Alteration of conserved alternative splicing in AMELX causes enamel defects

Genetic polymorphisms associated with developmental defects of enamel: A systematic review

BACKGROUND

Polymorphisms in genes related to enamel formation and mineralization may increase the risk of developmental defects of enamel (DDE).

AIM

To evaluate the existing literature on genetic polymorphisms associated with DDE.

DESIGN

This systematic review was registered in the PROSPERO (CRD42018115270). The literature search was performed in PubMed, Scopus, Web of Science, LILACS, BBO, Cochrane Library, and in the gray literature. Observational studies assessing the association between DDE and genetic polymorphism were included. The Newcastle-Ottawa Scale was used to assess the risk of bias.

RESULTS

One thousand one hundred and forty-six articles were identified, and 28 met the inclusion criteria. Five studies presented a low risk of bias. Ninety-two genes related to enamel development, craniofacial patterning morphogenesis, immune response, and hormone transcription/reception were included. Molar-incisor hypomineralization (MIH) and/or hypomineralization of primary second molars (HPSM) were associated with 80 polymorphisms of genes responsible for enamel development, immune response, morphogenesis, and xenobiotic detoxication. A significant association was found between the different clinical manifestations of dental fluorosis (DF) with nine polymorphisms of genes responsible for enamel development, craniofacial development, hormonal transcription/reception, and oxidative stress. Hypoplasia was associated with polymorphisms located in intronic regions.

CONCLUSION

MIH, HPSM, DF, and hypoplasia reported as having a complex etiology are significantly associated with genetic polymorphisms of several genes.

AMELX Mutations and Genotype-Phenotype Correlation in X-Linked Amelogenesis Imperfecta

AMELX mutations cause X-linked amelogenesis imperfecta (AI), known as AI types IE, IIB, and IIC in Witkop's classification, characterized by hypoplastic (reduced thickness) and/or hypomaturation (reduced hardness) enamel defects. In this study, we conducted whole exome analyses to unravel the disease-causing mutations for six AI families. Splicing assays, immunoblotting, and quantitative RT-PCR were conducted to investigate the molecular and cellular effects of the mutations. Four AMELX pathogenic variants (NM_182680.1:c.2T>C; c.29T>C; c.77del; c.145-1G>A) and a whole gene deletion (NG_012494.2:g.307534_403773del) were identified. The affected individuals exhibited enamel malformations, ranging from thin, poorly mineralized enamel with a "snow-capped" appearance to severe hypoplastic defects with minimal enamel. The c.145-1G>A mutation caused a -1 frameshift (NP_001133.1:p.Val35Cysfs*5). Overexpression of c.2T>C and c.29T>C AMELX demonstrated that mutant amelogenin proteins failed to be secreted, causing elevated endoplasmic reticulum stress and potential cell apoptosis. This study reveals a genotype-phenotype relationship for AMELX-associated AI: While amorphic mutations, including large deletions and 5' truncations, of AMELX cause hypoplastic-hypomaturation enamel with snow-capped teeth (AI types IIB and IIC) due to a complete loss of gene function, neomorphic variants, including signal peptide defects and 3' truncations, lead to severe hypoplastic/aplastic enamel (AI type IE) probably caused by "toxic" cellular effects of the mutant proteins.

Digenic inheritance accounts for phenotypic variability in amelogenesis imperfecta

Amelogenesis imperfecta (AI) represents a group of clinically and genetically heterogeneous disorders that affect enamel formation and mineralization. Although AI is commonly considered a monogenic disorder, digenic inheritance is rarely reported. In this study, we recruited two nonconsanguineous Chinese families exhibiting diverse phenotypes of enamel defects among affected family members. Digenic variants were discovered in both probands. In family 1, the proband inherited a paternal frameshift variant in LAMA3 (NM_198129.4:c.3712dup) and a maternal deletion encompassing the entire AMELX gene. This resulted in a combined hypoplastic and hypomineralized AI phenotype, which was distinct from the parents' manifestations. In family 2, whole-exome sequencing analysis revealed the proband carried a maternal heterozygous splicing variant in COL17A1 (NC_000010.11 (NM_000494.3): c.4156 + 2dup) and compound heterozygous variants in RELT (paternal: NM_032871.4:c.260A > T; maternal: NM_032871.4:c.521 T > G). These genetic changes caused the abundant irregular enamel defects observed in the proband, whereas other affected family members carrying heterozygous variants in both COL17A1 and RELT displayed only horizontal grooves as their phenotype. The pathogenicity of the novel COL17A1 splice site variant was confirmed through RT-PCR and minigene assay. This study enhances our understanding by highlighting the potential association between the co-occurrence of variants in two genes and variable phenotypes observed in AI patients.

