Type II collagen defect in two sibs with the Goldblatt syndrome, a chondrodysplasia with dentinogenesis imperfecta, and joint laxity

Translated Mutant DSPP mRNA Expression Level Impacts the Severity of Dentin Defects

Hereditary dentin defects are conventionally classified into three types of dentinogenesis imperfecta (DGI) and two types of dentin dysplasia (DD). Mutations in the dentin sialophosphoprotein (DSPP) gene have been identified to cause DGI type II and III and DD type II; therefore, these are not three different conditions, but rather allelic disorders. In this study, we recruited three families with varying clinical phenotypes from DGI-III to DD-II and performed mutational analysis by candidate gene analysis or whole-exome sequencing. Three novel mutations including a silent mutation (NM_014208.3: c.52-2del, c.135+1G>C, and c.135G>A; p.(Gln45=)) were identified, all of which affected pre-mRNA splicing. Comparison of the splicing assay results revealed that the expression level of the DSPP exon 3 deletion transcript correlated with the severity of the dentin defects. This study did not only expand the mutational spectrum of DSPP gene, but also advanced our understanding of the molecular pathogenesis impacting the severity of hereditary dentin defects.

The Genes Involved in Dentinogenesis

The development and repair of dentin are strictly regulated by hundreds of genes. Abnormal dentin development is directly caused by gene mutations and dysregulation. Understanding and mastering this signal network is of great significance to the study of tooth development, tissue regeneration, aging, and repair and the treatment of dental diseases. It is necessary to understand the formation and repair mechanism of dentin in order to better treat the dentin lesions caused by various abnormal properties, whether it is to explore the reasons for the formation of dentin defects or to develop clinical drugs to strengthen the method of repairing dentin. Molecular biology of genes related to dentin development and repair are the most important basis for future research.

Description of four patients with TRIP11 variants expand the clinical spectrum of odontochondroplasia (ODCD) and demonstrate the existence of common variants

More than two decades since the first clinical and radiological description of odontochondroplasia (ODCD) was reported, biallelic loss of function variants in the Thyroid hormone receptor interactor 11 gene (TRIP11) were identified, the same gene implicated in the lethal disorder achondrogenesis (ACG1A). Here we report the clinical and radiological follow-up of four ODCD patients, including two siblings and an adult who interestingly has the mildest form observed to date. Four TRIP11 variants were detected, two previously unreported. Subsequently, we review the clinical and radiological findings of the 14 reported ODCD patients. The majority of ODCD patients are compound heterozygotes for TRIP11 variants, 12/14 have a null allele and a splice variant whilst one is homozygous for an in-frame splicing variant, with the splice variants resulting in residual GMAP activity and hypothesized to explain why they have ODCD and not ACG1A. However, adult patient 4 has two potentially null alleles and it remains unknown why she has very mild clinical features. The c.586C>T; p.(Gln196*) variant, previously shown by mRNA studies to result in p.Val105_Gln196del, is the most frequent variant, present in seven individuals from four families, three from different regions of the world, suggesting that it may be a variant hotspot. Another variant, c.2993_2994del; p.(Lys998Serfs*5), has been observed in two individuals with a possible common ancestor. In summary, although there are clinical and radiological characteristics common to all individuals, we demonstrate that the clinical spectrum of TRIP11-associated dysplasias is even more diverse than previously described and that common genetic variants may exist.

Pathogenic variants in the TRIP11 gene cause a skeletal dysplasia spectrum from odontochondrodysplasia to achondrogenesis 1A

The thyroid hormone receptor interactor 11 (TRIP11) gene encodes the Golgi microtubule-associated protein 210 (GMAP-210), a protein essential for the operation of the Golgi apparatus. It is known that null mutations in TRIP11 disrupt Golgi function and cause a lethal skeletal dysplasia known as achondrogenesis type 1A (ACG1A), however recently, hypomorphic mutations in that gene have been linked to odontochondrodysplasia (ODCD), a nonlethal skeletal dysplasia characterized by skeletal changes in the spine and in the metaphyseal regions, associated with dentinogenesis imperfecta. Here we present two patients reflecting the phenotypic spectrum related to different TRIP11 variants. The first is a female child with ODCD, for whom a homozygous in-frame splicing mutation in intron 9 of TRIP11 was identified. The mutation appears to lead to the expression of an alternative TRIP11 transcript, that may explain the less severe radiological alterations in ODCD. The second is a fetus with classical form of ACG1A, associated with typical molecular findings (frameshift) in exon 11 of TRIP11, both novel mutations. The two patients reported here represent the TRIP11 spectrum of skeletal dysplasia ranging from mild to lethal phenotypes, thereby enabling one to suggest a genotype-phenotype correlation in these diseases.

