Dentinogenesis imperfecta 1 with or without progressive hearing loss is associated with distinct mutations in DSPP

Isolated dentinogenesis imperfecta: Novel DSPP variants and insights on genetic counselling

OBJECTIVE

Dentinogenesis imperfecta (DI) is an inherited dentin defect and may be isolated or associated with disorders such as osteogenesis imperfecta, odontochondrodysplasia Ehler-Danlos and others. Isolated DI is caused mainly by pathogenic variants in DSPP gene and around 50 different variants have been described in this gene. Herein, we report on 19 patients from two unrelated Egyptian families with isolated DI. Additionally, we focused on genetic counselling of the two families.

MATERIALS AND METHODS

The patients were examined clinically and dentally. Panoramic X-rays were done to some patients. Whole exome sequencing (WES) and Sanger sequencing were used.

RESULTS

WES revealed two new nonsense variants in DSPP gene, c.288T > A (p.Tyr96Ter) and c.255G > A (p.Trp85Ter). Segregation analysis by Sanger sequencing confirmed the presence of the first variant in all affected members of Family 1 while the second variant was confirmed to be de novo in the patient of Family 2.

CONCLUSIONS AND CLINICAL RELEVANCE

Our study extends the number of DSPP pathogenic variants and strengthens the fact that DSPP is the most common DI causative gene irrespective of patients' ethnicity. In addition, we provide insights on genetic counseling issues in patients with inherited DSPP variants taking into consideration the variable religion, culture and laws in our society.

Transcriptomic Analysis Identifies Candidate Genes for Differential Expression during Xenopus laevis Inner Ear Development

Background

The genes involved in inner ear development and maintenance of the adult organ have yet to be fully characterized. Previous genetic analysis has emphasized the early development that gives rise to the otic vesicle. This study aimed to bridge the knowledge gap and identify candidate genes that are expressed as the auditory and vestibular sensory organs continue to grow and develop until the systems reach postmetamorphic maturity.

Methods

Affymetrix microarrays were used to assess inner ear transcriptome profiles from three Xenopus laevis developmental ages where all eight endorgans comprise mechanosensory hair cells: larval stages 50 and 56, and the post-metamorphic juvenile. Pairwise comparisons were made between the three developmental stages and the resulting differentially expressed X. laevis Probe Set IDs (Xl-PSIDs) were assigned to four groups based on differential expression patterns. DAVID analysis was undertaken to impart functional annotation to the differentially regulated Xl-PSIDs.

Results

Analysis identified 1510 candidate genes for differential gene expression in one or more pairwise comparison. Annotated genes not previously associated with inner ear development emerged from this analysis, as well as annotated genes with established inner ear function, such as oncomodulin, neurod1, and sp7. Notably, 36% of differentially expressed Xl-PSIDs were unannotated.

Conclusions

Results draw attention to the complex gene regulatory patterns that characterize Xenopus inner ear development, and underscore the need for improved annotation of the X. laevis genome. Outcomes can be utilized to select candidate inner ear genes for functional analysis, and to promote Xenopus as a model organism for biomedical studies of hearing and balance.

Effects of Sr2+, BO33-, and SiO32- on Differentiation of Human Dental Pulp Stem Cells into Odontoblast-Like Cells

This study aimed to clarify the effects of strontium (Sr2+), borate (BO33-), and silicate (SiO32-) on cell proliferative capacity, the induction of differentiation into odontoblast-like cells (OLCs), and substrate formation of human dental pulp stem cells (hDPSCs). Sr2+, BO33-, and SiO32- solutions were added to the hDPSC culture medium at three different concentrations, totaling nine experimental groups. The effects of these ions on hDPSC proliferation, calcification, and collagen formation after 14, 21, and 28 days of culture were evaluated using a cell proliferation assay, a quantitative alkaline phosphatase (ALP) activity assay, and Alizarin Red S and Sirius Red staining, respectively. Furthermore, the effects of these ions on hDPSC differentiation into OLCs were assessed via quantitative polymerase chain reaction and immunocytochemistry. Sr2+ and SiO32- increased the expression of odontoblast markers; i.e., nestin, dentin matrix protein-1, dentin sialophosphoprotein, and ALP genes, compared with the control group. BO33- increased the ALP gene expression and activity. The results of this study suggested that Sr2+, BO33-, and SiO32- may induce hDPSC differentiation into OLCs.

Hereditary dentin defects with systemic diseases

OBJECTIVE

This review aimed to summarize recent progress on syndromic dentin defects, promoting a better understanding of systemic diseases with dentin malformations, the molecules involved, and related mechanisms.

SUBJECTS AND METHODS

References on genetic diseases with dentin malformations were obtained from various sources, including PubMed, OMIM, NCBI, and other websites. The clinical phenotypes and genetic backgrounds of these diseases were then summarized, analyzed, and compared.

RESULTS

Over 10 systemic diseases, including osteogenesis imperfecta, hypophosphatemic rickets, vitamin D-dependent rickets, familial tumoral calcinosis, Ehlers-Danlos syndrome, Schimke immuno-osseous dysplasia, hypophosphatasia, Elsahy-Waters syndrome, Singleton-Merten syndrome, odontochondrodysplasia, and microcephalic osteodysplastic primordial dwarfism type II were examined. Most of these are bone disorders, and their pathogenic genes may regulate both dentin and bone development, involving extracellular matrix, cell differentiation, and metabolism of calcium, phosphorus, and vitamin D. The phenotypes of these syndromic dentin defects various with the involved genes, part of them are similar to dentinogenesis imperfecta or dentin dysplasia, while others only present one or two types of dentin abnormalities such as discoloration, irregular enlarged or obliterated pulp and canal, or root malformation.

CONCLUSION

Some specific dentin defects associated with systemic diseases may serve as important phenotypes for dentists to diagnose. Furthermore, mechanistic studies on syndromic dentin defects may provide valuable insights into isolated dentin defects and general dentin development or mineralization.

Autosomal Dominant Non-Syndromic Hearing Loss (DFNA): A Comprehensive Narrative Review

Autosomal dominant non-syndromic hearing loss (HL) typically occurs when only one dominant allele within the disease gene is sufficient to express the phenotype. Therefore, most patients diagnosed with autosomal dominant non-syndromic HL have a hearing-impaired parent, although de novo mutations should be considered in all cases of negative family history. To date, more than 50 genes and 80 loci have been identified for autosomal dominant non-syndromic HL. DFNA22 (MYO6 gene), DFNA8/12 (TECTA gene), DFNA20/26 (ACTG1 gene), DFNA6/14/38 (WFS1 gene), DFNA15 (POU4F3 gene), DFNA2A (KCNQ4 gene), and DFNA10 (EYA4 gene) are some of the most common forms of autosomal dominant non-syndromic HL. The characteristics of autosomal dominant non-syndromic HL are heterogenous. However, in most cases, HL tends to be bilateral, post-lingual in onset (childhood to early adulthood), high-frequency (sloping audiometric configuration), progressive, and variable in severity (mild to profound degree). DFNA1 (DIAPH1 gene) and DFNA6/14/38 (WFS1 gene) are the most common forms of autosomal dominant non-syndromic HL affecting low frequencies, while DFNA16 (unknown gene) is characterized by fluctuating HL. A long audiological follow-up is of paramount importance to identify hearing threshold deteriorations early and ensure prompt treatment with hearing aids or cochlear implants.

Involvement of Dmp1 in the Precise Regulation of Hair Bundle Formation in the Developing Cochlea

Dentin matrix protein 1 (Dmp1) is a highly phosphorylated, extracellular matrix protein that is extensively expressed in bone and teeth but also found in soft tissues, including brain and muscle. However, the functions of Dmp1 in the mice cochlea are unknown. Our study showed that Dmp1 was expressed in auditory hair cells (HCs), with the role of Dmp1 in those cells identified using Dmp1 cKD mice. Immunostaining and scanning electron microscopy of the cochlea at P1 revealed that Dmp1 deficiency in mice resulted in an abnormal stereociliary bundle morphology and the mispositioning of the kinocilium. The following experiments further demonstrated that the cell-intrinsic polarity of HCs was affected without apparent effect on the tissue planer polarity, based on the observation that the asymmetric distribution of Vangl2 was unchanged whereas the Gαi3 expression domain was enlarged and Par6b expression was slightly altered. Then, the possible molecular mechanisms of Dmp1 involvement in inner ear development were explored via RNA-seq analysis. The study suggested that the Fgf23-Klotho endocrine axis may play a novel role in the inner ear and Dmp1 may regulate the kinocilium-stereocilia interaction via Fgf23-Klotho signaling. Together, our results proved the critical role of Dmp1 in the precise regulation of hair bundle morphogenesis in the early development of HCs.

