Genes and related proteins involved in amelogenesis imperfecta

Protein-mediated enamel mineralization

Enamel is a hard nanocomposite bioceramic with significant resilience that protects the mammalian tooth from external physical and chemical damages. The remarkable mechanical properties of enamel are associated with its hierarchical structural organization and its thorough connection with underlying dentin. This dynamic mineralizing system offers scientists a wealth of information that allows the study of basic principels of organic matrix-mediated biomineralization and can potentially be utilized in the fields of material science and engineering for development and design of biomimetic materials. This chapter will provide a brief overview of enamel hierarchical structure and properties and the process and stages of amelogenesis. Particular emphasis is given to current knowledge of extracellular matrix protein and proteinases, and the structural chemistry of the matrix components and their putative functions. The chapter will conclude by discussing the potential of enamel for regrowth.

Critical role for αvβ6 integrin in enamel biomineralization

Tooth enamel has the highest degree of biomineralization of all vertebrate hard tissues. During the secretory stage of enamel formation, ameloblasts deposit an extracellular matrix that is in direct contact with ameloblast plasma membrane. Although it is known that integrins mediate cell-matrix adhesion and regulate cell signaling in most cell types, the receptors that regulate ameloblast adhesion and matrix production are not well characterized. Thus, we hypothesized that αvβ6 integrin is expressed in ameloblasts where it regulates biomineralization of enamel. Human and mouse ameloblasts were found to express both β6 integrin mRNA and protein. The maxillary incisors of Itgb6−/− mice lacked yellow pigment and their mandibular incisors appeared chalky and rounded. Molars of Itgb6−/− mice showed signs of reduced mineralization and severe attrition. The mineral-to-protein ratio in the incisors was significantly reduced in Itgb6−/− enamel, mimicking hypomineralized amelogenesis imperfecta. Interestingly, amelogenin-rich extracellular matrix abnormally accumulated between the ameloblast layer of Itgb6−/− mouse incisors and the forming enamel surface, and also between ameloblasts. This accumulation was related to increased synthesis of amelogenin, rather than to reduced removal of the matrix proteins. This was confirmed in cultured ameloblast-like cells, which did not use αvβ6 integrin as an endocytosis receptor for amelogenins, although it participated in cell adhesion on this matrix indirectly via endogenously produced matrix proteins. In summary, integrin αvβ6 is expressed by ameloblasts and it plays a crucial role in regulating amelogenin deposition/turnover and subsequent enamel biomineralization.

Amelogenesis imperfecta: Report of a case and review of literature

Amelogenesis imperfecta (AI) is a diverse collection of inherited diseases that exhibit quantitative or qualitative tooth enamel defects in the absence of systemic manifestations. Also known by varied names such as Hereditary enamel dysplasia, Hereditary brown enamel, Hereditary brown opalescent teeth, this defect is entirely ectodermal, since mesodermal components of the teeth are basically normal. The AI trait can be transmitted by either autosomal dominant, autosomal recessive, or X-linked modes of inheritance. Genes implicated in autosomal forms are genes encoding enamel matrix proteins, namely: enamelin and ameloblastin, tuftelin, MMP-20 and kallikrein - 4. This article presents a case reported to Dr. D. Y. Patil, Dental College and Hospital, Pune, India, along with a review of this often seen clinical entity.

The use of mouse models to investigate shear bond strength in amelogenesis imperfecta

Patients with amelogenesis imperfecta (AI) have defective enamel; therefore, bonded restorations of patients with AI have variable success rates. To distinguish which cases of AI may have good clinical outcomes with bonded materials, we evaluated etching characteristics and bond strength of enamel in mouse models, comparing wild-type (WT) with those having mutations in amelogenin (Amelx) and matrix metalloproteinase-20 (Mmp20), which mimic 2 forms of human AI. Etched enamel surfaces were compared for roughness by scanning electron microscopy (SEM) images. Bonding was compared through shear bond strength (SBS) studies with 2 different systems (etch-and-rinse and self-etch). Etched enamel surfaces of incisors from Amelx knock-out (AmelxKO) mice appeared randomly organized and non-uniform compared with WT. Etching of Mmp20KO surfaces left little enamel, and the etching pattern was indistinguishable from unetched surfaces. SBS results were significantly different when AmelxKO and Mmp20KO enamel surfaces were compared. A significant increase in SBS was measured for all samples when the self-etch system was compared with the etch-and-rinse system. We have developed a novel system for testing shear bond strength of mouse incisors with AI variants, and analysis of these data may have important clinical implications for the treatment of patients with AI.