Hereditary Dentin Defects

By the Shields classification, articulated over 30 years ago, inherited dentin defects are divided into 5 types: 3 types of dentinogenesis imperfecta (DGI), and 2 types of dentin dysplasia (DD). DGI type I is osteogenesis imperfecta (OI) with DGI. OI with DGI is caused, in most cases, by mutations in the 2 genes encoding type I collagen. Many genes are required to generate the enzymes that catalyze collagen’s diverse post-translational modifications and its assembly into fibers, fibrils, bundles, and networks. Rare inherited diseases of bone are caused by defects in these genes, and some are occasionally found to include DGI as a feature. Appreciation of the complicated genetic etiology of DGI associated with bony defects splintered the DGI type I description into a multitude of more precisely defined entities, all with their own designations. In contrast, DD-II, DGI-II, and DGI-III, each with its own pattern of inherited defects limited to the dentition, have been found to be caused by various defects in DSPP (dentin sialophosphoprotein), a gene encoding the major non-collagenous proteins of dentin. Only DD-I, an exceedingly rare condition featuring short, blunt roots with obliterated pulp chambers, remains untouched by the revolution in genetics, and its etiology is still a mystery. A major surprise in the characterization of genes underlying inherited dentin defects is the apparent lack of roles played by the genes encoding the less-abundant non-collagenous proteins in dentin, such as dentin matrix protein 1 ( DMP1), integrin-binding sialoprotein ( IBSP), matrix extracellular phosphoglycoprotein ( MEPE), and secreted phosphoprotein-1, or osteopontin ( SPP1, OPN). This review discusses the development of the dentin extracellular matrix in the context of its evolution, and discusses the phenotypes and clinical classifications of isolated hereditary defects of tooth dentin in the context of recent genetic data respecting their genetic etiologies.

Odontoblasts: Specialized hard-tissue-forming cells in the dentin-pulp complex

Odontoblasts are specialized cells that produce dentin and exhibit unique morphological characteristics; i.e., they extend cytoplasmic processes into dentinal tubules. While osteoblasts, which are typical hard-tissue-forming cells, are generated from mesenchymal stem cells during normal and pathological bone metabolism, the induction of odontoblasts only occurs once during tooth development, and odontoblasts survive throughout the lives of healthy teeth. During the differentiation of odontoblasts, signaling molecules from the inner enamel epithelium are considered necessary for the differentiation of odontoblast precursors, i.e., peripheral dental papilla cells. If odontoblasts are destroyed by severe external stimuli, such as deep caries, the differentiation of dental pulp stem cells into odontoblast-like cells is induced. Various bioactive molecules, such as non-collagenous proteins, might be involved in this process, although the precise mechanisms responsible for odontoblast differentiation have not been fully elucidated. Recently, our knowledge about the other functional activities of odontoblasts (apart from dentin formation) has increased. For example, it has been suggested that odontoblasts might act as nociceptive receptors, and surveillance cells that detect the invasion of exogenous pathogens. The regeneration of the dentin-pulp complex has recently gained much attention as a promising future treatment modality that could increase the longevity of pulpless teeth. Finally, congenital dentin anomalies, which are concerned with the disturbance of odontoblast functions, are summarized.

Developmental defects of enamel and dentine: challenges for basic science research and clinical management

Abnormalities of enamel and dentine are caused by a variety of interacting factors ranging from genetic defects to environmental insults. The genetic changes associated with some types of enamel and dentine defects have been mapped, and many environmental influences, including medical illnesses that can damage enamel and dentine have been identified. Developmental enamel defects may present as enamel hypoplasia or hypomineralization while dentine defects frequently demonstrate aberrant calcifications and abnormalities of the dentine-pulp complex. Clinically, developmental enamel defects often present with problems of discolouration and aesthetics, tooth sensitivity, and susceptibility to caries, wear and erosion. In contrast, dentine defects are a risk for endodontic complications resulting from dentine hypomineralization and pulpal abnormalities. The main goals of managing developmental abnormalities of enamel and dentine are early diagnosis and improvement of appearance and function by preserving the dentition and preventing complications. However, despite major advances in scientific knowledge regarding the causes of enamel and dentine defects, further research is required in order to translate the knowledge gained in the basic sciences research to accurate clinical diagnosis and successful treatment of the defects.