Splicing mutations in AMELX and ENAM cause amelogenesis imperfecta

BACKGROUND

Amelogenesis imperfecta (AI) is a developmental enamel defect affecting the structure of enamel, esthetic appearance, and the tooth masticatory function. Gene mutations are reported to be relevant to AI. However, the mechanism underlying AI caused by different mutations is still unclear. This study aimed to reveal the molecular pathogenesis in AI families with 2 novel pre-mRNA splicing mutations.

METHODS

Two Chinese families with AI were recruited. Whole-exome sequencing and Sanger sequencing were performed to identify mutations in candidate genes. Minigene splicing assays were performed to analyze the mutation effects on mRNA splicing alteration. Furthermore, three-dimensional structures of mutant proteins were predicted by AlphaFold2 to evaluate the detrimental effect.

RESULTS

The affected enamel in family 1 was thin, rough, and stained, which was diagnosed as hypoplastic-hypomature AI. Genomic analysis revealed a novel splicing mutation (NM_001142.2: c.570 + 1G > A) in the intron 6 of amelogenin (AMELX) gene in family 1, resulting in a partial intron 6 retention effect. The proband in family 2 exhibited a typical hypoplastic AI, and the splicing mutation (NM_031889.2: c.123 + 4 A > G) in the intron 4 of enamelin (ENAM) gene was observed in the proband and her father. This mutation led to exon 4 skipping. The predicted structures showed that there were obvious differences in the mutation proteins compared with wild type, leading to impaired function of mutant proteins.

CONCLUSIONS

In this study, we identified two new splicing mutations in AMELX and ENAM genes, which cause hypoplastic-hypomature and hypoplastic AI, respectively. These results expand the spectrum of genes causing AI and broaden our understanding of molecular genetic pathology of enamel formation.

Mutations Causing X-Linked Amelogenesis Imperfecta Alter miRNA Formation from Amelogenin Exon4

Amelogenin plays a crucial role in tooth enamel formation, and mutations on X-chromosomal amelogenin cause X-linked amelogenesis imperfecta (AI). Amelogenin pre-messenger RNA (mRNA) is highly alternatively spliced, and during alternative splicing, exon4 is mostly skipped, leading to the formation of a microRNA (miR-exon4) that has been suggested to function in enamel and bone formation. While delivering the functional variation of amelogenin proteins, alternative splicing of exon4 is the decisive first step to producing miR-exon4. However, the factors that regulate the splicing of exon4 are not well understood. This study aimed to investigate the association between known mutations in exon4 and exon5 of X chromosome amelogenin that causes X-linked AI, the splicing of exon4, and miR-exon4 formation. Our results showed mutations in exon4 and exon5 of the amelogenin gene, including c.120T>C, c.152C>T, c.155C>G, and c.155delC, significantly affected the splicing of exon4 and subsequent miR-exon4 production. Using an amelogenin minigene transfected in HEK-293 cells, we observed increased inclusion of exon4 in amelogenin mRNA and reduced miR-exon4 production with these mutations. In silico analysis predicted that Ser/Arg-rich RNA splicing factor (SRSF) 2 and SRSF5 were the regulatory factors for exon4 and exon5 splicing, respectively. Electrophoretic mobility shift assay confirmed that SRSF2 binds to exon4 and SRSF5 binds to exon5, and mutations in each exon can alter SRSF binding. Transfection of the amelogenin minigene to LS8 ameloblastic cells suppressed expression of the known miR-exon4 direct targets, Nfia and Prkch, related to multiple pathways. Given the mutations on the minigene, the expression of Prkch has been significantly upregulated with c.155C>G and c.155delC mutations. Together, we confirmed that exon4 splicing is critical for miR-exon4 production, and mutations causing X-linked AI in exon4 and exon5 significantly affect exon4 splicing and following miR-exon4 production. The change in miR-exon4 would be an additional etiology of enamel defects seen in some X-linked AI.

Molecular-based phenotype variations in amelogenesis imperfecta

Amelogenesis imperfecta (AI) is one of the typical dental genetic diseases in human. It can occur isolatedly or as part of a syndrome. Previous reports have mainly clarified the types and mechanisms of nonsyndromic AI. This review aimed to compare the phenotypic differences among the hereditary enamel defects with or without syndromes and their underlying pathogenic genes. We searched the articles in PubMed with different strategies or keywords including but not limited to amelogenesis imperfecta, enamel defects, hypoplastic/hypomaturation/hypocalcified, syndrome, or specific syndrome name. The articles with detailed clinical information about the enamel and other phenotypes and clear genetic background were used for the analysis. We totally summarized and compared enamel phenotypes of 18 nonsyndromic AI with 17 causative genes and 19 syndromic AI with 26 causative genes. According to the clinical features, radiographic or ultrastructural changes in enamel, the enamel defects were basically divided into hypoplastic and hypomineralized (hypomaturated and hypocalcified) and presented a higher heterogeneity which were closely related to the involved pathogenic genes, types of mutation, hereditary pattern, X chromosome inactivation, incomplete penetrance, and other mechanisms.The gene-specific enamel phenotypes could be an important indicator for diagnosing nonsyndromic and syndromic AI.

