Amelogenesis Imperfecta Phenotype Research Articles

A novel mutation in GPR68 causes hypomaturation amelogenesis imperfecta

ObjectivesTo identify the genetic cause of a Chinese family with hypomaturation amelogenesis imperfecta (AI) and to characterize the structure of GPR68 mutated enamel in order to develop a deeper understanding of the role of the GPR68 protein during the intricate process of amelogenesis. DesignOne Chinese family with generalized hypomaturation AI was recruited. Two of the third molars from the proband were subjected to scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Whole exome sequencing (WES) was performed, and the identified mutation was confirmed by Sanger sequencing. Bioinformatics studies were further conducted to analyze the potential deleterious effects of the mutation. ResultsThe proband presented with a hypomaturation AI phenotype, characterized by fragile and discolored enamel surface. The AI enamel showed prismatic structure, which was sporadically obscured by areas of amorphous material and porous structure. EDX analysis showed the proband’s enamel demonstrated a significant decrease in calcium and phosphorus content and a significant increase in oxygen compared with normal enamel. A novel homozygous mutation of G protein-coupled receptor 68 (GPR68) (c .149 T > A, p.Ile50Asn) was identified in the proband. Bioinformatics analysis indicated that the mutation site displayed a high level of evolutionary conservation among species, and the mutation might impact the stability and conformation of the protein. ConclusionThe novel homozygous GPR68 mutation resulted in hypomaturation AI. We first described the effect of GPR68 mutation on enamel structure. Our results provide new genetic evidence that mutations involved in GPR68 contribute to hypomaturation AI.

Heterozygous COL17A1 variants are a frequent cause of amelogenesis imperfecta

BackgroundCollagen XVII is most typically associated with human disease when biallelic COL17A1 variants (>230) cause junctional epidermolysis bullosa (JEB), a rare, genetically heterogeneous, mucocutaneous blistering disease with amelogenesis imperfecta (AI),...

Tooth ultrastructure changes induced by a nonsense mutation in the FAM83H gene: insights into the diversity of amelogenesis imperfecta.

The current research on single-nucleotide polymorphism (SNP) mutation sites at different positions of the FAM83H gene and their phenotypic changes leading to amelogenesis imperfecta (AI) is inconsistent. We identified a previously reported heterozygous nonsense mutation c.1192C>T (p.Q398*) in the FAM83H gene and conducted a comprehensive analysis of the dental ultrastructure and chemical composition changes induced by this mutation. Additionally, we predicted the protein feature affected by this mutation site. The aim was to further deepen our understanding of the diversity of AI caused by different mutation sites in the FAM83H gene. Whole-exome sequencing (WES) and Sanger sequencing were used to confirm the mutation sites. Physical features of the patient's teeth were investigated using various methods including cone beam computer tomography (CBCT), scanning electron microscopy (SEM), contact profilometry (roughness measurement), and a nanomechanical tester (nanoindentation measurement). The protein features of wild-type and mutant FAM83H were predicted using bioinformatics methods. One previously discovered FAM83H heterozygous nonsense mutation c.1192C>T (p.Q398*) was detected in the patient. SEM revealed inconsistent dentinal tubules, and EDS showed that calcium and phosphorus were lower in the patient's dentin but higher in the enamel compared to the control tooth. Roughness measurements showed that AI patients' teeth had rougher occlusal surfaces than those of the control tooth. Nanoindentation measurements showed that the enamel and dentin hardness values of the AI patients' teeth were both significantly reduced compared to those of the control tooth. Compared to the wild-type FAM83H protein, the mutant FAM83H protein shows alterations in stability, hydrophobicity, secondary structure, and tertiary structure. These changes could underlie functional differences and AI phenotype variations caused by this mutation site. This study expands the understanding of the effects of FAM83H mutations on tooth structure. Our study enhances our understanding of the genetic basis of AI and may contribute to improved diagnostics and personalized treatment strategies for patients with FAM83H-related AI.