Tooth dentin defects reflect genetic disorders affecting bone mineralization

Several genetic disorders affecting bone mineralization may manifest during dentin mineralization. Dentin and bone are similar in several aspects, especially pertaining to the composition of the extracellular matrix (ECM) which is secreted by well-differentiated odontoblasts and osteoblasts, respectively. However, unlike bone, dentin is not remodelled and is not involved in the regulation of calcium and phosphate metabolism. In contrast to bone, teeth are accessible tissues with the shedding of deciduous teeth and the extractions of premolars and third molars for orthodontic treatment. The feasibility of obtaining dentin makes this a good model to study biomineralization in physiological and pathological conditions. In this review, we focus on two genetic diseases that disrupt both bone and dentin mineralization. Hypophosphatemic rickets is related to abnormal secretory proteins involved in the ECM organization of both bone and dentin, as well as in the calcium and phosphate metabolism. Osteogenesis imperfecta affects proteins involved in the local organization of the ECM. In addition, dentin examination permits evaluation of the effects of the systemic treatment prescribed to hypophosphatemic patients during growth. In conclusion, dentin constitutes a valuable tool for better understanding of the pathological processes affecting biomineralization.

Uncoupling of growth plate maturation and bone formation in mice lacking both Schnurri-2 and Schnurri-3

Formation and remodeling of the skeleton relies on precise temporal and spatial regulation of genes expressed in cartilage and bone cells. Debilitating diseases of the skeletal system occur when mutations arise that disrupt these intricate genetic regulatory programs. Here, we report that mice bearing parallel null mutations in the adapter proteins Schnurri2 (Shn2) and Schnurri3 (Shn3) exhibit defects in patterning of the axial skeleton during embryogenesis. Postnatally, these compound mutant mice develop a unique osteochondrodysplasia. The deletion of Shn2 and Shn3 impairs growth plate maturation during endochondral ossification but simultaneously results in massively elevated trabecular bone formation. Hence, growth plate maturation and bone formation can be uncoupled under certain circumstances. These unexpected findings demonstrate that both unique and redundant functions reside in the Schnurri protein family that are required for proper skeletal patterning and remodeling.

Hereditary dentine disorders: dentinogenesis imperfecta and dentine dysplasia

The hereditary dentine disorders, dentinogenesis imperfecta (DGI) and dentine dysplasia (DD), comprise a group of autosomal dominant genetic conditions characterised by abnormal dentine structure affecting either the primary or both the primary and secondary dentitions. DGI is reported to have an incidence of 1 in 6,000 to 1 in 8,000, whereas that of DD type 1 is 1 in 100,000. Clinically, the teeth are discoloured and show structural defects such as bulbous crowns and small pulp chambers radiographically. The underlying defect of mineralisation often results in shearing of the overlying enamel leaving exposed weakened dentine which is prone to wear. Currently, three sub-types of DGI and two sub-types of DD are recognised but this categorisation may change when other causative mutations are found. DGI type I is inherited with osteogenesis imperfecta and recent genetic studies have shown that mutations in the genes encoding collagen type 1, COL1A1 and COL1A2, underlie this condition. All other forms of DGI and DD, except DD-1, appear to result from mutations in the gene encoding dentine sialophosphoprotein (DSPP), suggesting that these conditions are allelic. Diagnosis is based on family history, pedigree construction and detailed clinical examination, while genetic diagnosis may become useful in the future once sufficient disease-causing mutations have been discovered. Differential diagnoses include hypocalcified forms of amelogenesis imperfecta, congenital erythropoietic porphyria, conditions leading to early tooth loss (Kostmann's disease, cyclic neutropenia, Chediak-Hegashi syndrome, histiocytosis X, Papillon-Lefevre syndrome), permanent teeth discolouration due to tetracyclines, Vitamin D-dependent and vitamin D-resistant rickets. Treatment involves removal of sources of infection or pain, improvement of aesthetics and protection of the posterior teeth from wear. Beginning in infancy, treatment usually continues into adulthood with a number of options including the use of crowns, over-dentures and dental implants depending on the age of the patient and the condition of the dentition. Where diagnosis occurs early in life and treatment follows the outlined recommendations, good aesthetics and function can be obtained.