Polymorphisms in genes expressed during amelogenesis and their association with dental caries: a case–control study

Objectives

Dental caries is a widespread multifactorial disease, caused by the demineralization of hard dental tissues. Susceptibility to dental caries is partially genetically conditioned; this study was aimed at finding an association of selected single nucleotide polymorphisms (SNPs) in genes encoding proteins involved in amelogenesis with this disease in children.

Materials and methods

In this case–control study, 15 SNPs in ALOX15, AMBN, AMELX, KLK4, TFIP11, and TUFT1 genes were analyzed in 150 children with primary dentition and 611 children with permanent teeth with/without dental caries from the European Longitudinal Study of Pregnancy and Childhood (ELSPAC) cohort.

Results

Dental caries in primary dentition was associated with SNPs in AMELX (rs17878486) and KLK4 (rs198968, rs2242670), and dental caries in permanent dentition with SNPs in AMELX (rs17878486) and KLK4 (rs2235091, rs2242670, rs2978642), (p ≤ 0.05). No significant differences between cases and controls were observed in the allele or genotype frequencies of any of the selected SNPs in ALOX15, AMBN, TFIP11, and TUFT1 genes (p > 0.05). Some KLK4 haplotypes were associated with dental caries in permanent dentition (p ≤ 0.05).

Conclusions

Based on this study, we found that although the SNPs in AMELX and KLK4 are localized in intronic regions and their functional significance has not yet been determined, they are associated with susceptibility to dental caries in children.

Clinical relevance

AMELX and KLK4 variants could be considered in the risk assessment of dental caries, especially in permanent dentition, in the European Caucasian population.

An Intron c.103-3T>C Variant of the AMELX Gene Causes Combined Hypomineralized and Hypoplastic Type of Amelogenesis Imperfecta: Case Series and Review of the Literature

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic disorders of dental enamel. X-linked AI results from disease-causing variants in the AMELX gene. In this paper, we characterise the genetic aetiology and enamel histology of female AI patients from two unrelated families with similar clinical and radiographic findings. All three probands were carefully selected from 40 patients with AI. In probands from both families, scanning electron microscopy confirmed hypoplastic and hypomineralised enamel. A neonatal line separated prenatally and postnatally formed enamel of distinctly different mineralisation qualities. In both families, whole exome analysis revealed the intron variant NM_182680.1: c.103-3T>C, located three nucleotides before exon 4 of the AMELX gene. In family I, an additional variant, c.2363G>A, was found in exon 5 of the FAM83H gene. This report illustrates a variant in the AMELX gene that was not previously reported to be causative for AI as well as an additional variant in the FAM83H gene with probably limited clinical significance.

Identification of a Novel FAM83H Mutation and Management of Hypocalcified Amelogenesis Imperfecta in Early Childhood

Amelogenesis imperfecta (AI) is a heterogeneous group of rare genetic disorders affecting amelogenesis during dental development. Therefore, the molecular genetic etiology of AI can provide information about the nature and progress of the disease. To confirm the genetic etiology of AI in a Korean family with an autosomal dominant inheritance, pedigree and mutational analyses were performed. DNA was isolated from the participating family members and whole-exome sequencing was performed with the DNA sample of the father of the proband. The identified mutation was confirmed by Sanger sequencing. The mutational analysis revealed a novel nonsense mutation in the FAM83H gene (NM_198488.5: c.1363C > T, p.(Gln455*)), confirming autosomal dominant hypocalcified AI. Full-mouth restorative treatments of the affected children were performed after the completion of the deciduous dentition. Early diagnosis of AI can be useful for understanding the nature of the disease and for managing the condition and treatment planning.

Spatiotemporal Changes in Transcriptome of Odontogenic and Non-odontogenic Regions in the Dental Arch of Mus musculus

Over the past 40 years, studies on tooth regeneration have been conducted. These studies comprised two main flows: some focused on epithelial-mesenchymal interaction in the odontogenic region, whereas others focused on creating a supernumerary tooth in the non-odontogenic region. Recently, the scope of the research has moved from conventional gene modification and molecular therapy to genome and transcriptome sequencing analyses. However, these sequencing data have been produced only in the odontogenic region. We provide RNA-Seq data of not only the odontogenic region but also the non-odontogenic region, which loses tooth-forming capacity during development and remains a rudiment. Sequencing data were collected from mouse embryos at three different stages of tooth development. These data will expand our understanding of tooth development and will help in designing developmental and regenerative studies from a new perspective.