Association between malocclusions and amelogenesis imperfecta genotype and phenotype: A systematic review

The aim of this systematic review (Prospero CRD42022323188) is to investigate whether an association exists in patients with amelogenesis imperfecta (AI) between occlusal characteristics and genotype on the one hand and enamel structural phenotype on the other. Reports up to May 2023 assessing occlusion of individuals with AI were browsed in a systematic search using Medline, Embase, ISI Web of Science, and the grey literature. Randomised control trials, case control studies, and case series specifying both occlusion, assessed by cephalometric or clinical analysis, and genotype or dental phenotype in patients with AI were included without any age limitation. Two authors independently selected the publications and extracted the data in accordance with the PRISMA statement. The risk of bias was assessed with the Critical Appraisal Checklists from the Johanna Briggs Institute. Twenty-five articles were chosen from the 261 results. Most of the included publications were case series (n=22) and case control studies (n=3). Thirteen studies reported both a genotype (ENAM, FAM83H, FAM20A, DLX3, CNMM4, WDR72) and occlusal diagnostic. The methodological quality of the studies was moderate. All AI phenotypes showed an open bite (OB) rate around 35%, except mixed form. The other malocclusions were not often mentioned. No correlation between occlusal phenotype and genotype or AI phenotype could be identified in patients with AI, as most studies had short occlusal descriptions and small sample sizes. OB malocclusions were more frequently reported in AI. This review highlighted the need for a more accurate description of orofacial features associated with AI, to better clarify the role of amelogenesis genes in the regulation of craniofacial morphogenesis and identify patients requiring orthognathic surgery at an early stage.

Amelogenesis imperfecta: Next-generation sequencing sheds light on Witkop's classification.

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic rare diseases disrupting enamel development (Smith et al., Front Physiol, 2017a, 8, 333). The clinical enamel phenotypes can be described as hypoplastic, hypomineralized or hypomature and serve as a basis, together with the mode of inheritance, to Witkop's classification (Witkop, J Oral Pathol, 1988, 17, 547-553). AI can be described in isolation or associated with others symptoms in syndromes. Its occurrence was estimated to range from 1/700 to 1/14,000. More than 70 genes have currently been identified as causative. Objectives: We analyzed using next-generation sequencing (NGS) a heterogeneous cohort of AI patients in order to determine the molecular etiology of AI and to improve diagnosis and disease management. Methods: Individuals presenting with so called "isolated" or syndromic AI were enrolled and examined at the Reference Centre for Rare Oral and Dental Diseases (O-Rares) using D4/phenodent protocol (www.phenodent.org). Families gave written informed consents for both phenotyping and molecular analysis and diagnosis using a dedicated NGS panel named GenoDENT. This panel explores currently simultaneously 567 genes. The study is registered under NCT01746121 and NCT02397824 (https://clinicaltrials.gov/). Results: GenoDENT obtained a 60% diagnostic rate. We reported genetics results for 221 persons divided between 115 AI index cases and their 106 associated relatives from a total of 111 families. From this index cohort, 73% were diagnosed with non-syndromic amelogenesis imperfecta and 27% with syndromic amelogenesis imperfecta. Each individual was classified according to the AI phenotype. Type I hypoplastic AI represented 61 individuals (53%), Type II hypomature AI affected 31 individuals (27%), Type III hypomineralized AI was diagnosed in 18 individuals (16%) and Type IV hypoplastic-hypomature AI with taurodontism concerned 5 individuals (4%). We validated the genetic diagnosis, with class 4 (likely pathogenic) or class 5 (pathogenic) variants, for 81% of the cohort, and identified candidate variants (variant of uncertain significance or VUS) for 19% of index cases. Among the 151 sequenced variants, 47 are newly reported and classified as class 4 or 5. The most frequently discovered genotypes were associated with MMP20 and FAM83H for isolated AI. FAM20A and LTBP3 genes were the most frequent genes identified for syndromic AI. Patients negative to the panel were resolved with exome sequencing elucidating for example the gene involved ie ACP4 or digenic inheritance. Conclusion: NGS GenoDENT panel is a validated and cost-efficient technique offering new perspectives to understand underlying molecular mechanisms of AI. Discovering variants in genes involved in syndromic AI (CNNM4, WDR72, FAM20A … ) transformed patient overall care. Unravelling the genetic basis of AI sheds light on Witkop's AI classification.

