Odontogenesis-associated phosphoprotein truncation blocks ameloblast transition into maturation in Odaph(C41*/C41*) mice

A novel ODAPH mutation causing amelogenesis imperfecta and its expression in human dental tissues

Background/purpose

Amelogenesis imperfecta (AI), an assemblage of genetic diseases with dental enamel malformations, is generally grouped into hypoplastic, hypomaturation, and hypocalcified types. This study aimed to identify the genetic etiology for a consanguineous Iranian family with autosomal recessive hypocalcified AI.

Materials and methods

Dental defects were characterized, and whole exome analysis conducted to search for disease-causing mutations. Minigene assay and RT-PCR were performed to evaluate molecular consequences of the identified mutation and expression of the causative gene in human dental tissues.

Results

The defective enamel of erupted teeth showed extensive post-eruptive failure and discoloration. Partial enamel hypoplasia and indistinct dentino-enamel junction were evident on unerupted teeth, resembling hypocalcified AI. A novel homozygous ODAPH (previously designated C4orf26) mutation of single-nucleotide deletion (NG_032974.1:g.5103del, NM_178497.5:c.67+1del) was identified to be disease-causing. The mutation would cause a frameshift to different ODAPH transcript variant (TV) products: p.(Ala23Hisfs∗29) for TV1 and p.(Gly23Aspfs∗140) for TV2. Both dental pulps of developing and exfoliating primary teeth expressed ODAPH TV2.

Conclusion

Loss-of-function ODAPH mutations can cause AI type IIIB (the hypocalcified, autosomal recessive type), rather than type IIA4 (the hypomaturation, pigmented autosomal recessive type). This study supports a hypothesis that the product of ODAPH TV2 is the single dominant ODAPH protein isoform critical for dental enamel formation and may also play an unappreciated role in development and homeostasis of dentin-pulp complex. Due to genetic heterogeneity and a nonideal genotype-phenotype correlation of AI, it is essential to perform genetic testing for patients with inherited enamel defects to make a definitive diagnosis.

Odontogenesis-Associated Phosphoprotein (ODAPH) Overexpression in Ameloblasts Disrupts Enamel Formation via Inducing Abnormal Mineralization of Enamel in Secretory Stage

Odontogenesis-associated phosphoprotein (ODAPH) is a recently discovered enamel matrix protein. Our previous study demonstrated that knockouting out Odaph in mice resulted in enamel hypomineralization. To further investigate the effect of Odaph on enamel mineralization, we constructed an Odaph overexpression mouse model, controlled by an amelogenin promoter. Our histological analysis of OdaphTg mice revealed that the enamel layer was thinner than in WT mice. An uneven, thinner enamel layer was confirmed using micro-computed tomography (uCT). It was subsequently found that the Tomes' processes lost their normal morphology, resulting in the loss of the enamel prism structure. These results indicate that Odaph overexpression in ameloblasts led to enamel dysplasia. In conjunction with this, Odaph overexpression hindered Amelx secretion, and may result in endoplasmic reticulum stress. Interestingly, uCT revealed that enamel had higher mineral density at the secretory stage; due to this, we did the histological staining for the mineralization-related proteins Alkaline phosphatase (ALPL) and Runt-related transcription factor 2 (RUNX2). It was observed that these proteins were up-regulated in OdaphTg mice versus WT mice, indicating that Odaph overexpression led to abnormal enamel mineralization. To confirm this, we transfected ameloblast-like cell line (ALC) with Odaph overexpression lentivirus in vitro and identified that both Alpl and Runx2 were strikingly upregulated in OE-mus-Odaph versus OE-NC cells. We concluded that the ectopic overexpression of Odaph in ameloblasts led to abnormal enamel mineralization. In summary, Odaph profoundly influences amelogenesis by participating in enamel mineralization.