The genetic basis of inherited anomalies of the teeth. Part 2: syndromes with significant dental involvement

Teeth are specialized structural components of the craniofacial skeleton. Developmental defects occur either alone or in combination with other birth defects. In this paper, we review the dental anomalies in several multiple congenital anomaly (MCA) syndromes, in which the dental component is pivotal in the recognition of the phenotype and/or the molecular basis of the disorder is known. We will consider successively syndromic forms of amelogenesis imperfecta or enamel defects, dentinogenesis imperfecta (i.e. osteogenesis imperfecta) and other dentine anomalies. Focusing on dental aspects, we will review a selection of MCA syndromes associated with teeth number and/or shape anomalies. A better knowledge of the dental phenotype may contribute to an earlier diagnosis of some MCA syndromes involving teeth anomalies. They may serve as a diagnostic indicator or help confirm a syndrome diagnosis.

Abnormal response to physical activity in femurs after heterozygous inactivation of one allele of the Col2a1 gene for type II collagen in mice

The objective of this study was to evaluate the influence of heterozygous inactivation of one allele of the type II collagen gene (Col2a1) on biomechanical properties and mineral density of bone under physical loading conditions. C57BL/6-TGN mice with heterozygous knockout (HZK) inactivation of Col2a1 gene and their nontransgenic littermate controls were housed in individual cages with running wheels for 9 and 15 months. The running activity of each mouse was monitored continuously throughout the experiment. Bone mineral density (BMD) of mice femora was measured using dual-energy X-ray absorptiometry (DXA) and peripheral quantitative computerized tomography (pQCT). Biomechanical properties were determined using three-point bending tests. Vertebral bone samples were prepared for quantitative polarized light microscopy and digital densitometry of proteoglycans. The concentration of total collagen and collagen cross-links were analyzed using high-performance liquid chromatograpy (HPLC). The average daily running distance was shorter for the HZK mice between the age of 4 and 15 months as compared with normal runners (P < 0.05). The ultimate breaking force was 14.8% and 23.6% (9 vs. 15 months) lower in HZK-runners than in wild-type runners. BMD of the femur was 6.1% lower in HZK-runners at the age of 9 months (P < 0.05). Physical activity increased cortical BMD in wild-type runners but not in the HZK runners at the age of 9 months. The collagen network of the HZK mice was less organized. There were only minor changes in BMD and mechanical and structural properties between sedentary HZK mice and their wild-type controls. Increased physical activity induced significantly lower bone density, mechanical properties, and organization of collagen fibers in male HZK mice. However, there were no major differences in biomechanical parameters between sedentary HZK and wild-type male mice. This suggests an important guiding role of collagen type II in bone remodelling and maturation.

Spondylometaphyseal dysplasia, east-African type: a new form of early, severe SMD with rounded vertebrae

Spondylometaphyseal dysplasias (SMD) are a heterogeneous group of bone dysplasias characterized by vertebral and metaphyseal changes of various severities. We report two unrelated patients of east African origin with a skeletal disorder consisting of 1) severe metaphyseal dysplasia of early onset, sparing hand bones, with bracket-shaped metaphyses; 2) dysplastic pelvis with irregular iliac rim; and 3) oval-shaped vertebral bodies. Contrasting with most types of SMD, the spinal dysplasia is limited to mild changes in the vertebral body shape that tend to soften with time, whereas the iliac rims have a striking lacy appearance. Except for the most common types (Kozlowski type and Schmidt type), most of the literature on SMD deals with single case reports, without longitudinal data, for which molecular definition is still lacking and classification remains unclear. These two patients could belongs to the A4 group in the classification of Maroteaux and Spranger [1991: Pediatr Radiol 2l:293-297], and illustrate the difficulties of a clinical classification of SMD.