Novel ITGB6 variants cause hypoplastic-hypomineralized amelogenesis imperfecta and taurodontism: characterization of tooth phenotype and review of literature

ObjectivesTo characterize phenotype and genotype of amelogenesis imperfecta (AI) in a Thai patient, and review of literature.Materials and methodsVariants were identified using trio-exome and Sanger sequencing. The ITGB6 protein level in patient’s gingival cells was measured. The patient’s deciduous first molar was investigated for surface roughness, mineral density, microhardness, mineral composition, and ultrastructure.ResultsThe patient exhibited hypoplastic-hypomineralized AI, taurodontism, and periodontal inflammation. Exome sequencing identified the novel compound heterozygous ITGB6 mutation, a nonsense c.625 G > T, p.(Gly209*) inherited from mother and a splicing c.1661-3 C > G from father, indicating AI type IH. The ITGB6 level in patient cells was significantly reduced, compared with controls. Analyses of a patient’s tooth showed a significant increase in roughness while mineral density of enamel and microhardness of enamel and dentin were significantly reduced. In dentin, carbon was significantly decreased while calcium, phosphorus, and oxygen levels were significantly increased. Severely collapsed enamel rods and a gap in dentinoenamel junction were observed. Of six affected families and eight ITGB6 variants that have been reported, our patient was the only one with taurodontism.ConclusionWe report the hypoplasia/hypomineralization/taurodontism AI patient with disturbed tooth characteristics associated with the novel ITGB6 variants and reduced ITGB6 expression, expanding genotype, phenotype, and understanding of autosomal recessive AI.

Deciphering the functions of Stromal Interaction Molecule-1 in amelogenesis using AmelX-iCre mice.

Introduction: The intracellular Ca2+ sensor stromal interaction molecule 1 (STIM1) is thought to play a critical role in enamel development, as its mutations cause Amelogenesis Imperfecta (AI). We recently established an ameloblast-specific (AmelX-iCre) Stim1 conditional deletion mouse model to investigate the role of STIM1 in controlling ameloblast function and differentiation in vivo (Stim1 cKO). Our pilot data (Said et al., J. Dent. Res., 2019, 98, 1002-1010) support our hypothesis for a broad role of Stim1 in amelogenesis. This paper aims to provide an in-depth characterization of the enamel phenotype observed in our Stim1 cKO model. Methods: We crossed AmelX-iCre mice with Stim1-floxed animals to develop ameloblast-specific Stim1 cKO mice. Scanning electron microscopy, energy dispersive spectroscopy, and micro- CT were used to study the enamel phenotype. RNAseq and RT-qPCR were utilized to evaluate changes in the gene expression of several key ameloblast genes. Immunohistochemistry was used to detect the amelogenin, matrix metalloprotease 20 and kallikrein 4 proteins in ameloblasts. Results: Stim1 cKO animals exhibited a hypomineralized AI phenotype, with reduced enamel volume, diminished mineral density, and lower calcium content. The mutant enamel phenotype was more severe in older Stim1 cKO mice compared to younger ones and changes in enamel volume and mineral content were more pronounced in incisors compared to molars. Exploratory RNAseq analysis of incisors' ameloblasts suggested that ablation of Stim1 altered the expression levels of several genes encoding enamel matrix proteins which were confirmed by subsequent RT-qPCR. On the other hand, RT-qPCR analysis of molars' ameloblasts showed non-significant differences in the expression levels of enamel matrix genes between control and Stim1-deficient cells. Moreover, gene expression analysis of incisors' and molars' ameloblasts showed that Stim1 ablation caused changes in the expression levels of several genes associated with calcium transport and mitochondrial kinetics. Conclusions: Collectively, these findings suggest that the loss of Stim1 in ameloblasts may impact enamel mineralization and ameloblast gene expression.

Novel KLK4 Mutations Cause Hypomaturation Amelogenesis Imperfecta.

Amelogenesis imperfecta (AI) is a group of rare genetic diseases affecting the tooth enamel. AI is characterized by an inadequate quantity and/or quality of tooth enamel and can be divided into three major categories: hypoplastic, hypocalcified and hypomaturation types. Even though there are some overlapping phenotypes, hypomaturation AI enamel typically has a yellow to brown discoloration with a dull appearance but a normal thickness indicating a less mineralized enamel matrix. In this study, we recruited four Turkish families with hypomaturation AI and performed mutational analysis using whole exome sequencing. These analyses revealed two novel homozygous mutations in the KLK4 gene: a nonsense mutation in exon 3 (NM_004917.4:c.170C>A, p.(Ser57*)) was found in families 1, 2 and 3 and a missense mutation in exon 6 (c.637T>C, p.(Cys213Arg)) in family 4. Functional analysis showed that the missense mutation transcript could not translate the mutant protein efficiently or generated an unstable protein that lacked functional activity. The two novel inactivating KLK4 mutations we identified caused a hypomaturation AI phenotype similar to those caused by the four previously described KLK4 nonsense and frameshift mutations. This study improves our understanding of the normal and pathologic mechanisms of enamel formation.

