Autosomal-dominant hypoplastic form of amelogenesis imperfecta caused by an enamelin gene mutation at the exon-intron boundary

ENAM Mutations Can Cause Hypomaturation Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a diverse group of inherited diseases featured by various presentations of enamel malformations that are caused by disturbances at different stages of enamel formation. While hypoplastic AI suggests a thickness defect of enamel resulting from aberrations during the secretory stage of amelogenesis, hypomaturation AI indicates a deficiency of enamel mineralization and hardness established at the maturation stage. Mutations in ENAM, which encodes the largest enamel matrix protein, enamelin, have been demonstrated to cause generalized or local hypoplastic AI. Here, we characterized 2 AI families with disparate hypoplastic and hypomaturation enamel defects and identified 2 distinct indel mutations at the same location of ENAM, c588+1del and c.588+1dup. Minigene splicing assays demonstrated that they caused frameshifts and truncation of ENAM proteins, p.Asn197Ilefs*81 and p.Asn197Glufs*25, respectively. In situ hybridization of Enam on mouse mandibular incisors confirmed its restricted expression in secretory stage ameloblasts and suggested an indirect pathogenic mechanism underlying hypomaturation AI. In silico analyses indicated that these 2 truncated ENAMs might form amyloid structures and cause protein aggregation with themselves and with wild-type protein through the added aberrant region at their C-termini. Consistently, protein secretion assays demonstrated that the truncated proteins cannot be properly secreted and impede secretion of wild-type ENAM. Moreover, compared to the wild-type, overexpression of the mutant proteins significantly increased endoplasmic reticulum stress and upregulated the expression of unfolded protein response (UPR)-related genes and TNFRSF10B, a UPR-controlled proapoptotic gene. Caspase, terminal deoxynucleotidyl transferase UTP nick-end labeling (TUNEL), and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assays further revealed that both truncated proteins, especially p.Asn197Ilefs*81, induced cell apoptosis and decreased cell survival, suggesting that the 2 ENAM mutations cause AI through ameloblast cell pathology and death rather than through a simple loss of function. This study demonstrates that an ENAM mutation can lead to generalized hypomaturation enamel defects and suggests proteinopathy as a potential pathogenesis for ENAM-associated AI.

Dental treatment approaches of amelogenesis imperfecta in children and young adults: A systematic review of the literature

OBJECTIVE

The aim of this review was to compare various types of restorations used in children and young adults affected with amelogenesis imperfecta (AI) to determine the most effective restorative treatment.

METHODS

This systematic review included randomized controlled trials, retrospective and prospective cohorts conducted on children and young adults diagnosed with amelogenesis imperfecta and written in French or English. A systematic search was conducted using four databases, namely Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE via PubMed, Science Direct and Scopus, using a selection of MeSH terms: "Amelogenesis Imperfecta," "Therapeutics," "Treatment Outcome," "Adult, young," "Child," "Dental Restoration, Permanent," "Dental Restoration, Temporary," and "Esthetics, Dental."

RESULTS

Out of 138 articles identified in the initial search, four articles met all the inclusion criteria. The results showed that ceramic restorations had better quality scores and longevity compared to other restorations.

CONCLUSION

Ceramic restorations could be considered the restorative treatment modality of choice for AI-affected children and young adults. However, more high-quality clinical trials involving young patients affected with AI are required to evaluate and compare the outcomes of different restorative approaches.

CLINICAL SIGNIFICANCE

Young patients affected with amelogenesis imperfecta usually suffer from low self-esteem, psychological problems and social avoidance, caused by the alteration of teeth such as discoloration, sensitivity, fractures and reduced size. For the dentist, selecting the appropriate restorative treatment for AI in young patients could be a veritable challenge. Therefore, it is important to have an evidence-based modality. For this reason, in this review, the different restorative approaches used in AI-affected young patients were compared to recommend the most effective treatment.

A novel ODAPH mutation causing amelogenesis imperfecta and its expression in human dental tissues

Background/purpose

Amelogenesis imperfecta (AI), an assemblage of genetic diseases with dental enamel malformations, is generally grouped into hypoplastic, hypomaturation, and hypocalcified types. This study aimed to identify the genetic etiology for a consanguineous Iranian family with autosomal recessive hypocalcified AI.

Materials and methods

Dental defects were characterized, and whole exome analysis conducted to search for disease-causing mutations. Minigene assay and RT-PCR were performed to evaluate molecular consequences of the identified mutation and expression of the causative gene in human dental tissues.

Results

The defective enamel of erupted teeth showed extensive post-eruptive failure and discoloration. Partial enamel hypoplasia and indistinct dentino-enamel junction were evident on unerupted teeth, resembling hypocalcified AI. A novel homozygous ODAPH (previously designated C4orf26) mutation of single-nucleotide deletion (NG_032974.1:g.5103del, NM_178497.5:c.67+1del) was identified to be disease-causing. The mutation would cause a frameshift to different ODAPH transcript variant (TV) products: p.(Ala23Hisfs∗29) for TV1 and p.(Gly23Aspfs∗140) for TV2. Both dental pulps of developing and exfoliating primary teeth expressed ODAPH TV2.

Conclusion

Loss-of-function ODAPH mutations can cause AI type IIIB (the hypocalcified, autosomal recessive type), rather than type IIA4 (the hypomaturation, pigmented autosomal recessive type). This study supports a hypothesis that the product of ODAPH TV2 is the single dominant ODAPH protein isoform critical for dental enamel formation and may also play an unappreciated role in development and homeostasis of dentin-pulp complex. Due to genetic heterogeneity and a nonideal genotype-phenotype correlation of AI, it is essential to perform genetic testing for patients with inherited enamel defects to make a definitive diagnosis.

Role of Dental Professionals in the Management of a Dentigerous Cyst in a Patient With Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a hereditary condition that affects the clinical features and structure of the enamel. The enamel formation diseases are inherited and might be X-linked, autosomal dominant, recessive, sex-related, or sporadic. Dental professionals should evaluate such patients completely, both clinically and radiographically, to detect any cysts associated with impacted or unerupted teeth. This report describes the case of a 10-year-old patient affected by amelogenesis imperfecta with a dentigerous cyst involving an impacted tooth and an unerupted tooth of the maxilla.

Exploring the Pool of Pathogenic Variants of Amelogenesis Imperfecta: An Approach to the Understanding of Its Genetic Architecture

Objective: To identify which genes are associated with the clinical phenotype of amelogenesis Imperfecta (AI) and to elucidate which of these genes participate in the determination of isolated and syndromic forms.

Methods: In this review, all data on mutations described in AI-related genes were obtained from HGMD® Professional. The data in relation to the mutations, inheritance, phenotype, type of AI and country were supplemented with information from the literature. The identity codes and frequency values were obtained from the dbSNP, ClinVar and OMIM databases. The percentage of specificity (PE) was determined for each gene.

