Dentinogenesis imperfecta 1 with or without progressive hearing loss is associated with distinct mutations in DSPP

Porcine dentin sialoprotein is a proteoglycan with glycosaminoglycan chains containing chondroitin 6-sulfate

Dentin sialoprotein (DSP) is a glycoprotein that is critical for proper tooth dentin formation, but little is known about the nature of its carbohydrate attachments and other post-translational modifications. We have isolated DSP from pig dentin and demonstrate that it is a proteoglycan. Polyclonal antibodies were raised in chicken against recombinant pig DSP, and used to identify native DSP in fractions of tooth dentin proteins extracted from developing pig molars. Amino acid analyses and characterization of lysylendopeptidase cleavage products confirmed that the purified protein was DSP, and that Arg391 is at the DSP C terminus. On SDS-PAGE and on urea gels, DSP appeared as a smear extending from 280 to 100 kDa, but in the presence of beta-mercaptoethanol the top of the DSP smear disappeared. The high molecular weight material was likely comprised of covalent DSP dimers connected by a disulfide bridge at Cys205. Oligosaccharides were released from DSP following N- and O-linked glycosidase digestions, but these digestions had little effect on the apparent molecular weight of DSP on SDS-PAGE, when compared with the significant reduction following chondroitinase ABC digestion. Glycosaminoglycanases with assorted glycosaminoglycan (GAG) cleavage specificities coupled with Western analyses of the cleaved GAG "stubs" demonstrated that the DSP GAG attachments contain chondroitin 6-sulfate, but not keratan sulfate, heparan sulfate, chondroitin, or chondroitin 4-sulfate. DSP binds biotin-labeled hyaluronic acid, and such binding is inhibited by the addition of unlabeled hyaluronic acid. We conclude that DSP is a proteoglycan and that GAG attachments are the predominant structural feature of porcine DSP.

The CCAAT enhancer-binding protein (C/EBP)beta and Nrf1 interact to regulate dentin sialophosphoprotein (DSPP) gene expression during odontoblast differentiation

Terminal differentiation of odontoblasts, the principal cells in dentin formation, proceeds by synthesis of type I collagen and noncollagenous proteins. DSP and DPP are specific markers for terminally differentiated odontoblasts and are encoded by a single gene DSPP (dentin sialophosphoprotein). In an attempt to understand the molecular mechanisms required for tissue-specific expression of the DSPP gene, we have identified a novel interaction between two bZIP transcription factors, Nrf1 and the CCAAT enhancer-binding protein (C/EBP)beta. This interaction was confirmed by both immunoprecipitation and chromatin immunoprecipitation assays. In undifferentiated odontoblasts, Nrf1 and C/EBPbeta repress DSPP promoter activity individually and synergistically by cooperatively interacting with each other. This mutual interaction is facilitated by the bZIP domains in both the proteins. The repression domain in both Nrf1 and C/EBPbeta was determined, and deletion of this domain abolished transcriptional repression. In fully differentiated odontoblasts, the loss of interaction between Nrf1 and C/EBPbeta results in an increased DSPP transcription. Further, this interaction was found to be dependent on phosphorylation at Ser(599) of Nrf1. Thus, the physical interaction between Nrf1 and C/EBPbeta provide a novel mechanism for the transcriptional regulation of DSPP in odontoblasts.

Regulation of the Cell Type-specific dentin sialophosphoprotein gene expression in mouse odontoblasts by a novel transcription repressor and an activator CCAAT-binding factor

Dentin sialophosphoprotein (DSPP) is an extracellular matrix protein that is cleaved into dentin sialoprotein (DSP) and dentin phosphoprotein (DPP) with a highly restricted expression pattern in tooth and bone. Mutations of the DSPP gene are associated with dentin genetic diseases. Regulation of tissue-specific DSPP expression has not been described. To define the molecular basis of this cell-specific expression, we characterized the promoter responsible for the cell-specific expression of the DSPP gene in odontoblasts. Within this region, DNase I footprinting and electrophoretic mobility shift assays delineated one element that contains an inverted CCAAT-binding factor site and a protein-DNA binding site using nuclear extracts from odontoblasts. A series of competitive electrophoretic mobility shift assay analyses showed that the protein-DNA binding core sequence, ACCCCCA, is a novel site sufficient for protein binding. These two protein-DNA binding sequences are conserved at the same proximal position in the mouse, rat, and human DSPP gene promoters and are ubiquitously present in the promoters of other tooth/bone genes. Mutations of the CCAAT-binding factor binding site resulted in a 5-fold decrease in promoter activity, whereas abolishment of the novel protein-DNA binding site increased promoter activity by about 4.6-fold. In contrast to DSPP, expression levels of the novel protein were significantly reduced during odontoblastic differentiation and dentin mineralization. The novel protein was shown to have a molecular mass of 72 kDa. This study shows that expression of the cell type-specific DSPP gene is mediated by the combination of inhibitory and activating mechanisms.

