Pathogenic mechanisms of tooth agenesis linked to paired domain mutations in human PAX9

Unveiling the Impact of Maternal Hyperthermia in the Late First Trimester: A Case Report of Anterior Esthetic Rehabilitation Utilizing Heterodontic Biologic Posts

The perinatal maternal environment is important for the normal development of the fetus. Epigenetic modifications that influence developmental control genes and signalling pathways for proper fetal development have been associated with maternal illnesses brought on by viruses, bacteria, or even parasitic protozoa. It is crucial to provide details on the onset, length, and timing of the mother's fever because these factors may influence the kind of certain abnormalities. Although fever is a primarily benign disease, it has been linked to negative health outcomes in children and has occasionally resulted in a substantial referral to critical care. This case report presents a 15-year-old female patient with repaired cleft palate and tetralogy of Fallot (TOF) who approached for esthetic rehabilitation of lower anterior teeth. The teeth (31, 32, 43) were tender on percussion. Radiographic evaluation showed the presence of periapical radiolucency. The root canal procedure was performed under local anaesthesia, and the supernumerary maxillary teeth were extracted. After cleaning and disinfecting, these teeth were used as biologic posts with respect to 32 and 33. A follow-up examination was performed after 12 months. The results of this case indicate that using autologous heterodontic biologic posts can lead to a favourable outcome.

The PAX Genes: Roles in Development, Cancer, and Other Diseases

Since their 1986 discovery in Drosophila, Paired box (PAX) genes have been shown to play major roles in the early development of the eye, muscle, skeleton, kidney, and other organs. Consistent with their roles as master regulators of tissue formation, the PAX family members are evolutionarily conserved, regulate large transcriptional networks, and in turn can be regulated by a variety of mechanisms. Losses or mutations in these genes can result in developmental disorders or cancers. The precise mechanisms by which PAX genes control disease pathogenesis are well understood in some cases, but much remains to be explored. A deeper understanding of the biology of these genes, therefore, has the potential to aid in the improvement of disease diagnosis and the development of new treatments.

Tooth agenesis patterns and variants in PAX9: A systematic review

Mutations in PAX9 are the most common genetic cause of tooth agenesis (TA). The aim of this study was to systematically review the profiles of the TA and PAX9 variants and establish their genotype-phenotype correlation. Forty articles were eligible for 178 patients and 61 mutations (26 in frame and 32 null mutations). PAX9 mutations predominantly affected molars, mostly the second molar, and the mandibular first premolar was the least affected. More missing teeth were found in the maxilla than the mandible, and with null mutations than in-frame mutations. The number of missing teeth was correlated with the locations of the in-frame mutations with the C-terminus mutations demonstrating the fewest missing teeth. The null mutation location did not influence the number of missing teeth. Null mutations in all locations predominantly affected molars. For the in-frame mutations, a missing second molar was commonly associated with mutations in the highly conserved paired DNA-binding domain, particularly the linking peptide (100% prevalence). In contrast, C-terminus mutations were rarely associated with missing second molars and anterior teeth, but were commonly related to an absent second premolar. These finding indicate that the mutation type and position contribute to different degrees of loss of PAX9 function that further differentially influences the manifestations of TA. This study provides novel information on the correlation of the PAX9 genotype-phenotype, aiding in the genetic counseling for TA.

Count Me in, Count Me out: Regulation of the Tooth Number via Three Directional Developmental Patterns

Tooth number anomalies, including hyperdontia and hypodontia, are common congenital dental problems in the dental clinic. The precise number of teeth in a dentition is essential for proper speech, mastication, and aesthetics. Teeth are ectodermal organs that develop from the interaction of a thickened epithelium (dental placode) with the neural-crest-derived ectomesenchyme. There is extensive histological, molecular, and genetic evidence regarding how the tooth number is regulated in this serial process, but there is currently no universal classification for tooth number abnormalities. In this review, we propose a novel regulatory network for the tooth number based on the inherent dentition formation process. This network includes three intuitive directions: the development of a single tooth, the formation of a single dentition with elongation of the continual lamina, and tooth replacement with the development of the successional lamina. This article summarizes recent reports on early tooth development and provides an analytical framework to classify future relevant experiments.

The phenotype and genotype of PAX9 mutations causing tooth agenesis

OBJECTIVES

The purpose of this study was to identify associations between PAX9 mutations and clinical features of non-syndromic tooth agenesis patients.