Epitope mapping of anti-amelogenin IgG in untreated celiac children

Children with untreated celiac disease (CeD) may develop enamel defects, and children with severe CeD have significantly increased levels of IgG to amelogenin, which may interfere with normal amelogenesis depending on which epitope(s) they bind. Children with untreated CeD (n = 42), for whom CeD had been confirmed either by biopsy (n = 17, cohort 1) or by the presence of particularly high serum levels of anti-transglutaminase 2 (TG2) IgA (n = 25, cohort 2), were selected from 146 children with CeD, and 10 controls were selected from 34 children who did not have CeD. Samples from these 52 children were used for detailed IgG anti-amelogenin, X isoform (AMELX) epitope mapping using 31 overlapping, 10-22mer peptides in ELISA. Although sera from both groups showed reactivity to peptides containing sequences from the N and C terminus of AMELX, sera from children with CeD reacted more strongly to peptides from the central region (amino acids 75-150) containing both a binding site for transforming growth factor-β (TGF-β), as well as the enzymatic cleavage sites for matrix metalloproteinase-20 and for kallikrein-4. Antigen-specific extraction revealed that only IgG to the central region cross-reacted to gliadin. Thus, cross-reactive anti-gliadin/amelogenin IgG may affect normal amelogenesis by interfering with enzymatic degradation, proper folding, and/or TGF-β signaling in children with untreated CeD.

Dimorphism in dental tissues: Sex differences in archaeological individuals for multiple tooth types

OBJECTIVES

Dimorphism in the dentition has been observed in human populations worldwide. However, research has largely focused on traditional linear crown measurements. As imaging systems, such as micro-computed tomography (micro-CT), become increasingly more accessible, new dental measurements such as dental tissue size and proportions can be obtained. This research investigates the variation of dental tissues and proportions by sex in archaeological samples.

MATERIALS AND METHODS

Upper and lower first incisor to second premolar tooth rows were obtained from 30 individuals (n = 300), from 3 archaeological samples. The teeth were micro-CT scanned and surface area and volumetric measurements were obtained from the surface meshes extracted. Dental wear was also recorded and differences between sexes determined.

RESULTS

Enamel and crown measurements were found to be larger in females. Conversely, dentine and root measurements were larger in males.

DISCUSSION

The findings support the potential use of dental tissues to estimate sex of individuals from archaeological samples, while also indicating that individuals aged using current dental aging methods may be underaged or overaged due to sex differences in enamel thickness.

Roles of heterogeneous nuclear ribonucleoprotein L in enamel organ development and the differentiation of ameloblasts

OBJECTIVE

We aimed to explore the role of Heterogeneous Nuclear Ribonucleoprotein L(hnRNP L) in enamel organ development through hnRNP L conditional knockout mice and knockdown of hnRNP L expression in mouse ameloblast-lineage cells (mALCs) METHODS: We created K14cre-mediated hnRNP L conditional knockout mice (hnRNP L^K14/fl) and silenced the expression of hnRNP L in mALCs to investigate the role of hnRNP L in enamel organ development.

RESULTS

We found that hnRNP L^K14/fl mice presented enamel organ development defects with reduced number of inner enamel epithelium (IEE) cells. The proliferation and differentiation of the IEE cells/ameloblasts were suppressed. The cell proliferation and mineralization ability were also decreased after hnRNP L knockdown. Further studies showed that Bone Morphogenetic Protein (BMP) signaling pathway was attenuated after the knockdown of hnRNP L expression both in vivo and in vitro.

CONCLUSIONS

These findings suggest that hnRNP L plays a critical role in enamel organ development by promoting the IEE cell/ameloblast proliferation and differentiation. BMP signaling pathway may be involved in the process.

Tooth Enamel and its Dynamic Protein Matrix

Tooth enamel is the outer covering of tooth crowns, the hardest material in the mammalian body, yet fracture resistant. The extremely high content of 95 wt% calcium phosphate in healthy adult teeth is achieved through mineralization of a proteinaceous matrix that changes in abundance and composition. Enamel-specific proteins and proteases are known to be critical for proper enamel formation. Recent proteomics analyses revealed many other proteins with their roles in enamel formation yet to be unraveled. Although the exact protein composition of healthy tooth enamel is still unknown, it is apparent that compromised enamel deviates in amount and composition of its organic material. Why these differences affect both the mineralization process before tooth eruption and the properties of erupted teeth will become apparent as proteomics protocols are adjusted to the variability between species, tooth size, sample size and ephemeral organic content of forming teeth. This review summarizes the current knowledge and published proteomics data of healthy and diseased tooth enamel, including advancements in forensic applications and disease models in animals. A summary and discussion of the status quo highlights how recent proteomics findings advance our understating of the complexity and temporal changes of extracellular matrix composition during tooth enamel formation.

