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.

Atypical expression of cleidocranial dysplasia: clinical and molecular-genetic analysis

Cleidocranial dysplasia (CCD) and the Rubinstein-Taybi syndrome (RTS) are two rare congenital syndromes that have many clinical signs in common. We present an 18-year-old-patient with untypical CCD expression who was misdiagnosed with RTS at the age of 2 years. An extensive craniofacial examination was carried out with respect to morphological and dental aspects. The molecular-genetic analysis of two underlying genes (CBFA1 and CBP) for CCD and RTS was performed using SSCP, direct sequencing and FISH. While the clinical examination showed uncharacteristic CCD symptoms with some findings common for RTS, the molecular-genetic analysis revealed a missense mutation in the CBFA1 gene, which is considered to be the etiological factor for CCD. Our findings with this patient presented clear evidence for the wide morphologic variety that can be related to a certain gene such as CBFA1. The diagnosis of rare diseases is currently based on the clinical phenomenology of small groups or single cases. The use of molecular-genetic biology extends the horizon of diagnostic and scientific possibilities. In this patient, it allowed us to compare the clinically diagnosis to molecular-genetic data. We conclude that molecular-genetic analysis may be a helpful tool in the differential diagnosis of many congenital diseases such as CCD and RTS.

Severe cleidocranial dysplasia can mimic hypophosphatasia

Cleidocranial dysplasia (OMIM 119600) is a skeletal dysplasia caused by mutations in the bone/cartilage specific osteoblast transcription factor RUNX2 gene. It is characterised by macrocephaly with persistently open sutures, absent or hypoplastic clavicles, dental anomalies, and delayed ossification of the pubic bones. A few patients have been reported with recurrent fractures or osteoporosis but these are not considered features of the disease. We report a patient with classical findings of cleidocranial dysplasia: markedly hypoplastic clavicles, delayed ossification of the pubic rami, multiple pseudoepiphyses of the metacarpals, and dental anomalies including delayed eruption of permanent dentition and multiple supernumerary teeth. The patient also had radiographic and biochemical features of hypophosphatasia (OMIM 241500, 146300) and was initially diagnosed with this condition. Serum alkaline phosphatase activity has been consistently reduced and specific enzyme substrates, phosphoethanolamine and pyridoxal-5'-phosphate, have been elevated. However, no mutations were found on direct sequencing of the tissue-nonspecific alkaline phosphatase ( TNSALP) gene using a protocol that detects up to 94% of all mutations causing hypophosphatasia.

CONCLUSION

We propose that a subset of patients with cleidocranial dysplasia have features of secondary hypophosphatasia due to decreased expression of the tissue-nonspecific alkaline phosphatase gene.

Cleidocranial dysplasia with decreased bone density and biochemical findings of hypophosphatasia

Cleidocranial dysplasia (CCD; MIM 119600) is an autosomal dominant skeletal dysplasia characterised by hypoplastic clavicles, patent fontanelles, short stature, tooth anomalies and other variable skeletal changes. Different mutations of the RUNX2/CBFA1 gene (MIM 600211) have been detected in patients with CCD. We investigated a mother and daughter with features of CCD presenting with reduced plasma alkaline phosphatase activity, increased urinary phosphoethanolamine excretion and decreased bone density. The latter findings were suggestive of hypophophatasia but mutation analysis showed no mutation in the tissue-nonspecific alkaline phosphatase gene (TNSALP; MIM 171760). However, a heterozygous mutation (Arg169Pro caused by nucleotide change 506G > C) was detected in the RUNX2 gene. Metabolic alterations gradually improved in both mother and daughter but bone-specific alkaline phosphatase remained low (less than 30% of normal) and mild phosphoethanolaminuria persisted. Recent studies in the Cbfa1 knock-out mouse showed decreased expression of alkaline phosphatase in differentiating bone.

CONCLUSION

we suggest that the observed metabolic alterations are secondary to the RUNX2 gene mutation affecting early bone maturation and turnover. This is the first description of biochemical findings of hypophosphatasia in patients with cleidocranial dysplasia.

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.