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.

A novel ENAM mutation causes hypoplastic amelogenesis imperfecta

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

Clinical management of Hypoplasic Amelogenesis Imperfecta: a challenge for multidisciplinary team. A case report

Amelogenesis imperfecta (AI) refers to a group of rare genetic disorders that involve tooth development and are passed down through families. Hypoplasic AI phenotypes include the absence of enamel as a result of a defect in the secretory stage. This case report describes the diagnosis and treatment of a patient with hypoplastic AI. The clinical implications include sensitive teeth, functional problems, and aesthetic complaining. The diagnosis was done through history, clinical examination and imaging. The intervention was performed by Direct Resin Veneers. This treatment showed to improve occlusion, esthetics, and selfimage of the teenager. The satisfactory clinical result has made it possible to avoid more invasive and expensive treatments.

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.

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.MethodsWhole 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.ConclusionNovel 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.

Phenotype and Variant Spectrum in the LAMB3 Form of Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a heterogeneous group of inherited disorders characterized by abnormal formation of dental enamel, either in isolation or as part of a syndrome. Heterozygous variants in laminin subunit beta 3 (LAMB3) cause AI with dominant inheritance in the absence of other cosegregating clinical features. In contrast, biallelic loss-of-function variants in LAMB3 cause recessive junctional epidermolysis bullosa, characterized by life-threatening skin fragility. We identified 2 families segregating autosomal dominant AI with variable degrees of a distinctive hypoplastic phenotype due to pathogenic variants in LAMB3. Whole exome sequencing revealed a nonsense variant (c.3340G>T, p.E1114*) within the final exon in family 1, while Sanger sequencing in family 2 revealed a variant (c.3383-1G>A) in the canonical splice acceptor site of the final exon. Analysis of cDNA from family 2 revealed retention of the final intron leading to a premature termination codon. Two unerupted third molar teeth from individual IV:5 in family 2 were subject to computerized tomography and scanning electron microscopy. LAMB3 molar teeth have a multitude of cusps versus matched controls. LAMB3 enamel was well mineralized but pitted. The architecture of the initially secreted enamel was abnormal, with cervical enamel appearing much less severely affected than coronal enamel. This study further defines the variations in phenotype-genotype correlation for AI due to variants in LAMB3, underlines the clustering of nonsense and frameshift variants causing AI in the absence of junctional epidermolysis bullosa, and highlights the shared AI phenotype arising from variants in genes coding for hemidesmosome proteins.

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.

Evolutionary Analysis Predicts Sensitive Positions of MMP20 and Validates Newly- and Previously-Identified MMP20 Mutations Causing Amelogenesis Imperfecta.

Amelogenesis imperfecta (AI) designates a group of genetic diseases characterized by a large range of enamel disorders causing important social and health problems. These defects can result from mutations in enamel matrix proteins or protease encoding genes. A range of mutations in the enamel cleavage enzyme matrix metalloproteinase-20 gene (MMP20) produce enamel defects of varying severity. To address how various alterations produce a range of AI phenotypes, we performed a targeted analysis to find MMP20 mutations in French patients diagnosed with non-syndromic AI. Genomic DNA was isolated from saliva and MMP20 exons and exon-intron boundaries sequenced. We identified several homozygous or heterozygous mutations, putatively involved in the AI phenotypes. To validate missense mutations and predict sensitive positions in the MMP20 sequence, we evolutionarily compared 75 sequences extracted from the public databases using the Datamonkey webserver. These sequences were representative of mammalian lineages, covering more than 150 million years of evolution. This analysis allowed us to find 324 sensitive positions (out of the 483 MMP20 residues), pinpoint functionally important domains, and build an evolutionary chart of important conserved MMP20 regions. This is an efficient tool to identify new- and previously-identified mutations. We thus identified six functional MMP20 mutations in unrelated families, finding two novel mutated sites. The genotypes and phenotypes of these six mutations are described and compared. To date, 13 MMP20 mutations causing AI have been reported, making these genotypes and associated hypomature enamel phenotypes the most frequent in AI.

Enamel and dental anomalies in latent-transforming growth factor beta-binding protein 3 mutant mice.