MATERIALS AND METHODS

Non-syndromic tooth agenesis patients were found to have mutations by whole exome sequencing (WES). Additionally, conservation analysis and three-dimensional structure prediction were also applied to identify mutated proteins.

RESULTS

Eight non-syndromic tooth agenesis probands were identified with PAX9 mutations (c.C112T; C.131_134del; c.G151A; c.189delG; c.305delT; c.C365A; c.394delG; c.A679C). All of the probands were missing more than six teeth (oligodontia). The mutations (c.131_134del,p.R44fs; c.189delG,p.T63fs; c.305delT,p.I102fs and c.394delG,p.G123fs) caused premature termination of the PAX9 protein. The c.C112T(p.R38X) mutation created a truncated protein. Bioinformatic prediction demonstrated that the three missense mutations change the PAX9 structure suggesting the corresponding functional impairments.

CONCLUSIONS

We reported that eight mutations of PAX9 caused non-syndromic tooth agenesis and analyzed the relationship between PAX9 mutations and non-syndromic tooth agenesis.

CLINICAL RELEVANCE

Our study revealed that PAX9 mutations might be the mutations most associated with non-syndromic tooth agenesis in humans, which greatly broadened the mutation spectrum of PAX9-related non-syndromic tooth agenesis.

BMPR2 Variants Underlie Nonsyndromic Oligodontia

Oligodontia manifests as a congenital reduction in the number of permanent teeth. Despite the major efforts that have been made, the genetic etiology of oligodontia remains largely unknown. Bone morphogenetic protein receptor type 2 (BMPR2) variants have been associated with pulmonary arterial hypertension (PAH). However, the genetic significance of BMPR2 in oligodontia has not been previously reported. In the present study, we identified a novel heterozygous variant (c.814C > T; p.Arg272Cys) of BMPR2 in a family with nonsyndromic oligodontia by performing whole-exome sequencing. In addition, we identified two additional heterozygous variants (c.1042G > A; p.Val348Ile and c.1429A > G; p.Lys477Glu) among a cohort of 130 unrelated individuals with nonsyndromic oligodontia by performing Sanger sequencing. Functional analysis demonstrated that the activities of phospho-SMAD1/5/8 were significantly inhibited in BMPR2-knockout 293T cells transfected with variant-expressing plasmids, and were significantly lower in BMPR2 heterozygosity simulation groups than in the wild-type group, indicating that haploinsufficiency may represent the genetic mechanism. RNAscope in situ hybridization revealed that BMPR2 transcripts were highly expressed in the dental papilla and adjacent inner enamel epithelium in mice tooth germs, suggesting that BMPR2 may play important roles in tooth development. Our findings broaden the genetic spectrum of oligodontia and provide clinical and genetic evidence supporting the importance of BMPR2 in nonsyndromic oligodontia.

An in vitro and computational validation of a novel loss-of-functional mutation in PAX9 associated with non-syndromic tooth agenesis

Congenital tooth agenesis (CTA) is one of the most common craniofacial anomalies. Its frequency varies among different population depending upon the genetic heterogeneity. CTA could be of familial or sporadic and syndromic or non-syndromic. Five major genes are found to be associated with non-syndromic CTA, namely PAX9, MSX1, EDA1, AXIN2, and WNT10A. Very few studies have been carried out so far on CTA on this Indian population making this study unique and important. This study was initiated to identify potential pathogenic variant associated with congenital tooth agenesis in an India family with molar tooth agenesis. CTA was investigated and a novel c.336C > G variation was identified in the exon 3 of PAX9, leading to substitution of evolutionary conserved Cys with Trp at 112th amino acid position located at the functionally significant DNA-binding paired domain region. Functional analysis revealed that p.Cys112Trp mutation did not prevent the nuclear localization although mutant protein had higher cytoplasmic retention. EMSA using e5 probe revealed that mutant protein was unable to bind with the paired-domain-binding site. Subsequently, GST pull-down assay revealed lower binding activity of the mutant protein with its known interactor MSX1. These in vitro results were consistent with the computational results. The in vitro and computational observations altogether suggest that c.336C > G (p.Cys112Trp) variation leads to loss of function of PAX9 leading to CTA in this family.