Expression of microRNAs in the stem cell niche of the adult mouse incisor

The mouse incisor is a valuable but under-utilized model organ for studying the behavior of adult stem cells. This remarkable tooth grows continuously throughout the animal's lifetime and houses two distinct epithelial stem cell niches called the labial and lingual cervical loop (laCL and liCL, respectively). These stem cells produce progeny that undergo a series of well-defined differentiation events en route to becoming enamel-producing ameloblasts. During this differentiation process, the progeny move out of the stem cell niche and migrate toward the distal tip of the tooth. Although the molecular pathways involved in tooth development are well documented, little is known about the roles of miRNAs in this process. We used microarray technology to compare the expression of miRNAs in three regions of the adult mouse incisor: the laCL, liCL, and ameloblasts. We identified 26 and 35 differentially expressed miRNAs from laCL/liCL and laCL/ameloblast comparisons, respectively. Out of 10 miRNAs selected for validation by qPCR, all transcripts were confirmed to be differentially expressed. In situ hybridization and target prediction analyses further supported the reliability of our microarray results. These studies point to miRNAs that likely play a role in the renewal and differentiation of adult stem cells during stem cell-fueled incisor growth.

Whole-Exome sequencing identifies FAM20A mutations as a cause of amelogenesis imperfecta and gingival hyperplasia syndrome

Amelogenesis imperfecta (AI) describes a clinically and genetically heterogeneous group of disorders of biomineralization resulting from failure of normal enamel formation. AI is found as an isolated entity or as part of a syndrome, and an autosomal-recessive syndrome associating AI and gingival hyperplasia was recently reported. Using whole-exome sequencing, we identified a homozygous nonsense mutation in exon 2 of FAM20A that was not present in the Single Nucleotide Polymorphism database (dbSNP), the 1000 Genomes database, or the Centre d'Etude du Polymorphisme Humain (CEPH) Diversity Panel. Expression analyses indicated that Fam20a is expressed in ameloblasts and gingivae, providing biological plausibility for mutations in FAM20A underlying the pathogenesis of this syndrome.

Adipose tissue-derived stem cell in vitro differentiation in a three-dimensional dental bud structure

Tooth morphogenesis requires sequential and reciprocal interactions between the cranial neural crest-derived mesenchymal cells and the stomadial epithelium, which regulate tooth morphogenesis and differentiation. We show how mesenchyme-derived single stem cell populations can be induced to transdifferentiate in vitro in a structure similar to a dental bud. The presence of stem cells in the adipose tissue has been previously reported. We incubated primary cultures of human adipose tissue-derived stem cells in a dental-inducing medium and cultured the aggregates in three-dimensional conditions. Four weeks later, cells formed a three-dimensional organized structure similar to a dental bud. Expression of dental tissue-related markers was tested assaying lineage-specific mRNA and proteins by RT-PCR, immunoblot, IHC, and physical-chemical analysis. In the induction medium, cells were positive for ameloblastic and odontoblastic markers as both mRNAs and proteins. Also, cells expressed epithelial, mesenchymal, and basement membrane markers with a positional relationship similar to the physiologic dental morphogenesis. Physical-chemical analysis revealed 200-nm and 50-nm oriented hydroxyapatite crystals as displayed in vivo by enamel and dentin, respectively. In conclusion, we show that adipose tissue-derived stem cells in vitro can transdifferentiate to produce a specific three-dimensional organization and phenotype resembling a dental bud even in the absence of structural matrix or scaffold to guide the developmental process.

PERP regulates enamel formation via effects on cell-cell adhesion and gene expression

Little is known about the role of cell-cell adhesion in the development of mineralized tissues. Here we report that PERP, a tetraspan membrane protein essential for epithelial integrity, regulates enamel formation. PERP is necessary for proper cell attachment and gene expression during tooth development, and its expression is controlled by P63, a master regulator of stratified epithelial development. During enamel formation, PERP is localized to the interface between the enamel-producing ameloblasts and the stratum intermedium (SI), a layer of cells subjacent to the ameloblasts. Perp-null mice display dramatic enamel defects, which are caused, in part, by the detachment of ameloblasts from the SI. Microarray analysis comparing gene expression in teeth of wild-type and Perp-null mice identified several differentially expressed genes during enamel formation. Analysis of these genes in ameloblast-derived LS8 cells upon knockdown of PERP confirmed the role for PERP in the regulation of gene expression. Together, our data show that PERP is necessary for the integrity of the ameloblast-SI interface and that a lack of Perp causes downregulation of genes that are required for proper enamel formation.

Fluoride reduces the expression of enamel proteins and cytokines in an ameloblast-derived cell line

OBJECTIVE

To investigate the effects of two different fluoride concentrations on the expression of enamel proteins, alkaline phosphatase (ALP), cytokines and interleukins by an ameloblast-derived cell line.

METHODS

Murine ameloblast-derived cells (LS-8), mouse odontogenic epithelia, were exposed to 1 or 5ppm sodium fluoride (NaF) (0.46 and 2.25ppm F, respectively) for 1, 3 and 7 days. The effect of NaF on the mRNA expression of enamel proteins was quantified; the secretion of cytokines, and interleukins, and the alkaline phosphatase (ALP) activity, into the cell culture medium was measured and compared to untreated controls. The effect on cell growth after 1- and 3-days in culture was measured using BrdU incorporation.