New mutations in the CBFA1 gene in two Mexican patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal disorder exhibiting a wide clinical spectrum ranging from minimal anomalies to classic CCD. Mutations scattered throughout the entire CBFA1 gene have been related to this disorder. However, it seems that most of them affect the highly conserved Runt domain, abolishing the DNA-binding ability of this transcription factor. Moreover, no systematic effect has been found to relate the type of mutation to the severity of the clinical features. In this paper, we studied two unrelated patients with classic CCD. DNA analysis revealed two novel mutations and three undescribed polymorphisms. One of the substitutions was a missense mutation in the Q/A domain leading to the replacement of a polar residue by a nonpolar one (158 A --> T [Q53L]). The second was an uncommon heterozygous stop codon mutation (1565 G --> C [X522S]) which theoretically results in a longer protein with 23 additional amino acids. This is the first report of this type of mutation in CBFA1. We discuss the possible consequences of these mutant sequences, although no phenotype-genotype correlation could be established. Our findings expand the existing number of allelic variants in this pathology.

Evidence of intrafamilial variability of CBFA1/RUNX2 expression in cleidocranial dysplasia--a family study

AIM

To investigate the phenotypical expression of an identical mutation of the CBFA1/RUNX2 gene within a family with cleidocranial dysplasia.

PATIENTS AND METHOD

A five-member family underwent clinical examination. Two members, father and son, showed dissimilar symptoms of cleidocranial dysplasia. The two affected patients were examined for syndrome-typical symptoms, and the genotype was determined by molecular-genetic analysis.

RESULTS

In both patients an identical missense mutation (G146R) in exon 2 of the CBFA1/RUNX2 gene was identified. In father and son the dental disturbances were similarly clearly expressed. However, the craniofacial skeleton of the son exhibited fewer dysostotic ossification features than that of the father. In the three clinically healthy family-members no mutation of the CBFA1/RUNX2 gene was found.

CONCLUSION

In two patients with cleidocranial dysplasia an identical missense mutation in the CBFA1/RUNX2 gene leading to a divergent craniofacial phenotype was determined. The results indicate marked variability in the phenotypical expression of CBFA1/RUNX2 mutations.

Mutations in the RUNX2 gene in patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a autosomal dominant disorder characterized by skeletal anomalies such as patent fontanels, late closure of cranial sutures with Wormian bones, late erupting secondary dentition, rudimentary clavicles, and short stature. The locus for this disease was mapped to chromosome 6p21. RUNX2 is a member of the runt family of transcription factors and its expression is restricted to developing osteoblasts and a subset of chondrocytes. Mutations in the RUNX2 gene have been shown to cause CCD. Chromosomal translocations, deletions, insertions, nonsense and splice-site mutations, as well as missense mutations of the RUNX2 gene have been described in CCD patients. Although there is a wide spectrum in phenotypic variability ranging from primary dental anomalies to all CCD features plus osteoporosis, no clear phenotype-genotype correlation has been established. However analysis of the three-dimensional structure of the DNA binding runt domain of the RUNX proteins and its interaction with DNA, as well as the cofactor CBFB, start to provide an insight into how missense mutations affect RUNX2 function.

A case of a Japanese patient with cleidocranial dysplasia possessing a mutation of CBFA1 gene

Cleidocranial dysplasia (CCD) is an autosomal dominant human bone disease characterized by hypoplastic or aplastic clavicles, wide cranial sutures, supernumerary teeth, short stature, and other skeletal disorders. Recently, various mutations of the core binding factor (CBFA1) gene have been detected in CCD patients. The CBFA1 gene is a member of the runt family of transcription factors. We experienced one Japanese case of CCD with open sutures, hypoplasia of clavicles and brachydactyly, combined with atlant-axis dislocation. We performed the sequence analysis of the CBFA1 gene and detected a missense mutation of R225W in exon 3.

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.

Genetic mutations in certain head and neck conditions of interest to the dentist

This article identifies certain syndromes of the head and neck, which a dentist may see in clinical practice, and relates these syndromes to their sites of mutation on involved genes. This paper is timely with the near completion of the Human Genome Project, the mapping of the entire human genetic material. Knowing the site of the genetic lesion is important in helping clinicians understand the genetic basis for these conditions, and may help in our future understanding of remedies and treatments.

Identification of a stop codon mutation in the CBFA1 runt domain from a patient with cleidocranial dysplasia and cleft lip

We examined a patient with cleidocranial dysplasia (CCD) and cleft lip and found a new stop codon mutation in CBFA1. This mutation was a heterozygous C-to-T transition in exon 3 of CBFA1. This nucleotide change converts a CAA codon to a TAA (stop) codon at amino acid position Gln195 in the runt domain of CBFA1.