Differential expression patterns of the dentin matrix proteins during mineralized tissue formation

Sequential and reciprocal interactions between the oral ectoderm and neural crest-derived mesenchyme are responsible for tooth development. During dentin formation, there are three components that are necessary for proper mineralization, namely, collagen which forms a scaffold, noncollagenous proteins that can specifically bind to the collagen template and function as a mineral nucleator and crystalline calcium phosphate deposited in an ordered manner. It is well established that noncollagenous proteins play an important role during mineralized tissue formation. Here we demonstrate by in situ hybridization techniques that the noncollagenous dentin matrix proteins 1, 2 (DMP1, 2) and dentin sialoprotein (DSP) have characteristic temporal and spatial expression patterns within odontogenic tissues during dentin mineralization. DMP1, DMP2 and DSP mRNA are expressed in the odontoblasts at specific and overlapping time points and are thus presumably used for different functions during dentin formation. In developing rat incisors and molars, high levels of expression of DMP2 mRNA were seen in polarized odontoblasts and preameloblasts, while DSP mRNA was expressed at significantly lower levels and was expressed by highly differentiated odontoblasts. However, their expression was continuously maintained during the mineralization of the organic matrix. In the adult rats, DMP2 and DSP mRNA was also detected in the osteoblasts. The expression of DMP1 mRNA was found to coincide with the start of the mineral nucleation process and gradually decreased during the maturation of the mineralized matrix during odontogenesis. In this study, we have also correlated the expression of these proteins relative to the presence of type I collagen and calcium phosphate crystals. Thus, the temporal and spatial differences between DMP1, DMP2 and DSP might implicate a direct demonstration of the functional difference between these three genes during calcified tissue formation.

Detection of dentin sialoprotein in rat periodontium

Cloning and sequencing of the cDNA indicates that dentin sialophosphoprotein (DSPP) is a precursor of both dentin sialoprotein (DSP) and dentin phosphoprotein (DPP). Dentin sialophosphoprotein must be proteolytically processed to form these two extracellular matrix (ECM) proteins. Numerous studies led us to conclude that DSP (and DSPP) are exclusively expressed by odontoblasts and preameloblasts. However, recent observations suggest a wider distribution. To test this hypothesis, we conducted systematic studies on rat first molar during root formation with immunohistochemical techniques using specific anti-DSP polyclonal and monoclonal antibodies. We also performed in situ hybridization, using high-stringency RNA probes to detect DSP transcripts. Immunohistochemical studies demonstrated that DSP is not only localized in odontoblasts, dentin ECM and preameloblasts, but also in alveolar bone, cellular cementum, osteocytes, cementocytes, and their matrices. The results of in situ hybridization were consistent with those from immunohistochemistry, showing the expression of DSP transcripts in osteoblasts of alveolar bone, fibroblasts in periodontal ligament and cementoblasts in cellular cementum. Together, these observations suggest that DSP is involved in formation of the periodontium as well as tooth structures.

Human genome research in China

Significant progress in human genome research has been made in China since 1994. This review aims to give a brief and incomplete introduction to the major research institutions and their achievements in human genome sequencing and functional genomics in medicine, with emphasis on the "1% Sequencing Project", the generation of single nucleotide polymorphism and haplotype maps of the human genome, disease gene identification, and the molecular characterization of leukemia and other diseases. Chinese efforts towards the sequencing of pathogenic microbial genomes and of the rice (Oryza sativa ssp. Indica) genome are also described.

Deletion of Dentin Matrix Protein-1 Leads to a Partial Failure of Maturation of Predentin into Dentin, Hypomineralization, and Expanded Cavities of Pulp and Root Canal during Postnatal Tooth Development

The dentin matrix protein-1 (DMP-1) gene is identified in odontoblasts during both embryonic and postnatal development. In vitro study suggests that this noncollagen acidic phosphoprotein plays a role in mineralization. However, deletion of the Dmp-1 gene has little effect on tooth development during embryogenesis. To address the role of DMP-1 in tooth during postnatal development, we analyzed changes of dentinogenesis in Dmp-1 null mice from 3 days after birth to 1 year. Here we show that Dmp-1 null mice postnatally develop a profound tooth phenotype characterized by a partial failure of maturation of predentin into dentin, enlarged pulp chambers, increased width of predentin zone with reduced dentin wall, and hypomineralization. The tooth phenotype of these mice is strikingly similar to that in dentin sialophosphoprotein (Dspp) null mice and shares some features of the human disease dentinogenesis imperfecta III. We have also demonstrated that DSPP levels are reduced in Dmp-1 null mice, suggesting that DSPP is probably regulated by DMP-1 during dentinogenesis. Finally, we show the absence or delayed development of the third molar in Dmp-1 null mice, which is probably secondary to defects in Dmp-1 null bone. Taken together, these studies suggest that DMP-1 is essential for later dentinogenesis during postnatal development.