Computational approach towards identification of pathogenic missense mutations in AMELX gene and their possible association with amelogenesis imperfecta

Amelogenin gene (AMEL-X) encodes an enamel protein called amelogenin, which plays a vital role in tooth development. Any mutations in this gene or the associated pathway lead to developmental abnormalities of the tooth. The present study aims to analyze functional missense mutations in AMEL-X genes and derive an association with amelogenesis imperfecta. The information on missense mutations of human AMEL-X gene was collected from Ensembl database (https://asia.ensembl.org). Three different computational tools viz., SIFT, PolyPhen and PROVEAN were used to identify the deleterious or pathogenic forms of mutations in the gene studied. I-Mutant Suit was used to identify the stability of the proteins identified as deleterious by the three tools. Further, MutPred analysis revealed the pathogenicity of these mutations. Among 96 missense variants reported in AMEL-X gene, 18 were found to be deleterious using the three prediction tools (SIFT, PolyPhen and PROVEAN). When these variants were subjected to protein stability analysis, about 14 missense variants showed decreased stability whereas the other 8 variants showed increased stability. Further, these variants were analyzed using MutPred which identified 9 variants to be highly pathogenic. ExAC database revealed that all the pathogenic mutations had a minor allele frequency less than 0.01. The in silico analysis revealed highly pathogenic mutations in amelogenin gene which could have a putative association with amelogenesis imperfecta. These mutations should be screened in patients for early diagnosis of susceptibility to AI.

Sexual dimorphism of the enamel and dentine dimensions of the permanent canines of the Middle Pleistocene hominins from Sima de los Huesos (Burgos, Spain)

Sexual dimorphism is an important component of the total variation seen in populations and plays a key role in taxonomic debates. In this study, microtomographic (microcomputed tomography) techniques were applied to a sample of hominin teeth from the Sima de los Huesos site (Spain). Dental tissue proportions of the permanent canines were assessed to characterize the pattern and degree of sexual dimorphism within this population. In addition, the possible similarities and differences with the Homo neanderthalensis remains from Krapina (Croatia) and with a recent modern human sample were evaluated. A combination of classical statistical approaches with more novel techniques allowed us not only to ratify the sex allocation of the individuals previously assigned in the literature but also to estimate the sex of the youngest individuals, which were not assessed in previous studies. Likewise, the sexes of certain extensively worn canines and isolated pieces were estimated. As a result, the sex ratio observed in our dental sample from the Sima de los Huesos population is 5:9 (N_m:N_f). In general terms, both Sima de los Huesos and Krapina dental samples have a degree of sexual dimorphism in their permanent canine tissue proportions that does not surpass that of modern humans. The marked dimorphic root volume of Sima de los Huesos mandibular canines is the exception, which surpasses the modern human mean, although it falls within the 95% confidence interval. Therefore, our results do not support that dental tissue proportions of the European Middle Pleistocene populations were more dimorphic than in modern humans. However, the differences in canine tissue proportions are great enough to allow sex estimation with a high degree of confidence.

Alteration of Exon Definition Causes Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a collection of genetic disorders affecting the quality and/or quantity of tooth enamel. More than 20 genes are, so far, known to be responsible for this condition. In this study, we recruited 3 Turkish families with hypomaturation AI. Whole-exome sequence analyses identified disease-causing mutations in each proband, and these mutations cosegregated with the AI phenotype in all recruited members of each family. The AI-causing mutations in family 1 were a novel AMELX mutation [NM_182680.1:c.143T>C, p.(Leu48Ser)] in the proband and a novel homozygous MMP20 mutation [NM_004771.3:c.616G>A, p.(Asp206Asn)] in the mother of the proband. Previously reported compound heterozygous MMP20 mutations [NM_004771.3:c.103A>C, p.(Arg35=) and c.389C>T, p.(Thr130Ile)] caused the AI in family 2 and family 3. Minigene splicing analyses revealed that the AMELX missense mutation increased exonic definition of exon 4 and the MMP20 synonymous mutation decreased exonic definition of exon 1. These mutations would trigger an alteration of exon usage during RNA splicing, causing the enamel malformations. These results broaden our understanding of molecular genetic pathology of tooth enamel formation.