Dentin defects caused by a Dspp-1 frameshift mutation are associated with the activation of autophagy

Dentin sialophosphoprotein (DSPP) is primarily expressed by differentiated odontoblasts (dentin-forming cells), and transiently expressed by presecretory ameloblasts (enamel-forming cells). Disease-causing DSPP mutations predominantly fall into two categories: 5' mutations affecting targeting and trafficking, and 3' - 1 frameshift mutations converting the repetitive, hydrophilic, acidic C-terminal domain into a hydrophobic one. We characterized the dental phenotypes and investigated the pathological mechanisms of DsppP19L and Dspp-1fs mice that replicate the two categories of human DSPP mutations. In DsppP19L mice, dentin is less mineralized but contains dentinal tubules. Enamel mineral density is reduced. Intracellular accumulation and ER retention of DSPP is observed in odontoblasts and ameloblasts. In Dspp-1fs mice, a thin layer of reparative dentin lacking dentinal tubules is deposited. Odontoblasts show severe pathosis, including intracellular accumulation and ER retention of DSPP, strong ubiquitin and autophagy activity, ER-phagy, and sporadic apoptosis. Ultrastructurally, odontoblasts show extensive autophagic vacuoles, some of which contain fragmented ER. Enamel formation is comparable to wild type. These findings distinguish molecular mechanisms underlying the dental phenotypes of DsppP19L and Dspp-1fs mice and support the recently revised Shields classification of dentinogenesis imperfecta caused by DSPP mutations in humans. The Dspp-1fs mice may be valuable for the study of autophagy and ER-phagy.

Novel dentin sialophosphoprotein gene frameshift mutations affect dentin mineralization

OBJECTIVE

This study aimed to identify candidate genes for inheritable dentin defects in three Chinese pedigrees and characterize the property of affected teeth.

DESIGN

Clinical and radiological features were recorded for the affected individuals. Genomic DNA obtained from peripheral venous blood or saliva were analyzed by whole-exome sequencing. The density and microhardness of affected dentin was measured. Scanning electron microscopy (SEM) was also performed to obtain the microstructure phenotype.

RESULTS

1) General appearance: the affected dentitions shared yellowish-brown or milky color. Radiographs showed that the pulp cavity and root canals were obliterated in varying degrees or exhibited a pulp aspect in the 'thistle tube'. Some patients exhibited periapical infections without pulpal exposure, and some affected individuals showed shortened, abnormally thin roots accompanied by severe alveolar bone loss. 2) Genomic analysis: three new frameshift mutations (NM_014208.3: c.2833delA, c.2852delGand c.3239delA) were identified in exon 5 of dentin sialophosphoprotein (DSPP) gene, altering dentin phosphoprotein (DPP) as result. In vitro studies showed that the density and microhardness of affected dentin were decreased, the dentinal tubules were sparse and arranged disorderly, and the dentinal-enamel-junction (DEJ) was abnormal.

CONCLUSIONS

In this study, we identified three novel frameshift mutations of dentin sialophosphoprotein gene related to inherited dentin defects. These mutations are speculated to cause abnormal coding of dentin phosphoprotein C-terminus, which affect dentin mineralization. These results expand the spectrum of dentin sialophosphoprotein gene mutations causing inheritable dentin defects and broaden our understanding of the biological mechanisms by which dentin forms.

Identification of DSPP novel variants and phenotype analysis in dentinogenesis dysplasia Shields type II patients

OBJECTIVES

To investigate the genetic causes and teeth characteristics of dentin dysplasia Shields type II(DD-II) in three Chinese families.

MATERIALS AND METHODS

Data from three Chinese families affected with DD-II were collected. Whole-exome sequencing (WES) and whole-genome sequencing (WGS) were conducted to screen for variations, and Sanger sequencing was used to verify mutation sites. The physical and chemical characteristics of the affected teeth including tooth structure, hardness, mineral content, and ultrastructure were investigated.

RESULTS

A novel frameshift deletion mutation c.1871_1874del(p.Ser624fs) in DSPP was found in families A and B, while no pathogenic mutation was found in family C. The affected teeth's pulp cavities were obliterated, and the root canals were smaller than normal teeth and irregularly distributed comprising a network. The patients' teeth also had reduced dentin hardness and highly irregular dentinal tubules. The Mg content of the teeth was significantly lower than that of the controls, but the Na content was obviously higher than that of the controls.

CONCLUSIONS

A novel frameshift deletion mutation, c.1871_1874del (p.Ser624fs), in the DPP region of the DSPP gene causes DD-II. The DD-II teeth demonstrated compromised mechanical properties and changed ultrastructure, suggesting an impaired function of DPP. Our findings expand the mutational spectrum of the DSPP gene and strengthen the understanding of clinical phenotypes related to the frameshift deletion in the DPP region of the DSPP gene.

CLINICAL RELEVANCE

A DSPP mutation can alter the characteristics of the affected teeth, including tooth structure, hardness, mineral content, and ultrastructure.

A Review of Dentinogenesis Imperfecta and Primary Dentin Disorders in Dogs

This review describes the clinical, radiographic and histologic characteristics of dentinogenesis imperfecta diagnosed in two unrelated young dogs without evidence of concurrent osteogenesis imperfecta. The dentition was noted to have generalized coronal discoloration ranging from grey-blue to golden brown. Clinical pulp exposure, coronal wear and fractures were observed as was radiographic evidence of endodontic disease, thin dentin walls or dystrophic obliteration of the pulp canal. The enamel was severely affected by attrition and abrasion despite histologically normal areas; loss was most likely due to poor adherence or support by the underlying abnormal dentin. Histologically, permanent and deciduous teeth examined showed thin, amorphous dentin without organized dentin tubules and odontoblasts had dysplastic cell morphology. Primary dentin disorders, including dentinogenesis imperfecta and dentin dysplasia, have been extensively studied and genetically characterized in humans but infrequently reported in dogs. Treatment in human patients is aimed at early recognition and multi-disciplinary intervention to restore and maintain normal occlusion, aesthetics, mastication and speech. Treatment in both humans and canine patients is discussed as is the documented genetic heritability of primary dentin disorders in humans.

BMP Signaling Pathway in Dentin Development and Diseases

BMP signaling plays an important role in dentin development. BMPs and antagonists regulate odontoblast differentiation and downstream gene expression via canonical Smad and non-canonical Smad signaling pathways. The interaction of BMPs with their receptors leads to the formation of complexes and the transduction of signals to the canonical Smad signaling pathway (for example, BMP ligands, receptors, and Smads) and the non-canonical Smad signaling pathway (for example, MAPKs, p38, Erk, JNK, and PI3K/Akt) to regulate dental mesenchymal stem cell/progenitor proliferation and differentiation during dentin development and homeostasis. Both the canonical Smad and non-canonical Smad signaling pathways converge at transcription factors, such as Dlx3, Osx, Runx2, and others, to promote the differentiation of dental pulp mesenchymal cells into odontoblasts and downregulated gene expressions, such as those of DSPP and DMP1. Dysregulated BMP signaling causes a number of tooth disorders in humans. Mutation or knockout of BMP signaling-associated genes in mice results in dentin defects which enable a better understanding of the BMP signaling networks underlying odontoblast differentiation and dentin formation. This review summarizes the recent advances in our understanding of BMP signaling in odontoblast differentiation and dentin formation. It includes discussion of the expression of BMPs, their receptors, and the implicated downstream genes during dentinogenesis. In addition, the structures of BMPs, BMP receptors, antagonists, and dysregulation of BMP signaling pathways associated with dentin defects are described.