Laminin α2 Is Essential for Odontoblast Differentiation Regulating Dentin Sialoprotein Expression

Laminin alpha2 is subunit of laminin-2 (alpha2beta1gamma1), which is a major component of the muscle basement membrane. Although the laminin alpha2 chain is expressed in the early stage of dental mesenchyme development and localized in the tooth germ basement membrane, its expression pattern in the late stage of tooth germ development and molecular roles are not clearly understood. We analyzed the role of laminin alpha2 in tooth development by using targeted mice with a disrupted lama2 gene. Laminin alpha2 is expressed in dental mesenchymal cells, especially in odontoblasts and during the maturation stage of ameloblasts, but not in the pre-secretory or secretory stages of ameloblasts. Lama2 mutant mice have thin dentin and a widely opened dentinal tube, as compared with wild-type and heterozygote mice, which is similar to the phenotype of dentinogenesis imperfecta. During dentin formation, the expression of dentin sialoprotein, a marker of odontoblast differentiation, was found to be decreased in odontoblasts from mutant mice. Furthermore, in primary cultures of dental mesenchymal cells, dentin matrix protein, and dentin sialophosphoprotein, mRNA expression was increased in laminin-2 coated dishes but not in those coated with other matrices, fibronectin, or type I collagen. Our results suggest that laminin alpha2 is essential for odontoblast differentiation and regulates the expression of dentin matrix proteins.

Clinical, histopathologic, and genetic investigation in two large families with dentinogenesis imperfecta type II

Dentinogenesis imperfecta (DI) type II, an inherited disorder affecting dentin, has been linked to mutations in the dentin sialophosphoprotein ( DSPP) gene on chromosome 4q21. The gene product is cleaved into two dentin-specific matrix proteins, dentin sialoprotein (DSP) and dentin phosphoprotein. The aim of this investigation was to study genotypes and phenotypes in two affected families with special reference to clinical, radiographic, and histopathologic manifestations. Seven affected members of Family A and five of Family B were documented clinically and radiographically; 14 and 10 teeth, respectively, were available for histopathologic investigation and prepared for ground sections, which were assessed semiquantitatively for dysplastic manifestations in the dentin according to the scoring system, dysplastic dentin score (DDS). Venous blood samples were collected from six affected and ten unaffected members of Family A, and from eight affected and six unaffected members of Family B. Genomic DNA was extracted and used for sequence analyses. The two families presented with different missense mutations. An Arg68Trp missense mutation in the DSP part of the gene was revealed in all six analyzed affected individuals in Family A. This mutation was not present in any of the ten healthy members. In Family B, an Ala15Val missense mutation involving the last residue of the signal peptide was found in all eight affected but in none of the six healthy members. The clinical and radiographic disturbances and DDS were more severe in Family B. The data indicate the presence of a genotype-phenotype correlation in DI type II.

Dentin matrix protein 1 immobilized on type I collagen fibrils facilitates apatite deposition in vitro

During bone and dentin mineralization, the crystal nucleation and growth processes are considered to be matrix regulated. Osteoblasts and odontoblasts synthesize a polymeric collagenous matrix, which forms a template for apatite initiation and elongation. Coordinated and controlled reaction between type I collagen and bone/dentin-specific noncollagenous proteins are necessary for well defined biogenic crystal formation. However, the process by which collagen surfaces become mineralized is not understood. Dentin matrix protein 1 (DMP1) is an acidic noncollagenous protein expressed during the initial stages of mineralized matrix formation in bone and dentin. Here we show that DMP1 bound specifically to type I collagen, with the binding region located at the N-telopeptide region of type I collagen. Peptide mapping identified two acidic clusters in DMP1 responsible for interacting with type I collagen. The collagen binding property of these domains was further confirmed by site-directed mutagenesis. Transmission electron microscopy analyses have localized DMP1 in the gap region of the collagen fibrils. Fibrillogenesis assays further demonstrated that DMP1 accelerated the assembly of the collagen fibrils in vitro and also increased the diameter of the reconstituted collagen fibrils. In vitro mineralization studies in the presence of calcium and phosphate ions demonstrated apatite deposition only at the collagen-bound DMP1 sites. Thus specific binding of DMP1 and possibly other noncollagenous proteins on the collagen fibril might be a key step in collagen matrix organization and mineralization.

Control of crystal size and lattice formation by starmaker in otolith biomineralization

The stone-like otoliths from the ears of teleost fishes are involved in balance and hearing and consist of calcium carbonate crystallites embedded in a protein framework. We report that a previously unknown gene, starmaker, is required in zebrafish for otolith morphogenesis. Reduction of starmaker activity by injection of modified antisense oligonucleotides causes a change in the crystal lattice structure and thus a change in otolith morphology. The expression pattern of starmaker, along with the presence of the protein on the growing otolith, suggest that the expression levels of starmaker control the shape of the otoliths.

Developmental biology. Rocks that roll zebrafish

The vestibular organs of the inner ear of higher vertebrates control balance, and their counterparts in fish control both balance and hearing. Essential to the operation of these sensory organs are the biomineralized structures--otoconia in higher vertebrates or otoliths in fish--that deflect the sensory hair bundles situated beneath them. In her Perspective, Fekete explores the fascinating world of otolith biomineralization in zebrafish; revealing the importance of a protein called Starmaker for coordinating the shape and type of crystal in fish otoliths ( Söllner et al.).

