Novel FAM20A mutations in hypoplastic amelogenesis imperfecta

Gingival proteomics reveals the role of TGF beta and YAP/TAZ signaling in Raine syndrome fibrosis

Raine syndrome (RNS) is a rare autosomal recessive osteosclerotic dysplasia. RNS is caused by loss-of-function disease-causative variants of the FAM20C gene that encodes a kinase that phosphorylates most of the secreted proteins found in the body fluids and extracellular matrix. The most common RNS clinical features are generalized osteosclerosis, facial dysmorphism, intracerebral calcifications and respiratory defects. In non-lethal RNS forms, oral traits include a well-studied hypoplastic amelogenesis imperfecta (AI) and a much less characterized gingival phenotype. We used immunomorphological, biochemical, and siRNA approaches to analyze gingival tissues and primary cultures of gingival fibroblasts of two unrelated, previously reported RNS patients. We showed that fibrosis, pathological gingival calcifications and increased expression of various profibrotic and pro-osteogenic proteins such as POSTN, SPARC and VIM were common findings. Proteomic analysis of differentially expressed proteins demonstrated that proteins involved in extracellular matrix (ECM) regulation and related to the TGFβ/SMAD signaling pathway were increased. Functional analyses confirmed the upregulation of TGFβ/SMAD signaling and subsequently uncovered the involvement of two closely related transcription cofactors important in fibrogenesis, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). Knocking down of FAM20C confirmed the TGFβ-YAP/TAZ interplay indicating that a profibrotic loop enabled gingival fibrosis in RNS patients. In summary, our in vivo and in vitro data provide a detailed description of the RNS gingival phenotype. They show that gingival fibrosis and calcifications are associated with, and most likely caused by excessed ECM production and disorganization. They furthermore uncover the contribution of increased TGFβ-YAP/TAZ signaling in the pathogenesis of the gingival fibrosis.

FAM20A is a golgi-localized Type II transmembrane protein

Family with sequence similarity 20, member A (FAM20A) is a pseudo-kinase in the secretory pathway and is essential for enamel formation in humans. Here we examine if FAM20A is a membrane-associated protein. We show that the full-length FAM20A can be purified from HEK293 cells transfected with a FAM20A-expresing construct. Further, it is only found in the membrane fraction, but not in the soluble fraction, of cell lysate. Consistently, it is not secreted out of the expressing cells. Moreover, it is co-localized with GM130, a cis-Golgi network marker, and membrane topology analysis indicates that it has its C-terminus oriented towards the lumen of the organelle. Our results support that FAM20A is a Type II transmembrane protein within the secretory compartments.

Navigating Complexity: A Case Report on a Comprehensive Dental Management Approach to Amelogenesis Imperfecta and Gingival Fibromatosis Syndrome

This clinical case report details the comprehensive diagnosis and dental management of a seven-year-old female patient diagnosed with the rare genetic disorder, amelogenesis imperfecta and gingival fibromatosis syndrome (AIGFS). The case initially presented as congenital adrenal hyperplasia and amelogenesis imperfecta, but further genetic analysis revealed the involvement of AIGFS due to a mutation in the FAM20A gene. Diagnosis, confirmed through whole exome sequencing, clinical assessment, and laboratory tests, necessitated a multidisciplinary approach to address the treatment of such cases. The article underscores the critical importance of diagnosing and managing dental manifestations in pediatric patients with complex genetic conditions, highlighting the difficulties of treating AIGFS in mixed dentition. This case also highlights the indispensable role of pediatric dentists in diagnosing and treating these cases, ultimately improving the quality of life for individuals with AIGFS.

Case report: Enamel renal syndrome: a case series from sub-Saharan Africa

Enamel Renal Syndrome (ERS) (OMIM # 204690) is a rare genetic condition characterised by hypoplastic amelogenesis imperfecta, failed tooth eruption, intra-pulpal calcifications, gingival enlargement and occasionally nephrocalcinosis. In this case series, we report on four unrelated patients with a confirmed molecular diagnosis of ERS (FAM20A pathogenic variants) from Sub-Saharan Africa. The pathognomonic oral profile of ERS was mostly fulfilled in these patients, with the notable addition of an odontoma in one patient. The cases presented a spectrum of phenotypic severity both dentally and systemically. One patient presented with nephrocalcinosis and abnormal kidney function, one had reduced kidney size with normal kidney function, and two had no renal abnormalities. Patients presenting with the oral profile of ERS should receive a prompt referral to a nephrologist and a geneticist. They should receive long-term management from a multidisciplinary medical and dental team.

FAM20A mutations and transcriptome analyses of dental pulp tissues of enamel renal syndrome

AIM

Biallelic loss-of-function FAM20A mutations cause amelogenesis imperfecta (AI) type IG, better known as enamel renal syndrome (ERS), characterized by severe enamel hypoplasia, delayed/failed tooth eruption, intrapulpal calcifications, gingival hyperplasia and nephrocalcinosis. FAM20A binds to FAM20C, the Golgi casein kinase (GCK) and potentiates its function to phosphorylate secreted proteins critical for biomineralization. While many FAM20A pathogenic mutations have been reported, the pathogeneses of orodental anomalies in ERS remain to be elucidated. This study aimed to identify disease-causing mutations for patients with ERS phenotypes and to discern the molecular mechanism underlying ERS intrapulpal calcifications.

METHODOLOGY

Phenotypic characterization and whole exome analyses were conducted for 8 families and 2 sporadic cases with hypoplastic AI. A minigene assay was performed to investigate the molecular consequences of a FAM20A splice-site variant. RNA sequencing followed by transcription profiling and gene ontology (GO) analyses were carried out for dental pulp tissues of ERS and the control.

RESULTS

Biallelic FAM20A mutations were demonstrated for each affected individual, including 7 novel pathogenic variants: c.590-5T>A, c.625T>A (p.Cys209Ser), c.771del (p.Gln258Argfs*28), c.832_835delinsTGTCCGACGGTGTCCGACGGTGTC CA (p.Val278Cysfs*29), c.1232G>A (p.Arg411Gln), c.1297A>G (p.Arg433Gly) and c.1351del (p.Gln451Serfs*4). The c.590-5T>A splice-site mutation caused Exon 3 skipping, which resulted in an in-frame deletion of a unique region of the FAM20A protein, p.(Asp197_Ile214delinsVal). Analyses of differentially expressed genes in ERS pulp tissues demonstrated that genes involved in biomineralization, particularly dentinogenesis, were significantly upregulated, such as DSPP, MMP9, MMP20 and WNT10A. Enrichment analyses indicated overrepresentation of gene sets associated with BMP and SMAD signalling pathways. In contrast, GO terms related to inflammation and axon development were underrepresented. Among BMP signalling genes, BMP agonists GDF7, GDF15, BMP3, BMP8A, BMP8B, BMP4 and BMP6 were upregulated, while BMP antagonists GREM1, BMPER and VWC2 showed decreased expression in ERS dental pulp tissues.

