Physiopathology of dental rickets in vitamin D receptor-ablated mice

Association of defects of enamel with polymorphisms in the vitamin D receptor and parathyroid hormone genes

This cross-sectional study aimed to investigate the association between developmental defects of enamel (DDE) and single nucleotide polymorphisms (SNPs) in the genes encoding the vitamin D receptor (VDR) and parathyroid hormone (PTH). Orthodontic patients receiving treatment at a dental school were selected through convenience sampling. Intra-oral photographs were used to assess DDE, which were classified according to the criteria proposed by Ghanim et al. (2015) by a single calibrated examiner (Kappa>0.80). Enamel hypoplasia, molar-incisor hypomineralization (MIH), hypomimineralized second primary molar (HSPM), and non-MIH/HSPM demarcated opacities were considered for the analysis. Genomic DNA was extracted from buccal cells. The SNPs in VDR (rs7975232) and PHT (rs694, rs6256, and rs307247) were genotyped using real-time polymerase chain reactions (PCR). Statistical analyses were performed using the PLINK software (version 1.03, designed by Shaun Purcell, EUA). Chi-square or Fisher's exact tests were performed at a significance level of 5%. Ninety-one (n=91) patients (49 females and 42 males) (mean age of 14.1±5.8 years) were included. The frequency of DDE was 38.5% (35 patients). Genotype distributions were in Hardy-Weinberg equilibrium. No significant statistical association was found between DDE and the SNPs evaluated. A borderline association (p=0.09) was observed between DDE and the CC haplotype for SNP rs7975232 in VDR. In conclusion, the selected SNPs in VDR and PTH genes were not associated with DDE in the studied samples.

Dental niche cells directly contribute to tooth reconstitution and morphogenesis

Mammalian teeth develop from the inductive epithelial-mesenchymal interaction, an important mechanism shared by many organs. The cellular basis for such interaction remains elusive. Here, we generate a dual-fluorescence model to track and analyze dental cells from embryonic to postnatal stages, in which Pitx2⁺ epithelium and Msx1⁺ mesenchyme are sufficient for tooth reconstitution. Single-cell RNA sequencing and spatial mapping further revealed critical cellular dynamics during molar development, where tooth germs are organized by Msx1⁺Sdc1⁺ dental papilla and surrounding dental niche. Surprisingly, niche cells are more efficient in tooth reconstitution and can directly regenerate papilla cells through interaction with dental epithelium. Finally, from the dental niche, we identify a group of previously unappreciated migratory Msx1⁺ Sox9⁺ cells as the potential cell origin for dental papilla. Our results indicate that the dental niche cells directly contribute to tooth organogenesis and provide critical insights into the essential cell composition for tooth engineering.

Identification by whole-exome sequencing of new single-nucleotide polymorphisms associated with molar-incisor hypomineralisation among the Lebanese population

OBJECTIVE

Molar-incisor hypomineralization (MIH) is a developmental qualitative enamel defect, causing a worldwide challenging dental problem. The etiology of this defect remains unclear. Here we identify by whole-exome sequencing (WES) new single-nucleotide polymorphisms (SNPs) in genes expressed during enamel mineralization and in those modulating prenatal, natal and postnatal risk factors among the Lebanese MIH children: immune system and xenobiotic detoxification.

DESIGN

Dental examination for MIH was performed based on the MIH index for diagnostic criteria. Saliva samples were collected from 37 non-related, MIH-diagnosed subjects for DNA extraction. WES was performed on the Illumina HiSeq2000 platform. The χ² test and Fisher's exact test were used to determine relationship between SNPs frequencies and MIH. OR and its 95% CI were used to report the strength of association. The significance threshold was set at 0.05.

RESULTS

Among the Lebanese population, 37 SNPs presented a significant association with MIH in the following genes: AMTN, MMP-20, STIM1, STIM2, ORAI1, SLC34A2, SLC34A3, VDR, PVALB, HSP90B1, TRPM7, SLC24A4, CA6, SLC4A2, TNFRSF11A, IL10RB, ARNT, ESR1 and CYP1B1.

CONCLUSION

This is the first WES study conducted in patients with MIH. Yet, interactions between polymorphisms in different gene categories are to be investigated for a better assessment of MIH susceptibility.

Vitamin D Review: The Low Hanging Fruit for Human Health

Vitamin D is an important hormone that is known for the regulation of calcium and phosphate metabolism. Vitamin D deficiency leads to rickets in children and osteoporosis in adults leading to poor bone mineralisation and can also lead to serious dental complications in the same population. Recent studies have shown vitamin D to work as a hormone needed not only in bone and teeth but also in other body organs from intrauterine life up to old age. It has been demonstrated that Vitamin D has various effects on biological processes that deal with cell growth, differentiation, cell death, immune regulation, DNA stability, and neuronal growth. Despite being readily formed in the body through the intervention of the sun, patients are still found to have low vitamin D levels. We review studies done to show how vitamin D works.