Functional mutagenesis of AML1/RUNX1 and PEBP2 beta/CBF beta define distinct, non-overlapping sites for DNA recognition and heterodimerization by the Runt domain

The Runt domain family of transcription factors play key roles in transcriptional regulation of definitive hematopoiesis and osteogenesis. This transcription factor family is characterized by a DNA-binding alpha-subunit harboring the Runt domain and a secondary subunit, beta, which binds to the Runt domain and enhances its interaction with DNA. Missense mutations in the Runt domain from either the blood or bone-related gene product are associated with the onset of acute human leukemia as well as a disease of skeletal patterning known as cleidocranial dysplasia. NMR "footprinting" analysis of Runt domain/beta/DNA ternary complexes in solution previously identified the likely residues that form the heterodimerization and DNA-binding surfaces of the Runt domain. Functional mutagenesis at 37 positions in the Runt domain or beta confirms the original identification of these interaction surfaces and reveals that the heterodimerization and DNA-binding surfaces of the Runt domain occur at distinct, non-overlapping sites within the domain. The analysis of an additional 21 disease-related missense mutations identified from patients with either blood or bone disease demonstrates that the primary defect in these patients is a failure in DNA-recognition by the Runt domain. The molecular basis for the DNA-binding defect is analyzed in the context of the three-dimensional structure of the Runt domain in binary and ternary protein/DNA complexes.

Transcriptional dysregulation in skeletal malformation syndromes

Normal skeletal development requires coordinated temporal and spatial gene expression patterns that specify the functions of various cell types. Transcription factors by definition coordinate this process and are themselves subject to hierarchical levels of regulation. Together they determine the context-dependent function of each transcription factor. Hence, loss-of-function and gain-of-function mutations within specific transcription factors cause dysregulation of broad transcriptional networks. Consequences are usually dominantly inherited skeletal malformation syndromes that can be broadly viewed as consequences of defects of cellular differentiation, proliferation, and survival versus defects in pattern formation. The study of human phenotypes and mutations can lead to hypotheses about targets within the respective transcriptional network. These targets can then be confirmed by combining mouse genetic and in vitro studies. Although this has been successful in a small group of skeletal dysplasias, the majority of transcriptional networks during skeletogenesis remain to be elucidated.

A RUNX2/PEBP2alphaA/CBFA1 mutation in cleidocranial dysplasia revealing the link between the gene and Smad

Cleidocranial dysplasia (CCD), an autosomal dominant human bone disease, is thought to be caused by heterozygous mutations in RUNX2/PEBP2alphaA/CBFA1. To understand the mechanism underlying the pathogenesis of CCD, we studied a novel mutant of RUNX2, namely CCDalphaA376, originally identified in a CCD patient. The nonsense mutation, which resulted in a truncated RUNX2 protein, severely impaired RUNX2 transactivation activity. We showed that signal transducers of transforming growth factor (TGF)-beta and bone morphogenetic protein (BMP) receptors, Smads, interact with RUNX2 in vivo and in vitro and enhance transactivation ability. The truncated RUNX2 protein failed to interact with Smads, and was unable to induce the osteoblast-like phenotype in C2C12 myoblasts following stimulation with BMP. Exogenous expression of Smads 1 and 4 in C2C12 cells stably expressing RUNX2 showed alkaline phosphatase (ALP) activity, suggesting a possible link between Smads and RUNX2, while in C2C12 stably expressing CCDalphaA376, a detectable level of ALP activity failed to be induced. The results suggest that CCDalphaA376 inhibited RUNX2 function in a dominant negative fashion.

Identification of a novel frameshift mutation (383insT) in the RUNX2 (PEBP2 alpha/CBFA1/AML3) gene in a Japanese patient with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder due to mutations in runt-related gene 2 (RUNX2)/polyomavirus enhancer-binding protein 2alphaA (PEBP2alphaA)/core-binding factor A1 (CBFA1)/acute myeloid leukemia 3 (AML3). To investigate the RUNX2 mutations in a Japanese patient with classic CCD, we analyzed the RUNX2 gene using polymerase chain reaction (PCR)-single-strand conformation polymorphism and PCR-restriction fragment length polymorphism. The patient had hypoplasia of the clavicles, patent fontanelles, short stature, supernumerary teeth, and retention of deciduous dentition. We identified a 1-bp insertion (383insT) at codon 128 of the RUNX2 gene. The 383T insertion affects the conserved residue in the runt domain and results in premature termination in the runt domain.

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.