Four Novel PAX9 Variants and the PAX9-Related Non-Syndromic Tooth Agenesis Patterns

The purpose of this research was to investigate and identify PAX9 gene variants in four Chinese families with non-syndromic tooth agenesis. We identified pathogenic gene variants by whole-exome sequencing (WES) and Sanger sequencing and then studied the effects of these variants on function by bioinformatics analysis and in vitro experiments. Four novel PAX9 heterozygous variants were identified: two missense variants (c.191G > T (p.G64V) and c.350T > G (p.V117G)) and two frameshift variants (c.352delC (p.S119Pfs*2) and c.648_649insC(p.Y217Lfs*100)). The bioinformatics analysis showed that these variants might be pathogenic. The tertiary structure analysis showed that these four variants could cause structural damage to PAX9 proteins. In vitro functional studies demonstrated that (1) the p.Y217Lfs*100 variant greatly affects mRNA stability, thereby affecting endogenous expression; (2) the p. S119Pfs* 2 variant impairs the subcellular localization of the nuclear expression of the wild-type PAX9 protein; and (3) the four variants (p.G64V, p.V117G, p.S119Pfs*2, and p.Y217Lfs*100) all significantly affect the downstream transcriptional activity of the BMP4 gene. In addition, we summarized and analyzed tooth missing positions caused by PAX9 variants and found that the maxillary second molar (84.11%) and mandibular second molar (84.11%) were the most affected tooth positions by summarizing and analyzing the PAX9-related non-syndromic tooth agenesis positions. Our results broaden the variant spectrum of the PAX9 gene related to non-syndromic tooth agenesis and provide useful information for future genetic counseling.

Detection of Novel Variant and Functional Study in a Chinese Family with Non-syndromic Oligodontia

OBJECTIVES

To investigate the pathogenic gene of a patient with non-syndromic oligodontia, and analyze its possible pathogenic mechanism.

SUBJECTS AND METHODS

The variant was detected by whole exome sequencing (WES) and Sanger sequencing in a family with oligodontia. Bioinformatic and structural analyses were used to analyze variant. Functional studies including western blotting and immunofluorescent analyses and luciferase reporter assay were conducted to explore the functional effects.

RESULTS

We identified a novel frameshift variant of PAX9 (c.491-510delGCCCT-ATCACGGCGGCGGCC, p.P165Qfs*145) outside the DNA-binding domain causing an autosomal-dominant non-syndromic oligodontia in a Chinese family. Bioinformatic and structural analyses revealed that the variant is pathogenic and conserved evolutionarily, and the changes might affect protein stability or folding. Functional studies demonstrate dramatically reduced ability in activating transcription activity of BMP4 promoter and a marked decrease in protein production, as evaluated by western blotting and immunofluorescent analyses.

CONCLUSIONS

We found a novel frameshift variant of PAX9 causing non-syndromic oligodontia in a Chinese family. Our findings indicate that frameshift variants cause loss of function of PAX9 protein during the patterning of the dentition and the subsequent tooth agenesis, providing new molecular insights into the role of frameshift variant of PAX9 and broaden the pathogenic spectrum of PAX9 variants.

Patterns of molar agenesis associated with p.P20L and p.R77Q variants in PAX9

Nonsyndromic tooth agenesis is associated with variants in several genes. There are numerous genotype-phenotype publications involving many patients and kindreds. Here, we identified six Thai individuals in two families with nonsyndromic tooth agenesis, performed exome sequencing, and conducted functional experiments. Family 1 had four affected members carrying the heterozygous PAX9 variant, c.59C>T (p.Pro20Leu). The p.Pro20Leu was previously reported in two families having four and three affected members. These seven cases and Proband-1 had agenesis of at least three third molars. Family 2 comprised two affected members with agenesis of all 12 molars. Both individuals were heterozygous for c.230G>A (p.Arg77Gln) in PAX9, which has not been reported previously. This variant is predicted to be damaging, evolutionarily conserved, and resides in the PAX9 linking peptide. The BMP4 RNA levels in Proband-1's leukocytes were not significantly different from those in the controls, whereas BMP4 levels observed in Proband-2 were significantly increased. Moreover, the p.Arg77Gln variant demonstrated nuclear localization similar to the wild-type but resulted in significantly impaired transactivation of BMP4, a PAX9 downstream gene. In conclusion, we demonstrate that the PAX9 p.Pro20Leu is highly associated with absent third molars, while the novel PAX9 p.Arg77Gln impairs BMP4 transactivation and is associated with total molar agenesis.

