testing different therapeutic strategies before they are introduced into the clinic. In the future, we believe that genetically engineered animal models will be indispensable for gaining important insights into the molecular mechanisms underlying development, as well as disease pathogenesis, diagnosis, prevention, and treatment.

Molecular ontogeny of the skeleton

From a traditional viewpoint, skeletal elements form by two distinct processes: endochondral ossification, during which a cartilage template is replaced by bone, and intramembranous ossification, whereby mesenchymal cells differentiate directly into osteoblasts. There are inherent difficulties with this historical classification scheme, not the least of which is that bones typically described as endochondral actually form bone through an intramembranous process, and that some membranous bones may have a transient chondrogenic phase. These innate contradictions can be circumvented if molecular and cellular, rather than histogenic, criteria are used to describe the process of skeletal tissue formation. Within the past decade, clinical examinations of human skeletal syndromes have led to the identification and subsequent characterization of regulatory molecules that direct chondrogenesis and osteogenesis in every skeletal element of the body. In this review, we survey these molecules and the tissue interactions that may regulate their expression. What emerges is a new paradigm, by which we can explain and understand the process of normal- and abnormal-skeletal development.