The Roles of SIBLING Proteins in Dental, Periodontal and Craniofacial Development

The majority of dental, periodontal, and craniofacial tissues are derived from the neural crest cells and ectoderm. Neural crest stem cells are pluripotent, capable of differentiating into a variety of cells. These cells can include osteoblasts, odontoblasts, cementoblasts, chondroblasts, and fibroblasts which are responsible for forming some of the tissues of the oral and craniofacial complex. The hard tissue forming cells deposit a matrix composed of collagen and non-collagenous proteins (NCPs) that later undergoes mineralization. The NCPs play a role in the mineralization of collagen. One such category of NCPs is the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family of proteins. This family is composed of dentin sialophosphosprotein (DSPP), osteopontin (OPN), dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), and matrix extracellular phosphoglycoprotein (MEPE). The SIBLING family is known to have regulatory effects in the mineralization process of collagen fibers and the maturation of hydroxyapatite crystals. It is well established that SIBLING proteins have critical roles in tooth development. Recent literature has described the expression and role of SIBLING proteins in other areas of the oral and craniofacial complex as well. The objective of the present literature review is to summarize and discuss the different roles the SIBLING proteins play in the development of dental, periodontal, and craniofacial tissues.

Translated Mutant DSPP mRNA Expression Level Impacts the Severity of Dentin Defects

Hereditary dentin defects are conventionally classified into three types of dentinogenesis imperfecta (DGI) and two types of dentin dysplasia (DD). Mutations in the dentin sialophosphoprotein (DSPP) gene have been identified to cause DGI type II and III and DD type II; therefore, these are not three different conditions, but rather allelic disorders. In this study, we recruited three families with varying clinical phenotypes from DGI-III to DD-II and performed mutational analysis by candidate gene analysis or whole-exome sequencing. Three novel mutations including a silent mutation (NM_014208.3: c.52-2del, c.135+1G>C, and c.135G>A; p.(Gln45=)) were identified, all of which affected pre-mRNA splicing. Comparison of the splicing assay results revealed that the expression level of the DSPP exon 3 deletion transcript correlated with the severity of the dentin defects. This study did not only expand the mutational spectrum of DSPP gene, but also advanced our understanding of the molecular pathogenesis impacting the severity of hereditary dentin defects.

The Modified Shields Classification and 12 Families with Defined DSPP Mutations

Mutations in Dentin Sialophosphoprotein (DSPP) are known to cause, in order of increasing severity, dentin dysplasia type-II (DD-II), dentinogenesis imperfecta type-II (DGI-II), and dentinogenesis imperfecta type-III (DGI-III). DSPP mutations fall into two groups: a 5′-group that affects protein targeting and a 3′-group that shifts translation into the −1 reading frame. Using whole-exome sequence (WES) analyses and Single Molecule Real-Time (SMRT) sequencing, we identified disease-causing DSPP mutations in 12 families. Three of the mutations are novel: c.53T>C/p.(Val18Ala); c.3461delG/p.(Ser1154Metfs*160); and c.3700delA/p.(Ser1234Alafs*80). We propose genetic analysis start with WES analysis of proband DNA to identify mutations in COL1A1 and COL1A2 causing dominant forms of osteogenesis imperfecta, 5′-DSPP mutations, and 3′-DSPP frameshifts near the margins of the DSPP repeat region, and SMRT sequencing when the disease-causing mutation is not identified. After reviewing the literature and incorporating new information showing distinct differences in the cell pathology observed between knockin mice with 5′-Dspp or 3′-Dspp mutations, we propose a modified Shields Classification based upon the causative mutation rather than phenotypic severity such that patients identified with 5′-DSPP defects be diagnosed as DGI-III, while those with 3′-DSPP defects be diagnosed as DGI-II.

Phenotype and molecular characterizations of a family with dentinogenesis imperfecta shields type II with a novel DSPP mutation

Background

Dentinogenesis imperfecta (DGI), Shields type-II is an autosomal dominant genetic disease which severely affects the function of the patients’ teeth. The dentin sialophosphoprotein (DSPP) gene is considered to be the pathogenic gene of DGI-II. In this study, a DGI-II family with a novel DSPP mutation were collected, functional characteristics of DGI cells and clinical features were analyzed to better understand the genotype-phenotype relationship of this disease.

Methods

Clinical data were collected, whole exome sequencing (WES) was conducted, and Sanger sequencing was used to verify the mutation sites. Physical characteristics of the patient’s teeth were examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The localization of green fluorescent protein (GFP)-fused wild-type (WT) dentin sialoprotein (DSP) and its variant were evaluated via an immunocytochemistry (ICC) assay. The behaviors of human dental pulp stem cells (hDPSCs) were investigated by flow cytometry, osteogenic differentiation, and quantitative real-time polymerase chain reaction (qRT-PCR).

Results

A novel heterozygous mutation c.53T > G (p. Val18Gly) in DSPP was found in this family. The SEM results showed that the participants’ teeth had reduced and irregular dentinal tubes. The EDS results showed that the Ca/P ratio of the patients’ teeth was significantly higher than that of the control group. The ICC assay showed that the mutant DSP was entrapped in the endoplasmic reticulum (ER), while the WT DSP located mainly in the Golgi apparatus. In comparison with normal cells, the patient’s cells exhibited significantly decreased mineralization ability and lower expression levels of DSPP and RUNX2.

Conclusions

The c.53T > G (p. Val18Gly) DSPP variant was shown to present with rare hypoplastic enamel defects. Functional analysis revealed that this novel variant disturbs dentinal characteristics and pulp cell behavior.

Effects of DSPP Gene Mutations on Periodontal Tissues

Dentin sialophosphoprotein ( DSPP ) gene mutations cause autosomal dominantly inherited diseases. DSPP gene mutations lead to abnormal expression of DSPP, resulting in a series of histological, morphological, and clinical abnormalities. A large number of previous studies demonstrated that DSPP is a dentinal-specific protein, and DSPP gene mutations lead to dentin dysplasia and dentinogenesis imperfecta. Recent studies have found that DSPP is also expressed in bone, periodontal tissues, and salivary glands. DSPP is involved in the formation of the periodontium as well as tooth structures. DSPP deficient mice present furcation involvement, cementum, and alveolar bone defect. We speculate that similar periodontal damage may occur in patients with DSPP mutations. This article reviewed the effects of DSPP gene mutations on periodontal status. However, almost all of the research is about animal study, there is no evidence that DSPP mutations cause periodontium defects in patients yet. We need to conduct systematic clinical studies on DSPP mutation families in the future to elucidate the effect of DSPP gene on human periodontium.

Molecular Evolution of Tooth-Related Genes Provides New Insights into Dietary Adaptations of Mammals

Mammals have evolved different tooth phenotypes that are hypothesized to be associated with feeding habits. However, the genetic basis for the linkage has not been well explored. In this study, we investigated 13 tooth-related genes, including seven enamel-related genes (AMELX, AMBN, ENAM, AMTN, ODAM, KLK4 and MMP20) and six dentin-related genes (DSPP, COL1A1, DMP1, IBSP, MEPE and SPP1), from 63 mammals to determine their evolutionary history. Our results showed that different evolutionary histories have evolved among divergent feeding habits in mammals. There was stronger positive selection for eight genes (ENAM, AMTN, ODAM, KLK4, DSPP, DMP1, COL1A1, MEPE) in herbivore lineages. In addition, AMELX, AMBN, ENAM, AMTN, MMP20 and COL1A1 underwent accelerated evolution in herbivores. While relatively strong positive selection was detected in IBSP, SPP1, and DSPP, accelerated evolution was only detected for MEPE and SPP1 genes among the carnivorous lineages. We found positive selection on AMBN and ENAM genes for omnivorous primates in the catarrhini clade. Interestingly, a significantly positive association between the evolutionary rate of ENAM, ODAM, KLK4, MMP20 and the average enamel thickness was found in primates. Additionally, we found molecular convergence in some amino acid sites of tooth-related genes among the lineages whose feeding habit are similar. The positive selection of related genes might promote the formation and bio-mineralization of tooth enamel and dentin, which would make the tooth structure stronger. Our results revealed that mammalian tooth-related genes have experienced variable evolutionary histories, which provide some new insights into the molecular basis of dietary adaptation in mammals.