Identification of novel candidate genes implicated in odontogenic potential in the developing mouse tooth germ using transcriptome analysis

BACKGROUND

In tooth bioengineering for replacement therapy of missing teeth, the utilized cells must possess an inductive signal-forming ability to initiate odontogenesis. This ability is called odontogenic potential. In mice, the odontogenic potential signal is known to be translocated from the epithelium to the mesenchyme at the early bud stage in the developing molar tooth germ. However, the identity of the molecular constituents of this process remains unclear.

OBJECTIVE

The purpose of this study is to determine the molecular identity of odontogenic potential and to provide a new perspective in the field of tooth development research.

METHODS

In this study, whole transcriptome profiles of the mouse molar tooth germ epithelium and mesenchyme were investigated using the RNA sequencing (RNA-seq) technique. The analyzed transcriptomes corresponded to two developmental stages, embryonic day 11.5 (E11.5) and 14.5 (E14.5), which represent the odontogenic potential shifts.

RESULTS

We identified differentially expressed genes (DEGs), which were specifically overexpressed in both the E11.5 epithelium and E14.5 mesenchyme, but not expressed in their respective counterparts. Of the 55 DEGs identified, the top three most expressed transcription factor genes (transcription factor AP-2 beta isoform 3 [TFAP2B], developing brain homeobox protein 2 [DBX2], and insulin gene enhancer protein ISL-1 [ISL1]) and three tooth development-related genes (transcription factor HES-5 [HES5], platelet-derived growth factor D precursor [PDGFD], semaphrin-3 A precursor [SEMA3A]) were selected and validated by quantitative RT-PCR. Using immunofluorescence staining, the TFAP2B protein expression was found to be localized only at the E11.5 epithelium and E14.5 mesenchyme.

CONCLUSIONS

Thus, our empirical findings in the present study may provide a new perspective into the characterization of the molecules responsible for the odontogenic potential and may have an implication in the cell-based whole tooth regeneration strategy.

The clinical significance and correlative signaling pathways of paired box gene 9 in development and carcinogenesis

Paired box 9 (PAX9) gene belongs to the PAX family, which encodes a family of metazoan transcription factors documented by a conserved DNA binding paired domain 128-amino-acids, critically essential for physiology and development. It is primarily expressed in embryonic tissues, such as the pharyngeal pouch endoderm, somites, neural crest-derived mesenchyme, and distal limb buds. PAX9 plays a vital role in craniofacial development by maintaining the odontogenic potential, mutations, and polymorphisms associated with the risk of tooth agenesis, hypodontia, and crown size in dentition. The loss-of-function of PAX9 in the murine model resulted in a short life span due to the arrest of cleft palate formation and skeletal abnormalities. According to recent studies, the PAX9 gene has a significant role in maintaining squamous cell differentiation, odontoblast differentiation of pluripotent stem cells, deregulation of which is associated with tumor initiation, and malignant transformation. Moreover, PAX9 contributes to promoter hypermethylation and alcohol- induced oro-esophageal squamous cell carcinoma mediated by downregulation of differentiation and apoptosis. Likewise, PAX9 activation is also reported to be associated with drug sensitivity. In summary, this current review aims to understand PAX9 function in the regulation of development, differentiation, and carcinogenesis, along with the underlying signaling pathways for possible cancer therapeutics.

Common variants of EDA are associated with non-syndromic hypodontia

OBJECTIVE

The aim of this case-control study was to investigate the association between non-syndromic hypodontia and nineteen common variants of candidate genes ectodysplasin A (EDA), paired box 9 (PAX9), msh homeobox 1 (MSX1) and axis inhibition protein 2 (AXIN2).

SETTINGS AND SAMPLE POPULATION

Sixty-one hypodontia cases were frequency-matched to 253 controls with no missing teeth (excluding the third molars).

MATERIAL AND METHODS

Self-report data and DNA samples were collected from each participant.

RESULTS

The sample had a mean age of 16.6 years (SD = 7.3), with most participants being female (59.6%), and of New Zealand European origin (75.4%). Using multiple logistic regression analysis, it was found that the T-allele of rs12853659 (EDA) and the G-allele of rs2428151 (EDA) were both associated with a higher risk of hypodontia (odds ratio, OR = 2.79, 95% CI = 1.11-7.01; and OR = 2.87, 95% CI = 1.04-7.94, respectively). The G-allele of rs2520378 (EDA) showed a protective effect with an OR of 0.61 (95% CI = 0.38-0.99). The EDA SNP findings were consistent with previous reports included in a meta-analysis. No associations were found with the PAX9, AXIN2 and MSX1 genes, after adjusting for sex and ethnicity.