CONCLUSIONS

Upregulation of BMP signalling underlies intrapulpal calcifications in ERS. FAM20A plays an essential role in pulp tissue homeostasis and prevention of ectopic mineralization in soft tissues. This critical function probably depends upon MGP (matrix Gla protein), a potent mineralization inhibitor that must be properly phosphorylated by FAM20A-FAM20C kinase complex.

Novel WDR72 Mutations Causing Hypomaturation Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a heterogeneous collection of hereditary enamel defects. The affected enamel can be classified as hypoplastic, hypomaturation, or hypocalcified in form. A better understanding of normal amelogenesis and improvements in our ability to diagnose AI through genetic testing can be realized through more complete knowledge of the genes and disease-causing variants that cause AI. In this study, mutational analysis was performed with whole exome sequencing (WES) to identify genetic etiology underlying the hypomaturation AI condition in affected families. Mutational analyses identified biallelic WDR72 mutations in four hypomaturation AI families. Novel mutations include a homozygous deletion and insertion mutation (NM_182758.4: c.2680_2699delinsACTATAGTT, p.(Ser894Thrfs*15)), compound heterozygous mutations (paternal c.2332dupA, p.(Met778Asnfs*4)) and (maternal c.1287_1289del, p.(Ile430del)) and a homozygous 3694 bp deletion that includes exon 14 (NG_017034.2:g.96472_100165del). A homozygous recurrent mutation variant (c.1467_1468delAT, p.(Val491Aspfs*8)) was also identified. Current ideas on WDR72 structure and function are discussed. These cases expand the mutational spectrum of WDR72 mutations causing hypomaturation AI and improve the possibility of genetic testing to accurately diagnose AI caused by WDR72 defects.

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.

A Recurrent FAM83H Mutation in an Extended Colombian Family and Variable Craniofacial Phenotypes

Amelogenesis imperfecta (AI) is a collection of rare genetic disorders affecting the quantity and/or quality of the tooth enamel. AI can be classified into three major types according to the clinical phenotype: hypoplastic, hypocalcified, and hypomatured. Among them, the hypocalcified type shows the weakest physical properties, leaving rough and discolored enamel surfaces after tooth eruption. To date, mutations in the FAM83H gene are responsible for the autosomal-dominant hypocalcified AI. In this study, we recruited a four-generation Colombian family with hypocalcified AI and identified a recurrent nonsense mutation in the FAM83H gene (NM_198488.5:c.1289C>A, p.(Ser430 *)) by candidate gene sequencing. Cephalometric analyses revealed the anterior open bite that occurred in the proband is not correlated with the AI in this family.

Pathogenesis of Enamel-Renal Syndrome Associated Gingival Fibromatosis: A Proteomic Approach

The enamel renal syndrome (ERS) is a rare disorder featured by amelogenesis imperfecta, gingival fibromatosis and nephrocalcinosis. ERS is caused by bi-allelic mutations in the secretory pathway pseudokinase FAM20A. How mutations in FAM20A may modify the gingival connective tissue homeostasis and cause fibromatosis is currently unknown. We here analyzed conditioned media of gingival fibroblasts (GFs) obtained from four unrelated ERS patients carrying distinct mutations and control subjects. Secretomic analysis identified 109 dysregulated proteins whose abundance had increased (69 proteins) or decreased (40 proteins) at least 1.5-fold compared to control GFs. Proteins over-represented were mainly involved in extracellular matrix organization, collagen fibril assembly, and biomineralization whereas those under-represented were extracellular matrix-associated proteins. More specifically, transforming growth factor-beta 2, a member of the TGFβ family involved in both mineralization and fibrosis was strongly increased in samples from GFs of ERS patients and so were various known targets of the TGFβ signaling pathway including Collagens, Matrix metallopeptidase 2 and Fibronectin. For the over-expressed proteins quantitative RT-PCR analysis showed increased transcript levels, suggesting increased synthesis and this was further confirmed at the tissue level. Additional immunohistochemical and western blot analyses showed activation and nuclear localization of the classical TGFβ effector phospho-Smad3 in both ERS gingival tissue and ERS GFs. Exposure of the mutant cells to TGFB1 further upregulated the expression of TGFβ targets suggesting that this pathway could be a central player in the pathogenesis of the ERS gingival fibromatosis. In conclusion our data strongly suggest that TGFβ -induced modifications of the extracellular matrix contribute to the pathogenesis of ERS. To our knowledge this is the first proteomic-based analysis of FAM20A-associated modifications.

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.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.

Lack of FAM20A, Ectopic Gingival Mineralization and Chondro/Osteogenic Modifications in Enamel Renal Syndrome

Enamel renal syndrome (ERS) is a rare recessive disorder caused by loss-of-function mutations in FAM20A (family with sequence similarity 20 member A, OMIM #611062). Enamel renal syndrome is characterized by amelogenesis imperfecta, delayed or failed tooth eruption, intrapulpal calcifications, gingival overgrowth and nephrocalcinosis. Although gingival overgrowth has consistently been associated with heterotopic calcifications the pathogenesis, structure and interactions of the mineral deposits with the surrounding connective tissue are largely unknown. We here report a novel FAM20A mutation in exon 1 (c.358C > T) introducing a premature stop codon (p.Gln120*) and resulting in a complete loss of FAM20A. In addition to the typical oral findings and nephrocalcinosis, ectopic calcified nodules were also seen in the cervical and thoracic vertebrae regions. Histopathologic analysis of the gingiva showed an enlarged papillary layer associated with aberrant angiogenesis and a lamina propria displaying significant changes in its extracellular matrix composition, including disruption of the collagen I fiber network. Ectopic calcifications were found throughout the connective gingival tissue. Immunomorphological and ultrastructural analyses indicated that the calcification process was associated with epithelial degeneration and transformation of the gingival fibroblasts to chondro/osteoblastic-like cells. Mutant gingival fibroblasts cultures were prone to calcify and abnormally expressed osteoblastic markers such as RUNX2 or PERIOSTIN. Our findings expand the previously reported phenotypes and highlight some aspects of ERS pathogenesis.