A newly recognized syndrome of skeletal dysplasia with opalescent and rootless teeth

A Thai girl with skeletal dysplasia and dental anomalies was seen. Her anomalies consisted of disproportionately short stature, short neck, broad and depressed nasal bridge, broad chest in the anteroposterior dimension, kyphosis, widely spaced nipples, and protruded abdomen. Radiographic testing indicated that she had a large sella turcica, platyspondyly, hypoplastic acetabulum, and a small body of mandible. Both her deciduous and permanent teeth were equally opalescent, and most were rootless, with root development of the mandibular teeth more severely affected. Some maxillary roots were extremely short and tapered. Hypodontia was also observed. These findings represent a unique and hitherto undescribed syndrome of skeletal dysplasia with concomitant dental anomalies.

[Odontochondrodysplasia]

BACKGROUND

Dentinogenesis imperfecta is exceptionally associated to chondrodysplasia. The aim of this work is to present four cases of such an association.

CASE REPORT

These four children (three boys; one girl) suffered from growth retardation, ligamentous hyperlaxity, scoliosis. Main features were present since the first months of life. Dentinogenesis imperfecta was more marked on the first teeth. On X-rays, all patients had short tubular bones, more pronounced at the level of the middle segment of their limbs, with irregular metaphyses. Cone-shaped epiphyses were present on the hands. Iliac wings were square-shaped and vertebral bodies had a posterior wedging.

CONCLUSION

These four cases, including two previously published as Goldblatt's syndrome, share the same findings as another case described by this author. We propose the name of odontochondrodysplasia for this apparently unfortuitous association: dentinogenesis imperfecta and chondrodysplasia.

Type II collagenopathies: are there additional family members?

The type II collagenopathies represent a group of chondrodysplasias sharing clinical and radiological manifestations which are expressed as a continuous spectrum of phenotypes, ranging from perinatally lethal to very mild conditions. Their common molecular bases are mutations in the type II collagen gene (COL2A1). We describe one case of lethal platyspondylic dysplasia, Torrance type, and a variant of lethal Kniest dysplasia, neither of which has been reported as a type II collagenopathy. Biochemical studies of cartilage collagens and morphological analysis of cartilage sections suggest that abnormalities of type II collagen structure and biosynthesis are the main pathogenetic factors in both cases. Thus, the phenotypic spectrum of type II collagenopathies might be greater than hitherto suspected.

Human COL2A1-directed SV40 T antigen expression in transgenic and chimeric mice results in abnormal skeletal development

The ability of SV40 T antigen to cause abnormalities in cartilage development in transgenic mice and chimeras has been tested. The cis-regulatory elements of the COL2A1 gene were used to target expression of SV40 T antigen to differentiating chondrocytes in transgenic mice and chimeras derived from embryonal stem (ES) cells bearing the same transgene. The major phenotypic consequences of transgenic (pAL21) expression are malformed skeleton, disproportionate dwarfism, and perinatal/neonatal death. Expression of T antigen was tissue specific and in the main characteristic of the mouse alpha 1(II) collagen gene. Chondrocyte densities and levels of alpha 1(II) collagen mRNAs were reduced in the transgenic mice. Islands of cells which express cartilage characteristic genes such as type IIB procollagen, long form alpha 1(IX) collagen, alpha 2(XI) collagen, and aggrecan were found in the articular and growth cartilages of pAL21 chimeric fetuses and neonates. But these cells, which were expressing T antigen, were not properly organized into columns of proliferating chondrocytes. Levels of alpha 1(II) collagen mRNA were reduced in these chondrocytes. In addition, these cells did not express type X collagen, a marker for hypertrophic chondrocytes. The skeletal abnormality in pAL21 mice may therefore be due to a retardation of chondrocyte maturation or an impaired ability of chondrocytes to complete terminal differentiation and an associated paucity of some cartilage matrix components.

The type II collagenopathies: a spectrum of chondrodysplasias

With the application of molecular techniques the aetiopathogenesis of skeletal dysplasias is gradually elucidated. Recent advances show that some bone dysplasias result from defects in the biosynthesis of type II (cartilage) collagen. Clinical entities caused by mutations in the COL2A1 gene coding for type II collagen comprise achondrogenesis II, hypochondrogenesis, spondyloepiphyseal dysplasia congenita, Kniest dysplasia, Stickler arthroophthalmopathy and mild dominant spondyloarthropathy. The mutations are expressed in the heterozygous state, and inheritance of type II collagenopathies is autosomal dominant. The wide range of clinical manifestations is not well understood but characterization of the basic defect may provide clues to establish specific genotype-phenotype correlations.