Non-Syndromic Dentinogenesis Imperfecta Caused by Mild Mutations in COL1A2

Hereditary dentin defects can be categorized as a syndromic form predominantly related to osteogenesis imperfecta (OI) or isolated forms without other non-oral phenotypes. Mutations in the gene encoding dentin sialophosphoprotein (DSPP) have been identified to cause dentinogenesis imperfecta (DGI) Types II and III and dentin dysplasia (DD) Type II. While DGI Type I is an OI-related syndromic phenotype caused mostly by monoallelic mutations in the genes encoding collagen type I alpha 1 chain (COL1A1) and collagen type I alpha 2 chain (COL1A2). In this study, we recruited families with non-syndromic dentin defects and performed candidate gene sequencing for DSPP exons and exon/intron boundaries. Three unrelated Korean families were further analyzed by whole-exome sequencing due to the lack of the DSPP mutation, and heterozygous COL1A2 mutations were identified: c.3233G>A, p.(Gly1078Asp) in Family 1 and c.1171G>A, p.(Gly391Ser) in Family 2 and 3. Haplotype analysis revealed different disease alleles in Families 2 and 3, suggesting a mutational hotspot. We suggest expanding the molecular genetic etiology to include COL1A2 for isolated dentin defects in addition to DSPP.

DSPP dosage affects tooth development and dentin mineralization

Dentin Sialoprotein (DSP) and phosphophoryn (PP) are two most dominant non-collagenous proteins in dentin, which are the cleavage products of the DSPP (dentin sialophosphoprotein) precursor protein. The absence of the DSPP gene in DSPP knock-out (KO) mice results in characteristics that are consistent with dentinogenesis imperfecta type III in humans. Symptoms include thin dentin, bigger pulp chamber with frequent pulp exposure as well as abnormal epithelial-mesenchymal interactions, and the appearance of chondrocyte-like cells in dental pulp. To better understand how DSPP influences tooth development and dentin formation, we used a bacterial artificial chromosome transgene construct (BAC-DSPP) that contained the complete DSPP gene and promoter to generate BAC-DSPP transgenic mice directly in a mouse DSPP KO background. Two BAC-DSPP transgenic mouse strains were generated and characterized. DSPP mRNA expression in BAC-DSPP Strain A incisors was similar to that from wild-type (wt) mice. DSPP mRNA expression in BAC-DSPP Strain B animals was only 10% that of wt mice. PP protein content in Strain A incisors was 25% of that found in wt mice, which was sufficient to completely rescue the DSPP KO defect in mineral density, since microCT dentin mineral density analysis in 21-day postnatal animal molars showed essentially identical mineral density in both strain A and wt mice. Strain B mouse incisors, with 5% PP expression, only partially rescued the DSPP KO defect in mineral density, as microCT scans of 21-day postnatal animal molars indicated a reduced dentin mineral density compared to wt mice, though the mineral density was still increased over that of DSPP KO. Furthermore, our findings showed that DSPP dosage in Strain A was sufficient to rescue the DSPP KO defect in terms of epithelial-mesenchymal interactions, odontoblast lineage maintenance, along with normal dentin thickness and normal mineral density while DSPP gene dosage in Strain B only partially rescued the aforementioned DSPP KO defect.

Identification of Novel Candidate Genes and Variants for Hearing Loss and Temporal Bone Anomalies

Background: Hearing loss remains an important global health problem that is potentially addressed through early identification of a genetic etiology, which helps to predict outcomes of hearing rehabilitation such as cochlear implantation and also to mitigate the long-term effects of comorbidities. The identification of variants for hearing loss and detailed descriptions of clinical phenotypes in patients from various populations are needed to improve the utility of clinical genetic screening for hearing loss. Methods: Clinical and exome data from 15 children with hearing loss were reviewed. Standard tools for annotating variants were used and rare, putatively deleterious variants were selected from the exome data. Results: In 15 children, 21 rare damaging variants in 17 genes were identified, including: 14 known hearing loss or neurodevelopmental genes, 11 of which had novel variants; and three candidate genes IST1, CBLN3 and GDPD5, two of which were identified in children with both hearing loss and enlarged vestibular aqueducts. Patients with variants within IST1 and MYO18B had poorer outcomes after cochlear implantation. Conclusion: Our findings highlight the importance of identifying novel variants and genes in ethnic groups that are understudied for hearing loss.

Dentin Sialophosphoprotein Deletion Leads to Femoral Head Cartilage Attenuation and Subchondral Bone Ill-mineralization

Dentin sialophosphoprotein (DSPP), which expresses and synthesizes in odontoblasts of dental pulp, is a critical protein for normal teeth mineralization. Originally, DSPP was identified as a dentin-specific protein. In 2010, DSPP was also found in femoral head cartilage, and it is still unclear what roles DSPP play in femoral head cartilage formation, growth, and maintenance. To reveal biological functions of DSPP in the femoral head cartilage, we examined Dspp null mice compared with wild-type (WT) mice to observe DSPP expression as well as localization in WT mice and to uncover differences of femoral head cartilage, bone morphology, and structure between these two kinds of mice. Expression data demonstrated that DSPP had heterogeneous fragments, expressed in each layer of femoral head cartilage and subchondral bone of WT mice. Dspp null mice exhibited a significant reduction in the thickness of femoral head cartilage, with decreases in the amount of proliferating cartilage cells and increases in apoptotic cells. In addition, the subchondral bone mineralization decreased, and the expressions of vessel markers (vascular endothelial growth factor [VEGF] and CD31), osteoblast markers (Osterix and dentin matrix protein 1 [DMP1]), osteocyte marker (sclerostin [SOST]), and osteoclast marker (tartrate-resistant acid phosphatase [TRAP]) were remarkably altered. These indicate that DSPP deletion can affect the proliferation of cartilage cells in the femoral head cartilage and endochondral ossification in subchondral bone. Our data clearly demonstrate that DSPP plays essential roles in the femoral head cartilage growth and maintenance and subchondral biomineralization.

Physicochemical properties of dentinogenesis imperfecta with a known DSPP mutation

AIM

To investigate the chemical and mechanical properties of teeth affected by a 1-bp deletion (c.2688delT) in the DSPP gene.

METHODS AND MATERIALS

Maxillary first premolars were extracted from the affected individual at age 9 years due to the orthodontic reason for crowding. A sample was imbedded in epoxy resin and sectioned buccolingually, after micro-computerized tomography (μCT) images were taken. Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS) and Vickers microhardness testing were also performed.

RESULTS

μCT reconstruction and analysis showed an irregularly obliterated pulp chamber and an extremely small pulpal volume in the DGI-II sample. The mineral density and microhardness scores were smaller in the dentin of the DGI-II sample compared to the wild-type. Mg content was lower in the dentin of the DGI-II sample compared to the wild-type.

CONCLUSION

This study shows that dentin affected by a 1-bp deletion in DSPP has a reduced mineral density, diminished microhardness and reduced Mg content.

Genetics of Postlingual Sensorineural Hearing Loss

Literature and clinical practice around adult-onset hearing loss (HL) has traditionally focused on environmental risk factors, including noise exposure, ototoxic drug exposure, and cardiovascular disease. The most common diagnosis in adult-onset HL is presbycusis. However, the age of onset of presbycusis varies, and patients often describe family history of HL as well as individual variation in progression and severity. In recent years, there has been accumulating evidence of gene-environment interactions underlying adult cases of HL. Susceptibility loci for age-related HL have been identified, and genes related to postlingual nonsyndromic HL continue to be discovered through individual reports and genome-wide association studies. This review will outline main concepts in genetics as related to HL, identify implicated genes, and discuss clinical implications. Laryngoscope, 131:401-409, 2021.

Hypophosphatemic osteosclerosis, hyperostosis, and enthesopathy associated with novel homozygous mutations of DMP1 encoding dentin matrix protein 1 and SPP1 encoding osteopontin: The first digenic SIBLING protein osteopathy?