Two new families with enamel renal syndrome: A novel FAM20A gene mutation and review of literature

Enamel renal syndrome (ERS) or so-called amelogenesis imperfecta type IG is a very rare disorder characterized by the triad of amelogenesis imperfecta, gingival enlargement and nephrocalcinosis. It is caused by biallelic mutations in the FAM20A gene. Herein, we report two unrelated patients with ERS. Our patients presented with the characteristic features of the syndrome, and amelogenesis imperfecta and gingival hyperplasia were the main complaint. Strikingly, they both had long face, thick lips, notched upper central incisors, and thick alveolar ridge which have never been reported before in patients with ERS. Gingival biopsy showed psammomatous calcifications, and renal ultrasound revealed bilateral nephrocalcinosis in the two patients. Mutational analysis of the FAM20A gene identified two homozygous mutations including a novel one (c.915_918delCTTT, p.Phe305Leufs*76 and c.1219 + 3_1219+6delAGGT). Our data expand the phenotypic and mutational spectrum of FAM20A gene and reinforce the importance of kidney examination and follow up for all patients with amelogenesis imperfecta unless FAM20A mutations were ruled out.

Syndromes with gingival fibromatosis: A systematic review

OBJECTIVE

The aim of systematic review was to describe the phenotypes and molecular profiles of syndromes with gingival fibromatosis (GF).

METHODS

A comprehensive search of PubMed, LILACS, Livivo, Scopus, and Web of Science was conducted using key terms relevant to the research questions and supplemented by a gray literature search. The Methodological Quality and Synthesis of Case Series and Case Reports in association with the Case Series and Prevalence Studies from the Joanna Briggs Institute critical appraisal tools were used for the risk of bias. We followed the PRISMA checklist guidelines.

RESULTS

Eighty-four studies reporting GF as an oral manifestation of a syndrome were identified in this review. Enamel renal syndrome was the most frequently reported syndrome with GF, represented by 54 individuals in 19 studies, followed by Zimmermann-Laband syndrome with 24 individuals in 15 studies and Costello syndrome, which was presented in a case series study with 41 individuals. Among reported cases, other clinical manifestations such as hypertrichosis, ectopic gingival calcification, and cherubism were described.

CONCLUSIONS

The results emphasize the need of systematic oro-dental-facial phenotyping for future descriptions as well as further molecular analysis in order to better understand the occurrence of syndromic GF.

Nonsyndromic hereditary gingival fibromatosis: Characterization of a family and review of genetic etiology

Our aim is to describe a family with a nonsyndromic form of hereditary gingival fibromatosis (HGF) and discuss genetic characteristics of this rare disease by reviewing reported cases. A mother and three descendants were diagnosed with HGF. There was marked variable expressivity: from severe generalized gingival overgrowth in a 16-year-old boy (the proband) to minimal manifestations in the mother. The proband was submitted to gingivectomy and gingivoplasty. In younger siblings, the disease remained stable for 5 years, suggesting that clinical surveillance is a good option. The diagnosis was supported by histopathological examination. Analysis of this family and literature-reported cases supports that HGF most frequently shows an autosomal dominant inheritance with high penetrance and variable expressivity. Neomutations and gonadal mosaicism do not seem to be a rare event. Although five loci have been mapped by linkage analysis, only two genes, SOS1 and REST, were identified in four families.

Four novel mutations of FAM20A in amelogenesis imperfecta type IG and review of literature for its genotype and phenotype spectra

Amelogenesis imperfecta type IG (AI1G) is caused by mutations in FAM20A. Genotypic and phenotypic features of AI1G are diverse and their full spectra remain to be characterized. The aim of this study was to identify and summarize variants in FAM20A in a broad population of patients with AI1G. We identified a Thai female (Pt-1) and a Saudi male (Pt-2) affected with AI1G. Both had hypoplastic enamel, gingival hyperplasia, and intrapulpal calcification. Pt-1 also had rapidly progressive embedding of unerupted teeth, early eruption of permanent teeth, and spontaneous dental infection. Uniquely, Pt-2 had all permanent teeth erupted which was uncommon in AI1G patients. Whole exome sequencing (WES) identified that Pt-1 was heterozygous for FAM20A, c.758A > G (p.Tyr253Cys), inherited from her father. The mutation on maternal allele was not detected by WES. Pt-2 possessed compound heterozygous mutations, c.1248dupG (p.Phe417Valfs*7); c.1081C > T (p.Arg361Cys) in FAM20A. Array comparative genomic hybridization (aCGH), cDNA sequencing, and whole genome sequencing successfully identified 7531 bp deletion on Pt-1's maternal allele. This was the largest FAM20A deletion ever found. A review of all 70 patients from 50 independent families with AI1G (including two families in this study) showed that the penetrance of hypoplastic enamel and gingival hyperplasia was complete. Unerupted permanent teeth were found in all 70 patients except Pt-2. Exons 1 and 11 were mutation-prone. Most mutations were frameshift. Certain variants showed founder effect. To conclude, this study reviews and expands phenotypic and genotypic spectra of AI1G. A large deletion missed by WES can be detected by WGS. Hypoplastic enamel, gingival hyperplasia, and unerupted permanent teeth prompt genetic testing of FAM20A. Screening of nephrocalcinosis, early removal of embedded teeth, and monitoring of dental infection are recommended.

Transcriptome analysis of gingival tissues of enamel-renal syndrome

BACKGROUND AND OBJECTIVE

Biallelic loss-of-function mutations of human FAM20A have been known to cause enamel-renal syndrome (ERS), featured by agenesis of dental enamel, nephrocalcinosis, and other orodental abnormalities, including gingival hyperplasia. However, while the histopathology of this gingival anomaly has been analyzed, its underlying molecular mechanism remains largely unknown. This study aimed to unravel the pathogenesis of gingival hyperplasia in ERS.

METHODS

Whole-exome sequencing was conducted for an ERS case. Transcriptome analyses, using RNA sequencing, of the patient's gingiva were performed to unravel dysregulated molecules and aberrant biological processes underlying the gingival pathology of ERS, which was further confirmed by histology and immunohistochemistry.

RESULTS

Two novel frameshift FAM20A mutations in Exon 1 (g.5417delG; c.129delG; p.Cys44Alafs*101) and Exon 5 (g.62248_62249delAG; c.734_735delAG; p.Glu245Glyfs*11) were identified. Transcriptional profiling of patient's gingival tissue revealed a total of 1683 genes whose expression had increased (1129 genes) or decreased (554 genes) at least 2-fold compared to control gingival tissues. There were 951 gene ontology (GO) terms of biological process being significantly over-represented or under-represented. While GOs involved in extracellular matrix organization, angiogenesis, biomineralization, and epithelial cell proliferation appeared to be activated in ERS gingiva, genes related to keratinocyte differentiation, epithelial development, and keratinization were of decreased expression. FAM20A immunohistochemistry revealed a strong reactivity at the suprabasal layers of epithelium in control gingiva but showed a significantly diminished and scattered signal in ERS tissues. For genes showing significant over-expression in the transcriptome analyses, namely ALPL, SPARC, and ACTA2, an increased immunoreactivity was observed.