Results: HGMD® describes 27 genes involved in AI, which we propose to group into 5 categories: (1) genes whose mutations are associated only with isolated AI, (2) genes whose mutations cause only syndromic AI, (3) genes with both mutations that cause isolated AI and mutations responsible for other pathologies, (4) genes with mutations responsible for syndromic AI and mutations that cause other pathologies, and (5) genes with mutations that cause isolated AI and mutations that cause AI associated with syndromes and other pathologies. Using the PE calculation, the genes were ranked into 5 specificity groups. The genes of category 1 are specific for isolated AI, while the genes of categories 2 and 4 are non-specific. Interestingly, we observed that mutations in some genes were associated with different types of cancer.

Conclusion: The ACP4, AMTN, MMP20, ODAPH, RELT, SLC24A4 and SP6 genes participate in causing isolated AI, and the CNNM4, DLX3 and FAM20A genes participate in causing syndromic forms of AI.

Molecular Cloning of Mouse Homologue of Enamel Protein C4orf26 and Its Phosphorylation by FAM20C

It is widely accepted that cellular processes are controlled by protein phosphorylation and has become increasingly clear that protein degradation, localization and conformation as well as protein-protein interaction are the examples of subsequent cellular events modulated by protein phosphorylation. Enamel matrix proteins belong to members of the secretory calcium binding phosphoprotein (SCPP) family clustered on chromosome 4q21, and most of the SCPP phosphoproteins have at least one S-X-E motifs (S; serine, X; any amino acid, E; glutamic acid). It has been reported that mutations in C4orf26 gene, located on chromosome 4q21, are associated with autosomal recessive type of Amelogenesis Imperfecta (AI), a hereditary condition that affects enamel formation/mineralization. The enamel phenotype observed in patients with C4orf26 mutations is hypomineralized and partially hypoplastic, indicating that C4orf26 protein may function at both secretory and maturation stages of amelogenesis. The previous in vitro study showed that the synthetic phosphorylated peptide based on C4orf26 protein sequence accelerates hydroxyapatite nucleation. Here we show the molecular cloning of Gm1045, mouse homologue of C4orf26, which has 2 splicing isoforms. Immunohistochemical analysis demonstrated that the immunolocalization of Gm1045 is mainly observed in enamel matrix in vivo. Our report is the first to show that FAM20C, the Golgi casein kinase, phosphorylates C4orf26 and Gm1045 in cell cultures. The extracellular localization of C4orf26/Gm1045 was regulated by FAM20C kinase activity. Thus, our data point out the biological importance of enamel matrix-kinase control of SCPP phosphoproteins and may have a broad impact on the regulation of amelogenesis and AI.

Restorative Treatment of Amelogenesis Imperfecta with Prefabricated Composite Veneers

This case report presents the use of prefabricated composite veneers for restorative treatment of amelogenesis imperfecta (AI). This technique bridges the gap between a conventional direct technique and a conventional indirect technique and introduces an alternative semidirect restorative technique for AI patients. The aim of this case report is to describe restoration of a young girl with severe AI using prefabricated composite veneers and to discuss the benefits and limitations of this technique compared to the alternative restorative techniques.

Translational Attenuation by an Intron Retention in the 5' UTR of ENAM Causes Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a collection of rare genetic conditions affecting tooth enamel. The affected enamel can be of insufficient quantity and/or altered quality, impacting structural content, surface integrity and coloration. Heterozygous mutations in ENAM result in hypoplastic AI without other syndromic phenotypes, with variable expressivity and reduced penetrance, unlike other AI-associated genes. In this study, we recruited a Caucasian family with hypoplastic AI. Mutational analysis (using whole exome sequencing) revealed a splicing donor site mutation (NM_031889.3: c. -61 + 1G > A). Mutational effects caused by this variant were investigated with a minigene splicing assay and in vitro expression analysis. The mutation resulted in a retention of intron 1 and exon 2 (a normally skipped exon), and this elongated 5' UTR sequence attenuated the translation from the mutant mRNA. Structure and translation predictions raised the possibility that the long complex structures-especially a hairpin structure located right before the translation initiation codon of the mutant mRNA-caused reduced protein expression. However, there could be additional contributing factors, including additional uORFs. For the first time, we determined that a mutation altered the ENAM 5' UTR, but maintained the normal coding amino acid sequence, causing hypoplastic AI.

A novel ENAM mutation causes hypoplastic amelogenesis imperfecta

OBJECTIVES

To identify the genetic cause of one Chinese family with hypoplastic amelogenesis imperfecta (AI) and explore the relationship between genotype and its phenotype.

MATERIAL AND METHODS

One Chinese family with generalized hypoplastic AI was recruited. One deciduous tooth from the proband was subjected to scanning electron microscopy. Whole-exome sequencing was performed and identified mutation was confirmed by Sanger sequencing. Bioinformatics studies were further conducted to analyze potential deleterious effects of the mutation.

RESULTS

The proband presented a typical hypoplastic AI phenotype whose teeth in deciduous and permanent dentitions showed thin, yellow, and hard enamel surface. The affected enamel in deciduous tooth showed irregular, broken, and collapsing enamel rods with borders of the enamel prisms undulated and structural shapes of prisms irregular. A novel homozygous nonsense mutation in the last exon of the enamelin (ENAM) gene (NM_031889.3; c.2078C>G) was identified in the proband, which was predicted to produce a highly truncated protein (NP_114095.2; p.(Ser693*)). This mutation was also identified in the proband's parents in heterozygous form. Surprisingly, the clinical phenotype of the heterozygous parents varied from a lack of penetrance to mild enamel defects. Additional bioinformatics studies demonstrated that the detected mutation could change the 3D structure of the ENAM protein and severely damaged the function of ENAM.

CONCLUSION

The novel homozygous ENAM mutation resulted in hypoplastic AI in the present study. Our results provide new genetic evidence that mutations involved in ENAM contribute to hypoplastic AI.

Genetic polymorphisms influence shear bond resistance of orthodontic brackets

OBJECTIVES

The purpose of this study was to determine if shear bond resistance of orthodontic brackets bonded to enamel is associated with genes implicated in the enamel mineralization process.

METHODS

Ninety-two permanent, caries-free premolars extracted for orthodontic purposes and their associated saliva samples were obtained. Eighteen single nucleotide polymorphisms (SNPs) were studied for association with shear bond resistance. The genes of interest in this study were those previously associated with dental caries by our group. All tooth samples were bonded on the buccal surface with metallic lower lateral brackets, and then subjected to physical debonding. The force required to debond the bracket was recorded in Newtons (N) and converted to a shear bond resistance value in Megapascals (N/mm²). The data were analyzed for statistical significance as compared with the mean shear bond resistance value via PLINK whole genome analysis software.

RESULTS

Associations were found between the SNPs for tuftelin (rs7526319, P = 0.004) and amelogenin (rs17878486, P = 0.04) and a higher shear bond resistance.

CONCLUSION

The collected data support the proposed hypothesis that genes involved in the mineralization process affect the bonding of orthodontic brackets, and such an association is of value for the field of orthodontics, particularly in evaluating the efficacy of enamel-resin bond strength for patients receiving treatment.