The SIBLINGs are a subfamily of the secreted calcium-binding phosphoproteins and comprise five small integrin-binding ligand N-linked glycoproteins [dentin matrix protein-1 (DMP1), secreted phosphoprotein-1 (SPP1) also called osteopontin (OPN), integrin-binding sialoprotein (IBSP) also called bone sialoprotein (BSP), matrix extracellular phosphoglycoprotein (MEPE), and dentin sialophosphoprotein (DSPP)]. Each SIBLING has at least one "acidic, serine- and aspartic acid-rich motif" (ASARM) and multiple Ser-x-Glu/pSer sequences that when phosphorylated promote binding of the protein to hydroxyapatite for regulation of biomineralization. Mendelian disorders from loss-of-function mutation(s) of the genes that encode the SIBLINGs thus far involve DSPP causing various autosomal dominant dysplasias of dentin but without skeletal disease, and DMP1 causing autosomal recessive hypophosphatemic rickets, type 1 (ARHR1). No diseases have been reported from gain-of-function mutation(s) of DSPP or DMP1 or from alterations of SPP1, IBSP, or MEPE. Herein, we describe severe hypophosphatemic osteosclerosis and hyperostosis associated with skeletal deformity, short stature, enthesopathy, tooth loss, and high circulating FGF23 levels in a middle-aged man and young woman from an endogamous family living in southern India. Both shared novel homozygous mutations within two genes that encode a SIBLING protein: stop-gain ("nonsense") DMP1 (c.556G>T,p.Glu186Ter) and missense SPP1 (c.769C>T,p.Leu266Phe). The man alone also carried novel heterozygous missense variants within two additional genes that condition mineral homeostasis and are the basis for autosomal recessive disorders: CYP27B1 underlying vitamin D dependent rickets, type 1, and ABCC6 underlying both generalized arterial calcification of infancy, type 2 and pseudoxanthoma elasticum (PXE). By immunochemistry, his bone contained high amounts of OPN, particularly striking surrounding osteocytes. We review how our patients' disorder may represent the first digenic SIBLING protein osteopathy.

Biallelic TANGO1 mutations cause a novel syndromal disease due to hampered cellular collagen secretion

The transport and Golgi organization 1 (TANGO1) proteins play pivotal roles in the secretory pathway. Full length TANGO1 is a transmembrane protein localised at endoplasmic reticulum (ER) exit sites, where it binds bulky cargo within the ER lumen and recruits membranes from the ER Golgi intermediate compartment to create an exit route for their export. Here we report the first TANGO1-associated syndrome in humans. A synonymous substitution that results in exon eight skipping in most mRNA molecules, ultimately leading to a truncated TANGO1 protein was identified as disease-causing mutation. The four homozygously affected sons of a consanguineous family display severe dentinogenesis imperfecta, short stature, various skeletal abnormalities, insulin-dependent diabetes mellitus, sensorineural hearing loss, and mild intellectual disability. Functional studies in HeLa and U2OS cells revealed that the corresponding truncated TANGO1 protein is dispersed in the ER and its expression in cells with intact endogenous TANGO1 impairs cellular collagen I secretion.

Bioinspired surface modification of orthopedic implants for bone tissue engineering

Biomedical implants have been widely used in various orthopedic treatments, including total hip arthroplasty, joint arthrodesis, fracture fixation, non-union, dental repair, etc. The modern research and development of orthopedic implants have gradually shifted from traditional mechanical support to a bioactive graft in order to endow them with better osteoinduction and osteoconduction. Inspired by structural and mechanical properties of natural bone, this review provides a panorama of current biological surface modifications for facilitating the interaction between medical implants and bone tissue and gives a future outlook for fabricating the next-generation multifunctional and smart implants by systematically biomimicking the physiological processes involved in formation and functioning of bones.

The functional significance of dentin sialoprotein-phosphophoryn and dentin sialoprotein

Phosphophoryn (PP) and dentin sialoprotein (DSP) are the most dominant non-collagenous proteins in dentin. PP is an extremely acidic protein that can function as a mineral nucleator for dentin mineralization. DSP was first identified in 1981, yet its functional significance is still controversial. Historically, these two proteins were considered to be independently synthesized and secreted by dental pulp cells into the developing dentin matrix. However, with the identification of the DSP coding sequence in 1994, followed 2 years later by the finding that the PP coding sequence was located immediately downstream from the DSP sequence, it became immediately clear that DSP and PP proteins were derived from a single DSP-PP (i.e., dentin sialophosphoprotein, DSPP) transcript. Since DSPP cDNA became available, tremendous progress has been made in studying DSP-PP mRNA distribution and DSP generation from the DSP-PP precursor protein at specific cleavage sites by protease tolloid-related-1 (TLR1) or bone morphogenetic protein 1 (BMP1). The functions of DSP-PP and DSP were investigated via DSP-PP knockout (KO) and DSP knockin in DSP-PP KO mice. In addition, a number of in vitro studies aimed to elucidate DSPP and DSP function in dental pulp cells.

Along with phosphophoryn (PP), dental sialoprotein (DSP) is the dominant non-collagen protein in dentin, and in vitro studies have demonstrated that DSP is involved in inducing the differentiation of dental pulp cells into odontoblast-like cells, which form dentin. PP is known to be involved in the mineralization of dentin, but the functional significance of DSP had been controversial. Helena Ritchie of the University of Michigan School of Dentistry conducted a review of studies investigating the derivation, function and distribution of PP and DSP. It was originally thought that PP and DSP were synthesized independently; later, it became evident that they derive from a single DSP-PP gene. Wider DSP-PP distribution in various tissues, including kidney and salivary glands, and DSP or PP expression in non-mineralized tissues suggest that the proteins may have functions other than mineralization.

Structural features, processing mechanism and gene splice variants of dentin sialophosphoprotein

Dentin sialophosphoprotein (DSPP) plays an important role in the formation of dentin. Understanding its structure and function would provide important insights into the regulation of dentin mineralization. For the past 15 years, we have been studying DSPP-derived proteins isolated from pig dentin. Porcine DSPP is synthesized and secreted by odontoblasts and processed into three proteins, i.e., dentin sialoprotein (DSP), dentin glycoprotein (DGP), and dentin phosphoprotein (DPP), by bone morphogenetic protein 1 and matrix metalloproteinase-20 and -2. DSP is a proteoglycan that forms covalent dimers, DGP is a phosphorylated glycoprotein, and DPP is a highly phosphorylated intrinsically disordered protein with genetic polymorphisms. Furthermore, DPP is not detected in dental pulp. This is possibly due to the existence of two mRNA variants of the DSPP gene: one that encodes the DSP region alone and another that encodes full-length DSPP. The mRNA variant encoding DSP alone is expressed in dental pulp and odontoblasts, but the variant encoding full-length DSPP is predominantly expressed in odontoblasts and barely in dental pulp.

Familial nonsyndromic hearing loss with incomplete partition type II caused by novel DSPP gene mutations

BACKGROUND

Familial nonsyndromic hearing loss (NSHL) with incomplete partition type II (IP-II) is a very rare condition.

AIMS/OBJECTIVES

To determine the audiological feature, inheritance patterns and genetic etiology of familial NSHL with IP-II in a Chinese family with eight family members.

MATERIAL AND METHODS

Clinical data were collected from all eight family members, selected deafness genes were sequenced in proband and whole genome sequencing of seven family members was performed.

RESULTS

The proband were a pair of male nonidentical twins (III:1, III:2). Three patients in this family, including the twins and their father (II:1), were diagnosed with bilateral NSHL with IP-II, and no mutation was found in the genes of SLC26A4, GJB2, GJB3, mitochondrial 12S rRNA, and MITF. Whole genome sequencing data indicated de novo mutations of the gene DSPP, c.3085A > G and c.3087C > T, which resulted in p.N1029D and co-segregated with deafness phenotype, were the underlying genetic etiology.

CONCLUSION AND SIGNIFICANCE

Familial NSHL with IP-II is extremely rare. In this family, de novo DSPP gene mutations, were considered to be the most probable genetic etiology. And this is the first report to reveal DSPP gene mutations leading to familial NSHL with IP-II.