RESULTS

Fluoride at 2.25ppm reduced mRNA expression of the structural enamel matrix proteins amelogenin (amel), ameloblastin (ambn), enamelin (enam), and the enamel protease matrix metallopeptidase-20 (MMP-20). Similarly several vascularisation factors (vascular endothelial growth factor (VEGF), monocyte chemoattractant proteins (MCP-1) and interferon inducible protein 10 (IP-10), was also reduced by 2.25ppm fluoride. ALP activity and proliferation were stimulated by 0.46ppm fluoride but inhibited by 2.25ppm fluoride.

CONCLUSIONS

These results indicate that fluoride may impact on the expression of structural enamel proteins and the protease responsible for processing these proteins during the secretory stage of amelogenesis and go some way to explaining the mineralization defect that characterises fluorotic enamel.

MMP1 and MMP20 contribute to tooth agenesis in humans

OBJECTIVE

Variations in genes that are critical for tooth formation may contribute to the tooth agenesis. MMPs are potential candidate genes for dental alterations based on the roles they play during embryogenesis. The aim of this study was to investigate the possible association between MMP1, MMP3, and MMP20 and tooth agenesis.

METHODS

One hundred sixty-seven nuclear families from two different populations were analysed, 116 from Brazil and 51 from Turkey. Probands had at least one congenitally missing tooth. DNA samples were obtained from blood or saliva samples and genotyping was performed using TaqMan chemistry. In addition, Mmp20 was selected for quantitative real-time polymerase chain reaction analysis with SYBR Green I Dye in mouse tooth development.

RESULTS

Associations between tooth agenesis and MMP1 (p=0.007), and MMP20 (p=0.03) were found in Brazilian families. In the total dataset, MMP20 continued to be associated with tooth agenesis (p=0.01). Mmp20 was not expressed during the initial stages of tooth development.

CONCLUSION

Our findings provide evidence that MMP1 and MMP20 play a role in human tooth agenesis.

Defining a new candidate gene for amelogenesis imperfecta: from molecular genetics to biochemistry

Amelogenesis imperfecta is a group of genetic conditions that affect the structure and clinical appearance of tooth enamel. The types (hypoplastic, hypocalcified, and hypomature) are correlated with defects in different stages of the process of enamel synthesis. Autosomal dominant, recessive, and X-linked types have been previously described. These disorders are considered clinically and genetically heterogeneous in etiology, involving a variety of genes, such as AMELX, ENAM, DLX3, FAM83H, MMP-20, KLK4, and WDR72. The mutations identified within these causal genes explain less than half of all cases of amelogenesis imperfecta. Most of the candidate and causal genes currently identified encode proteins involved in enamel synthesis. We think it is necessary to refocus the search for candidate genes using biochemical processes. This review provides theoretical evidence that the human SLC4A4 gene (sodium bicarbonate cotransporter) may be a new candidate gene.

Prospects and Pits on the Path of Biomimetics: The case of tooth enamel

This review presents a discourse on challenges in understanding and imitating the process of amelogenesis in vitro on the molecular scale. In light of the analysis of imitation of the growth of dental enamel, it also impends on the prospects and potential drawbacks of the biomimetic approach in general. As the formation of enamel proceeds with the protein matrix guiding the crystal growth, while at the same time conducting its own degradation and removal, it is argued that three aspects of amelogenesis need to be induced in parallel: a) crystal growth; b) protein assembly; c) proteolytic degradation. A particular emphasis is therefore placed on ensuring conditions for proteolysis-coupled protein-guided crystallization to occur. Discussed are structural and functional properties of the protein species involved in amelogenesis, mainly amelogenin and enamelysin, the main protein and the protease of the developing enamel matrix, respectively. A model of enamel growth based on controlled delivery of constituent ions or crystalline or amorphous building blocks by means of amelogenin is proposed. The importance of high viscosity of the enamel matrix and a more intricate role that water may play in such a gelatinous medium are also touched upon. The tendency of amelogenin to self-assemble into fibrous and rod-shaped morphologies is considered as potentially important in explaining the formation of elongated apatite crystals. The idea that a preassembling protein matrix serves as a template for the uniaxial growth of apatite crystals in enamel is finally challenged with the one based on co-assembly of the protein and the mineral phases.

Matrix metalloproteinase activity assays: Importance of zymography

INTRODUCTION

Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases capable of degrading extracellular matrix, including the basement membrane. MMPs are associated with various physiological processes such as morphogenesis, angiogenesis, and tissue repair. Moreover, due to the novel non-matrix related intra- and extracellular targets of MMPs, dysregulation of MMP activity has been implicated in a number of acute and chronic pathological processes, such as arthritis, acute myocardial infarction, chronic heart failure, chronic obstructive pulmonary disease, inflammation, and cancer metastasis. MMPs are considered as viable drug targets in the therapy of the above diseases.

METHODS

For the development of selective MMP inhibitor molecules, reliable methods are necessary for target validation and lead development. Here, we discuss the major methods used for MMP assays, focusing on substrate zymography. We highlight some problems frequently encountered during sample preparations, electrophoresis, and data analysis of zymograms.