ENAM mutations and digenic inheritance

BACKGROUND

ENAM mutations cause autosomal dominant or recessive amelogenesis imperfecta (AI) and show a dose effect: enamel malformations are more severe or only penetrant when both ENAM alleles are defective.

METHODS

Whole exome sequences of recruited AI probands were initially screened for mutations in known AI candidate genes. Sanger sequencing was used to confirm sequence variations and their segregation with the disease phenotype. The co-occurrence of ENAM and LAMA3 mutations in one family raised the possibility of digenic inheritance. Enamel formed in Enam+/+ Ambn+/+ , Enam+/- , Ambn+/- , and Enam+/- Ambn+/- mice was characterized by dissection and backscattered scanning electron microscopy (bSEM).

RESULTS

ENAM mutations segregating with AI in five families were identified. Two novel ENAM frameshift mutations were identified. A single-nucleotide duplication (c.395dupA/p.Pro133Alafs*13) replaced amino acids 133-1142 with a 12 amino acid (ATTKAAFEAAIT*) sequence, and a single-nucleotide deletion (c.2763delT/p.Asp921Glufs*32) replaced amino acids 921-1142 with 31 amino acids (ESSPQQASYQAKETAQRRGKAKTLLEMMCPR*). Three families were heterozygous for a previously reported single-nucleotide ENAM deletion (c.588+1delG/p.Asn197Ilefs*81). One of these families also harbored a heterozygous LAMA3 mutation (c.1559G>A/p.Cys520Tyr) that cosegregated with both the AI phenotype and the ENAM mutation. In mice, Ambn+/- maxillary incisors were normal. Ambn+/- molars were also normal, except for minor surface roughness. Ambn+/- mandibular incisors were sometimes chalky and showed minor chipping. Enam+/- incisor enamel was thinner than normal with ectopic mineral deposited laterally. Enam+/- molars were sometimes chalky and rough surfaced. Enam+/- Ambn+/- enamel was thin and rough, in part due to ectopic mineralization, but also underwent accelerated attrition.

CONCLUSION

Novel ENAM mutations causing AI were identified, raising to 22 the number of ENAM variations known to cause AI. The severity of the enamel phenotype in Enam+/- Ambn+/- double heterozygous mice is caused by composite digenic effects. Digenic inheritance should be explored as a cause of AI in humans.

Hypoplastic AI with Highly Variable Expressivity Caused by ENAM Mutations

Tooth enamel, the hardest tissue in the human body, is formed after a complex series of interactions between dental epithelial tissue and the underlying ectomesenchyme. Nonsyndromic amelogenesis imperfecta (AI) is a rare genetic disorder affecting tooth enamel without other nonoral symptoms. In this study, we identified 2 novel ENAM mutations in 2 families with hypoplastic AI by whole exome sequencing. Family 1 had a heterozygous splicing donor site mutation in intron 4, NM_031889; c.123+2T>G. Affected individuals had hypoplastic enamel with or without the characteristic horizontal hypoplastic grooves in some teeth. Family 2 had a nonsense mutation in the last exon, c.1842C>G, p.(Tyr614*), that was predicted to truncate the protein by 500 amino acids. Participating individuals had at least 1 mutant allele, while the proband had a homozygous mutation. Most interestingly, the clinical phenotype of the individuals harboring the heterozygous mutation varied from a lack of penetrance to a mild hypoplastic enamel defect. We believe that these findings will broaden our understanding of the clinical phenotype of AI caused by ENAM mutations.

Amelogenesis imperfecta caused by N-terminal enamelin point mutations in mice and men is driven by endoplasmic reticulum stress

‘Amelogenesis imperfecta’ (AI) describes a group of inherited diseases of dental enamel that have major clinical impact. Here, we identify the aetiology driving AI in mice carrying a p.S55I mutation in enamelin; one of the most commonly mutated proteins underlying AI in humans. Our data indicate that the mutation inhibits the ameloblast secretory pathway leading to ER stress and an activated unfolded protein response (UPR). Initially, with the support of the UPR acting in pro-survival mode, Enam^p.S55I heterozygous mice secreted structurally normal enamel. However, enamel secreted thereafter was structurally abnormal; presumably due to the UPR modulating ameloblast behaviour and function in an attempt to relieve ER stress. Homozygous mutant mice failed to produce enamel. We also identified a novel heterozygous ENAM^p.L31R mutation causing AI in humans. We hypothesize that ER stress is the aetiological factor in this case of human AI as it shared the characteristic phenotype described above for the Enam^p.S55I mouse. We previously demonstrated that AI in mice carrying the Amelx^p.Y64H mutation is a proteinopathy. The current data indicate that AI in Enam^p.S55I mice is also a proteinopathy, and based on comparative phenotypic analysis, we suggest that human AI resulting from the ENAM^p.L31R mutation is another proteinopathic disease. Identifying a common aetiology for AI resulting from mutations in two different genes opens the way for developing pharmaceutical interventions designed to relieve ER stress or modulate the UPR during enamel development to ameliorate the clinical phenotype.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Novel FAM83H mutations in patients with amelogenesis imperfecta

Amelogenesis imperfecta (AI), characterized by a deficiency in the quantity and/or quality of dental enamel, is genetically heterogeneous and phenotypically variable. The most severe type, hypocalcified AI, is mostly caused by truncating mutations in the FAM83H gene. This study aimed to identify genetic mutations in four Chinese families with hypocalcified AI. We performed mutation analysis by sequencing the candidate FAM83H gene. Three novel mutations (c.931dupC, p.V311Rfs*13; c.1130_1131delinsAA, p.S377X; and c.1147 G > T, p.E383X) and one previously reported mutation (c.973 C > T, p.R325X) in the last exon of FAM83H gene were identified. Furthermore, constructs expressing Green fluorescent protein (GFP)-tagged wild-type and three novel mutant FAM83Hs were transfected into rat dental epithelial cells (SF2 cells). Wild-type FAM83H-GFP was localized exclusively in the cytoplasm, especially in the area surrounding the nucleus, while the mutant FAM83H-GFPs (p.V311Rfs*13, p.S377X, and p.E383X) were localized predominantly in the nucleus, with lower levels in the cytoplasm.

Significance of genetic variations in developmental enamel defects of primary dentition in Polish children

OBJECTIVES

The aim of the study was to reveal the association between developmental defects of enamel (DDE) and single nucleotide polymorphisms (SNPs) in the ENAM, AMELX, AMBN, TUFT1, and TFIP11 genes.