CONCLUSION

Our results unraveled a potential role for FAM20A in homeostasis of both gingival epithelium and connective tissues.

Candidate gene sequencing reveals mutations causing hypoplastic amelogenesis imperfecta

OBJECTIVE

Amelogenesis imperfecta (AI) is a rare hereditary disorder affecting the quality and quantity of the tooth enamel. The purpose of this study was to identify the genetic etiology of hypoplastic AI families based on the candidate gene approach.

MATERIALS AND METHODS

We recruited three Turkish families with hypoplastic AI and performed a candidate gene screening based on the characteristic clinical feature to find the pathogenic genetic etiology.

RESULTS

The candidate gene sequencing of the LAMB3 gene for family 1 revealed a heterozygous nonsense mutation in the last exon [c.3431C > A, p.(Ser1144*)]. FAM20A gene sequencing for families 2 and 3 identified a homozygous deletion [c.34_35delCT, p.(Leu12Alafs*67)] and a homozygous deletion-insertion (c.1109 + 3_1109 + 7delinsTGGTC) mutation, respectively.

CONCLUSION

The candidate gene approach can be successfully used to identify the genetic etiology of the AI in some cases with characteristic clinical features.

CLINICAL RELEVANCE

Identification of the genetic etiology of the AI will help both the family members and dentist understand the nature of the disorder. Characteristic clinical feature can suggest possible genetic causes.

Structure of Fam20A reveals a pseudokinase featuring a unique disulfide pattern and inverted ATP-binding

Mutations in FAM20A cause tooth enamel defects known as Amelogenesis Imperfecta (AI) and renal calcification. We previously showed that Fam20A is a secretory pathway pseudokinase and allosterically activates the physiological casein kinase Fam20C to phosphorylate secreted proteins important for biomineralization (Cui et al., 2015). Here we report the nucleotide-free and ATP-bound structures of Fam20A. Fam20A exhibits a distinct disulfide bond pattern mediated by a unique insertion region. Loss of this insertion due to abnormal mRNA splicing interferes with the structure and function of Fam20A, resulting in AI. Fam20A binds ATP in the absence of divalent cations, and strikingly, ATP is bound in an inverted orientation compared to other kinases. Fam20A forms a dimer in the crystal, and residues in the dimer interface are critical for Fam20C activation. Together, these results provide structural insights into the function of Fam20A and shed light on the mechanism by which Fam20A mutations cause disease.

DOI:http://dx.doi.org/10.7554/eLife.23990.001

Periodontal disease and FAM20A mutations

Enamel-renal-gingival syndrome (ERGS; OMIM #204690), a rare autosomal recessive disorder caused by mutations in FAM20A, is characterized by nephrocalcinosis, nephrolithiasis, amelogenesis imperfecta, hypoplastic type, gingival fibromatosis and other dental abnormalities, including hypodontia and unerupted teeth with large dental follicles. We report three patients and their families with findings suggestive of ERGS. Mutation analysis of FAM20A was performed in all patients and their family members. Patients with homozygous frameshift and compound heterozygous mutations in FAM20A had typical clinical findings along with periodontitis. The other had a novel homozygous missense mutation in exon 10, mild gingival fibromatosis and renal calcifications. The periodontitis in our patients may be a syndrome component, and similar findings in previous reports suggest more than coincidence. Fam20a is an allosteric activator that increases Fam20c kinase activity. It is hypothesized that lack of FAM20A activation of FAM20C in our patients with FAM20A mutations might have caused amelogenesis imperfecta, abnormal bone remodeling and periodontitis. Nephrocalcinosis appears not to be a consistent finding of the syndrome and the missense mutation may correlate with mild gingival fibromatosis. Here we report three patients with homozygous or compound heterozygous mutations in FAM20A and findings that extend the phenotypic spectrum of this disorder, showing that protein truncation is associated with greater clinical severity.

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.

Loss of epithelial FAM20A in mice causes amelogenesis imperfecta, tooth eruption delay and gingival overgrowth

FAM20A has been studied to a very limited extent. Mutations in human FAM20A cause amelogenesis imperfecta, gingival fibromatosis and kidney problems. It would be desirable to systemically analyse the expression of FAM20A in dental tissues and to assess the pathological changes when this molecule is specifically nullified in individual tissues. Recently, we generated mice with a Fam20A-floxed allele containing the beta-galactosidase reporter gene. We analysed FAM20A expression in dental tissues using X-Gal staining, immunohistochemistry and in situ hybridization, which showed that the ameloblasts in the mouse mandibular first molar began to express FAM20A at 1 day after birth, and the reduced enamel epithelium in erupting molars expressed a significant level of FAM20A. By breeding K14-Cre mice with Fam20A^flox/flox mice, we created K14-Cre;Fam20A^flox/flox (conditional knock out, cKO) mice, in which Fam20A was inactivated in the epithelium. We analysed the dental tissues of cKO mice using X-ray radiography, histology and immunohistochemistry. The molar enamel matrix in cKO mice was much thinner than normal and was often separated from the dentinoenamel junction. The Fam20A-deficient ameloblasts were non-polarized and disorganized and were detached from the enamel matrix. The enamel abnormality in cKO mice was consistent with the diagnosis of amelogenesis imperfecta. The levels of enamelin and matrix metalloproteinase 20 were lower in the ameloblasts and enamel of cKO mice than the normal mice. The cKO mice had remarkable delays in the eruption of molars and hyperplasia of the gingival epithelium. The findings emphasize the essential roles of FAM20A in the development of dental and oral tissues.