RESULTS AND DISCUSSION

Zymography is a widely used technique to study extracellular matrix-degrading enzymes, such as MMPs, from tissue extracts, cell cultures, serum or urine. This simple and sensitive technique identifies MMPs by the degradation of their substrate and by their molecular weight and therefore helps to understand the widespread role of MMPs in different pathologies and cellular pathways.

A mutation in the mouse Amelx tri-tyrosyl domain results in impaired secretion of amelogenin and phenocopies human X-linked amelogenesis imperfecta

Amelogenesis imperfecta (AI) describes a broad group of clinically and genetically heterogeneous inherited defects of dental enamel bio-mineralization. Despite identification of a number of genetic mutations underlying AI, the precise causal mechanisms have yet to be determined. Using a multi-disciplinary approach, we describe here a mis-sense mutation in the mouse Amelx gene resulting in a Y --> H substitution in the tri-tyrosyl domain of the enamel extracellular matrix protein amelogenin. The enamel in affected animals phenocopies human X-linked AI where similar mutations have been reported. Animals affected by the mutation have severe defects of enamel bio-mineralization associated with absence of full-length amelogenin protein in the developing enamel matrix, loss of ameloblast phenotype, increased ameloblast apoptosis and formation of multi-cellular masses. We present evidence to demonstrate that affected ameloblasts express but fail to secrete full-length amelogenin leading to engorgement of the endoplasmic reticulum/Golgi apparatus. Immunohistochemical analysis revealed accumulations of both amelogenin and ameloblastin in affected cells. Co-transfection of Ambn and mutant Amelx in a eukaryotic cell line also revealed intracellular abnormalities and increased cytotoxicity compared with cells singly transfected with wild-type Amelx, mutant Amelx or Ambn or co-transfected with both wild-type Amelx and Ambn. We hypothesize that intracellular protein-protein interactions mediated via the amelogenin tri-tyrosyl motif are a key mechanistic factor underpinning the molecular pathogenesis in this example of AI. This study therefore successfully links phenotype with underlying genetic lesion in a relevant murine model for human AI.

Mutations in the beta propeller WDR72 cause autosomal-recessive hypomaturation amelogenesis imperfecta

Healthy dental enamel is the hardest and most highly mineralized human tissue. Though acellular, nonvital, and without capacity for turnover or repair, it can nevertheless last a lifetime. Amelogenesis imperfecta (AI) is a collective term for failure of normal enamel development, covering diverse clinical phenotypes that typically show Mendelian inheritance patterns. One subset, known as hypomaturation AI, is characterised by near-normal volumes of organic enamel matrix but with weak, creamy-brown opaque enamel that fails prematurely after tooth eruption. Mutations in genes critical to enamel matrix formation have been documented, but current understanding of other key events in enamel biomineralization is limited. We investigated autosomal-recessive hypomaturation AI in a consanguineous Pakistani family. A whole-genome SNP autozygosity screen identified a locus on chromosome 15q21.3. Sequencing candidate genes revealed a point mutation in the poorly characterized WDR72 gene. Screening of WDR72 in a panel of nine additional hypomaturation AI families revealed the same mutation in a second, apparently unrelated, Pakistani family and two further nonsense mutations in Omani families. Immunohistochemistry confirmed intracellular localization in maturation-stage ameloblasts. WDR72 function is unknown, but as a putative β propeller is expected to be a scaffold for protein-protein interactions. The nearest homolog, WDR7, is involved in vesicle mobilization and Ca²⁺-dependent exocytosis at synapses. Vesicle trafficking is important in maturation-stage ameloblasts with respect to secretion into immature enamel and removal of cleaved enamel matrix proteins via endocytosis. This raises the intriguing possibility that WDR72 is critical to ameloblast vesicle turnover during enamel maturation.

Expression patterns of the Fam83h gene during murine tooth development

AIM

Recently a novel gene, FAM83H, was identified by a genetic linkage study in the hypocalcified form of the amelogenesis imperfecta family with an autosomal dominant hereditary pattern. Little is known about this novel gene, and so we investigated the expression pattern of Fam83h in murine tooth development using serial sectional in situ hybridisation.

METHODS AND MATERIALS

Using mandibles of ICR mouse at specific developmental stages, in situ hybridisation was performed by DIG-labeled RNA probe.

RESULTS

Faint expression was detected in limited cells at embryonic day 14 (E14) in the molar. At the bell stage, E16, Fam83h was localised in the outer and inner enamel epithelium, as well as dental papilla. Fam83h expression begins on E15 in the developing incisor. At E18, Fam83h was expressed in the inner enamel epithelium of the apical bud, ameloblasts and odontoblasts. The expression was stronger in the presecretory stages than the secretory stages.

CONCLUSION

Fam83h was detected in the ameloblasts from the presecretory to the secretory stage, and also the odontoblasts layer and surrounding alveolar bone.