MATERIAL AND METHODS

The molecular analysis was carried out in 52 children, aged 10-42 months, from four nursery schools situated in the region of Poznan, Poland (26 individuals with hypomineralization and/or hypoplasia of enamel - "cases" and 26 unaffected children - "controls"), chosen from 262 individuals that had prior dental examination. Six selected SNP variants (rs17878486 in AMELX, rs4694075 in AMBN, rs3796704 in ENAM, rs134136 and rs5997096 in TFIP11, and rs3790506 in TUFT1) were genotyped by the TaqMan probes assay. Genotype and allele frequencies were calculated, and a standard chi-squared analysis was used to test for deviation from Hardy-Weinberg equilibrium. The association between genetic variations and developmental defects of enamel was assessed by the Fisher's exact test and p ≤ 0.05 was considered statistically significant.

RESULTS

Statistically significant positive correlations were found between the rare T allele (p = 0.005) and the TT genotype (p = 0.0052) for rs17878486 in AMELX and occurrence of developmental enamel defects in primary dentition of children. For rs4694075 in AMBN, a higher incidence of the rare T allele (p = 0.0157) was observed in controls compared to DDE cases, whereas the wild-type CC homozygote was more frequent in DDE cases than in controls (p = 0.0062).

CONCLUSIONS

The study showed that the single nucleotide polymorphisms in the AMELX and AMBN genes may be genetic variants that contribute to developmental defects of enamel in primary dentition of children.

CLINICAL RELEVANCE

The single nucleotide polymorphisms of enamel formation genes may increase the risk for developmental defects of enamel (DDE) occurrence in primary dentition in children.

MiR-153 Regulates Amelogenesis by Targeting Endocytotic and Endosomal/lysosomal Pathways-Novel Insight into the Origins of Enamel Pathologies

Amelogenesis imperfecta (AI) is group of inherited disorders resulting in enamel pathologies. The involvement of epigenetic regulation in the pathogenesis of AI is yet to be clarified due to a lack of knowledge about amelogenesis. Our previous genome-wide microRNA and mRNA transcriptome analyses suggest a key role for miR-153 in endosome/lysosome-related pathways during amelogenesis. Here we show that miR-153 is significantly downregulated in maturation ameloblasts compared with secretory ameloblasts. Within ameloblast-like cells, upregulation of miR-153 results in the downregulation of its predicted targets including Cltc, Lamp1, Clcn4 and Slc4a4, and a number of miRNAs implicated in endocytotic pathways. Luciferase reporter assays confirmed the predicted interactions between miR-153 and the 3'-UTRs of Cltc, Lamp1 (in a prior study), Clcn4 and Slc4a4. In an enamel protein intake assay, enamel cells transfected with miR-153 show a decreased ability to endocytose enamel proteins. Finally, microinjection of miR-153 in the region of mouse first mandibular molar at postnatal day 8 (PN8) induced AI-like pathologies when the enamel development reached maturity (PN12). In conclusion, miR-153 regulates maturation-stage amelogenesis by targeting key genes involved in the endocytotic and endosomal/lysosomal pathways, and disruption of miR-153 expression is a potential candidate etiologic factor contributing to the occurrence of AI.

Induction of Apatite by the Cooperative Effect of Amelogenin and the 32-kDa Enamelin

Extracellular matrix proteins are considered to play essential roles in controlling the nucleation, growth, and organization of hydroxyapatite crystals during enamel formation. The effects of amelogenin and the 32-kDa enamelin proteins on apatite nucleation were investigated by a steady-state gel diffusion device containing 10% gelatin gels loaded with 0, 0.75%, and 1.5% (w/w) native porcine amelogenins. It was found that the induction time for hydroxyapatite precipitation was strongly increased by the presence of amelogenins, suggesting an inhibitory effect of apatite nucleation. Addition of 18 μg/mL of 32-kDa enamelin to 10% gelatin also caused inhibition of nucleation. Remarkably, addition of 18 and 80 μg/mL of 32-kDa enamelin in gels containing 1.5% amelogenin accelerated the nucleation process in a dose-dependent manner. Our observations strongly suggest that the 32-kDa enamelin and amelogenins cooperate to promote nucleation of apatite crystals and propose a possible novel mechanism of mineral nucleation during enamel biomineralization.

A Novel Missense Mutation (p.P52R) in Amelogenin Gene Causing X-linked Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a hereditary disease with abnormal dental enamel formation. Here we report a Japanese family with X-linked AI transmitted over at least four generations. Mutation analysis revealed a novel mutation (p.P52R) in exon 5 of the amelogenin gene. The mutation was detected as heterozygous in affected females and as hemizygous in their affected father. The affected sisters exhibited vertical ridges on the enamel surfaces, whereas the affected father had thin, smooth, yellowish enamel with distinct widening of inter-dental spaces. To study the pathological cause underlying the disease in this family, we synthesized the mutant amelogenin p.P52R protein and evaluated it in vitro. Furthermore, we studied differences in the chemical composition between normal and affected teeth by x-ray diffraction analysis and x-ray fluorescence analysis. We believe that these results will greatly aid our understanding of the pathogenesis of X-linked AI.

Amelogenesis Imperfecta in a New Animal Model—a Mutation in Chromosome 5 (human 4q21)

Candidate genes for amelogenesis imperfecta (AI) and dentinogenesis imperfecta (DI) are located on 4q21 in humans. We tested our hypothesis that mutations in the portion of mouse chromosome 5 corresponding to human chromosome 4q21 would cause enamel and dentin abnormalities. Male C3H mice were injected with ethylnitrosourea (ENU). Within a dominant ENU mutagenesis screen, a mouse mutant was isolated with an abnormal tooth enamel (ATE) phenotype. The structure and ultrastructure of teeth were studied. The mutation was located on mouse chromosome 5 in an interval of 9 cM between markers D5Mit18 and D5Mit10. Homozygotic mutants showed total enamel aplasia with exposed dentinal tubules, while heterozygotic mutants showed a significant reduction in enamel width. Dentin of mutant mice showed a reduced content of mature collagen cross-links. We were able to demonstrate that a mutation on chromosome 5 corresponding to human chromosome 4q21 can cause amelogenesis imperfecta and changes in dentin composition.

ENAM mutations with incomplete penetrance

Amelogenesis imperfecta (AI) is a genetic disease affecting tooth enamel formation. AI can be an isolated entity or a phenotype of syndromes. To date, more than 10 genes have been associated with various forms of AI. We have identified 2 unrelated Turkish families with hypoplastic AI and performed mutational analysis. Whole-exome sequencing identified 2 novel heterozygous nonsense mutations in the ENAM gene (c.454G>T p.Glu152* in family 1, c.358C>T p.Gln120* in family 2) in the probands. Affected individuals were heterozygous for the mutation in each family. Segregation analysis within each family revealed individuals with incomplete penetrance or extremely mild enamel phenotype, in spite of having the same mutation with the other affected individuals. We believe that these findings will broaden our understanding of the clinical phenotype of AI caused by ENAM mutations.