FAM20A binds to and regulates FAM20C localization

Mutations in the Family with sequence similarity (FAM) 20 gene family are associated with mineralized tissue phenotypes in humans. Among these genes, FAM20A mutations are associated with Amelogenesis Imperfecta (AI) with gingival hyperplasia and nephrocalcinosis, while FAM20C mutations cause Raine syndrome, exhibiting bone and craniofacial/dental abnormalities. Although it has been demonstrated that Raine syndrome associated-FAM20C mutants prevented FAM20C kinase activity and secretion, overexpression of the catalytically inactive D478A FAM20C mutant was detected in both cell extracts and the media. This suggests that FAM20C secretion doesn’t require its kinase activity, and that another molecule(s) may control the secretion. In this study, we found that extracellular FAM20C localization was increased when wild-type (WT), but not AI-forms of FAM20A was co-transfected. On the other hand, extracellular FAM20C was absent in the conditioned media of mouse embryonic fibroblasts (MEFs) derived from Fam20a knock-out (KO) mouse, while it was detected in the media from WT MEFs. We also showed that cells with the conditioned media of Fam20a WT MEFs mineralized, but those with the conditioned media of KO MEFs failed to mineralize in vitro. Our data thus demonstrate that FAM20A controls FAM20C localization that may assist in the extracellular function of FAM20C in mineralized tissues.

Claudin-16 Deficiency Impairs Tight Junction Function in Ameloblasts, Leading to Abnormal Enamel Formation

Claudin-16 protein (CLDN16) is a component of tight junctions (TJ) with a restrictive distribution so far demonstrated mainly in the kidney. Here, we demonstrate the expression of CLDN16 also in the tooth germ and show that claudin-16 gene (CLDN16) mutations result in amelogenesis imperfecta (AI) in the 5 studied patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC). To investigate the role of CLDN16 in tooth formation, we studied a murine model of FHHNC and showed that CLDN16 deficiency led to altered secretory ameloblast TJ structure, lowering of extracellular pH in the forming enamel matrix, and abnormal enamel matrix protein processing, resulting in an enamel phenotype closely resembling human AI. This study unravels an association of FHHNC owing to CLDN16 mutations with AI, which is directly related to the loss of function of CLDN16 during amelogenesis. Overall, this study indicates for the first time the importance of a TJ protein in tooth formation and underlines the need to establish a specific dental follow-up for these patients.

A targeted next-generation sequencing assay for the molecular diagnosis of genetic disorders with orodental involvement

BACKGROUND

Orodental diseases include several clinically and genetically heterogeneous disorders that can present in isolation or as part of a genetic syndrome. Due to the vast number of genes implicated in these disorders, establishing a molecular diagnosis can be challenging. We aimed to develop a targeted next-generation sequencing (NGS) assay to diagnose mutations and potentially identify novel genes mutated in this group of disorders.

METHODS

We designed an NGS gene panel that targets 585 known and candidate genes in orodental disease. We screened a cohort of 101 unrelated patients without a molecular diagnosis referred to the Reference Centre for Oro-Dental Manifestations of Rare Diseases, Strasbourg, France, for a variety of orodental disorders including isolated and syndromic amelogenesis imperfecta (AI), isolated and syndromic selective tooth agenesis (STHAG), isolated and syndromic dentinogenesis imperfecta, isolated dentin dysplasia, otodental dysplasia and primary failure of tooth eruption.

RESULTS

We discovered 21 novel pathogenic variants and identified the causative mutation in 39 unrelated patients in known genes (overall diagnostic rate: 39%). Among the largest subcohorts of patients with isolated AI (50 unrelated patients) and isolated STHAG (21 unrelated patients), we had a definitive diagnosis in 14 (27%) and 15 cases (71%), respectively. Surprisingly, COL17A1 mutations accounted for the majority of autosomal-dominant AI cases.

CONCLUSIONS

We have developed a novel targeted NGS assay for the efficient molecular diagnosis of a wide variety of orodental diseases. Furthermore, our panel will contribute to better understanding the contribution of these genes to orodental disease.

TRIAL REGISTRATION NUMBERS

NCT01746121 and NCT02397824.

Gingival fibromatosis: clinical, molecular and therapeutic issues

Gingival fibromatosis is a rare and heterogeneous group of disorders that develop as slowly progressive, local or diffuse enlargements within marginal and attached gingiva or interdental papilla. In severe cases, the excess tissue may cover the crowns of the teeth, thus causing functional, esthetic, and periodontal problems, such as bone loss and bleeding, due to the presence of pseudopockets and plaque accumulation. It affects both genders equally. Hereditary, drug-induced, and idiopathic gingival overgrowth have been reported. Hereditary gingival fibromatosis can occur as an isolated condition or as part of a genetic syndrome. The pathologic manifestation of gingival fibromatosis comprises excessive accumulation of extracellular matrix proteins, of which collagen type I is the most prominent example. Mutation in the Son-of-Sevenless-1 gene has been suggested as one possible etiological cause of isolated (non-syndromic) hereditary gingival fibromatosis, but mutations in other genes are also likely to be involved, given the heterogeneity of this condition. The most attractive concept of mechanism for drug-induced gingival overgrowth is epithelial-to-mesenchymal transition, a process in which interactions between gingival cells and the extracellular matrix are weakened as epithelial cells transdifferentiate into fibrogenic fibroblast-like cells. The diagnosis is mainly made on the basis of the patient's history and clinical features, and on histopathological evaluation of affected gingiva. Early diagnosis is important, mostly to exclude oral malignancy. Differential diagnosis comprises all pathologies in the mouth with excessive gingival overgrowth. Hereditary gingival fibromatosis may present as an autosomal-dominant or less commonly autosomal-recessive mode of inheritance. If a systemic disease or syndrome is suspected, the patient is directed to a geneticist for additional clinical examination and specialized diagnostic tests. Treatments vary according to the type of overgrowth and the extent of disease progression, thus, scaling of teeth is sufficient in mild cases, while in severe cases surgical intervention is required. Prognosis is precarious and the risk of recurrence exists.

A distinctive oral phenotype points to FAM20A mutations not identified by Sanger sequencing

Biallelic FAM20A mutations cause two conditions where Amelogenesis Imperfecta (AI) is the presenting feature: Amelogenesis Imperfecta and Gingival Fibromatosis Syndrome; and Enamel Renal Syndrome. A distinctive oral phenotype is shared in both conditions. On Sanger sequencing of FAM20A in cases with that phenotype, we identified two probands with single, likely pathogenic heterozygous mutations. Given the recessive inheritance pattern seen in all previous FAM20A mutation‐positive families and the potential for renal disease, further screening was carried out to look for a second pathogenic allele. Reverse transcriptase‐PCR on cDNA was used to determine transcript levels. CNVseq was used to screen for genomic insertions and deletions. In one family, FAM20A cDNA screening revealed only a single mutated FAM20A allele with the wild‐type allele not transcribed. In the second family, CNV detection by whole genome sequencing (CNVseq) revealed a heterozygous 54.7 kb duplication encompassing exons 1 to 4 of FAM20A. This study confirms the link between biallelic FAM20A mutations and the characteristic oral phenotype. It highlights for the first time examples of FAM20A mutations missed by the most commonly used mutation screening techniques. This information informed renal assessment and ongoing clinical care.