Ctip2/Bcl11b controls ameloblast formation during mammalian odontogenesis

The transcription factor Ctip2/Bcl11b plays essential roles in developmental processes of the immune and central nervous systems and skin. Here we show that Ctip2 also plays a key role in tooth development. Ctip2 is highly expressed in the ectodermal components of the developing tooth, including inner and outer enamel epithelia, stellate reticulum, stratum intermedium, and the ameloblast cell lineage. In Ctip2(-/-) mice, tooth morphogenesis appeared to proceed normally through the cap stage but developed multiple defects at the bell stage. Mutant incisors and molars were reduced in size and exhibited hypoplasticity of the stellate reticulum. An ameloblast-like cell population developed ectopically on the lingual aspect of mutant lower incisors, and the morphology, polarization, and adhesion properties of ameloblasts on the labial side of these teeth were severely disrupted. Perturbations of gene expression were also observed in the mandible of Ctip2(-/-) mice: expression of the ameloblast markers amelogenin, ameloblastin, and enamelin was down-regulated, as was expression of Msx2 and epiprofin, transcription factors implicated in the tooth development and ameloblast differentiation. These results suggest that Ctip2 functions as a critical regulator of epithelial cell fate and differentiation during tooth morphogenesis.

Regeneration of bone and periodontal ligament induced by recombinant amelogenin after periodontitis

Regeneration of mineralized tissues affected by chronic diseases comprises a major scientific and clinical challenge. Periodontitis, one such prevalent disease, involves destruction of the tooth-supporting tissues, alveolar bone, periodontal-ligament and cementum, often leading to tooth loss. In 1997, it became clear that, in addition to their function in enamel formation, the hydrophobic ectodermal enamel matrix proteins (EMPs) play a role in the regeneration of these periodontal tissues. The epithelial EMPs are a heterogeneous mixture of polypeptides encoded by several genes. It was not clear, however, which of these many EMPs induces the regeneration and what mechanisms are involved. Here we show that a single recombinant human amelogenin protein (rHAM⁺), induced in vivo regeneration of all tooth-supporting tissues after creation of experimental periodontitis in a dog model. To further understand the regeneration process, amelogenin expression was detected in normal and regenerating cells of the alveolar bone (osteocytes, osteoblasts and osteoclasts), periodontal ligament, cementum and in bone marrow stromal cells. Amelogenin expression was highest in areas of high bone turnover and activity. Further studies showed that during the first 2 weeks after application, rHAM⁺ induced, directly or indirectly, significant recruitment of mesenchymal progenitor cells, which later differentiated to form the regenerated periodontal tissues. The ability of a single protein to bring about regeneration of all periodontal tissues, in the correct spatio-temporal order, through recruitment of mesenchymal progenitor cells, could pave the way for development of new therapeutic devices for treatment of periodontal, bone and ligament diseases based on rHAM⁺.

Msx and dlx homeogene expression in epithelial odontogenic tumors

Epithelial odontogenic tumors are rare jaw pathologies that raise clinical diagnosis and prognosis dilemmas notably between ameloblastomas and clear cell odontogenic carcinomas (CCOCs). In line with previous studies, the molecular determinants of tooth development-amelogenin, Msx1, Msx2, Dlx2, Dlx3, Bmp2, and Bmp4-were analyzed by RT-PCR, ISH, and immunolabeling in 12 recurrent ameloblastomas and in one case of CCOC. Although Msx1 expression imitates normal cell differentiation in these tumors, other genes showed a distinct pattern depending on the type of tumor and the tissue involved. In benign ameloblastomas, ISH localized Dlx3 transcripts and inconstantly detected Msx2 transcripts in epithelial cells. In the CCOC, ISH established a lack of both Dlx3 and Msx2 transcripts but allowed identification of the antisense transcript of Msx1, which imitates the same scheme of distribution between mesenchyme and epithelium as in the cup stage of tooth development. Furthermore, while exploring the expression pattern of signal molecules by RT-PCR, Bmp2 was shown to be completely inactivated in the CCOC and irregularly noticeable in ameloblastomas. Bmp4 was always expressed in all the tumors. Based on the established roles of Msx and Dlx transcription factors in dental cell fates, these data suggest that their altered expression is a proposed trail to explain the genesis and/or the progression of odontogenic tumors.

Identifying promoter elements necessary for enamelin tissue-specific expression

Enamel development requires the strictly regulated spatiotemporal expression of genes encoding enamel matrix proteins. The mechanisms orchestrating the initiation and termination of gene transcription at each specific stage of amelogenesis are unknown. In this study, we identify cis- regulatory regions necessary for normal enamelin (Enam) expression. Sequence analysis of the Enam promoter 5'-noncoding region identified potentially important cis-regulatory elements located within 5.2 kb upstream of the Enam translation initiation site. DNA constructs containing 5.2 or 3.9 kb upstream of the Enam translation initiation site were linked to an LacZ reporter gene and used to generate transgenic mice. The 3.9-kb Enam-LacZ transgenic lines showed no expression in ameloblasts, but ectopic LacZ staining was detected in osteoblasts. In contrast, the 5.2-kb Enam-LacZ construct was sufficient to mimic the endogenous Enam ameloblast-specific expression pattern. Our study provides new insights into the molecular control of Enam cell- and stage-specific expression.