Amelogenesis imperfecta and anterior open bite: Etiological, classification, clinical and management interrelationships

Although amelogenesis imperfecta is not a common dental pathological condition, its etiological, classification, clinical and management aspects have been addressed extensively in the scientific literature. Of special clinical consideration is the frequent co-existence of amelogenesis imperfecta with the anterior open bite. This paper provides an updated review on amelogenesis imperfecta as well as anterior open bite, in general, and documents the association of these two separate entities, in particular. Diagnosis and treatment of amelogenesis imperfecta patients presenting also with anterior open bite require a lengthy, comprehensive and multidisciplinary approach, which should aim to successfully address all dental, occlusal, developmental, skeletal and soft tissue problems associated with these two serious clinical conditions.

Interventions for the restorative care of amelogenesis imperfecta in children and adolescents

BACKGROUND

Amelogenesis imperfecta (AI) is a tooth development disorder in which the teeth are covered with thin, abnormally formed enamel. This enamel is easily fractured and damaged, which affects the appearance of the teeth, especially if left untreated. Negative psychological outcomes, due to compromised appearance and function, in patients with AI, have been found to compromise a person's attractiveness and reduce social interaction. The treatment used depends on the severity of the problem.

OBJECTIVES

To compare the success rates of different restorative materials and techniques used for the restoration of anterior and posterior teeth with AI in terms of patient satisfaction (aesthetics and sensitivity) and function.

SEARCH METHODS

We searched the Cochrane Oral Health Group's Trials Register (to 18 April 2013), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2013, Issue 3), MEDLINE via OVID (1946 to 18 April 2013), EMBASE via OVID (1980 to 18 April 2013), CINAHL via EBSCO (1980 to 18 April 2013), Abstracts of the Conference Proceedings of the International Association for Dental Research (2001 to 18 April 2013) and reference lists of relevant articles. There were no restrictions on language or date of publication in the electronic searches.

SELECTION CRITERIA

Randomised controlled trials where children and adolescents with AI who required restoration of teeth were allocated to different restoration techniques would have been selected. Outcomes which would have been evaluated were patient satisfaction, aesthetics, masticatory function and longevity of restorations.

DATA COLLECTION AND ANALYSIS

Two review authors would have extracted data and assessed the risk of bias in included studies independently. Disagreement between the two authors would have been resolved by consulting a third review author. First authors were contacted for additional information and unpublished data.

MAIN RESULTS

No studies met the inclusion criteria for this review.

AUTHORS' CONCLUSIONS

We found no randomised controlled trials of restorative treatments for children and adolescents with AI, and therefore there is no evidence as to which is the best restoration. Well defined randomised controlled trials which recruit children and adolescents and focus on the type and severity of the disorder should be undertaken to determine the best intervention for restoring teeth affected by AI.

Characterization of periodontal structures of enamelin-null mice

BACKGROUND

Enamelin-null (ENAM(-/-)) mice have no enamel. When characterizing ENAM(-/-) mice, alveolar bone height reduction was observed, and it was hypothesized that enamel defects combined with diet are associated with the periodontal changes of ENAM(-/-)mice. The aim of the present study is to compare the dimension of interradicular bone of ENAM(-/-) (knock-out [KO]) with wild-type (WT) mice, maintained on hard (HC) or soft (SC) chow.

METHODS

A total of 100 animals divided into four groups were studied at 3, 8, and 24 weeks of age: 1) KO/HC; 2) KO/SC; 3) WT/HC; and 4) WT/SC. Microcomputed tomography was performed, and the following measurements were made between mandibular first (M1) and second (M2) molars: relative alveolar bone height (RBH), crestal bone width (CBW), bone volume (BV), bone mineral content (BMC), and bone mineral density (BMD). The position of M1 and M2 in relation to the inferior border of the mandible was also determined at 24 weeks. All variables were analyzed by one-way analysis of variance and Dunnett test for pairwise comparisons. Morphologic analyses were conducted on hematoxylin and eosin-stained sections.

RESULTS

Radiographically, the enamel layer was absent in ENAM(-/-) mice. Interproximal open contacts were observed exclusively in ENAM(-/-) mice, and the prevalence decreased over time, suggesting that a shifting of tooth position had occurred. Additionally, in the two ENAM(-/-) groups, RBH was significantly lower at 8 and 24 weeks (P <0.02); CBW, BV, and BMC were significantly less (P <0.05) at 24 weeks. No differences in BMD were found among the four groups. The molars migrated to a more coronal position in ENAM(-/-) mice and mice on HC. Histologic findings were consistent with radiographic observations. After eruption, the junctional epithelium was less organized in ENAM(-/-) mice.

CONCLUSION

The interdental bone density was not affected in the absence of enamelin, but its volume was, which is likely a consequence of alternations in tooth position.

Bodyweight assessment of enamelin null mice

The Enam null mice appear to be smaller than wild-type mice, which prompted the hypothesis that enamel defects negatively influence nutritional intake and bodyweight gain (BWG). We compared the BWG of Enam^−/− and wild-type mice from birth (D0) to Day 42 (D42). Wild-type (WT) and Enam^−/− (N) mice were given either hard chow (HC) or soft chow (SC). Four experimental groups were studied: WTHC, WTSC, NHC, and NSC. The mother's bodyweight (DBW) and the average litter bodyweight (ALBW) were obtained from D0 to D21. After D21, the pups were separated from the mother and provided the same type of food. Litter bodyweights were measured until D42. ALBW was compared at 7-day intervals using one-way ANOVA, while the influence of DBW on ALBW was analyzed by mixed-model analyses. The ALBW of Enam^−/− mice maintained on hard chow (NHC) was significantly lower than the two WT groups at D21 and the differences persisted into young adulthood. The ALBW of Enam^−/− mice maintained on soft chow (NSC) trended lower, but was not significantly different than that of the WT groups. We conclude that genotype, which affects enamel integrity, and food hardness influence bodyweight gain in postnatal and young adult mice.

Multiple unerupted teeth with amelogenesis imperfecta in siblings

Amelogenesis imperfecta encompasses a group of inherited abnormalities that are generally considered to primarily affect the formation and/or calcification of enamel. This case report describes the unusual presentation of amelogenesis imperfecta in siblings as multiple unerupted teeth, multiple pulpal calcifications, and multiple dilacerations of roots along with the defect in the enamel. The intent of our report is to highlight a rare co-occurrence of amelogenesis imperfecta with multiple morphologic alterations in siblings.

Clinical and molecular analysis of the enamelin gene ENAM in Colombian families with autosomal dominant amelogenesis imperfecta

In this study, we analyzed the phenotype, clinical characteristics and presence of mutations in the enamelin gene ENAM in five Colombian families with autosomal dominant amelogenesis imperfecta (ADAI). 22 individuals (15 affected and seven unaffected) belonging to five Colombian families with ADAI and eight individuals (three affected and five unaffected) belonging to three Colombian families with autosomal recessive amelogenesis imperfecta (ARAI) that served as controls for molecular alterations and inheritance patterns were studied. Clinical, radiographic and genetic evaluations were done in all individuals. Eight exons and three intron-exon boundaries were sequenced for mutation analysis. Two of the five families with ADAI had the hypoplasic phenotype, two had the hypocalcified phenotype and one had the hypomaturative phenotype. Anterior open bite and mandibular retrognathism were the most frequent skeletal abnormalities in the families with ADAI. No mutations were found. These findings suggest that ADAI in these Colombian families was unrelated to previously described mutations in the ENAM gene. These results also indicate that other regions not included in this investigation, such as the promoter region, introns and other genes should be considered as potential ADAI candidates.