The secretory pathway kinases

Protein phosphorylation is a nearly universal post-translation modification involved in a plethora of cellular events. Even though phosphorylation of extracellular proteins had been observed, the identity of the kinases that phosphorylate secreted proteins remained a mystery until only recently. Advances in genome sequencing and genetic studies have paved the way for the discovery of a new class of kinases that localize within the endoplasmic reticulum, Golgi apparatus and the extracellular space. These novel kinases phosphorylate proteins and proteoglycans in the secretory pathway and appear to regulate various extracellular processes. Mutations in these kinases cause human disease, thus underscoring the biological importance of phosphorylation within the secretory pathway. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases.

A secretory kinase complex regulates extracellular protein phosphorylation

Although numerous extracellular phosphoproteins have been identified, the protein kinases within the secretory pathway have only recently been discovered, and their regulation is virtually unexplored. Fam20C is the physiological Golgi casein kinase, which phosphorylates many secreted proteins and is critical for proper biomineralization. Fam20A, a Fam20C paralog, is essential for enamel formation, but the biochemical function of Fam20A is unknown. Here we show that Fam20A potentiates Fam20C kinase activity and promotes the phosphorylation of enamel matrix proteins in vitro and in cells. Mechanistically, Fam20A is a pseudokinase that forms a functional complex with Fam20C, and this complex enhances extracellular protein phosphorylation within the secretory pathway. Our findings shed light on the molecular mechanism by which Fam20C and Fam20A collaborate to control enamel formation, and provide the first insight into the regulation of secretory pathway phosphorylation.

DOI:http://dx.doi.org/10.7554/eLife.06120.001

Some proteins must be modified in order to work effectively. One common modification is to add a phosphate group to the protein, which is performed by enzymes called protein kinases. Although most of the protein kinases work on proteins inside the cell, it was discovered recently that a small group of kinases work within the ‘secretory pathway’ and modify proteins that are released (or secreted) out of cells.

One such secretory pathway kinase—called Fam20C—phosphorylates a wide range of secreted proteins and helps to ensure the proper development of bones and teeth. Specifically, Fam20C and a closely related protein called Fam20A are important for forming enamel, the hardest substance in human body, which makes up the outer surface of teeth. However, the exact role of Fam20A is unknown.

Cui et al. now show that Fam20A binds to Fam20C, and this increases the ability of Fam20C to phosphorylate the proteins that form the ‘matrix’ that guides the deposition of the enamel minerals. Furthermore, mutations in Fam20A lead to the inefficient phosphorylation of enamel matrix proteins by Fam20C, and prevent proper enamel formation. The results raise the possibility that similar mechanisms of secretory kinase activation may also be important in other biological processes where many secreted proteins need to be phosphorylated rapidly.

DOI:http://dx.doi.org/10.7554/eLife.06120.002

Novel FAM20A mutation causes autosomal recessive amelogenesis imperfecta

OBJECTIVE

To relate the peculiar phenotype of amelogenesis imperfecta in a large Bedouin family to the genotype determined by whole genome linkage analysis.

DESIGN

Amelogenesis 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.

RESULTS

The sequencing unravelled a novel homozygous deletion mutation in exon 11 (c.1523delC), predicted to insert a premature stop codon (p.Thr508Lysfs*6).

CONCLUSIONS

We 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.

Further evidence for causal FAM20A mutations and first case of amelogenesis imperfecta and gingival hyperplasia syndrome in Morocco: a case report

Background

Amelogenesis imperfecta represents a group of developmental conditions, clinically and genetically heterogeneous, that affect the structure and clinical appearance of enamel. Amelogenesis imperfecta occurred as an isolated trait or as part of a genetic syndrome. Recently, disease-causing mutations in the FAM20A gene were identified, in families with an autosomal recessive syndrome associating amelogenesis imperfecta and gingival fibromatosis.

Case presentation

We report, the first description of a Moroccan patient with amelogenesis imperfecta and gingival fibromatosis, in whom we performed Sanger sequencing of the entire coding sequence of FAM20A and identified a homozygous mutation in the FAM20A gene (c.34_35delCT), already reported in a family with this syndrome.

Conclusion

Our finding confirms that the mutations of FAM20A gene are causative for amelogenesis imperfecta and gingival fibromatosis and underlines the recurrent character of the c.34_35delCT in two different ethnic groups.

Clinical findings and long-term managements of patients with amelogenesis imperfecta

The aim of this clinical case series is to present a diagnosis and different treatment methods of patients in different ages with amelogenesis imperfecta (AI) as well as further treatments during a 3-6 years follow-up period. A number of 31 patients (16 female, 15 male with a mean age of 10.77 ± 2.65 years) with AI have been examined for the study group between 2007 and 2010 years. A detailed anamnesis was recorded, followed by a clinical and radiological assessment of oral health. The types of AI classified for each patient according to clinical and radiographic evaluation. The main complaints of patients, presence of dental caries and dental anomalies were noted. Necessary treatments had been planned for the individual cases of AI. A number of 19 patients had hypoplastic (HP) form, and 10 patients showed hypomaturation (HM) form of AI, while one patient showed hypocalcified form of AI and one patient had HM-HP form with taurodontism. Main complaints were chiefly related to dissatisfactory esthetics and dental sensitivity. Caries prevalence index was 93.5%. Mean decayed, missing, filling permanent teeth (DMF) and DMF surface (DMFS) were found as 2.74 ± 1.71 and 6.23 ± 3.99; df (decayed, filling primary teeth) and dfs (decayed, filling primary teeth surface) were found as 3.12 ± 2.85 and 5.24 ± 4.97, respectively. All patients received individual clinical care, including preventive, restorative, and prosthetic treatments. Patients have scheduled for regular follow-up in every 3 months. Composite restorations were used as the most common treatment (25 patients, 80.6%). The treatment plan should be based on patient's age, type of defects and individual needs of the patients. Necessary treatment plan is essential, not only due to functional and aesthetic reasons, but also for the positive psychological impact on young patients.

Alteration of conserved alternative splicing in AMELX causes enamel defects

Tooth enamel is the most highly mineralized tissue in vertebrates. Enamel crystal formation and elongation should be well controlled to achieve an exceptional hardness and a compact microstructure. Enamel matrix calcification occurs with several matrix proteins, such as amelogenin, enamelin, and ameloblastin. Among them, amelogenin is the most abundant enamel matrix protein, and multiple isoforms resulting from extensive but well-conserved alternative splicing and postsecretional processing have been identified. In this report, we recruited a family with a unique enamel defect and identified a silent mutation in exon 4 of the AMELX gene. We show that the mutation caused the inclusion of exon 4, which is almost always skipped, in the mRNA transcript. We further show, by generating and characterizing a transgenic animal model, that the alteration of the ratio and quantity of the developmentally conserved alternative splicing repertoire of AMELX caused defects in enamel matrix mineralization.