A Novel AMELX Mutation, Its Phenotypic Features, and Skewed X Inactivation

Amelogenesis imperfecta (AI) is a group of genetic disorders of defective dental enamel. Mutation of AMELX encoding amelogenin on the X chromosome is a major cause of AI. Here we report a Chinese family with hypoplastic and hypomineralized AI. Whole exome analysis revealed a novel mutation c.185delC in exon 5 of AMELX causing the frame shift p.Pro62ArgfsTer47 (or p.Pro62Argfs*47). By sequencing of polymerase chain reaction products and T-vector clones, the mutation was confirmed as homozygous in the proband, hemizygous in her father, and heterozygous in her mother. The proband and her father had small and yellowish teeth with thin and rough enamel that was radiographically indistinguishable from the underlying dentin. Scanning electronic microscopy of 1 maternal tooth showed cracks and exposed loosely packed enamel prisms in affected areas. Consistent with a 25:75 skewing of X inactivation in the peripheral blood DNA as measured by androgen receptor allele methylation, the surface of the mother’s tooth had alternating vertical ridges of transparent normal and white chalky enamel in a 34:66 ratio. In summary, this study provides one of the few phenotypic comparisons of hemizygous and homozygous AMELX mutations and suggests that the skewing of X inactivation in AI contributes to the phenotypic variations in heterozygous carriers of X-linked AI.

Characterization of inter-crystallite peptides in human enamel rods reveals contribution by the Y allele of amelogenin

Proteins of the inter-rod sheath and peptides within the narrow inter-crystallite space of the rod structure are considered largely responsible for visco-elastic and visco-plastic properties of enamel. The present study was designed to investigate putative peptides of the inter-crystallite space. Entities of 1-6 kDa extracted from enamel rods of erupted permanent teeth were analysed by mass spectrometry (MS) and shown to comprise N-terminal amelogenin (AMEL) peptides either containing or not containing exon 4 product. Other dominant entities consisted of an N-terminal peptide from ameloblastin (AMBN) and a series of the most hydrophobic peptides from serum albumin (ALBN). Amelogenin peptides encoded by the Y-chromosome allele were strongly detected in Enamel from male teeth. Location of N-terminal AMEL peptides as well as AMBN and ALBN, between apatite crystallites, was disclosed by immunogold scanning electron microscopy (SEM). Density plots confirmed the relative abundance of these products including exon 4+ AMEL peptides that have greater capacity for binding to hydroxyapatite. Hydrophilic X and Y peptides encoded in exon 4 differ only in substitution of non-polar isoleucine in Y for polar threonine in X with reduced disruption of the hydrophobic N-terminal structure in the Y form. Despite similarity of X and Y alleles of AMEL the non-coding region upstream from exon 4 shows significant variation with implications for segregation of processing of transcripts from exon 4. Detection of fragments from multiple additional proteins including keratins (KER), fetuin A (FETUA), proteinases and proteinase inhibitors, likely reflect biochemical events during enamel formation.

Dose-Dependent Rescue of KO Amelogenin Enamel by Transgenes in Vivo

Mice lacking amelogenin (KO) have hypoplastic enamel. Overexpression of the most abundant amelogenin splice variant M180 and LRAP transgenes can substantially improve KO enamel, but only ~40% of the incisor thickness is recovered and the prisms are not as tightly woven as in WT enamel. This implies that the compositional complexity of the enamel matrix is required for different aspects of enamel formation, such as organizational structure and thickness. The question arises, therefore, how important the ratio of different matrix components, and in particular amelogenin splice products, is in enamel formation. Can optimal expression levels of amelogenin transgenes representing both the most abundant splice variants and cleavage product at protein levels similar to that of WT improve the enamel phenotype of KO mice? Addressing this question, our objective was here to understand dosage effects of amelogenin transgenes (Tg) representing the major splice variants M180 and LRAP and cleavage product CTRNC on enamel properties. Amelogenin KO mice were mated with M180Tg, CTRNCTg and LRAPTg mice to generate M180Tg and CTRNCTg double transgene and M180Tg, CTRNCTg, LRAPTg triple transgene mice with transgene hemizygosity (on one allelle) or homozygosity (on both alleles). Transgene homo- vs. hemizygosity was determined by qPCR and relative transgene expression confirmed by Western blot. Enamel volume and mineral density were analyzed by microCT, thickness and structure by SEM, and mechanical properties by Vickers microhardness testing. There were no differences in incisor enamel thickness between amelogenin KO mice with three or two different transgenes, but mice homozygous for a given transgene had significantly thinner enamel than mice hemizygous for the transgene (p < 0.05). The presence of the LRAPTg did not improve the phenotype of M180Tg/CTRNCTg/KO enamel. In the absence of endogenous amelogenin, the addition of amelogenin transgenes representing the most abundant splice variants and cleavage product can rescue abnormal enamel properties and structure, but only up to a maximum of ~80% that of molar and ~40% that of incisor wild-type enamel.

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.