CONCLUSIONS

Common variants of the EDA genes are associated with specific phenotypes of non-syndromic hypodontia, thus confirming their role in the regulatory pathways of normal tooth development. However, larger samples are needed to investigate the association further.

Graphene-Based Nanomaterials for Tissue Engineering in the Dental Field

The world of dentistry is approaching graphene-based nanomaterials as substitutes for tissue engineering. Apart from its exceptional mechanical strength, electrical conductivity and thermal stability, graphene and its derivatives can be functionalized with several bioactive molecules. They can also be incorporated into different scaffolds used in regenerative dentistry, generating nanocomposites with improved characteristics. This review presents the state of the art of graphene-based nanomaterial applications in the dental field. We first discuss the interactions between cells and graphene, summarizing the available in vitro and in vivo studies concerning graphene biocompatibility and cytotoxicity. We then highlight the role of graphene-based nanomaterials in stem cell control, in terms of adhesion, proliferation and differentiation. Particular attention will be given to stem cells of dental origin, such as those isolated from dental pulp, periodontal ligament or dental follicle. The review then discusses the interactions between graphene-based nanomaterials with cells of the immune system; we also focus on the antibacterial activity of graphene nanomaterials. In the last section, we offer our perspectives on the various opportunities facing the use of graphene and its derivatives in associations with titanium dental implants, membranes for bone regeneration, resins, cements and adhesives as well as for tooth-whitening procedures.

Characterization of PAX9 variant P20L identified in a Japanese family with tooth agenesis

Transcription factors PAX9 and MSX1 play crucial roles in the development of permanent teeth at the bud stage, and their loss-of-function variants have been associated with congenital tooth agenesis. We sequenced the coding regions of the PAX9 and MSX1 genes from nine patients with non-syndromic tooth agenesis, and identified a missense mutation, P20L, of PAX9 in a single familial case involving three patients in two generations. Identical mutation was previously reported by other authors, but has not been characterized in detail. The mutation was located in a highly conserved N-terminal subdomain of the paired domain and co-segregated as a heterozygote with tooth agenesis. The patients showed defects primarily in the first and second molars, which is typical for cases attributable to PAX9 mutation. Luciferase reporter assay using the 2.3-kb promoter region of BMP4 and electrophoretic mobility shift assay using the CD19-2(A-ins) sequence revealed that P20L substitution eliminated most of the transactivation activity and specific DNA binding activity of PAX9 under the experimental conditions we employed, while some residual activity of the mutant was evident in the former assay. The hypomorphic nature of the variant may explain the relatively mild phenotype in this case, as compared with other PAX9 pathogenic variants such as R26W.

[Risk factors for teeth aplasia and hypoplasia in cleft lip and palate children]

The aim of the study was to assess the significance of environmental risk factors for teeth aplasia and hypoplasia in cleft lip and palate children. Two hundred and forty-seven cleft lip and palate (CLP) children were enrolled in the study including 105 (42.5%) with bilateral CLP and 57.5% with unilateral CLP. The mean age was 11.2±4.9 years. Teeth condition was assessed clinically and radiologically. The impact of risk factors for teeth anomalies was analyzed by retrospective data obtained from computer database (absence of preoperative orthopedic treatment, palatal defects after primary palatoplasty and type of primary procedures). Surgical trauma by early periosteoplasty (at the age of 3-4 months), excessive scarring and tissue traction due to absence of early orthopedic treatment and palatal defect were associated with significantly higher incidence of incisors hypoplasia (both developmental enamel defects and microdentia) and aplasia of central incisors not seen in the other study subgroups. Incisors aplasia and hypoplasia in CLP patients do not always have disembryogenic origin but may depend on external environmental factors, including surgical trauma.

Transcription Factors in Craniofacial Development: From Receptor Signaling to Transcriptional and Epigenetic Regulation

Craniofacial morphogenesis is driven by spatial-temporal terrains of gene expression, which give rise to stereotypical pattern formation. Transcription factors are key cellular components that control these gene expressions. They are information hubs that integrate inputs from extracellular factors and environmental cues, direct epigenetic modifications, and define transcriptional status. These activities allow transcription factors to confer specificity and potency to transcription regulation during development.