Casein kinase: the triple meaning of a misnomer

The term 'casein kinase' has been widely used for decades to denote protein kinases sharing the ability to readily phosphorylate casein in vitro. These fall into three main classes: two of them, later renamed as protein kinases CK1 (casein kinase 1, also known as CKI) and CK2 (also known as CKII), are pleiotropic members of the kinome functionally unrelated to casein, whereas G-CK, or genuine casein kinase, responsible for the phosphorylation of casein in the Golgi apparatus of the lactating mammary gland, has only been identified recently with Fam20C [family with sequence similarity 20C; also known as DMP-4 (dentin matrix protein-4)], a member of the four-jointed family of atypical protein kinases, being responsible for the phosphorylation of many secreted proteins. In hindsight, therefore, the term 'casein kinase' is misleading in every instance; in the case of CK1 and CK2, it is because casein is not a physiological substrate, and in the case of G-CK/Fam20C/DMP-4, it is because casein is just one out of a plethora of its targets, and a rather marginal one at that. Strikingly, casein kinases altogether, albeit representing a minimal proportion of the whole kinome, appear to be responsible for the generation of up to 40-50% of non-redundant phosphosites currently retrieved in human phosphopeptides database. In the present review, a short historical explanation will be provided accounting for the usage of the same misnomer to denote three unrelated classes of protein kinases, together with an update of our current knowledge of these pleiotropic enzymes, sharing the same misnomer while playing very distinct biological roles.

Pathognomonic oral profile of Enamel Renal Syndrome (ERS) caused by recessive FAM20A mutations

Amelogenesis imperfecta (AI) is a genetically and clinically heterogeneous group of inherited dental enamel defects. Commonly described as an isolated trait, it may be observed concomitantly with other orodental and/or systemic features such as nephrocalcinosis in Enamel Renal Syndrome (ERS, MIM#204690), or gingival hyperplasia in Amelogenesis Imperfecta and Gingival Fibromatosis Syndrome (AIGFS, MIM#614253). Patients affected by ERS/AIGFS present a distinctive orodental phenotype consisting of generalized hypoplastic AI affecting both the primary and permanent dentition, delayed tooth eruption, pulp stones, hyperplastic dental follicles, and gingival hyperplasia with variable severity and calcified nodules. Renal exam reveals a nephrocalcinosis which is asymptomatic in children affected by ERS. FAM20A recessive mutations are responsible for both syndromes. We suggest that AIGFS and ERS are in fact descriptions of the same syndrome, but that the kidney phenotype has not always been investigated fully in AIGFS. The aim of this review is to highlight the distinctive and specific orodental features of patients with recessive mutations in FAM20A. We propose ERS to be the preferred term for all the phenotypes arising from recessive FAM20A mutations. A differential diagnosis has to be made with other forms of AI, isolated or syndromic, where only a subset of the clinical signs may be shared. When ERS is suspected, the patient should be assessed by a dentist, nephrologist and clinical geneticist. Confirmed cases require long-term follow-up. Management of the orodental aspects can be extremely challenging and requires the input of multi-disciplinary specialized dental team, especially when there are multiple unerupted teeth.

Exonal Deletion of SLC24A4 Causes Hypomaturation Amelogenesis Imperfecta

Amelogenesis imperfecta is a heterogeneous group of genetic conditions affecting enamel formation. Recently, mutations in solute carrier family 24 member 4 ( SLC24A4) have been identified to cause autosomal recessive hypomaturation amelogenesis imperfecta. We recruited a consanguineous family with hypomaturation amelogenesis imperfecta with generalized brown discoloration. Sequencing of the candidate genes identified a 10-kb deletion, including exons 15, 16, and most of the last exon of the SLC24A4 gene. Interestingly, this deletion was caused by homologous recombination between two 354-bp-long homologous sequences located in intron 14 and the 3′ UTR. This is the first report of exonal deletion in SLC24A4 providing confirmatory evidence that the function of SLC24A4 in calcium transport has a crucial role in the maturation stage of amelogenesis.

FAM20A mutations associated with enamel renal syndrome

We identified two families with an autosomal-recessive disorder manifested by severe enamel hypoplasia, delayed and failed tooth eruption, misshapen teeth, intrapulpal calcifications, and localized gingival hyperplasia. Genetic analyses identified novel FAM20A mutations associated with the disease phenotype in both families. The proband of Family 1 had an altered splice junction in Intron 1 (g.502011G>C; c.405-1G>C) and a missense mutation in Exon 8 (g.65094G>A; c.1207G>A; p.D403N). The missense mutation is notable because D(403) is strictly conserved among FAM20A homologues, and the corresponding defect in FAM20C caused osteosclerotic bone dysplasia and a loss of kinase activity. The proband at age 12 yrs tested negative for nephrocalcinosis. The proband and her affected father in Family 2 were homozygous for a single nucleotide deletion that altered a splice junction in Intron 10 (g.66622del; c.1361+4del). Minigene analyses demonstrated that this alteration precluded normal splicing. Immunohistochemistry (IHC) of mouse maxillary first molars localized FAM20A in secretory-stage ameloblasts, in odontoblasts, and in the eruption pathway. IHC of kidneys localized FAM20A in the renal tubules. We conclude that FAM20A is likely a secretory pathway kinase and that loss-of-function mutations cause pathology where its phosphorylations are necessary for normal development or homeostasis.

Phosphorylation of substrates destined for secretion by the Fam20 kinases

Since the discovery of protein kinases, protein phosphorylation has emerged as a key regulatory mechanism. The majority of phosphoproteins reside within the nucleus and cytoplasm; however, many secreted proteins are phosphorylated by unknown kinases located within the secretory pathway and/or in the extracellular space. The Fam20 kinases are emerging as the enzymes responsible for phosphorylating secreted proteins and proteoglycans. Evolutionary analysis reveals that these kinases are exclusively present in metazoans and contain conserved features that are common among all eukaryotic protein kinases. Mutations in the Fam20 family members cause disorders of biomineralization in humans that highlight the physiological significance of secreted protein phosphorylation.