SCPP gene evolution and the dental mineralization continuum

Many genes critical to vertebrate skeletal mineralization are members of the secretory calcium-binding phosphoprotein (SCPP) gene family, which has evolved by gene duplication from a single ancestral gene. In humans, mutations in some of these SCPP genes have been associated with various diseases related to dentin or enamel hypoplasia. Recently, systematic searches for SCPP genes of various species have allowed us to investigate the history of phylogenetically variable dental tissues as a whole. One important conclusion is that not all disease-associated SCPP genes are present in tetrapods, and teleost fish probably have none, even in toothed species, having acquired their complement of SCPP genes through an independent duplication history. Here, we review comparative analyses of mineralized dental tissues, with particular emphasis on the use of SCPPs, within and between tetrapods and teleosts. Current knowledge suggests a close relationship among bone, dentin, teleost fish enameloid (enamel-like hard tissue), and tetrapod enamel. These tissues thus form a mineralized-tissue continuum. Contemporary dental tissues have evolved from an ancestral continuum through lineage-specific modifications.

Zebrafish dentition in comparative context

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.

New insights into the functional mechanisms and clinical applications of the kallikrein-related peptidase family

The Kallikrein-related peptidase (KLK) family consists of fifteen conserved serine proteases that form the largest contiguous cluster of proteases in the human genome. While primarily recognized for their clinical utilities as potential disease biomarkers, new compelling evidence suggests that this family plays a significant role in various physiological processes, including skin desquamation, semen liquefaction, neural plasticity, and body fluid homeostasis. KLK activation is believed to be mediated through highly organized proteolytic cascades, regulated through a series of feedback loops, inhibitors, auto-degradation and internal cleavages. Gene expression is mainly hormone-dependent, even though transcriptional epigenetic regulation has also been reported. These regulatory mechanisms are integrated with various signaling pathways to mediate multiple functions. Dysregulation of these pathways has been implicated in a large number of neoplastic and non-neoplastic pathological conditions. This review highlights our current knowledge of structural/phylogenetic features, functional role and regulatory/signaling mechanisms of this important family of enzymes.

Enamel formation and amelogenesis imperfecta

Dental enamel is the epithelial-derived hard tissue covering the crowns of teeth. It is the most highly mineralized and hardest tissue in the body. Dental enamel is acellular and has no physiological means of repair outside of the protective and remineralization potential provided by saliva. Enamel is comprised of highly organized hydroxyapatite crystals that form in a defined extracellular space, the contents of which are supplied and regulated by ameloblasts. The entire process is under genetic instruction. The genetic control of amelogenesis is poorly understood, but requires the activities of multiple components that are uniquely important for dental enamel formation. Amelogenesis imperfecta (AI) is a collective designation for the variety of inherited conditions displaying isolated enamel malformations, but the designation is also used to indicate the presence of an enamel phenotype in syndromes. Recently, genetic studies have demonstrated the importance of genes encoding enamel matrix proteins in the etiology of isolated AI. Here we review the essential elements of dental enamel formation and the results of genetic analyses that have identified disease-causing mutations in genes encoding enamel matrix proteins. In addition, we provide a fresh perspective on the roles matrix proteins play in catalyzing the biomineralization of dental enamel.

A mutation in the enamelin gene in a mouse model

Amelogenesis imperfecta is an inherited disorder affecting tooth enamel formation. We previously isolated a mouse strain with an amelogenesis imperfecta phenotype (ATE1 mice) from a dominant ethylnitrosourea screen and mapped the disease-causing defect to a 9-cM region of mouse chromosome 5. In the current study, we tested the hypothesis that there is a mutation in enamelin (ENAM) or ameloblastin (AMBN), both of which are located within the linkage region, by sequencing these two candidate genes. Analysis of our data shows that the amelogenesis imperfecta phenotype is linked to a C > T transition in exon 8 of the enamelin gene. The mutation predicts a C826T transition, which is present in the enamelin transcript and changes the glutamine (Gln) codon at position 176 into a premature stop codon (Gln176X). Conversely, no mutation could be detected in the ameloblastin gene. These results define the ATE1 mice as a model for local hypoplastic autosomal-dominant amelogenesis imperfecta (AIH2), which is caused by enamelin truncation mutations in humans.

Rat wct mutation induces a hypo-mineralization form of amelogenesis imperfecta and cyst formation in molar teeth

Our previous findings have demonstrated that the rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induces enamel defects resembling those of human amelogenesis imperfecta (AI) in continuously growing incisor teeth. The present study clarifies the effect of the wct mutation on the morphogenesis and calcification of rat molar teeth. Formalin-fixed maxillae obtained from animals aged 4-30 days were examined by electron probe micro-analysis (EPMA) and by immunocytochemistry for amelogenin, ameloblastin, and enamelin. There were no distinct differences in the calcium and phosphorous contents and the amount of enamel between homozygous mutant and wild-type teeth during postnatal days 4-11. Although the mineral density in the enamel matrix considerably increased in the wild-type teeth until day 15, no changes occurred in mutant teeth during days 11-30. The immunoreactivity for enamel proteins in the secretory-stage ameloblasts in mutant teeth was similar to that in the wild-type teeth, and subsequently mutant maturation-stage ameloblasts became detached from the enamel surface, resulting in odontogenic cyst formation between the enamel organ and matrix until day 7 and the expansion of the cyst around the whole tooth crown on day 15. On day 30, the erupted mutant teeth presented morphological changes such as enamel destruction and tertiary dentin formation in addition to low mineral density in the enamel. Thus, the wct mutation prevents mineral transport without disturbing the synthesis of enamel proteins in molar teeth because of the absence of maturation-stage ameloblasts, in addition to the occurrence of odontogenic cysts.