Dentin Phosphoprotein Mimetic Peptide Nanofibers Promote Biomineralization

Dentin phosphoprotein (DPP) is a major component of the dentin matrix playing crucial role in hydroxyapatite deposition during bone mineralization, making it a prime candidate for the design of novel materials for bone and tooth regeneration. The bioactivity of DPP-derived proteins is controlled by the phosphorylation and dephosphorylation of the serine residues. Here an enzyme-responsive peptide nanofiber system inducing biomineralization is demonstrated. It closely emulates the structural and functional properties of DPP and facilitates apatite-like mineral deposition. The DPP-mimetic peptide molecules self-assemble through dephosphorylation by alkaline phosphatase (ALP), an enzyme participating in tooth and bone matrix mineralization. Nanofiber network formation is also induced through addition of calcium ions. The gelation process following nanofiber formation produces a mineralized extracellular matrix like material, where scaffold properties and phosphate groups promote mineralization. It is demonstrated that the DPP-mimetic peptide nanofiber networks can be used for apatite-like mineral deposition for bone regeneration.

Transgenic expression of dentin phosphoprotein (DPP) partially rescued the dentin defects of DSPP-null mice

Mutations in the dentin sialophosphoprotein (DSPP) gene cause dentinogenesis imperfecta. After synthesis, DSPP is proteolytically processed into NH2- and COOH-terminal fragments. The NH2-terminal fragment of DSPP is highly glycosylated but not phosphorylated, whereas the COOH-terminal fragment (named "dentin phosphoprotein" or "DPP") is highly phosphorylated but not glycosylated. These two fragments are believed to perform distinct roles in dentin formation. To analyze the functions of DPP in dentinogenesis, we created "Dspp-/-;DPP Tg mice", which expressed transgenic DPP driven by a Type I collagen promoter but lacked the endogenous Dspp gene. We characterized the dentin of the Dspp-/-;DPP Tg mice using X-ray radiography, histology, scanning electron microscopy, double fluorochrome labeling, immunohistochemistry and in situ hybridization. Micro-computed tomography analyses revealed that at postnatal 6 months, the transgenic expression of DPP increased the dentin thickness of the Dspp-null mice by 97.1% and restored the dentin material density by 29.5%. Histological analyses showed that the Dspp-null mice manifested an abnormal widening of the predentin while the predentin in Dspp-/-;DPP Tg mice was narrower than in the Dspp-null mice. Scanning electron microscopy analyses showed that the dentinal tubules in the Dspp-/-;DPP Tg mice were better organized than in the Dspp-null mice. The double fluorochrome labeling analyses demonstrated that the dentin mineral deposition rate in the Dspp-/-;DPP Tg mice was significantly improved compared to that in the Dspp-null mice. These findings indicate that the transgenic expression of DPP partially rescued the dentin defects of the DSPP-null mice, suggesting that DPP may promote dentin formation and that the coordinated actions between DPP and the NH2-terminal fragment of DSPP may be necessary for dentinogenesis.

Bone morphogenetic protein 7 promotes odontogenic differentiation of dental pulp stem cells in vitro

AIMS

in vitro effects of bone morphogenetic protein 7 (BMP-7) on proliferation and differentiation of dental pulp stem cells (DPSCs) have not been investigated, nor has an appropriate dose been established.

MAIN METHODS

Human DPSCs obtained from healthy volunteers were cultured with BMP-7 at 25, 50, and 100 ng/ml. Cell viability was measured by Cell Counting Kit-8 assay. Expression profiles of selected odontogenic differentiation-related markers in DPSCs were evaluated using quantitative reverse transcription polymerase chain reaction (qRT-PCR), immunocytochemistry, and western blot analysis. Mineralization of DPSCs was evaluated by alizarin red staining. The Smad5 signaling pathway was examined by qRT-PCR and western blot analysis.

KEY FINDINGS

Diminished cell viability was found in DPSCs induced with 25, 50, and 100 ng/ml of BMP-7 for 7 days, showing a dose-response effect (P-trend = 0.03). DSPP, OCN, DMP-1, and RUNX2 were upregulated by BMP-7 induction after 7 and 14 days, especially at 50 and 100 ng/ml (P < 0.05). Immunocytochemical staining revealed strong expression of DSPP, DMP-1 and ALP in DPSCs induced by BMP-7, whereas null or weak expression in untreated cells. Western blot analysis confirmed over-expression of DSPP in cells induced by BMP-7. Alizarin red staining confirmed formation of mineralized nodules 4 weeks after BMP-7 induction. BMP-7 treated cells showed dose-dependently increased expression of BMPR1A, Smad5, and p-Smad5.

SIGNIFICANCE

Our data indicated that BMP-7 at 50 ng/ml and 100 ng/ml was capable to induce DPSCs toward odontogenic differentiation through the Smad5 signaling pathway and not dramatically halt cell proliferation in vitro.

Dentinogenesis imperfecta type II- genotype and phenotype analyses in three Danish families

BACKGROUND

Dentinogenesis imperfecta (DI) is a rare debilitating hereditary disorder affecting dentin formation and causing loss of the overlying enamel. Clinically, DI sufferers have a discolored and weakened dentition with an increased risk of fracture. The aims of this study were to assess genotype-phenotype findings in three families with DI-II with special reference to mutations in the DSPP gene and clinical, histological, and imaging manifestations.

METHODS

Nine patients participated in the study (two from family A, four from family B, and three from family C). Buccal swab samples were collected from all participants and extracted for genomic DNA. Clinical and radiographic examinations had been performed longitudinally, and the dental status was documented using photographic images. Four extracted and decalcified tooth samples were prepared for histological analysis to assess dysplastic manifestations in the dentin. Optical coherence tomography (OCT) was applied to study the health of enamel tissue from in vivo images and the effect of the mutation on the function and structure of the DSPP gene was analyzed using bioinformatics software programs.

RESULTS

The direct DNA sequence analysis revealed three distinct mutations, one of which was a novel finding. The mutations caused dominant phenotypes presumably by interference with signal peptide processing and protein secretion. The clinical and radiographic disturbances in the permanent dentition indicated interfamilial variability in DI-II manifestations, however, no significant intrafamilial variability was observed.

CONCLUSION

The different mutations in the DSPP gene were accompanied by distinct phenotypes. Enamel defects suggested deficit in preameloblast function during the early stages of amelogenesis.

The role of bone morphogenetic proteins 2 and 4 in mouse dentinogenesis

OBJECTIVE

The bone morphogenetic proteins (BMPs) play crucial roles in tooth development. However, several BMPs retain expression in the dentin of the fully patterned and differentiated tooth. We hypothesized that BMP signaling therefore plays a role in the function of the differentiated odontoblast, the job of which is to lay down and mineralize the dentin matrix.

DESIGN

We generated mice deficient in Bmp2 and 4 using a dentin matrix protein 1 (Dmp1) promoter-driven cre recombinase that was expressed in differentiated odontoblasts.

RESULTS

The first and second molars of these Bmp2 and Bmp4 double conditional knockout (DcKO) mice displayed reduced dentin and enlarged pulp chambers compared to cre-negative littermate controls. DcKO mouse dentin in first molars was characterized by small, disorganized dentinal fibers, a wider predentin layer, and reduced expression of dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), and bone sialoprotein (BSP). DcKO mouse odontoblasts demonstrated increased type I collagen mRNA production, indicating that the loss of BMP signaling altered the rate of collagen gene expression in these cells. Bmp2 and Bmp4 single Dmp1-cre knockout mice displayed no discernable dentin phenotype.

CONCLUSIONS

These data demonstrate that BMP signaling in differentiated odontoblasts is necessary for proper dentin production in mature teeth.