Novel genetic linkage of rat Sp6 mutation to Amelogenesis imperfecta

Background

Amelogenesis imperfecta (AI) is an inherited disorder characterized by abnormal formation of tooth enamel. Although several genes responsible for AI have been reported, not all causative genes for human AI have been identified to date. AMI rat has been reported as an autosomal recessive mutant with hypoplastic AI isolated from a colony of stroke-prone spontaneously hypertensive rat strain, but the causative gene has not yet been clarified. Through a genetic screen, we identified the causative gene of autosomal recessive AI in AMI and analyzed its role in amelogenesis.

Methods

cDNA sequencing of possible AI-candidate genes so far identified using total RNA of day 6 AMI rat molars identified a novel responsible mutation in specificity protein 6 (Sp6). Genetic linkage analysis was performed between Sp6 and AI phenotype in AMI. To understand a role of SP6 in AI, we generated the transgenic rats harboring Sp6 transgene in AMI (Ami/Ami + Tg). Histological analyses were performed using the thin sections of control rats, AMI, and Ami/Ami + Tg incisors in maxillae, respectively.

Results

We found the novel genetic linkage between a 2-bp insertional mutation of Sp6 gene and the AI phenotype in AMI rats. The position of mutation was located in the coding region of Sp6, which caused frameshift mutation and disruption of the third zinc finger domain of SP6 with 11 cryptic amino acid residues and a stop codon. Transfection studies showed that the mutant protein can be translated and localized in the nucleus in the same manner as the wild-type SP6 protein. When we introduced the CMV promoter-driven wild-type Sp6 transgene into AMI rats, the SP6 protein was ectopically expressed in the maturation stage of ameloblasts associated with the extended maturation stage and the shortened reduced stage without any other phenotypical changes.

Conclusion

We propose the addition of Sp6 mutation as a new molecular diagnostic criterion for the autosomal recessive AI patients. Our findings expand the spectrum of genetic causes of autosomal recessive AI and sheds light on the molecular diagnosis for the classification of AI. Furthermore, tight regulation of the temporospatial expression of SP6 may have critical roles in completing amelogenesis.

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.

Target gene analyses of 39 amelogenesis imperfecta kindreds

Previously, mutational analyses identified six disease-causing mutations in 24 amelogenesis imperfecta (AI) kindreds. We have since expanded the number of AI kindreds to 39, and performed mutation analyses covering the coding exons and adjoining intron sequences for the six proven AI candidate genes [amelogenin (AMELX), enamelin (ENAM), family with sequence similarity 83, member H (FAM83H), WD repeat containing domain 72 (WDR72), enamelysin (MMP20), and kallikrein-related peptidase 4 (KLK4)] and for ameloblastin (AMBN) (a suspected candidate gene). All four of the X-linked AI families (100%) had disease-causing mutations in AMELX, suggesting that AMELX is the only gene involved in the aetiology of X-linked AI. Eighteen families showed an autosomal-dominant pattern of inheritance. Disease-causing mutations were identified in 12 (67%): eight in FAM83H, and four in ENAM. No FAM83H coding-region or splice-junction mutations were identified in three probands with autosomal-dominant hypocalcification AI (ADHCAI), suggesting that a second gene may contribute to the aetiology of ADHCAI. Six families showed an autosomal-recessive pattern of inheritance, and disease-causing mutations were identified in three (50%): two in MMP20, and one in WDR72. No disease-causing mutations were found in 11 families with only one affected member. We conclude that mutation analyses of the current candidate genes for AI have about a 50% chance of identifying the disease-causing mutation in a given kindred.

The cooperation of enamelin and amelogenin in controlling octacalcium phosphate crystal morphology

Enamel matrix proteins, including the most abundant amelogenin and lesser amounts of enamelin, ameloblastin, and proteinases, play vital roles in controlling crystal nucleation and growth during enamel formation. The cooperative action between amelogenin and the 32-kDa enamelin is critical to regulating the growth morphology of octacalcium phosphate crystals. Using biophysical methods, we investigated the interaction between the 32-kDa enamelin and recombinant pig amelogenin 148 (rP148) at pH 6.5 in phosphate-buffered saline (PBS). Dynamic light scattering results showed a trend of increasing particle size in the mixture with the addition of enamelin to amelogenin. Upon addition of the 32-kDa enamelin, the shift and intensity decrease in the ellipticity minima of rP148 in the circular dichroism spectra of rP148 illustrated a direct interaction between the 2 proteins. In the fluorescence spectra, the maximum emission of rP148 was blue shifted from 335 to 333 nm in the presence of enamelin as a result of complexation of the 2 proteins. Our results demonstrate that the 32-kDa enamelin has a close association with amelogenin at pH 6.5 in PBS buffer. Our present study provides novel insights into the possible cooperation between enamelin and amelogenin in macromolecular coassembly and in controlling enamel mineral formation.

Tooth enamel proteins enamelin and amelogenin cooperate to regulate the growth morphology of octacalcium phosphate crystals

To examine the hypothetical cooperative role of enamelin and amelogenin in controlling the growth morphology of enamel crystals in the post-secretory stage, we applied a cation selective membrane system for the growth of octacalcium phosphate (OCP) in the truncated recombinant porcine amelogenin (rP148) with and without the 32kDa enamelin fragment. Enamelin alone inhibited the growth in the c-axis direction more than rP148, yielding OCP crystals with the smallest aspect ratio of all conditions tested. When enamelin was added to the amelogenin "gel-like matrix", the inhibitory action of the protein mixture on the growth of OCP in the c-axis direction was diminished, while that in the b-axis direction was increased. As a result, the length to width ratio (aspect ratio) of OCP crystal was markedly increased. Addition of enamelin to amelogenin enhanced the potential of amelogenin to stabilize the amorphous calcium phosphate (ACP) transient phase. The ratio of enamelin and amelogenin was crucial for stabilization of ACP and the growth of OCP crystals with larger aspect ratio. The cooperative regulatory action of enamelin and amelogenin was attained, presumably, through co-assembling of enamelin and amelogenin. These results have important implications in understanding the growth mechanism of enamel crystals with large aspect ratio.