Enamel defects in Acp4R110C/R110C mice and human ACP4 mutations

Human ACP4 (OMIM*606362) encodes a transmembrane protein that belongs to histidine acid phosphatase (ACP) family. Recessive mutations in ACP4 cause non-syndromic hypoplastic amelogenesis imperfecta (AI1J, OMIM#617297). While ACP activity has long been detected in developing teeth, its functions during tooth development and the pathogenesis of ACP4-associated AI remain largely unknown. Here, we characterized 2 AI1J families and identified a novel ACP4 disease-causing mutation: c.774_775del, p.Gly260Aspfs*29. To investigate the role of ACP4 during amelogenesis, we generated and characterized Acp4R110C mice that carry the p.(Arg110Cys) loss-of-function mutation. Mouse Acp4 expression was the strongest at secretory stage ameloblasts, and the protein localized primarily at Tomes' processes. While Acp4 heterozygous (Acp4+/R110C) mice showed no phenotypes, incisors and molars of homozygous (Acp4R110C/R110C) mice exhibited a thin layer of aplastic enamel with numerous ectopic mineralized nodules. Acp4R110C/R110C ameloblasts appeared normal initially but underwent pathology at mid-way of secretory stage. Ultrastructurally, sporadic enamel ribbons grew on mineralized dentin but failed to elongate, and aberrant needle-like crystals formed instead. Globs of organic matrix accumulated by the distal membranes of defective Tomes' processes. These results demonstrated a critical role for ACP4 in appositional growth of dental enamel probably by processing and regulating enamel matrix proteins around mineralization front apparatus.

Molecular evolutionary analyses of tooth genes support sequential loss of enamel and teeth in baleen whales (Mysticeti)

The loss of teeth and evolution of baleen racks in Mysticeti was a profound transformation that permitted baleen whales to radiate and diversify into a previously underutilized ecological niche of bulk filter-feeding on zooplankton and other small prey. Ancestral state reconstructions suggest that postnatal teeth were lost in the common ancestor of crown Mysticeti. Genomic studies provide some support for this hypothesis and suggest that the genetic toolkit for enamel production was inactivated in the common ancestor of living baleen whales. However, molecular studies to date have not provided direct evidence for the complete loss of teeth, including their dentin component, on the stem mysticete branch. Given these results, several questions remain unanswered: (1) Were teeth lost in a single step or did enamel loss precede dentin loss? (2) Was enamel lost early or late on the stem mysticete branch? (3) If enamel and dentin/tooth loss were decoupled in the ancestry of baleen whales, did dentin loss occur on the stem mysticete branch or independently in different crown mysticete lineages? To address these outstanding questions, we compiled and analyzed complete protein-coding sequences for nine tooth-related genes from cetaceans with available genome data. Seven of these genes are associated with enamel formation (ACP4, AMBN, AMELX, AMTN, ENAM, KLK4, MMP20) whereas two other genes are either dentin-specific (DSPP) or tooth-specific (ODAPH) but not enamel-specific. Molecular evolutionary analyses indicate that all seven enamel-specific genes have inactivating mutations that are scattered across branches of the mysticete tree. Three of the enamel genes (ACP4, KLK4, MMP20) have inactivating mutations that are shared by all mysticetes. The two genes that are dentin-specific (DSPP) or tooth-specific (ODAPH) do not have any inactivating mutations that are shared by all mysticetes, but there are shared mutations in Balaenidae as well as in Plicogulae (Neobalaenidae + Balaenopteroidea). These shared mutations suggest that teeth were lost at most two times. Shared inactivating mutations and dN/dS analyses, in combination with cetacean divergence times, were used to estimate inactivation times of genes and by proxy enamel and tooth phenotypes at ancestral nodes. The results of these analyses are most compatible with a two-step model for the loss of teeth in the ancestry of living baleen whales: enamel was lost very early on the stem Mysticeti branch followed by the independent loss of dentin (and teeth) in the common ancestors of Balaenidae and Plicogulae, respectively. These results imply that some stem mysticetes, and even early crown mysticetes, may have had vestigial teeth comprised of dentin with no enamel. Our results also demonstrate that all odontocete species (in our study) with absent or degenerative enamel have inactivating mutations in one or more of their enamel genes.