Latent‐transforming growth factor beta‐binding protein 3 (LTBP‐3) is important for craniofacial morphogenesis and hard tissue mineralization, as it is essential for activation of transforming growth factor‐β (TGF‐β). To investigate the role of LTBP‐3 in tooth formation we performed micro‐computed tomography (micro‐CT), histology, and scanning electron microscopy analyses of adult Ltbp3‐/‐ mice. The Ltbp3‐/‐ mutants presented with unique craniofacial malformations and reductions in enamel formation that began at the matrix formation stage. Organization of maturation‐stage ameloblasts was severely disrupted. The lateral side of the incisor was affected most. Reduced enamel mineralization, modification of the enamel prism pattern, and enamel nodules were observed throughout the incisors, as revealed by scanning electron microscopy. Molar roots had internal irregular bulbous‐like formations. The cementum thickness was reduced, and microscopic dentinal tubules showed minor nanostructural changes. Thus, LTBP‐3 is required for ameloblast differentiation and for the formation of decussating enamel prisms, to prevent enamel nodule formation, and for proper root morphogenesis. Also, and consistent with the role of TGF‐β signaling during mineralization, almost all craniofacial bone components were affected in Ltbp3‐/‐ mice, especially those involving the upper jaw and snout. This mouse model demonstrates phenotypic overlap with Verloes Bourguignon syndrome, also caused by mutation of LTBP3, which is hallmarked by craniofacial anomalies and amelogenesis imperfecta phenotypes.

Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a clinically and genetically heterogeneous group of diseases characterized by enamel defects. The authors have identified a large consanguineous Moroccan family segregating different clinical subtypes of hypoplastic and hypomineralized AI in different individuals within the family. Using targeted next-generation sequencing, the authors identified a novel heterozygous nonsense mutation in COL17A1 (c.1873C>T, p.R625*) segregating with hypoplastic AI and a novel homozygous 8-bp deletion in C4orf26 (c.39_46del, p.Cys14Glyfs*18) segregating with hypomineralized-hypoplastic AI in this family. This study highlights the phenotypic and genotypic heterogeneity of AI that can exist even within a single consanguineous family. Furthermore, the identification of novel mutations in COL17A1 and C4orf26 and their correlation with distinct AI phenotypes can contribute to a better understanding of the pathophysiology of AI and the contribution of these genes to amelogenesis.

Bmp2 deletion causes an amelogenesis imperfecta phenotype via regulating enamel gene expression.

Although Bmp2 is essential for tooth formation, the role of Bmp2 during enamel formation remains unknown in vivo. In this study, the role of Bmp2 in regulation of enamel formation was investigated by the Bmp2 conditional knock out (Bmp2 cKO) mice. Teeth of Bmp2 cKO mice displayed severe and profound phenotypes with asymmetric and misshaped incisors as well as abrasion of incisors and molars. Scanning electron microscopy analysis showed that the enamel layer was hypoplastic and enamel lacked a typical prismatic pattern. Teeth from null mice were much more brittle as tested by shear and compressive moduli. Expression of enamel matrix protein genes, amelogenin, enamelin, and enamel-processing proteases, Mmp-20 and Klk4 was reduced in the Bmp2 cKO teeth as reflected in a reduced enamel formation. Exogenous Bmp2 up-regulated those gene expressions in mouse enamel organ epithelial cells. This result for the first time indicates Bmp2 signaling is essential for proper enamel development and mineralization in vivo.

Novel FAM20A mutation causes autosomal recessive amelogenesis imperfecta

ObjectiveTo relate the peculiar phenotype of amelogenesis imperfecta in a large Bedouin family to the genotype determined by whole genome linkage analysis. DesignAmelogenesis imperfecta (AI) is a broad group of inherited pathologies affecting enamel formation, characterized by variability in phenotypes, causing mutations and modes of inheritance. Autosomal recessive or compound heterozygous mutations in FAM20A, encoding sequence similarity 20, member A, have been shown to cause several AI phenotypes. Five members from a large consanguineous Bedouin family presented with hypoplastic amelogenesis imperfecta with unerupted and resorbed permanent molars. Following Soroka Medical Center IRB approval and informed consent, blood samples were obtained from six affected offspring, five obligatory carriers and two unaffected siblings. Whole genome linkage analysis was performed followed by Sanger sequencing of FAM20A. ResultsThe sequencing unravelled a novel homozygous deletion mutation in exon 11 (c.1523delC), predicted to insert a premature stop codon (p.Thr508Lysfs*6). ConclusionsWe provide an interesting case of novel mutation in this rare disorder, in which the affected kindred is unique in the large number of family members sharing a similar phenotype.