Enamel-renal-gingival syndrome and FAM20A mutations

The enamel-renal syndrome of amelogenesis imperfecta (AI) and nephrocalcinosis, and the amelogenesis imperfecta-gingival fibromatosis syndrome have both been associated with mutations in FAM20A. We report on two unrelated Thai patients with three novel and one previously reported mutations in FAM20A with findings suggesting both disorders, including hypoplastic AI, gingival fibromatosis, unerupted teeth, aggressive periodontitis, and nephrocalcinosis/nephrolithiasis. Additional findings consisted of a supernumerary premolar, localized aggressive periodontitis, thin alveolar bone, vitamin D deficiency-associated hyperparathyroidism, and heterotopic calcification in other tissues, including lungs, dental pulp, gingiva, dental follicles, and periodontal tissues, and early cessation of limited menstruation. Greater promotory activity of urine on calcium oxalate crystal growth compared to controls may help to explain the pathogenesis, and suggest that FAM20A mutations can contribute to nephrocalcinosis/nephrolithiasis. Our findings expand the phenotypic spectrum of FAM20A mutations. Since both of our patients and a large number of previously reported cases had all the important features of both syndromes, including AI, renal anomalies, and gingival fibromatosis, we are convinced that these two disorders actually are the same entity. The name of enamel-renal-gingival syndrome is suggested.

Crystal structure of the Golgi casein kinase

The family with sequence similarity 20 (Fam20) kinases phosphorylate extracellular substrates and play important roles in biomineralization. Fam20C is the Golgi casein kinase that phosphorylates secretory pathway proteins within Ser-x-Glu/pSer motifs. Mutations in Fam20C cause Raine syndrome, an osteosclerotic bone dysplasia. Here we report the crystal structure of the Fam20C ortholog from Caenorhabditis elegans. The nucleotide-free and Mn/ADP-bound structures unveil an atypical protein kinase-like fold and highlight residues critical for activity. The position of the regulatory αC helix and the lack of an activation loop indicate an architecture primed for efficient catalysis. Furthermore, several distinct elements, including the presence of disulfide bonds, suggest that the Fam20 family diverged early in the evolution of the protein kinase superfamily. Our results reinforce the structural diversity of protein kinases and have important implications for patients with disorders of biomineralization.

FAM20A mutations can cause enamel-renal syndrome (ERS)

Enamel-renal syndrome (ERS) is an autosomal recessive disorder characterized by severe enamel hypoplasia, failed tooth eruption, intrapulpal calcifications, enlarged gingiva, and nephrocalcinosis. Recently, mutations in FAM20A were reported to cause amelogenesis imperfecta and gingival fibromatosis syndrome (AIGFS), which closely resembles ERS except for the renal calcifications. We characterized three families with AIGFS and identified, in each case, recessive FAM20A mutations: family 1 (c.992G>A; g.63853G>A; p.Gly331Asp), family 2 (c.720-2A>G; g.62232A>G; p.Gln241_Arg271del), and family 3 (c.406C>T; g.50213C>T; p.Arg136* and c.1432C>T; g.68284C>T; p.Arg478*). Significantly, a kidney ultrasound of the family 2 proband revealed nephrocalcinosis, revising the diagnosis from AIGFS to ERS. By characterizing teeth extracted from the family 3 proband, we demonstrated that FAM20A(-/-) molars lacked true enamel, showed extensive crown and root resorption, hypercementosis, and partial replacement of resorbed mineral with bone or coalesced mineral spheres. Supported by the observation of severe ectopic calcifications in the kidneys of Fam20a null mice, we conclude that FAM20A, which has a kinase homology domain and localizes to the Golgi, is a putative Golgi kinase that plays a significant role in the regulation of biomineralization processes, and that mutations in FAM20A cause both AIGFS and ERS.

Nephrocalcinosis (enamel renal syndrome) caused by autosomal recessive FAM20A mutations

BACKGROUND/AIMS

Calcium homeostasis requires regulated cellular and interstitial systems interacting to modulate the activity and movement of this ion. Disruption of these systems in the kidney results in nephrocalcinosis and nephrolithiasis, important medical problems whose pathogenesis is incompletely understood.

METHODS

We investigated 25 patients from 16 families with unexplained nephrocalcinosis and characteristic dental defects (amelogenesis imperfecta, gingival hyperplasia, impaired tooth eruption). To identify the causative gene, we performed genome-wide linkage analysis, exome capture, next-generation sequencing, and Sanger sequencing.

RESULTS

All patients had bi-allelic FAM20A mutations segregating with the disease; 20 different mutations were identified.

CONCLUSIONS

This autosomal recessive disorder, also known as enamel renal syndrome, of FAM20A causes nephrocalcinosis and amelogenesis imperfecta. We speculate that all individuals with biallelic FAM20A mutations will eventually show nephrocalcinosis.

Secreted protein kinases

Protein kinases constitute one of the largest gene families and control many aspects of cellular life. In retrospect, the first indication for their existence was reported 130 years ago when the secreted protein, casein, was shown to contain phosphate. Despite its identification as the first phosphoprotein, the responsible kinase has remained obscure. This conundrum was solved with the discovery of a novel family of atypical protein kinases that are secreted and appear to phosphorylate numerous extracellular proteins, including casein. Fam20C, the archetypical member, phosphorylates secreted proteins within Ser-x-Glu/pSer motifs. This discovery has solved a 130-year-old mystery and has shed light on several human disorders of biomineralization.

Typical Features of Amelogenesis Imperfecta in Two Patients with Bartter's Syndrome

BACKGROUND/AIMS

Amelogenesis imperfecta (AI) is due to many inherited defects of enamel formation that affect the quantity and quality of enamel, leading to delay in tooth eruption and cosmetic consequences. AI has been described in association with nephrocalcinosis, which is called the enamel-renal syndrome. The aim of this case report is to describe typical features of AI in 2 patients with Bartter's syndrome (BS) for the first time.

METHODS

-Eight patients with confirmed BS were systematically screened for dental abnormalities as part of protocol. Those with suggestive clinical features of AI were submitted to panoramic X-ray and decayed teeth were analyzed by scanning electron microscopy.

RESULTS

Typical features of AI were detected in 2 girls with BS. These 2 patients showed nephrocalcinosis, and diagnosis and adequate clinical control were delayed. Genetic analysis detected the mutation responsible for BS in 1 of these patients. In this case, BS was due to a homozygous mutation of exon 5 of the KCNJ1 gene resulting in a substitution of valine for alanine at the codon 214 (A214V).

CONCLUSIONS

The finding of typical features of AI in BS might constitute preliminary evidence that abnormalities of the biomineralization process found in patients with renal tubular disorders might also affect calcium deposition in dental tissues.