Malfunction of airway basal stem cells plays a crucial role in pathophysiology of tracheobronchopathia osteoplastica

Understanding disease-associated stem cell abnormality has major clinical implications for prevention and treatment of human disorders, as well as for regenerative medicine. Here we report a multifaceted study on airway epithelial stem cells in Tracheobronchopathia Osteochondroplastica (TO), an under-detected tracheobronchial disorder of unknown etiology and lack of specific treatment. Epithelial squamous metaplasia and heterotopic bone formation with abnormal cartilage proliferation and calcium deposits are key pathological hallmarks of this disorder, but it is unknown whether they are coincident or share certain pathogenic mechanisms in common. By functional evaluation and genome-wide profiling at both transcriptional and epigenetic levels, we reveal a role of airway basal cells in TO progression by acting as a repository of inflammatory and TGFβ-BMP signals, which contributes to both epithelial metaplasia and mesenchymal osteo-chondrogenesis via extracellular signaling and matrix remodeling. Restoration of microenvironment by cell correction or local pathway intervention may provide therapeutic benefits.

Tracheobronchopathia osteoplastica (TO), is an underreported affliction characterized by squamous metaplasia and heterotopic bone formation in trachea and bronchi. Here the authors apply functional, as well as genome-wide transcriptional and epigenetic profiling to identify airway basal cells dysfunction underlying TO.

The synergistic effects of TGF-β1 and RUNX2 on enamel mineralization through regulating ODAPH expression during the maturation stage

Transforming growth factor β1 (TGF-β1) and Runt-related transcription factor 2 (RUNX2) are critical factors promoting enamel development and maturation. Our previous studies reported that absence of TGF-β1 or RUNX2 resulted in abnormal secretion and absorption of enamel matrix proteins. However, the mechanism remained enigmatic. In this study, TGF-β1^-/-Runx2^-/- and TGF-β1^+/-Runx2^+/- mice were successfully generated to clarify the relationship between TGF-β1 and RUNX2 during amelogenesis. Lower mineralization was observed in TGF-β1^-/-Runx2^-/- and TGF-β1^+/-Runx2^+/- mice than single gene deficient mice. Micro-computed tomography (μCT) revealed a lower ratio of enamel to dentin density in TGF-β1^-/-Runx2^-/- mice. Although μCT elucidated a relatively constant enamel thickness, variation was identified by scanning electron microscopy, which revealed that TGF-β1^-/-Runx2^-/- mice were more vulnerable to acid etching with lower degree of enamel mineralization. Furthermore, the double gene knock-out mice exhibited more serious enamel dysplasia than the single gene deficient mice. Hematoxylin-eosin staining revealed abnormalities in ameloblast morphology and arrangement in TGF-β1^-/-Runx2^-/- mice, which was accompanied by the absence of atypical basal lamina (BL) and the ectopic of enamel matrix. Odontogenesis-associated phosphoprotein (ODAPH) has been identified as a component of an atypical BL. The protein and mRNA expression of ODAPH were down-regulated. In summary, TGF-β1 and RUNX2 might synergistically regulate enamel mineralization through the downstream target gene Odaph. However, the specific mechanism by which TGF-β1 and RUNX2 promote mineralization remains to be further studied.