BMP-2 induced Dspp transcription is mediated by Dlx3/Osx signaling pathway in odontoblasts

Dentin sialophosphoprotein (Dspp) as a differentiation marker of odontoblasts is regulated by BMP-2. However, the intimate mechanism is still unknown. Transcription factors Dlx3 and Osx are essential for odontoblasts differentiation. We hypothesized that BMP-2 regulation of Dspp transcription was mediated by Dlx3 and/or Osx in odontoblasts. In the present investigation, we found that BMP-2 stimulated expression and nuclear translocation of Dlx3 and Osx in odontoblasts both in vitro and in vivo. Osx was a downstream target of Dlx3 and both of them stimulated Dsp expression. Both Dlx3 and Osx were able to activate Dspp promoter from nucleotides (nt) -318 to +54 by transfections of luciferase reports containing different lengths of mouse Dspp promoters. The binding of Dlx3 and Osx with nt -318 to +54 of Dspp promoter was verified by chromatin immunoprecipitation in vivo. Two Dlx3 binding sites and one Osx binding site on Dspp promoter were found by EMSA. Furthermore, the exact biological function of these binding sites was confirmed by site-directed mutagenesis. At last, the protein-protein interaction between Dlx3 and Osx in odontoblasts was detected by co-immunoprecipitation. In conclusion, in this study we found a novel signaling pathway in which BMP-2 activates Dspp gene transcription via Dlx3/Osx pathway.

Genetics of Tinnitus: Still in its Infancy

Tinnitus is the perception of a phantom sound that affects between 10 and 15% of the general population. Despite this considerable prevalence, treatments for tinnitus are presently lacking. Tinnitus exhibits a diverse array of recognized risk factors and extreme clinical heterogeneity. Furthermore, it can involve an unknown number of auditory and non-auditory networks and molecular pathways. This complex combination has hampered advancements in the field. The identification of specific genetic factors has been at the forefront of several research investigations in the past decade. Nine studies have examined genes in a case-control association approach. Recently, a genome-wide association study has highlighted several potentially significant pathways that are implicated in tinnitus. Two twin studies have calculated a moderate heritability for tinnitus and disclosed a greater concordance rate in monozygotic twins compared to dizygotic twins. Despite the more recent data alluding to genetic factors in tinnitus, a strong association with any specific genetic locus is lacking and a genetic study with sufficient statistical power has yet to be designed. Future research endeavors must overcome the many inherent limitations in previous study designs. This review summarizes the previously embarked upon tinnitus genetic investigations and summarizes the hurdles that have been encountered. The identification of candidate genes responsible for tinnitus may afford gene based diagnostic approaches, effective therapy development, and personalized therapeutic intervention.

Klf5 Mediates Odontoblastic Differentiation through Regulating Dentin-Specific Extracellular Matrix Gene Expression during Mouse Tooth Development

Klf5, a member of the Krüppel-like transcription factor family, has essential roles during embryonic development, cell proliferation, differentiation, migration and apoptosis. This study was to define molecular mechanism of Klf5 during the odontoblastic differentiation. The expression of Klf5, odontoblast-differentiation markers, Dspp and Dmp1 was co-localized in odontoblastic cells at different stages of mouse tooth development and mouse dental papilla mesenchymal cells. Klf5 was able to promote odontoblastic differentiation and enhance mineral formation of mouse dental papilla mesenchymal cells. Furthermore, overexpression of Klf5 could up-regulate Dspp and Dmp1 gene expressions in mouse dental papilla mesenchymal cells. In silico analysis identified that several putative Klf5 binding sites in the promoter and first intron of Dmp1 and Dspp genes that are homologous across species lines. Electrophoretic mobility shift assay and chromatin immunoprecipitation analysis indicated that Klf5 bound to these motifs in vitro and in intact cells. The responsible regions of Dmp1 gene were located in the promoter region while effect of Klf5 on Dspp activity was in the first intron of Dspp gene. Our results identify Klf5 as an activator of Dmp1 and Dspp gene transcriptions by different mechanisms and demonstrate that Klf5 plays a pivotal role in odontoblast differentiation.

Dentin sialoprotein facilitates dental mesenchymal cell differentiation and dentin formation

Dentin sialoprotein (DSP) is a dentin extracellular matrix protein. It is involved in dental mesenchymal cell lineages and dentin formation through regulation of its target gene expression. DSP mutations cause dentin genetic diseases. However, mechanisms of DSP in controlling dental mesenchymal cell differentiation are unknown. Using DSP as bait, we screened a protein library from mouse odontoblastic cells and found that DSP is a ligand and binds to cell surface receptor, occludin. Further study identified that the C-terminal DSP domain^{aa 363–458} interacts with the occludin extracellular loop 2^{aa 194–241}. The C-terminal DSP domain induced phosphorylation of occludin Ser⁴⁹⁰ and focal adhesion kinase (FAK) Ser⁷²² and Tyr⁵⁷⁶. Coexpression of DSP, occludin and FAK was detected in dental mesenchymal cells during tooth development. Occludin physically interacts with FAK, and occludin and FAK phosphorylation can be blocked by DSP and occludin antibodies. This DSP domain facilitates dental mesenchymal cell differentiation and mineralization. Furthermore, transplantation and pulp-capping procedures revealed that this DSP domain induces endogenous dental pulp mesenchymal cell proliferation, differentiation and migration, while stimulating blood vessel proliferation. This study elucidates the mechanism of DSP in dental mesenchymal lineages and implies that DSP may serve as a therapeutic agent for dentin-pulp complex regeneration in dental caries.

Dentin Sialoprotein is a Novel Substrate of Matrix Metalloproteinase 9 in vitro and in vivo

Dentin sialoprotein (DSP) is essential for dentinogenesis and processed into fragments in the odontoblast-like cells and the tooth compartments. Matrix metalloproteinase 9 (MMP9) is expressed in teeth from early embryonic to adult stage. Although MMP9 has been reported to be involved in some physiological and pathological conditions through processing substrates, its role in tooth development and whether DSP is a substrate of MMP9 remain unknown. In this study, the function of MMP9 in the tooth development was examined by observation of Mmp9 knockout (Mmp9−/−) mouse phenotype, and whether DSP is a substrate of MMP9 was explored by in vitro and in vivo experiments. The results showed that Mmp9−/− teeth displayed a phenotype similar to dentinogenesis imperfecta, including decreased dentin mineral density, abnormal dentin architecture, widened predentin and irregular predentin-dentin boundary. The distribution of MMP9 and DSP overlapped in the odontoblasts, the predentin, and the mineralized dentin, and MMP9 was able to specifically bind to DSP. MMP9 highly efficiently cleaved DSP into distinct fragments in vitro, and the deletion of Mmp9 caused improper processing of DSP in natural teeth. Therefore, our findings demonstrate that MMP9 is important for tooth development and DSP is a novel target of MMP9 during dentinogenesis.

Phenotype and genotype analyses in seven families with dentinogenesis imperfecta or dentin dysplasia

OBJECTIVE

Hereditary dentin defects can be categorised into two classes according to their clinical manifestations: dentinogenesis imperfecta (DGI), which includes three types (DGI-I, DGI-II and DGI-III), and dentin dysplasia (DD), which includes two types (DD-I and DD-II). This study investigated the phenotypic characteristics and genetic causes of hereditary dentin defects in seven Chinese families.

MATERIALS AND METHODS

Seven families affected with DGI-II, DGI-III or DD-II were enrolled. Clinical examinations were performed to determine the phenotypic characteristics, and DNA samples were collected for Sanger sequencing.

RESULTS

Clinical diagnoses revealed DGI-II in five families, DGI-III in one family and DD-II in one family. Variants of the dentin sialophosphoprotein (DSPP) gene were found in six of the seven families. Of these, c.52G>T was identified in two families. Each of the remaining four families had a different variant: c.2684delG, c.52-2A>G, c.1874-1877delACAG and c.3509-3521del13bp; the last three variants were novel.

CONCLUSIONS

This is the first study to analyse all three important types of hereditary dentin defect and include comprehensive genetic analyses of both dentin sialoprotein and dentin phosphoprotein in Chinese families. This study expands the spectrum of DSPP variants, highlighting their associated phenotypic continuum.