Amelogenesis Imperfecta; Genes, Proteins, and Pathways

Amelogenesis imperfecta (AI) is the name given to a heterogeneous group of conditions characterized by inherited developmental enamel defects. AI enamel is abnormally thin, soft, fragile, pitted and/or badly discolored, with poor function and aesthetics, causing patients problems such as early tooth loss, severe embarrassment, eating difficulties, and pain. It was first described separately from diseases of dentine nearly 80 years ago, but the underlying genetic and mechanistic basis of the condition is only now coming to light. Mutations in the gene AMELX, encoding an extracellular matrix protein secreted by ameloblasts during enamel formation, were first identified as a cause of AI in 1991. Since then, mutations in at least eighteen genes have been shown to cause AI presenting in isolation of other health problems, with many more implicated in syndromic AI. Some of the encoded proteins have well documented roles in amelogenesis, acting as enamel matrix proteins or the proteases that degrade them, cell adhesion molecules or regulators of calcium homeostasis. However, for others, function is less clear and further research is needed to understand the pathways and processes essential for the development of healthy enamel. Here, we review the genes and mutations underlying AI presenting in isolation of other health problems, the proteins they encode and knowledge of their roles in amelogenesis, combining evidence from human phenotypes, inheritance patterns, mouse models, and in vitro studies. An LOVD resource (http://dna2.leeds.ac.uk/LOVD/) containing all published gene mutations for AI presenting in isolation of other health problems is described. We use this resource to identify trends in the genes and mutations reported to cause AI in the 270 families for which molecular diagnoses have been reported by 23rd May 2017. Finally we discuss the potential value of the translation of AI genetics to clinical care with improved patient pathways and speculate on the possibility of novel treatments and prevention strategies for AI.

Recessive Mutations in ACPT, Encoding Testicular Acid Phosphatase, Cause Hypoplastic Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic disorders affecting tooth enamel. The affected enamel can be hypoplastic and/or hypomineralized. In this study, we identified ACPT (testicular acid phosphatase) biallelic mutations causing non-syndromic, generalized hypoplastic autosomal-recessive amelogenesis imperfecta (AI) in individuals from six apparently unrelated Turkish families. Families 1, 4, and 5 were affected by the homozygous ACPT mutation c.713C>T (p.Ser238Leu), family 2 by the homozygous ACPT mutation c.331C>T (p.Arg111Cys), family 3 by the homozygous ACPT mutation c.226C>T (p.Arg76Cys), and family 6 by the compound heterozygous ACPT mutations c.382G>C (p.Ala128Pro) and 397G>A (p.Glu133Lys). Analysis of the ACPT crystal structure suggests that these mutations damaged the activity of ACPT by altering the sizes and charges of key amino acid side chains, limiting accessibility of the catalytic core, and interfering with homodimerization. Immunohistochemical analysis confirmed localization of ACPT in secretory-stage ameloblasts. The study results provide evidence for the crucial function of ACPT during amelogenesis.

Enamel ribbons, surface nodules, and octacalcium phosphate in C57BL/6 Amelx-/- mice and Amelx+/- lyonization

BACKGROUND

Amelogenin is required for normal enamel formation and is the most abundant protein in developing enamel.

METHODS

Amelx^+/+, Amelx^+/- , and Amelx^-/- molars and incisors from C57BL/6 mice were characterized using RT-PCR, Western blotting, dissecting and light microscopy, immunohistochemistry (IHC), transmission electron microscopy (TEM), scanning electron microscopy (SEM), backscattered SEM (bSEM), nanohardness testing, and X-ray diffraction.

RESULTS

No amelogenin protein was detected by Western blot analyses of enamel extracts from Amelx^-/- mice. Amelx^-/- incisor enamel averaged 20.3 ± 3.3 μm in thickness, or only 1/6th that of the wild type (122.3 ± 7.9 μm). Amelx^-/- incisor enamel nanohardness was 1.6 Gpa, less than half that of wild-type enamel (3.6 Gpa). Amelx^+/- incisors and molars showed vertical banding patterns unique to each tooth. IHC detected no amelogenin in Amelx^-/- enamel and varied levels of amelogenin in Amelx^+/- incisors, which correlated positively with enamel thickness, strongly supporting lyonization as the cause of the variations in enamel thickness. TEM analyses showed characteristic mineral ribbons in Amelx^+/+ and Amelx^-/- enamel extending from mineralized dentin collagen to the ameloblast. The Amelx^-/- enamel ribbons were not well separated by matrix and appeared to fuse together, forming plates. X-ray diffraction determined that the predominant mineral in Amelx^-/- enamel is octacalcium phosphate (not calcium hydroxyapatite). Amelx^-/- ameloblasts were similar to wild-type ameloblasts except no Tomes' processes extended into the thin enamel. Amelx^-/- and Amelx^+/- molars both showed calcified nodules on their occlusal surfaces. Histology of D5 and D11 developing molars showed nodules forming during the maturation stage.