Diagnosis and management of unusual dental abscesses in children

Although the majority of dental abscesses in children originate from dental caries or trauma, a few are associated with unusual conditions which challenge diagnosis and management. Recent research findings have shed light on these unusual entities and greatly improved understanding of their clinical implications. These conditions include developmental abnormalities such as dens invaginatus in which there is an invagination of dental tissues into the pulp chamber and dens evaginatus in which a tubercle containing pulp is found on the external surface of a tooth crown. In addition, inherited conditions which show abnormal dentine such as dentine dysplasia, dentinogenesis imperfecta, and osteogenesis imperfecta predispose the dentition to abscess formation. Furthermore, 'spontaneous' dental abscesses are frequently encountered in familial hypophosphataemia, also known as vitamin D-resistant rickets, in which there is hypomineralization of dentine and enlargement of the pulp. In addition to developmental conditions, there are also acquired conditions which may cause unusual dental abscesses. These include pre-eruptive intracoronal resorption which was previously known as 'pre-eruptive caries' or the 'fluoride bomb'. In addition, some undiagnosed infections associated with developing teeth are now thought to be the mandibular infected buccal cysts which originate from infection of the developing dental follicles. In the present paper, these relatively unknown entities which cause unusual abscesses in children are reviewed with the aim of updating the general practitioner in their diagnosis and management.

Nonsyndromic hearing loss

The past decade has seen extremely rapid progress in the field of hereditary hearing loss. To date, 80 loci for nonsyndromic hearing loss have been mapped to the human genome. Furthermore, 30 genes have been identified. These genes belong to a wide variety of protein classes: from myosins and other cytoskeletal proteins, over channel and gap junction components, to transcription factors, extracellular matrix proteins and genes with an unknown function. The identification of these genes has enabled geneticists to offer DNA diagnostic tests for some types of nonsyndromic hearing loss. Moreover, it holds the promise to significantly improve the molecular knowledge on the auditory and vestibular organs and on the pathological mechanisms leading to hearing loss. This opens perspectives for future therapeutic and/or preventive measures for hearing loss. This review attempts to give an overview of the current knowledge of the genes responsible for nonsyndromic hearing loss, their expression and functions in the cochlea.

Nucleation of apatite crystals in vitro by self-assembled dentin matrix protein 1

Bones and teeth are biocomposites that require controlled mineral deposition during their self-assembly to form tissues with unique mechanical properties. Acidic extracellular matrix proteins play a pivotal role during biomineral formation. However, the mechanisms of protein-mediated mineral initiation are far from understood. Here we report that dentin matrix protein 1 (DMP1), an acidic protein, can nucleate the formation of hydroxyapatite in vitro in a multistep process that begins by DMP1 binding calcium ions and initiating mineral deposition. The nucleated amorphous calcium phosphate precipitates ripen and nanocrystals form. Subsequently, these expand and coalesce into microscale crystals elongated in the c-axis direction. Characterization of the functional domains in DMP1 demonstrated that intermolecular assembly of acidic clusters into a beta-sheet template was essential for the observed mineral nucleation. Protein-mediated initiation of nanocrystals, as discussed here, might provide a new methodology for constructing nanoscale composites by self-assembly of polypeptides with tailor-made peptide sequences.

Dentin sialophosphoprotein knockout mouse teeth display widened predentin zone and develop defective dentin mineralization similar to human dentinogenesis imperfecta type III

Dentin sialophosphoprotein (Dspp) is mainly expressed in teeth by the odontoblasts and preameloblasts. The Dspp mRNA is translated into a single protein, Dspp, and cleaved into two peptides, dentin sialoprotein and dentin phosphoprotein, that are localized within the dentin matrix. Recently, mutations in this gene were identified in human dentinogenesis imperfecta II (Online Mendelian Inheritance in Man (OMIM) accession number 125490) and in dentin dysplasia II (OMIM accession number 125420) syndromes. Herein, we report the generation of Dspp-null mice that develop tooth defects similar to human dentinogenesis imperfecta III with enlarged pulp chambers, increased width of predentin zone, hypomineralization, and pulp exposure. Electron microscopy revealed an irregular mineralization front and a lack of calcospherites coalescence in the dentin. Interestingly, the levels of biglycan and decorin, small leucine-rich proteoglycans, were increased in the widened predentin zone and in void spaces among the calcospherites in the dentin of null teeth. These enhanced levels correlate well with the defective regions in mineralization and further indicate that these molecules may adversely affect the dentin mineralization process by interfering with coalescence of calcospherites. Overall, our results identify a crucial role for Dspp in orchestrating the events essential during dentin mineralization, including potential regulation of proteoglycan levels.

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.

Dentin sialoprotein isoforms: detection and characterization of a high molecular weight dentin sialoprotein

Dentin sialoprotein (DSP) is a glycoprotein accounting for 5-8% of the dentin non-collagenous proteins. The cDNA sequence predicts that rat DSP has 13 potential casein kinase phosphorylation sites and six potential N-linked glycosylation sites. However, its total phosphorylation level, as well as the nature and locations of the carbohydrate moieties, are unknown. Our findings in the present study show that rat DSP has 6.2 phosphates per molecule and that the majority of carbohydrates are attached to the protein through N-linked glycosylations. During our separation of dentin non-collagenous proteins with ion-exchange chromatography, we observed high molecular weight components eluting late in the salt gradient that were recognized by anti-DSP antibodies. We have purified these high molecular weight components using a monoclonal anti-DSP antibody affinity column. Data from amino acid analysis, phosphate level measurements and Edman degradation of tryptic peptides unequivocally proved that the very acidic, high molecular weight components are isoforms of DSP (designated HMW-DSP). Deglycosylation analysis indicates that the slower migration rate of HMW-DSP on SDS-PAGE results from its higher level of carbohydrate modifications.