Altered enamelin phosphorylation site causes amelogenesis imperfecta

Defects in the enamelin gene (ENAM) cause amelogenesis imperfecta (AI). Our objective was to identify the genetic etiology of enamel hypoplasia in a Caucasian proband. Our hypothesis was that ENAM was defective. The proband and his father have an AG insertion (g.13185_13186insAG; p.422FsX448) in ENAM previously identified in AI kindreds from Slovenia and Turkey. The proband, his brother, and his mother have a novel missense mutation (g.12573C>T) that substitutes leucine for a phosphorylated serine (p.S216L) in the 32-kDa enamelin cleavage product. In this family, a defect in one ENAM allele caused minor pitting or localized enamel hypoplasia, whereas defects in both alleles caused severe enamel malformations, with little or no mineral covering dentin. Ser(216) is one of two serines on the 32-kDa enamelin that is phosphorylated by Golgi casein kinase and is thought to mediate calcium binding. We propose that phosphorylation of enamelin is critical for its function.

Exclusion of candidate genes in seven Turkish families with autosomal recessive amelogenesis imperfecta

Amelogenesis imperfectas (AI) are a group of inherited defects of dental enamel formation that show both clinical and genetic heterogeneity. Seven Turkish families segregating autosomal recessive AI (ARAI) were evaluated for evidence of a genetic etiology of AI for the seven major candidate gene loci (AMBN, AMELX, ENAM, FAM83H, KLK4, MMP20, and TUFT1). Dental and periodontal characteristics of the affected members of these families were also described. The mean scores of DMFS and dfs indices were 9.7 and 9.6, respectively. The mean PPD was 2.2 mm and the percentage of the sites with plaque and BOP were 87.8% and 72.4%, respectively. The exons and intron/exon junctions of the candidate genes were sequenced and no gene mutations were identified in any individuals. These findings support the existence of an additional gene(s) that are etiologic for ARAI in these families.

Candidate gene strategy reveals ENAM mutations

Amelogenesis imperfecta (AI) is a genetically and phenotypically heterogeneous genetic disorder affecting tooth enamel without other non-oral syndromic conditions. Based on a review of the literature, the authors constructed a candidate-gene-based mutational analysis strategy. To test the strategy, they identified two Turkish families with hypoplastic enamel without any other non-oral syndromic phenotype. The authors analyzed all exons and exon/intron boundaries of the enamelin (ENAM) gene for family 1 and the DLX3 and ENAM genes for family 2, to identify the underlying genetic etiology. The analysis revealed 2 ENAM mutations (autosomal-dominant g.14917delT and autosomal-recessive g.13185-13186insAG mutations). A single T deletion in exon 10 is a novel deletional mutation (g.14917delT, c.2991delT), which is predicted to result in a frameshift with a premature termination codon (p.L998fsX1062). This result supports the use of a candidate-gene-based strategy to study the genetic basis for AI.

Dietary change and adaptive evolution of enamelin in humans and among primates

Scans of the human genome have identified many loci as potential targets of recent selection, but exploration of these candidates is required to verify the accuracy of genomewide scans and clarify the importance of adaptive evolution in recent human history. We present analyses of one such candidate, enamelin, whose protein product operates in tooth enamel formation in 100 individuals from 10 populations. Evidence of a recent selective sweep at this locus confirms the signal of selection found by genomewide scans. Patterns of polymorphism in enamelin correspond with population-level differences in tooth enamel thickness, and selection on enamel thickness may drive adaptive enamelin evolution in human populations. We characterize a high-frequency nonsynonymous derived allele in non-African populations. The polymorphism occurs in codon 648, resulting in a nonconservative change from threonine to isoleucine, suggesting that the allele may affect enamelin function. Sequences of exons from 12 primate species show evidence of positive selection on enamelin. In primates, it has been documented that enamel thickness correlates with diet. Our work shows that bursts of adaptive enamelin evolution occur on primate lineages with inferred dietary changes. We hypothesize that among primate species the evolved differences in tooth enamel thickness are correlated with the adaptive evolution of enamelin.

A novel autosomal-recessive mutation, whitish chalk-like teeth, resembling amelogenesis imperfecta, maps to rat chromosome 14 corresponding to human 4q21

A rat mutant, whitish chalk-like teeth (wct), with white, chalk-like abnormal incisors, was discovered and morphologically and genetically characterized. The mutant rats showed tooth enamel defects that were similar to those of human amelogenesis imperfecta. The wct mutation was found to disturb the morphological transition of ameloblasts from secretory to maturation stages and to induce cyst formation. This mutation also disturbs the transfer of iron into the enamel, resulting in the whitish chalk-like incisors. A genetic linkage study indicated that the wct locus maps to a specific interval of rat chromosome 14 between D14Got13 and D14Wox2. Interestingly, the human chromosomal region orthologous to wct, a 5.5-Mb interval in human chromosome 4q21, is a critical region for the locus of human amelogenesis imperfecta AIH2. These results strongly suggest that this wct mutant is a useful model for the identification of genes responsible for amelogenesis imperfecta and molecular mechanisms of tooth development.

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.

Developmental biology and genetics of dental malformations

The synthesis of tooth development biology with human studies focusing on inherited conditions that specifically interfere with tooth development is improving our understanding of normal and pathological tooth formation. The type of inherited dental malformations observed in a given kindred relate to when, during odontogenesis, the defective gene is critically expressed. Information about the protein encoded by the defective gene and the resulting dental phenotype helps us understand the major processes underway at different stages during tooth development. Genes affecting early tooth development (PAX9, MSX1, and AXIN2) are associated with familial tooth agenesis or oligodontia. Genes expressed by odontoblasts (COL1A1, COL1A2, and DSPP), and ameloblasts (AMELX, ENAM, MMP20, and KLK4) during the crown formation stage, are associated with dentinogenesis imperfecta, dentin dysplasia, and amelogenesis imperfecta. Late genes expressed during root formation (ALPL and DLX3) are associated with cementum agenesis (hypophosphatasia) and taurodontism. Understanding the relationships between normal tooth development and the dental pathologies associated with inherited diseases improves our ability to diagnose and treat patients suffering the manifestations of inherited dental disorders.

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.

Exclusion of known gene for enamel development in two Brazilian families with amelogenesis imperfecta

Amelogenesis imperfecta (AI) is a genetically heterogeneous group of diseases that result in defective development of tooth enamel. Mutations in several enamel proteins and proteinases have been associated with AI. The object of this study was to evaluate evidence of etiology for the six major candidate gene loci in two Brazilian families with AI. Genomic DNA was obtained from family members and all exons and exon-intron boundaries of the ENAM, AMBN, AMELX, MMP20, KLK4 and Amelotin gene were amplified and sequenced. Each family was also evaluated for linkage to chromosome regions known to contain genes important in enamel development. The present study indicates that the AI in these two families is not caused by any of the known loci for AI or any of the major candidate genes proposed in the literature. These findings indicate extensive genetic heterogeneity for non-syndromic AI.

The molecular etiologies and associated phenotypes of amelogenesis imperfecta

The amelogenesis imperfectas (AIs) are a clinically and genetically diverse group of conditions that are caused by mutations in a variety of genes that are critical for normal enamel formation. To date, mutations have been identified in four genes (AMELX, ENAM, KLK4, MMP20) known to be involved in enamel formation. Additional yet to be identified genes also are implicated in the etiology of AI based on linkage studies. The diverse and often unique phenotypes resulting from the different allelic and non-allelic mutations in these genes provide an opportunity to better understand the role of these genes and their related proteins in enamel formation. Understanding the AI phenotypes also provides an aid to clinicians in directing molecular studies aimed at delineating the genetic basis underlying these diverse clinical conditions. Our current knowledge of the known mutations and associated phenotypes of the different AI subtypes are reviewed.