Novel PAX9 Mutations Cause Non-syndromic Tooth Agenesis

PAX9 is a transcription factor expressed in the tooth mesenchyme during tooth morphogenesis. In Pax9-null mice, tooth development is arrested at the bud stage. In humans, heterozygous mutations in PAX9 have been associated with non-syndromic tooth agenesis, predominantly in the molars. Here, we report 2 novel mutations in the paired domain of PAX9, a three-nucleotide deletion (73-75 delATC) and a missense mutation (C146T), in two unrelated Japanese patients with non-syndromic tooth agenesis. The individual with the 73-75del ATC mutation was missing all maxillary molars and mandibular second and third molars. The individual with the C146T mutation was missing the mandibular central incisors, maxillary second premolars, and first molars, along with all second and third molars. Both mutations affected amino acids that are highly conserved among different species and are critical for DNA binding. When both mutants were transfected to COS7 cells, nuclear localization of PAX9 proteins was not affected. However, reduced expression of the mutant proteins and almost no transcriptional activity of the target BMP4 gene were observed, suggesting haploinsufficiency of PAX9 as the cause of non-syndromic tooth agenesis.

Contrasting evolutionary dynamics of the developmental regulator PAX9, among bats, with evidence for a novel post-transcriptional regulatory mechanism

Morphological evolution can be the result of natural selection favoring modification of developmental signaling pathways. However, little is known about the genetic basis of such phenotypic diversity. Understanding these mechanisms is difficult for numerous reasons, yet studies in model organisms often provide clues about the major developmental pathways involved. The paired-domain gene, PAX9, is known to be a key regulator of development, particularly of the face and teeth. In this study, using a comparative genetics approach, we investigate PAX9 molecular evolution among mammals, focusing on craniofacially diversified (Phyllostomidae) and conserved (Vespertilionidae) bat families, and extend our comparison to other orders of mammal. Open-reading frame analysis disclosed signatures of selection, in which a small percentage of residues vary, and lineages acquire different combinations of variation through recurrent substitution and lineage specific changes. A few instances of convergence for specific residues were observed between morphologically convergent bat lineages. Bioinformatic analysis for unknown PAX9 regulatory motifs indicated a novel post-transcriptional regulatory mechanism involving a Musashi protein. This regulation was assessed through fluorescent reporter assays and gene knockdowns. Results are compatible with the hypothesis that the number of Musashi binding-elements in PAX9 mRNA proportionally regulates protein translation rate. Although a connection between morphology and binding element frequency was not apparent, results indicate this regulation would vary among craniofacially divergent bat species, but be static among conserved species. Under this model, Musashi's regulatory control of alternative human PAX9 isoforms would also vary. The presence of Musashi-binding elements within PAX9 of all mammals examined, chicken, zebrafish, and the fly homolog of PAX9, indicates this regulatory mechanism is ancient, originating basal to much of the animal phylogeny.

Novel PAX9 and COL1A2 missense mutations causing tooth agenesis and OI/DGI without skeletal abnormalities

Inherited dentin defects are classified into three types of dentinogenesis imperfecta (DGI) and two types of dentin dysplasia (DD). The genetic etiology of DD-I is unknown. Defects in dentin sialophosphoprotein (DSPP) cause DD type II and DGI types II and III. DGI type I is the oral manifestation of osteogenesis imperfecta (OI), a systemic disease typically caused by defects in COL1A1 or COL1A2. Mutations in MSX1, PAX9, AXIN2, EDA and WNT10A can cause non-syndromic familial tooth agenesis. In this study a simplex pattern of clinical dentinogenesis imperfecta juxtaposed with a dominant pattern of hypodontia (mild tooth agenesis) was evaluated, and available family members were recruited. Mutational analyses of the candidate genes for DGI and hypodontia were performed and the results validated. A spontaneous novel mutation in COL1A2 (c.1171G>A; p.Gly391Ser) causing only dentin defects and a novel mutation in PAX9 (c.43T>A; p.Phe15Ile) causing hypodontia were identified and correlated with the phenotypic presentations in the family. Bone radiographs of the proband's dominant leg and foot were within normal limits. We conclude that when no DSPP mutation is identified in clinically determined isolated DGI cases, COL1A1 and COL1A2 should be considered as candidate genes. PAX9 mutation p.Phe15Ile within the N-terminal β-hairpin structure of the PAX9 paired domain causes tooth agenesis.