It's only teeth - are there limits to genetic testing?

Dental genetic disorders can cause severe social and psychological effects in affected individuals. The cost of treatment can be considerable, not only in financial terms but also in time spent during treatment. In theory it is, or will soon be, possible to use advances in molecular genetics for pre-natal testing, for selection of embryos using in vitro fertilization techniques, and for gene therapy. The questions we pose are whether these approaches are appropriate. We hope that this review will stimulate debate on these issues.

MEPE/OF45, a new dentin/bone matrix protein and candidate gene for dentin diseases mapping to chromosome 4q21

Substantial progress has been made regarding the molecular etiology of the dentin diseases dentinogenesis imperfecta types II and III, and dentin dysplasia type II. Genetic linkage studies have identified the critical loci for these diseases on human chromosome 4q21. Located within an overlapping segment of these disease loci is a dentin/bone gene cluster that includes osteopontin, bone sialoprotein, dentin matrix protein 1, dentin sialophosphoprotein, and a new gene MEPE also known as OF45, renamed osteoregulin. In this paper, we report the location of MEPE/OF45 in relationship to the other members of the dentin/bone gene cluster as well as the genomic organization of the human gene. For the first time, MEPE/OF45 expression was shown in dental tissue, in particular odontoblasts, by reverse-transcription polymerase chain reaction (RT-PCR) amplification and characterization of a partial mouse cDNA. Our data provide the first evidence that MEPE/OF45 is expressed during odontogenesis and should be considered as a candidate gene for dentin structural diseases mapping to human chromosome 4q21.

Spatial expression of Cbfa1/Runx2 isoforms in teeth and characterization of binding sites in the DSPP gene

Cbfa1/Runx2 is an essential transcription factor for osteoblast and odontoblast differentiation. Heterogeneous mutations of Cbfa1 gene result in cleidocranial dysplasia, an autosomal dominant syndrome, characterized by abnormal skeletal genesis and dental disorders. Recently three Cbfa1/Runx isoforms (Pebp2 alpha A/type I, til-1/type II, and Osf2/type III) have been identified that differ in their amino-terminal sequences. The precise roles of Cbfa1/Runx2 isoforms in odontoblast development are not known. The purpose of this study was to determine and compare expression patterns of the three Cbfa1/Runx2 isoforms in newborn tooth organs. Toward this aim, we developed three probes: type I and type II, which specifically hybridize with Pebp2 alpha A and til-1, respectively, and type II/III, which hybridizes with osf2 and partially with til-1. In addition, Cbfa1/Runx2 binding sites were identified in the regulatory elements of mouse dentin sialophosphoprotein (mDSPP) gene, which encodes a matrix protein expressed during odontogenesis. In situ hybridization performed with the specific Cbfa1/Runx2 isoform probes demonstrated that all isoforms are expressed in teeth and bone. The type I isoform was expressed at higher levels than isoforms type II and type II/III in developing newborn mouse incisors. Genomic mDSPP clones were isolated and characterized containing approximately 2.6 kb of the promoter region. Computer analysis of the promoter segment and intron 1 revealed a number of potential transcriptional factor binding sites including five Cbfa1/Runx2 binding sites, three in the promoter region and two within intron 1. DNA-protein assay and antibody supershift experiments showed that these binding sites interact with nuclear extracts isolated from the mouse odontoblast cell line MO6-G3. Further characterization of the functional role of Cbfa1/Runx2 in the regulation of the mDSPP gene expression is being investigated.

Extracellular matrix proteins and the dynamics of dentin formation

Dentinogenesis involves controlled reactions that result in conversion of unmineralized predentin to dentin when apatite crystals are formed. This process is dynamic: Maturation events occur within predentin beginning at the proximal layer and progressing to the predentin-dentin (PD) border. One type of controlled reaction is the proteolytic processing of dentin sialophosphoprotein (DSPP) to dentin sialoprotein (DSP) and dentin phosphoprotein (DPP), by cleavage of at least three highly conserved peptide bonds. We postulate that this processing event represents an activation step, resulting in release of DPP, which is active in its effects on formation and growth of apatite crystals. Dentin matrix protein 1 (DPM1), present as a processed fragment (57-kD protein) in bone, is seen in dentin on sodium dodecyl sulfate polyacrylamide gel electrophoresis as one intact protein of 150-200 kD. Anti-57-kD antibodies elicit immunoreactivity in bone, dentin, and cellular cementum. In bone, the reactivity is associated with osteocytes and their cell processes. Similarly, dentin shows reactivity in odontoblasts, predentin, and the odontoblast processes. In summary, the processing of large sialic acid-rich proteins into smaller fragments may be an important part of the controlled conversion of predentin to dentin and osteoid to bone.