How do enamelysin and kallikrein 4 process the 32-kDa enamelin?

The activities of two proteases--enamelysin (MMP-20) and kallikrein 4 (KLK4)--are necessary for dental enamel to achieve its high degree of mineralization. We hypothesize that the selected enamel protein cleavage products which accumulate in the secretory-stage enamel matrix do so because they are resistant to further cleavage by MMP-20. Later, they are degraded by KLK4. The 32-kDa enamelin is the only domain of the parent protein that accumulates in the deeper enamel. Our objective was to identify the cleavage sites of 32-kDa enamelin that are generated by proteolysis with MMP-20 and KLK4. Enamelysin, KLK4, the major amelogenin isoform (P173), and the 32-kDa enamelin were isolated from developing porcine enamel. P173 and the 32-kDa enamelin were incubated with MMP-20 or KLK4 for up to 48 h. Then, the 32-kDa enamelin digestion products were fractionated by reverse-phase high-performance liquid chromatography (RP-HPLC) and characterized by Edman sequencing, amino acid analysis, and mass spectrometry. Enamelysin cleaved the 32-kDa enamelin only after it was deglycosylated. Kallikrein 4 digestion of the 32-kDa enamelin generated nine major cleavage products, six of which were successfully characterized. After 12 h of digestion with KLK4, all of the 32-kDa enamelin had been cleaved, but some cleavage products persisted after 48 h of digestion.

The use of animal models to explore amelogenin variants in amelogenesis imperfecta

Amelogenin proteins are secreted by ameloblast cells during tooth development. Because of extensive alternative splicing of the amelogenin primary RNA transcript, and because systematic proteolysis results in many additional peptides during enamel maturation, it has been difficult to assign function to individual amelogenins. Targeted deletions and transgenic mice have been generated in order to better understand amelogenin protein function in vivo. From these murine models, we have determined that amelogenins are responsible for normal enamel thickness and structure, but not for initiation of enamel mineral formation at the dentin-enamel junction. Although it is now clear that the amelogenin (AmelX) gene exists in a nested orientation and that AmelX is expressed at a low level in various developing tissues, the significance of these findings is incompletely understood. Future studies are expected to answer remaining questions concerning structure/function relationships among these 'enamel proteins'.

Proteomics and genetics of dental enamel

The initiation of enamel crystals at the dentino-enamel junction is associated with the expression of dentin sialophosphoprotein (DSPP, a gene normally linked with dentin formation), three 'structural' enamel proteins--amelogenin (AMELX), enamelin (ENAM), and ameloblastin (AMBN)--and a matrix metalloproteinase, enamelysin (MMP20). Enamel formation proceeds with the steady elongation of the enamel crystals at a mineralization front just beneath the ameloblast distal membrane, where these proteins are secreted. As the crystal ribbons lengthen, enamelysin processes the secreted proteins. Some of the cleavage products accumulate in the matrix, others are reabsorbed back into the ameloblast. Once crystal elongation is complete and the enamel layer reaches its final thickness, kallikrein 4 (KLK4) facilitates the breakdown and reabsorption of accumulated enamel matrix proteins. The importance of the extracellular matrix proteins to proper tooth development is best illustrated by the dramatic dental phenotypes observed in the targeted knockouts of enamel matrix genes in mice (Dspp, Amelx, Ambn, Mmp20) and in human kindreds with defined mutations in the genes (DSPP, AMELX, ENAM, MMP20, KLK4) encoding these matrix proteins. However, ablation studies alone cannot give specific mechanistic information on how enamel matrix proteins combine to catalyze the formation of enamel crystals. The best approach for determining the molecular mechanism of dental enamel formation is to reconstitute the matrix and synthesize enamel crystals in vitro. Here, we report refinements to the procedures used to isolate porcine enamel and dentin proteins, recent advances in the characterization of enamel matrix protein posttranslational modifications, and summarize the results of human genetic studies that associate specific mutations in the genes encoding matrix proteins with a range of dental phenotypes.

Mutational analysis of candidate genes in 24 amelogenesis imperfecta families

Amelogenesis imperfecta (AI) is a heterogeneous group of inherited defects in dental enamel formation. The malformed enamel can be unusually thin, soft, rough and stained. The strict definition of AI includes only those cases where enamel defects occur in the absence of other symptoms. Currently, there are seven candidate genes for AI: amelogenin, enamelin, ameloblastin, tuftelin, distal-less homeobox 3, enamelysin, and kallikrein 4. To identify sequence variations in AI candidate genes in patients with isolated enamel defects, and to deduce the likely effect of each sequence variation on protein expression and structure, families with isolated enamel defects were recruited. The coding exons and nearby intron sequences were amplified for each of the AI candidate genes by using genomic DNA from the proband as template. The amplification products for the proband were sequenced. Then, other family members were tested to determine their genotype with respect to each sequence variation. All subjects received an oral examination, and intraoral photographs and dental radiographs were obtained. Out of 24 families with isolated enamel defects, only six disease-causing mutations were identified in the AI candidate genes. This finding suggests that many additional genes potentially contribute to the etiology of AI.

A case of amelogenesis imperfecta, cleft lip and palate and polycystic kidney disease

OBJECTIVE

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic disorders characterized by developmental abnormalities of tooth enamel. The AI is also seen as part of multi-organ abnormalities, e.g. with cone-rod dystrophy, hypothalamo-hypophyseal insufficiency and renal failure. The present patient with AI and nephrocalcinosis exhibited a phenotype different from previous cases with renal failure. To highlight the characteristics of this rare case, extensive analysis that included histological, biochemical and genetic examinations was performed.

PATIENT

The present Japanese male patient exhibited dentition with AI and bilateral cleft lip and palate. Ground sections of his extracted tooth showed that it was hypomaturation-type AI, unlike previous cases with nephrocalcinosis were hypoplastic-type. He showed nephrocalcinosis and hematuria at 15 years of age but these symptoms appeared to be secondary to polycystic kidney disease. He showed skeletal Class II pattern with a retrognathic profile and retroclined incisors of both arches. A dolicofacial appearance was seen with an enlarged gonial angle. Biochemical makers including serum alkaline phosphatase, parathyroid hormone, calcitonin, calcium, and phosphate, were all in the normal range. Sequence analysis of the genes encoding amelogenin and enamelin, which are known to be responsible for hypoplastic-type AI, did not reveal any mutations. Since mouse null mutant of homeobox transcription factor, Msx2, exhibits a phenotype resembling AI, the human homolog of this gene, MSX2, was sequenced. There was a missense mutation of T447C that resulted in the conversion of methionine to threonine at 129.