Rehabilitation of a patient with amelogenesis imperfecta using all-ceramic crowns: A clinical report

This article presents a patient with amelogenesis imperfecta rehabilitated with all-ceramic crowns following surgical and orthodontic intervention. The 6-year evaluation of the esthetics and function of the restorations showed evidence of isolated pulp exposure and crown fractures in the posterior areas.

Rat wct mutation prevents differentiation of maturation-stage ameloblasts resulting in hypo-mineralization in incisor teeth

A recent study provided genetic and morphological evidence that rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induced tooth enamel defects resembling those of human amelogenesis imperfecta (AI). The wct locus maps to a specific interval of rat chromosome 14 corresponding to human chromosome 4q21 where the ameloblastin and enamelin genes exist, although these genes are not included in the wct locus. The effect of the wct gene mutation on the enamel matrix synthesis and calcification remains to be elucidated. This study clarifies how the wct gene mutation influences the synthesis of enamel matrix and its calcification by immunocytochemistry for amelogenin, ameloblastin and enamelin, and by electron probe micro-analysis (EPMA). The immunoreactivity for enamel proteins such as amelogenin, ameloblastin, and enamelin in the ameloblasts in the homozygous teeth was the same as that in the heterozygous teeth from secretory to transitional stages, although the homozygous ameloblasts became detached from the enamel matrix in the transitional stage. The flattened ameloblasts in the maturation stage of the homozygous samples contained enamel proteins in their cytoplasm. Thus, the wct mutation was found to prevent the morphological transition of ameloblasts from secretory to maturation stages without disturbing the synthesis of enamel matrix proteins, resulting in the hypo-mineralization of incisor enamel and cyst formation between the enamel organ and matrix. This mutation also prevents the transfer of iron into the enamel.

The Origin and Evolution of Enamel Mineralization Genes

BACKGROUND/AIMS

Enamel and enameloid were identified in early jawless vertebrates, about 500 million years ago (MYA). This suggests that enamel matrix proteins (EMPs) have at least the same age. We review the current data on the origin, evolution and relationships of enamel mineralization genes.

METHODS AND RESULTS

Three EMPs are secreted by ameloblasts during enamel formation: amelogenin (AMEL), ameloblastin (AMBN) and enamelin (ENAM). Recently, two new genes, amelotin (AMTN) and odontogenic ameloblast associated (ODAM), were found to be expressed by ameloblasts during maturation, increasing the group of ameloblast-secreted proteins to five members. The evolutionary analysis of these five genes indicates that they are related: AMEL is derived from AMBN, AMTN and ODAM are sister genes, and all are derived from ENAM. Using molecular dating, we showed that AMBN/AMEL duplication occurred >600 MYA. The large sequence dataset available for mammals and reptiles was used to study AMEL evolution. In the N- and C-terminal regions, numerous residues were unchanged during >200 million years, suggesting that they are important for the proper function of the protein.

CONCLUSION

The evolutionary analysis of AMEL led to propose a dataset that will be useful to validate AMEL mutations leading to X- linked AI.

Amelogenesis imperfecta

Amelogenesis imperfecta (AI) represents a group of developmental conditions, genomic in origin, which affect the structure and clinical appearance of enamel of all or nearly all the teeth in a more or less equal manner, and which may be associated with morphologic or biochemical changes elsewhere in the body. The prevalence varies from 1:700 to 1:14,000, according to the populations studied. The enamel may be hypoplastic, hypomineralised or both and teeth affected may be discoloured, sensitive or prone to disintegration. AI exists in isolation or associated with other abnormalities in syndromes. It may show autosomal dominant, autosomal recessive, sex-linked and sporadic inheritance patterns. In families with an X-linked form it has been shown that the disorder may result from mutations in the amelogenin gene, AMELX. The enamelin gene, ENAM, is implicated in the pathogenesis of the dominant forms of AI. Autosomal recessive AI has been reported in families with known consanguinity. Diagnosis is based on the family history, pedigree plotting and meticulous clinical observation. Genetic diagnosis is presently only a research tool. The condition presents problems of socialisation, function and discomfort but may be managed by early vigorous intervention, both preventively and restoratively, with treatment continued throughout childhood and into adult life. In infancy, the primary dentition may be protected by the use of preformed metal crowns on posterior teeth. The longer-term care involves either crowns or, more frequently these days, adhesive, plastic restorations.