TGF beta-1 downregulates DMP-1 and DSPP in odontoblasts

Transforming growth factor beta-1 (TGF beta-1) is a multifunctional growth factor that is expressed in numerous cell types. It has been shown to induce secretion of dentin extracellular matrix components associated with primary dentinogenesis and to play a role in tertiary or reparative dentinogenesis. In this study, we investigated the potential transcriptional regulation by TGF beta-1 of two dentin matrix proteins: dentin matrix protein 1 (DMP-1), and dentin sialophosphoprotein (DSPP). In vitro promoter studies were performed using plasmid constructs containing mouse DMP-1 and DSPP promoter sequences fused to the luciferase reporter gene. Constructs were transiently transfected in the mouse odontoblast cell line M06-G3 and cultured in the presence or absence of TGF beta-1. The integrity of the TGF beta-1 signaling pathway was investigated in the M06-G3 cells by identifying known key effectors of TGF beta-1 signal transduction. Transient transfection studies demonstrate for the first time that TGF beta-1 downregulates both DMP-1 and DSPP genes. Our findings indicate that the TGF beta-1 type I receptor ALK5 is expressed by odontoblasts as well as the signal transduction proteins Smad2, Smad3, and Smad4. These results suggest that TGF beta-1 regulates two key dentin proteins involved in matrix mineralization most likely mediated through the type I ALK5 receptor and transduced by Smads 2, 3, and 4.

Assessment of dysplastic dentin in osteogenesis imperfecta and dentinogenesis imperfecta

Two semiquantitative scoring systems, Clinical Radiographic Score (CRS) and Dysplastic Dentin Score (DDS), were introduced for analyzing degree of dysplastic manifestations in dentin. The utility of both systems was demonstrated in a large material of teeth from patients with dentinogenesis imperfecta (DI) and osteogenesis imperfecta (OI). Twenty teeth from healthy controls, 81 teeth from 40 patients with OI, and 18 teeth with DI without OI (DI type II) were examined. The degree of dysplasia was correlated with type and form of OI and type of DI. The median DDS did not differ between DI associated with OI (DI type I) and DI type II. DDS in OI patients without clinical signs of DI was above that of control teeth. Both circumpulpal and mantle dentin showed increased DDS, although circumpulpal dentin was more severely affected. The median DDS was highest for the most severe type of non-lethal OI (type III). DDS increased significantly with form (severity) of OI. A significant association between DDS and CRS was found, although diagnosis of DI in less severe cases was not possible based on radiographic or clinical signs alone. Thus, the DDS system proved valuable when the CRS system based on radiographic/clinical manifestations failed, the most significant finding being subclinical histological manifestations of DI in patients with OI but without clinical or radiographic signs of DI. These subtle dysplastic changes are most likely an expression of genetic disturbances associated with OI and should not be diagnosed as DI, but rather be termed histologic manifestations of dysplastic dentin associated with OI.

Characterization of porcine dentin sialoprotein (DSP) and dentin sialophosphoprotein (DSPP) cDNA clones

Dentin sialophosphoprotein (DSPP) is a chimeric glycoprotein with dentin sialoprotein (DSP) on its N-terminus and dentin phosphoprotein (DPP) on its C-terminus. We have constructed and screened a unidirectional cDNA library derived from the pulp organ of developing pig teeth, and isolated cDNA clones encoding DSP-only, as well as two DSPP clones with alternative sequences in their 3' coding regions. The DSP-only transcript has an open reading frame of 386 codons, and is generated through the use of a polyadenylation signal within intron 4, immediately following the DSP coding region. the use of this polyadenylation signal deletes the DPP coding region and places a TGA translation termination signal as the fourth codon following the exon 4-encoded segment. The DSPP cDNAs contain open reading frames of 593 and 600 codons. Northern blots hybridized to radiolabeled DSP probes showed bands at 1.4, 2.5, 4.4, and 4.8 kb. Cloning and characterization of reverse transcriptase polymerase chain reaction products confirmed the existence of mRNA encoding pDSP386, pDSPP593, and pDSPP600in vivo, but also suggested that DNA sequence redundancies in the DSPP coding region make it prone to cloning artifacts.

Odontoblast-specific expression of cre recombinase successfully deletes gene segments flanked by loxP sites in mouse teeth

Embryonic or neonatal lethality of mice with targeted disruption of critical genes preclude them from further characterization of specific roles of these genes during postnatal development and aging. In order to study the molecular roles of such genes in teeth, we generated transgenic mouse lines expressing bacteriophage Cre recombinase under the control of the mouse dentin sialophosphoprotein (dspp) gene promoter. The expression of Cre recombinase protein was mainly detected in the nucleus of the odontoblasts. The efficiency of Cre activity was analyzed by crossing the Dspp-Cre mice with ROSA26 reporter (R26R) mice. The offspring with both genotypes have shown specific deletion of intervening sequences flanked by loxP sites upstream of the reporter gene, thereby facilitating the expression of the beta-galactosidase (beta-gal) gene in the teeth. The activity of beta-gal was initially observed in the odontoblasts of 1-day-old mice and increased with tooth development. Almost all of the odontoblasts have shown lacZ activity by 3 weeks of age. We could not detect Cre recombinase activity in any other cells, including ameloblasts. These studies indicate that the Dspp-Cre transgenic mice will be valuable to generate odontoblast-specific gene knockout mice so as to gain insight into the molecular roles of critical genes in the odontoblasts during dentinogenesis.