Genetic basis of non-syndromic anomalies of human tooth number

Teeth organogenesis develops through a well-ordered series of inductive events involving genes and BMP, FGF, SHH and WNT represent the main signalling pathways that regulate epithelial-mesenchymal interactions. Moreover, progress in genetics and molecular biology indicates that more than 300 genes are involved in different phases of teeth development. Mutations in genes involved in odontogenesis are responsible for many dental anomalies, including a number of dental anomalies that can be associated with other systemic skeletal or organic manifestations (syndromic dental anomalies) or not (non-syndromic dental anomalies). The knowledge of the genetic development mechanisms of the latter is of major interest. Understanding the mechanisms of pathogenesis of non-syndromic teeth anomalies would also clarify the role of teeth in craniofacial development, and this would represent an important contribution to the diagnosis, treatment and prognosis of congenital malformations, and the eventual association to other severe diseases. Future research in this area is likely to lead to the development of tests for doctors to formulate an early diagnosis of these anomalies.

Novel missense mutations in PAX9 causing oligodontia

OBJECTIVE

We investigated the disease-causing gene of oligodontia in Chinese families and analysed the pathogenesis of mutations of this gene that results in oligodontia.

METHODS

Two families with oligodontia, but of different descent and 100 unrelated healthy controls were enrolled in our study. Genomic DNA was isolated from blood samples. Mutation analysis was performed by amplifying MSX1 and PAX9 exons and sequencing the products. After identifying the mutations, we performed site-directed mutagenesis to generate mutated vectors. The wild-type and mutated PAX9 vectors were then transfected separately to NIH3T3 cells. Immunolocalization, electrophoretic mobility shift assay (EMSA) and luciferase reporter assay were performed to analyse the effects of mutations on protein function.

RESULTS

We identified two novel missense mutations, Leu27Pro (L27P) and Ile29Thr (I29T) in the paired-domain of PAX9. Analysis of homologous PAX proteins indicated that these two substitutions may affect the function of the PAX9 protein. Results of immunofluorescence and western blot showed that the mutations did not alter the nuclear localization of PAX9. EMSA and luciferase reporter assays indicated that both the mutated proteins could not bind DNA or transactivate the BMP4 promoter.

CONCLUSIONS

Two novel missense mutations in PAX9 have been indentified in Chinese families causing oligodontia.

A novel nonsense mutation in PAX9 is associated with sporadic hypodontia

The most important events during the regulation of tooth development were inductive interactions between the epithelial and mesenchymal tissues. The expression of Pax9 had been shown to specifically mark the mesenchymal regions at the prospective sites of all teeth prior to any morphological manifestations. Here, we investigated the PAX9 gene as a candidate gene for hypodontia in five unrelated Chinese patients with tooth agenesis. Direct sequencing and restriction enzyme analysis revealed a novel heterozygous mutation c.480C>G (p.160Tyr>X, Y160X) in a patient who was missing 20 permanent teeth (the third molars excluded) and 6 primary teeth. The mutation was a nonsense mutation, leading to a premature stop codon in exon 2 of PAX9 gene. PCR analysis of complementary DNA from cultured lymphocytes of the affected individual could not indicate the complete degradation of the mutated transcript. Promoter reporter assays revealed reduced transcriptional activity of the mutated PAX9 protein suggesting that the severe phenotype may result from haploinsufficiency of PAX9. In another patient with 15 missing permanent teeth (the third molars excluded), we found the c.219insG mutation previously reported by Stockton.

Regulation of bmp4 expression in odontogenic mesenchyme: from simple to complex

For many years the molecular mechanisms governing bone morphogenetic protein 4 (Bmp4) expression in tooth bud mesenchyme could be explained by an uncomplicated model involving the interaction of the homeobox gene Msx1 and the paired domain gene Pax9 and a limited proximal promoter segment of Bmp4. New insights have led to major revisions, but we are still far from understanding the role of Msx1 and Pax9 in the complex processes that result in the expression of Bmp4 in the mesenchymal layer of the developing tooth bud. The objective of these studies was to gain further insight into the molecular relationship between Pax9, Msx1, and Bmp4 in dental mesenchyme and explore its association with nonsyndromic tooth agenesis in humans.