CONCLUSION

Amelogenin forms a resorbable matrix that separates and supports, but does not shape early secretory-stage enamel ribbons. Amelogenin may facilitate the conversion of enamel ribbons into hydroxyapatite by inhibiting the formation of octacalcium phosphate. Amelogenin is necessary for thickening the enamel layer, which helps maintain ribbon organization and development and maintenance of the Tomes' process.

Amelogenin Exon4 Forms a Novel miRNA That Directs Ameloblast and Osteoblast Differentiation

Amelogenins constitute the major portion of secretory enamel matrix proteins and are known to be highly alternative spliced. Of all the alternatively spliced forms of amelogenins, exon4 is most commonly spliced out. Our analyses of the exon4 sequence led us to hypothesize that when spliced out, exon4 may generate a novel mature miRNA. To explore this possibility, we used in vivo mouse models (wild-type and Amel knockout mice) and in vitro cell culture to investigate the presence and function of a mature miRNA derived from exon4 (miR-exon4). When ameloblast-like cells (LS8) were transfected with an amelogenin minigene to increase amelogenin synthesis, the transfected cells synthesized miR-exon4. Introduction of a mutation in the conserved CNNC sequence required for primary miRNA recognition, downstream of the mature miR-exon4 sequence, resulted in a significantly reduced production of miR-exon4 in the transfected cells. In vivo, miR-exon4 was most highly amplified from wild-type mouse enamel organs at the secretory stage. In Amel knockout mice, an in vivo model for reduced amelogenin synthesis, we found reduced miR-exon4, with no changes in expression of enamel matrix-related genes. However, expression of Runx2 and its downstream genes Odam and Amtn were significantly downregulated. Transfection of miR-exon4 mimic to the LS8 cells also significantly upregulated Runx2. The mature miR-exon4 as well as Runx2 was also present in mouse osteoblasts with no apparent change in expression level between wild-type and Amel knockout mice. However, transfecting miR-exon4 inhibitor to the MC3T3-E1 osteoblastic cells resulted in a significant downregulation of Runx2 expression. These data indicate that when exon4 is spliced out, as occurs most of the time during alternative splicing of amelogenin pre-mRNA, a novel mature miRNA is generated from exon4. This miR-exon4 may contribute to the differentiation of ameloblasts and osteoblasts through regulation of Runx2 expression.

Exon4 amelogenin transcripts in enamel biomineralization

Amelogenins are proteins formed by alternative splicing of the amelogenin gene, and are essential for tooth enamel formation. However, the unique functions of various alternatively spliced amelogenins in enamel formation are not well understood. In this study, we determined the spatiotemporal location of amelogenins derived from transcripts containing exon4 (AMG+4) in the enamel matrix, and the relative binding of recombinant AMG+4 to hydroxyapatite (HAP). Immunohistochemistry and mass spectrometry analyses showed that AMG+4 proteins were secreted into the enamel matrix at the early maturation stage. A stage-specific increase in the synthesis of AMG+4 was further supported by our observation that in mice overexpressing leucine-rich amelogenin peptide (TgLRAP), in which ameloblasts differentiate earlier, AMG+4 transcripts were also upregulated earlier. In vitro binding studies, supported by in silico modeling of protein binding to calcium and phosphate, showed that more recombinant AMG+4 bound to hydroxyapatite (HAP) as compared with recombinant AMG-4. The temporal and spatial localization of amelogenins containing exon4 peptide, and their functional differences in HAP binding, suggests that the unique properties of amelogenins containing exon4 cause a specific enhancement of biomineralization related to stabilization of early-formed HAP at the maturation stage.

Novel ITGB6 mutation in autosomal recessive amelogenesis imperfecta

Objective

Hereditary defects in tooth enamel formation, amelogenesis imperfecta (AI), can be non-syndromic or syndromic phenotype. Integrins are signaling proteins that mediate cell–cell and cell–extracellular matrix communication, and their involvement in tooth development is well known. The purposes of this study were to identify genetic cause of an AI family and molecular pathogenesis underlying defective enamel formation.

Materials and Methods

We recruited a Turkish family with isolated AI and performed mutational analyses to clarify the underlying molecular genetic etiology.

Results

Autozygosity mapping and exome sequencing identified a novel homozygous ITGB6 transversion mutation in exon 4 (c.517G>C, p.Gly173Arg). The glycine at this position in the middle of the βI-domain is conserved among a wide range of vertebrate orthologs and human paralogs. Clinically, the enamel was generally thin and pitted with pigmentation. Thicker enamel was noted at the cervical area of the molars.

Conclusions

In this study, we identified a novel homozygous ITGB6 mutation causing isolated AI, and this advances the understanding of normal and pathologic enamel development.