Bone matrix proteins: their function, regulation, and relationship to osteoporosis

Bone is a unique tissue composed of numerous cell types entombed within a mineralized matrix each with its own unique functions. While the majority of the matrix is composed of inorganic materials, study of the organic components has yielded most of the insights into the roles and regulation of cell and tissue specific functions. The goal of this review will be to describe some of the major known organic components of the bone matrix and discuss their functions as currently perceived. The potential usefulness of bone matrix protein assays for diagnosing the status of bone diseases and our current understanding of how these proteins could be related to diseases such as osteoporosis will also be reviewed.

Phosphate and calcium uptake by rat odontoblast-like MRPC-1 cells concomitant with mineralization

It has been suggested that odontoblasts are instrumental in translocating Ca2+ and inorganic phosphate (Pi) ions during the mineralization of dentin. The aim of this study was to characterize cellular Pi and Ca2+ uptake in the novel rat odontoblast-like cell line mineralizing rat pulpal cell line (MRPC) 1 during mineralization to see if changes in the ion transport activity would occur as the cultures develop and begin forming a mineralized matrix. MRPC-1 cells were cultured in chemically defined medium containing ascorbate and Pi, and cultures were specifically analyzed for cellular P, and Ca2+ uptake activities and expression of type II high-capacity Na+-Pi cotransporters. The odontoblast-like phenotype of the cell line was ascertained by monitoring the expression of collagen type I and dentin phosphopoprotein (DPP). Mineralized nodule formation started at day 9 after confluency and then rapidly increased. Ca2+ uptake by the cells showed a maximum during the end of the proliferative phase (days 5-7). Pi uptake declined to a basal level during proliferation and then was up-regulated simultaneously with the onset of mineralization to a level fourfold of the basal uptake, suggesting an initiating and regulatory role for cellular Pi uptake in mineral formation. This up-regulation coincided with a conspicuously increased glycosylation of NaPi-2a, indicating an activation of this Na+-Pi cotransporter. The study showed that MRPC-1 cells express an odontoblast-like phenotype already at the onset of culture, but that to mineralize the collagenous extracellular matrix (ECM) that formed, a further differentiation involving their ion transporters is necessary.

Identification of differentially expressed cDNA clones from gerbil cochlear outer hair cells

In order to identify genes that are associated with outer hair cell(OHC)-specific function, a plasmid library enriched with OHC-specific gene products was constructed using single cell-type-specific complementary DNA (cDNA) and a PCR subtractive hybridization strategy. As a first step, we created separate OHC and inner hair cell (IHC) cDNA pools from individually collected cells using a nonspecific reverse transcription polymerase chain reaction. Next, the OHC cDNA was subtracted against IHC cDNA using a PCR-based subtractive technique. IHCs and OHCs share many common features, making IHC cDNA an ideal 'driver' to 'subtract away' common hair cell gene products and enrich differentially expressed cDNAs, including OHC-specific genes. The subtracted OHC cDNAs were then cloned to generate an OHC - IHC subtracted cDNA plasmid library. Finally, a differential screening procedure was performed, resulting in 477 differentially positive clones. After analysis of these 477 clones, 50 known genes were identified, including two previously known OHC-specific proteins: oncomodulin and the recently described motor protein prestin. An additional 84 novel clones were also found. As this library of cDNA fragments represents differentially expressed genes in OHCs, it can be used as starting material for isolation and characterization of a complete set of OHC gene products, an important step in investigating normal and abnormal cochlear function.

Continuing to break the sound barrier: genes in hearing

The past year has seen major advances in our understanding of the genes involved in Usher syndrome, as well as the discovery of a myriad of other genes expressed specifically in hair cells. Mouse models continue to be invaluable in illuminating our knowledge of how mutations in genes lead to deafness. The role of mitochondrial genes in the hearing process has also contributed to elucidating the workings of the auditory system.

The expression of dentin sialophosphoprotein gene in bone

Dentin sialoprotein (DSP) and dentin phosphoprotein (DPP) are expressed as a single mRNA transcript coding for a large precursor protein termed dentin sialophosphoprotein (DSPP). DSP, DPP, and DSPP have been considered to be tooth-specific. To test for the expression of the dspp gene in bone, we performed Western immunoblots and reverse-transcription polymerase chain-reaction (RT-PCR). With Western immunoblots, we detected DSP in the Gdm/EDTA extracts of rat long bone, at a level of about 1/400 of that in dentin. Using RT-PCR, we detected DSPP mRNA in mouse calvaria. Similar to Western immunoblots, the results of RT-PCR indicated that the dspp gene is expressed at a lower level in bone than in dentin and odontoblasts. Analysis of the data shows that DSPP is not a tooth-specific protein, and that dramatically different regulatory mechanisms governing DSPP expression are involved in the bone and dentin.