The amyloid protein APin is highly expressed during enamel mineralization and maturation in rat incisors

This study investigated the expression and localization of APin (which was previously identified and cloned from a rat odontoblast cDNA library), during ameloblast differentiation in rat incisors, by using in situ hybridization and immunohistochemistry. The subcellular localization of APin varied during ameloblast differentiation, but was stage-specific. APin mRNA was not expressed in pre-ameloblasts, was weakly expressed in secretory ameloblasts, and was strongly expressed in maturation-stage ameloblasts as well as in the junctional epithelium attached to the enamel of erupted molars. In the maturation-stage ameloblasts, APin protein was conspicuous in the supranuclear area (Golgi complex) of smooth-ended ameloblasts as well as in both the supranuclear area and the ruffle end of ruffle-ended ameloblasts. During ameloblast-lineage cell culture, APin was expressed at a low level in the early stages of culture, but at a high level in the late stage of culture, which was equivalent to the maturation stage. APin protein was efficiently secreted from transfected cells in culture. Furthermore, its overexpression and inactivation caused an increase and decrease in matrix metalloproteinase-20 (MMP-20) and tuftelin expression, respectively. These findings indicate a functional role for APin in the mineralization and maturation of enamel that is mediated by the expression of MMP-20 and tuftelin.

A new locus for autosomal dominant amelogenesis imperfecta on chromosome 8q24.3

Amelogenesis imperfecta (AI) is a collective term used to describe phenotypically diverse forms of defective tooth enamel development. AI has been reported to exhibit a variety of inheritance patterns, and several loci have been identified that are associated with AI. We have performed a genome-wide scan in a large Brazilian family segregating an autosomal dominant form of AI and mapped a novel locus to 8q24.3. A maximum multipoint LOD score of 7.5 was obtained at marker D8S2334 (146,101,309 bp). The disease locus lies in a 1.9 cM (2.1 Mb) region according to the Rutgers Combined Linkage-Physical map, between a VNTR marker (at 143,988,705 bp) and the telomere (146,274,826 bp). Ten candidate genes were identified based on gene ontology and microarray-facilitated gene selection using the expression of murine orthologues in dental tissue, and examined for the presence of a mutation. However, no causative mutation was identified.

Buonocore Memorial Lecture

For many years, operative dentistry has been using regenerative approaches to treat dental disease. The use of calcium hydroxide to stimulate reparative or reactionary dentin is clearly an example of such a therapeutic strategy. The advent of tissue engineering is allowing dentistry to move forward in the use of regeneration as an underlying principle for the treatment of dental disease. Tissue engineering is a multi-disciplinary science that brings together biology, engineering and clinical sciences with developing new tissues and organs. It is based on fundamental principles that involve the identification of appropriate cells, the development of conducive scaffolds and an understanding of the morphogenic signals required to induce cells to regenerate the tissues that were lost. This review is focused on the presentation and discussion of existing literature that covers the engineering of enamel, dentin and pulp, as well on the engineering of entire teeth. There are clearly major roadblocks to overcome before such strategies move to the clinic and are used regularly to treat patients. However, existing evidence strongly suggests that the engineering of new dental structures to replace tissues lost during the process of caries or trauma will have a place in the future of operative dentistry.

Cloning of rat amelotin and localization of the protein to the basal lamina of maturation stage ameloblasts and junctional epithelium

Formation of tooth enamel is a very complex process in which a specific set of proteins secreted by ameloblasts play a primordial role. As part of a screening procedure to identify novel proteins secreted by EO (enamel organ) cells of rat incisors, we isolated a partial cDNA fragment (EO-017) that is the homologue of the recently described mouse Amtn (amelotin) gene [Iwasaki, Bajenova, Somogyi-Ganss, Miller, Nguyen, Nourkeyhani, Gao, Wendel and Ganss (2005) J. Dent. Res. 84, 1127-1132]. Presented herein is the cloning of rat and pig full-length cDNAs with their deduced protein sequences. Detailed expression profiling by Northern-blot analysis and RT (reverse transcriptase)-PCR on rat and mouse tissues revealed highest expression in the mandible, more specifically in the maturation stage of the EO. Among all tissues tested, low expression was detected only in periodontal ligament, lung, thymus and gingiva. In silico analyses revealed that the Amtn gene is highly conserved in seven other mammals, but is absent from fish, birds and amphibians. The Amtn protein is enriched in proline, leucine, glutamine and threonine (52% of total) and contains a perfectly conserved protein kinase CK2 phosphorylation site. Transient transfection experiments in HEK-293 cells (human embryonic kidney cells) showed that secreted Amtn is post-translationally modified possibly through O-linked oligosaccharides on threonine residues. In concordance with its predominant expression site, immunofluorescence localization within the rat and mouse mandibles revealed Amtn localized to the basal lamina of maturation stage ameloblasts of incisors and unerupted molars. Intense Amtn protein expression was also detected in the internal basal lamina of junctional epithelium in molars. The peculiar and unique cellular localization of Amtn suggests a role in cell adhesion.