Post-translational Modifications of SIBLING Proteins and Their Roles in Osteogenesis and Dentinogenesis

The extracellular matrix (ECM) of bone and dentin contains several non-collagenous proteins. One category of non-collagenous protein is termed the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family, that includes osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE). These polyanionic SIBLING proteins are believed to play key biological roles in the mineralization of bone and dentin. Although the specific mechanisms involved in controlling bone and dentin formation are still unknown, it is clear that some functions of the SIBLING family members are dependent on the nature and extent of post-translational modifications (PTMs), such as phosphorylation, glycosylation, and proteolytic processing, since these PTMs would have significant effects on their structure. OPN and BSP are present in the ECM of bone and dentin as full-length forms, whereas amino acid sequencing indicates that DMP1 and DSPP exist as proteolytically processed fragments that result from scission of X-Asp bonds. We hypothesized that the processing of DMP1 and DSPP is catalyzed by the PHEX enzyme, since this protein, an endopeptidase that is predominantly expressed in bone and tooth, has a strong preference for cleavage at the NH2-terminus of aspartyl residue. We envision that the proteolytic processing of DMP1 and DSPP may be an activation process that plays a significant, crucial role in osteogenesis and dentinogenesis, and that a failure in this processing would cause defective mineralization in bone and dentin, as observed in X-linked hypophosphatemic rickets.

Hereditary Dentin Defects

By the Shields classification, articulated over 30 years ago, inherited dentin defects are divided into 5 types: 3 types of dentinogenesis imperfecta (DGI), and 2 types of dentin dysplasia (DD). DGI type I is osteogenesis imperfecta (OI) with DGI. OI with DGI is caused, in most cases, by mutations in the 2 genes encoding type I collagen. Many genes are required to generate the enzymes that catalyze collagen’s diverse post-translational modifications and its assembly into fibers, fibrils, bundles, and networks. Rare inherited diseases of bone are caused by defects in these genes, and some are occasionally found to include DGI as a feature. Appreciation of the complicated genetic etiology of DGI associated with bony defects splintered the DGI type I description into a multitude of more precisely defined entities, all with their own designations. In contrast, DD-II, DGI-II, and DGI-III, each with its own pattern of inherited defects limited to the dentition, have been found to be caused by various defects in DSPP (dentin sialophosphoprotein), a gene encoding the major non-collagenous proteins of dentin. Only DD-I, an exceedingly rare condition featuring short, blunt roots with obliterated pulp chambers, remains untouched by the revolution in genetics, and its etiology is still a mystery. A major surprise in the characterization of genes underlying inherited dentin defects is the apparent lack of roles played by the genes encoding the less-abundant non-collagenous proteins in dentin, such as dentin matrix protein 1 ( DMP1), integrin-binding sialoprotein ( IBSP), matrix extracellular phosphoglycoprotein ( MEPE), and secreted phosphoprotein-1, or osteopontin ( SPP1, OPN). This review discusses the development of the dentin extracellular matrix in the context of its evolution, and discusses the phenotypes and clinical classifications of isolated hereditary defects of tooth dentin in the context of recent genetic data respecting their genetic etiologies.

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

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

Mutation in SSUH2 Causes Autosomal-Dominant Dentin Dysplasia Type I

Dentin dysplasia type I (DDI) is an autosomal-dominant genetic disorder resulting from dentin defects. The molecular basis of DDI remains unclear. DDI exhibits unique characteristics with phenotypes featuring obliteration of pulp chambers and diminutive root, thus providing a useful model for understanding the genetics of tooth formation. Using a large Chinese family with 14 DDI patients, we mapped the gene locus responsible for DDI to 3p26.1-3p24.3 and further identified a missense mutation, c.353C>A (p.P118Q) in the SSUH2 gene on 3p26.1, which co-segregated with DDI. We showed that SSUH2 (p.P118Q) perturbed the structure and significantly reduced levels of mutant (MT) protein and mRNA compared with wild-type SSUH2. Furthermore, MT P141Q knock-in mice (+/- and -/-) had a unique partial obliteration of the pulp cavity and upregulation or downregulation of six major genes involved in odontogenesis: Dspp, Dmp1, Runx2, Pax9, Bmp2, and Dlx2. The phenotype of missing teeth was determined in zebrafish with morpholino gene knockdowns and rescued by injection of normal human mRNA. Taken together, our observations demonstrate that SSUH2 disrupts dental formation and that this novel gene, together with other odontogenesis genes, is involved in tooth development.

The Dentin Sialoprotein (DSP) Domain Regulates Dental Mesenchymal Cell Differentiation through a Novel Surface Receptor

Dentin sialophosphoprotein (DSPP) is a dentin extracellular matrix protein that is processed into dentin sialoprotein (DSP), dentin glycoprotein (DGP) and dentin phosphoprotein (DPP). DSP is mainly expressed in odontoblasts. We hypothesized that DSP interacts with cell surface receptors and subsequently activates intracellular signaling. Using DSP as bait for screening a protein library, we demonstrate that DSP acts as a ligand and binds to integrin β6. The 36 amino acid residues of DSP are sufficient to bind to integrin β6. This peptide promoted cell attachment, migration, differentiation and mineralization of dental mesenchymal cells. In addition, DSP (aa183-219) stimulated phosphorylation of ERK1/2 and P38 kinases. This activation was inhibited by an anti-integrin β6 antibody and siRNA. Furthermore, we demonstrate that this DSP fragment induces SMAD1/5/8 phosphorylation and nuclear translocation via ERK1/2 and P38 signaling. SMAD1/5/8 binds to SMAD binding elements (SBEs) in the DSPP gene promoter. SBE mutations result in a decrease in DSPP transcriptional activity. Endogenous DSPP expression was up-regulated by DSP (aa183-219) in dental mesenchymal cells. The data in the current study demonstrate for the first time that this DSP domain acts as a ligand in a RGD-independent manner and is involved in intracellular signaling via interacting with integrin β6. The DSP domain regulates DSPP expression and odontoblast homeostasis via a positive feedback loop.

N-terminal Dentin Sialoprotein fragment induces type I collagen production and upregulates dentinogenesis marker expression in osteoblasts

Bone and dentin are mineralized extracellular matrices produced by osteoblasts and odontoblasts, respectively, and their major organic portion is type I collagen. Dentinogenesis Imperfecta (DGI) is one of the most common clinically- and genetically-based disturbances of dentin formation, causing irreversible dentin defects. Among several types of DGI, patients with DGI type II exhibit opalescent dentin with partial or complete pulp obliteration. It has been previously reported that the non-sense mutation (c.133C>T) in Dentin Sialophosphoprotein (DSPP) was identified in DGI type II patients at glutamine residue 45, resulting in the premature stop codon (p.Q45X). DSPP is known to be synthesized as a single gene product and further processed at Gly⁴⁶²-Asp⁴⁶³, resulting in the production of Dentin Sialoprotein (DSP) and Dentin Phosphoprotein (DPP). We hypothesized that the shorter form (Q45X) of N-terminal Dentin Sialoprotein (N-DSP) may cause over-production of type I collagen protein as obliterated pulp is occupied by dentin. To test this hypothesis, we generated mouse recombinant Glutathione-S-Transferase (GST)-N-DSP fusion protein, and the effect of GST-N-DSP was investigated in calvarial bone explant culture and MC3T3-E1 osteoblastic culture systems. Here we show that a significant increase in calvarial bone formation is observed by GST-N-DSP. GST-N-DSP accelerates MC3T3-E1 osteoblast cell growth and proliferation and subsequent osteoblast differentiation by inducing the expression of certain osteogenic markers such as type I collagen, Runx2, Osterix and ATF4. Interestingly, GST-N-DSP significantly enhances dentinogenesis marker gene expression including Dspp and Dmp1 gene expression in non-odontogenic MC3T3-E1 cells. To rule out any artificial effect of GST-tag, we also used the synthetic peptide of N-DSP and confirmed the results of N-DSP peptide were essentially similar to those of GST-N-DSP. Taken together, our data suggest that N-DSP promotes bone formation by accelerating osteoblast cell proliferation and subsequent osteoblast differentiation accompanied by marked up-regulation of the dentin matrix markers, such as Dspp and Dmp1 genes.

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Recombinant N-terminal DSP (N-DSP) protein was generated.

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N-DSP mimics the non-sense mutation form of Dentinogenesis Imperfecta type II.

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N-DSP enhances bone formation in clavarial ex vivo cultures.

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N-DSP accelerates osteoblast proliferation.

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N-DSP upregulates type I collagen and Dspp expression in non-odontogenic osteoblasts.