Dental aberrations in children and adolescents with osteogenesis imperfecta

The aim of this investigation was to study dental aberrations in a large sample of unrelated patients with different types and forms of osteogenesis imperfecta (OI). Sixty-eight non-related index patients aged 0.3 to 20 years (mean, 10 years) were examined clinically. Panoramic radiographs from 49 patients were analyzed. Dentinogenesis imperfecta (DI) type I was found in 27 of 65 patients and was significantly more common in OI type III than in types I and IV and in patients with a severe form of the disease. The presence or absence of DI showed almost complete accordance between affected parents and children and between affected siblings. Moreover, agenesis was found in 11 of 49 patients, apically extended pulp chambers in 20 of 48 patients, and impaction of second permanent molars in 7 of 19 patients older than 15 years. The percentage of patients with no apparent dental aberrations was approximately the same in patients with OI type I and type III and in patients with mild and severe form of the disease. The high prevalence of dental aberrations in OI stresses the importance of clinical and radiographic odontologic examination as part of the clinical investigation. In patients with mild forms of the disease, in whom the medical diagnosis is uncertain, demonstration of disturbances in dental development can be crucial for establishing the OI diagnosis. C

Drugs, the human genome, and individual-based medicine

The so-called "Genomic Revolution" has made possible the high-resolution sequencing of the DNA making up the human genome. One of the main conclusions of the currently available sequencing data is that individuals differ genetically from one another via sequence variations in their genomes. When affected genes are transcribed and translated, some of these sequence variations result in protein products that may affect the functioning of the proteins. This has led to widespread optimism that information on an individual's pattern of sequence variations will lead to drugs that target that individual's variant proteins and make "individual-based medicine" possible. I this chapter some of the assumptions underlying the proposed production of individual drug treatments are examined. The assumptions are viewed in the light of very recent experimental evidence about the sequence patterns found in humans. Also discussed are ancillary ethical problems in cataloging and using databases containing individuals' sequence data, what human genomic sequences are revealing about the use of animal models in developing drugs, and how evidence is mounting that the human genome is only one element serving to maintain an organism's interaction with its environment.

Molecular diagnostics in China

Molecular diagnostics is changing the face of clinical laboratories and laboratory medicine. The case of China is no exception. In the present paper, a brief description on this promising discipline is given first, followed by an overview of the development of molecular diagnostics in China. Work done in the author's own laboratory is introduced in the third part and a short discussion on the challenges ahead is provided last.

A novel rat dentin mRNA coding only for dentin sialoprotein

Dentin sialoprotein (DSP) is a major glycoprotein present in the mineralized dentin matrix that is expressed mainly by young and mature odontoblasts. Mutations in the DSP coding regions are linked to Dentinogenesis imperfecta I and II. indicating the importance of DSP in tooth formation. Previous studies have identified multiple mRNA transcripts in dentin that code for both DSP and phosphophoryns (PPs). Using reverse transcriptase-polymerase chain reaction (RT-PCR) to characterize these mRNA transcripts, we have identified a cDNA that codes for DSP, but not PP. This cDNA codes for a protein with 324 amino acids, 303 amino acids being identical to the published rat DSP sequence. However, the subsequent 21 amino acids are unique to this cDNA. Based on the coding sequence, the core protein is predicted to have a pI=4.24, a net charge of -34, and to contain four potential N-glycosylation sites and six potential sites for phosphorylation by casein kinase. That the corresponding mRNA was present in day 5 molar tooth germs was confirmed using RNA protection assays. These data, therefore, identify a novel transcript in rat tooth germs that codes only for DSP (designated as DSPII).

A novel rat 523 amino acid phosphophoryn: nucleotide sequence and genomic organization

Phosphophoryns (PP), the major noncollagenous proteins (NCPs) in dentin, are believed to play a crucial role in mineral nucleation and hydroxyapatite growth during dentin mineralization. Previously we identified two mature rat PP transcripts, one coding for a 240 amino acid protein (designated as PP(240)) (H.H. Ritchie, L.-H. Wang, J. Biol. Chem. 271 (1996) 21695-21698), and another coding for a 171 amino acid protein (PP(171)) (H. Ritchie, L. Wang, Biochim. Biophys. Acta 1493 (2000) 27-32). We now have identified a third novel dentin sialoprotein (DSP)-PP cDNA transcript that encodes a 523 amino acid protein (PP(523)) with typical PP characteristics including DSS and DS motifs suitable as potential casein kinase I and II phosphorylation sites. Based on amino acid composition, the PP(523) protein product is identical to native rat HP2. We also show that the PP(523) sequence is identical to the corresponding genomic DNA sequence. Taken together, the existence of multiple DSP-PP transcripts, each significantly different from the other in net negative charge, suggests that dentin mineralization processes may be under fine-tune control by these PP protein isoforms.

Soundbites

Dentinogenesis imperfecta (DGI) is characterized by discolored teeth with an opalescent sheen and dentin that fails to support enamel, causing it to easily chip. Two new studies show that DGI is associated with mutations in DSPP, a gene encoding dentin sialophosphoprotein that is processed into two proteins: dentin sialoprotein (DSP) and dentin phosphoprotein (DPP). These are key components of the dentin extracellular matrix (DECM). Notably, missense mutations in DSPP are also associated with progressive hearing loss.