Expression and localization of amelotin, laminin γ2 and odontogenesis-associated phosphoprotein (ODAPH) on the basal lamina and junctional epithelium

At maturation stage of enamel development, a specialized basal lamina (sBL) was built between ameloblasts and enamel. After the teeth eruption, the ameloblasts transform into the inner cell layer of junctional epithelium. The inner cell layer forms the internal basal lamina of junctional epithelium. However, the composition of the sBL and internal basal lamina was not clarified. The objective of our study was to make a description of the localization of amelotin (AMTN), laminin γ2 (LAMC2) and Odontogenesis-associated phosphoprotein (ODAPH) on the sBL and internal basal lamina. In immunohistochemical study, AMTN, LAMC2 and ODAPH were detected on the sBL at maturation stage. AMTN was also detected in ameloblasts at maturation stage. The expression of AMTN decreased from early-to-late maturation stage. In contrast, the expression of LAMC2 and ODAPH was stable. Immunofluorescence double-staining showed the localization of AMTN was close to enamel surface. However, the localization of ODAPH was close to ameloblasts. LAMC2 and ODAPH were observed on internal basal lamina of junctional epithelium. In contrast, no expression of AMTN was detected on internal basal lamina of junctional epithelium. Our results suggested that ODAPH might participate in enamel maturation and periodontal health, which might provide a better understanding of enamel defects and periodontal disease in clinic.

Mouse Dspp frameshift model of human dentinogenesis imperfecta

Non-syndromic inherited defects of tooth dentin are caused by two classes of dominant negative/gain-of-function mutations in dentin sialophosphoprotein (DSPP): 5' mutations affecting an N-terminal targeting sequence and 3' mutations that shift translation into the - 1 reading frame. DSPP defects cause an overlapping spectrum of phenotypes classified as dentin dysplasia type II and dentinogenesis imperfecta types II and III. Using CRISPR/Cas9, we generated a Dspp^-1fs mouse model by introducing a FLAG-tag followed by a single nucleotide deletion that translated 493 extraneous amino acids before termination. Developing incisors and/or molars from this mouse and a Dspp^P19L mouse were characterized by morphological assessment, bSEM, nanohardness testing, histological analysis, in situ hybridization and immunohistochemistry. Dspp^P19L dentin contained dentinal tubules but grew slowly and was softer and less mineralized than the wild-type. Dspp^P19L incisor enamel was softer than normal, while molar enamel showed reduced rod/interrod definition. Dspp^-1fs dentin formation was analogous to reparative dentin: it lacked dentinal tubules, contained cellular debris, and was significantly softer and thinner than Dspp^+/+ and Dspp^P19L dentin. The Dspp^-1fs incisor enamel appeared normal and was comparable to the wild-type in hardness. We conclude that 5' and 3' Dspp mutations cause dental malformations through different pathological mechanisms and can be regarded as distinct disorders.

Maturation stage enamel defects in Odontogenesis-associated phosphoprotein (Odaph) deficient mice

BACKGROUND

Mutation in Odontogenesis-associated phosphoprotein (ODAPH) has been reported to cause recessive hypomineralized amelogenesis imperfecta (AI) in human. However, the exact role of ODAPH in amelogenesis is still unknown.

RESULTS

ODAPH was identified as a novel constituent of the atypical basal lamina located at the interface between maturation ameloblasts and the enamel by dual immunofluorescence staining of ODAPH and LAMC2. Odaph knockout mice were generated to explore the function of ODAPH in amelogenesis. Odaph^-/- mice teeth showed severely attrition and reduced enamel mineralization. Histological analysis showed from transition or early-maturation stage, ameloblasts were rapidly shortened, lost cell polarity, and exhibited cell pathology. Abundant enamel matrix marked by amelogenin was retained. Temporary cyst-like structures were formed between flattened epithelial cells and the enamel from maturation stage to eruption. The integrity of the atypical basal lamina was impaired indicated by the reduced diffuse expression of LAMC2 and AMTN. The expression of maturation stage related genes of Amtn, Klk4, Integrinβ6 and Slc24a4 were significantly decreased.

CONCLUSIONS

Our results suggested Odaph played vital roles during amelogenesis by maintaining the integrity of the atypical basal lamina in maturation stage, which may contribute to a better understanding of the pathophysiology of human AI.