Clinical and functional data implicate the Arg(151)Ser variant of MSX1 in familial hypodontia

Multiple previous reports confirm that several missense alleles of MSX1 exhibit Mendelian inheritance of an oligodontia phenotype (agenesis of more than six secondary teeth besides third molars). However, the extent to which missense MSX1 alleles contribute to common, multifactorial disorders is less certain. It is still not yet clear whether multiple non-synonomous MSX1-coding variants identified among patients with oral clefting are merely neutral polymorphisms or whether any of these might represent real mutations with mild effects. The present work steps toward resolving these issues for at least one MSX1 allele: R151S, previously identified in a single Japanese proband with unilateral cleft lip and palate. Candidate gene sequencing within a patient cohort demonstrating mild tooth agenesis (loss of six or less secondary teeth besides third molars, hypodontia), secondarily identified this same MSX1 variant, functioning as a mildly deleterious, moderately penetrant allele. Four of five heterozygous R151S individuals from one Japanese family exhibited the hypodontia phenotype. The in vitro functional assays of the variant protein display partial repression activity with normal nuclear localization. These data establish that the MSX1-R151S allele is a low-frequency, mildly deleterious allele for familial hypodontia that alone is insufficient to cause oral facial clefting. Yet, as this work also establishes its hypomorphic nature, it suggests that it may in fact contribute to the likelihood of common birth disorder phenotypes, such as partial tooth agenesis and oral facial clefting. Nevertheless, the exact mechanism in which differential pleiotropy is manifested will need further and deeper clinical and functional analyses.

Msx1 mutations: how do they cause tooth agenesis?

Mutations in the transcription factors PAX9 and MSX1 cause selective tooth agenesis in humans. In tooth bud mesenchyme of mice, both proteins are required for the expression of Bmp4, which is the key signaling factor for progression to the next step of tooth development. We have previously shown that Pax9 can transactivate a 2.4-kb Bmp4 promoter construct, and that most tooth-agenesis-causing PAX9 mutations impair DNA binding and Bmp4 promoter activation. We also found that Msx1 by itself represses transcription from this proximal Bmp4 promoter, and that, in combination with Pax9, it acts as a potentiator of Pax9-induced Bmp4 transactivation. This synergism of Msx1 with Pax9 is significant, because it is currently the only documented mechanism for Msx1-mediated activation of Bmp4. In this study, we investigated whether the 5 known tooth-agenesis-causing MSX1 missense mutations disrupt this Pax9-potentiation effect, or if they lead to deficiencies in protein stability, protein-protein interactions, nuclear translocation, and DNA-binding. We found that none of the studied molecular mechanisms yielded a satisfactory explanation for the pathogenic effects of the Msx1 mutations, calling for an entirely different approach to the investigation of this step of odontogenesis on the molecular level.

Non-syndromic Oligodontia with a Novel Mutation of PAX9

Agenesis of the permanent teeth is a congenital anomaly that is frequently seen in humans. Oligodontia is a severe type of tooth agenesis involving 6 or more congenitally missing teeth, excluding the third molars. Previous studies have indicated that mutations in the homeobox gene MSX1, paired domain transcription factor PAX9, and EDA are associated with non-syndromic oligodontia. This study reports a Japanese family (eight of 14 family members affected) with non-syndromic oligodontia who preferentially lacked molar teeth. In this family, a novel frameshift mutation (321_322insG) was identified in the paired domain of PAX9. The frameshift mutation caused altered amino acids in the paired domain and premature termination of translation by 26 amino acids. When transfected into COS-7 cells, the mRNA expression of 321_322insG PAX9 was comparable with that of wild-type PAX9. However, the mRNA of 321_322insG PAX9 was more unstable than that of wild-type PAX9. This mRNA instability caused a marked decrease in protein production, as evaluated by Western blot analysis and immunostaining. These findings suggest that the 321_322insG mutation causes insufficient function of PAX9 protein and haploinsufficiency as a genetic model of familial non-syndromic oligodontia with a PAX9 mutation.

Genetics and human malformations

Genetics gains more and more importance in all areas of health care including craniofacial surgery and dentistry. This does not mean that every patient will benefit from genetic advances, but for many health problems, we will see progress in explaining disease pathogenesis, establishing diagnosis, guiding therapy, predicting prognosis, and achieving prevention. In this report, we briefly review the roles of the PAX9, MSX1, AXIN2, and EDA genes in the causation of congenital tooth agenesis and the promise of molecular genetic research for the improvement of patient care.