Probing the Scope and Mechanisms of Calcitriol Actions Using Genetically Modified Mouse Models

Genetically modified mice have provided novel insights into the mechanisms of activation and inactivation of vitamin D, and in the process have provided phenocopies of acquired human disease such as rickets and osteomalacia and inherited diseases such as pseudovitamin D deficiency rickets, hereditary vitamin D resistant rickets, and idiopathic infantile hypercalcemia. Both global and tissue-specific deletion studies leading to decreases of the active form of vitamin D, calcitriol [1,25(OH)2D], and/or of the vitamin D receptor (VDR), have demonstrated the primary role of calcitriol and VDR in bone, cartilage and tooth development and in the regulation of mineral metabolism and of parathyroid hormone (PTH) and FGF23, which modulate calcium and phosphate fluxes. They have also, however, extended the spectrum of actions of calcitriol and the VDR to include, among others: modulation, jointly and independently, of skin metabolism; joint regulation of adipose tissue metabolism; cardiovascular function; and immune function. Genetic studies in older mice have also shed light on the molecular mechanisms underlying the important role of the calcitriol/VDR pathway in diseases of aging such as osteoporosis and cancer. In the course of these studies in diverse tissues, important upstream and downstream, often tissue-selective, pathways have been illuminated, and intracrine, as well as endocrine actions have been described. Human studies to date have focused on acquired or genetic deficiencies of the prohormone vitamin D or the (generally inactive) precursor metabolite 25-hyrodxyvitamin D, but have yet to probe the pleiotropic aspects of deficiency of the active form of vitamin D, calcitriol, in human disease. © 2020 American Society for Bone and Mineral Research © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Inadequate tissue mineralization promotes cancer cell attachment

Bone metastases are a frequent complication in prostate cancer, and several studies have shown that vitamin D deficiency promotes bone metastases. However, while many studies focus on vitamin D’s role in cell metabolism, the effect of chronically low vitamin D levels on bone tissue, i.e. insufficient mineralization of the tissue, has largely been ignored. To investigate, whether poor tissue mineralization promotes cancer cell attachment, we used a fluorescence based adhesion assay and single cell force spectroscopy to quantify the adhesion of two prostate cancer cell lines to well-mineralized and demineralized dentin, serving as biomimetic bone model system. Adhesion rates of bone metastases-derived PC3 cells increased significantly on demineralized dentin. Additionally, on mineralized dentin, PC3 cells adhered mainly via membrane anchored surface receptors, while on demineralized dentin, they adhered via cytoskeleton-anchored transmembrane receptors, pointing to an interaction via exposed collagen fibrils. The adhesion rate of lymph node derived LNCaP cells on the other hand is significantly lower than that of PC3 and not predominately mediated by cytoskeleton-linked receptors. This indicates that poor tissue mineralization facilitates the adhesion of invasive cancer cells by the exposure of collagen and emphasizes the disease modifying effect of sufficient vitamin D for cancer patients.

The relationship between molar incisor hypomineralization, dental caries, socioeconomic factors, and polymorphisms in the vitamin D receptor gene: a population-based study

OBJECTIVES

The purpose of this cross-sectional study was to investigate whether polymorphisms in vitamin D receptor (VDR) genes increase the prevalence of dental caries, molar incisor hypomineralization (MIH), and hypomineralized primary second molars (HPSM).

MATERIAL AND METHODS

A representative population-based sample of 731 schoolchildren, 8 years of age, was randomly selected in Curitiba, Paraná, Brazil. MIH, HPSM, and dental caries were clinically assessed by four calibrated examiners (kappa > 0.80) using European Academy of Pediatric Dentistry (2003) criteria, the modified Developmental Defects of Enamel (DDE) index, and the Decayed, Missing, or Filled Teeth (DMFT) index by the World Health Organization (2013), respectively. The VDR rs739837 and rs2228570 polymorphisms were genotyped using real-time polymerase chain reaction. Associations were analyzed by Poisson regression with robust variance (α = 0.05).

RESULTS

Schoolchildren with MIH presented a higher prevalence of dental caries (DMFT > 1, PR = 2.52, confidence interval = 1.60-3.97, p ≤ 0.001). No association was observed between MIH, HPSM, and dental caries, with rs739837 and rs2228570 polymorphisms. Individuals with the GT/GG genotype in rs739837 polymorphism presented a higher prevalence of MIH in molars and incisors than individuals TT (PR = 2.34, confidence interval = 1.08-5.07, p = 0.03).

CONCLUSION

Children with MIH presented a significant higher prevalence of dental caries than children without MIH. To carry at least one G allele in rs739837 was associated to higher prevalence of MIH in molars and incisors.

CLINICAL RELEVANCE

Our findings suggested that more severe cases with incisors affected by MIH could be associated with polymorphism in VDR gene.

The Associations of Maternal and Neonatal Vitamin D with Dental Development in Childhood

BACKGROUND

Vitamin D influences the formation and mineralization of teeth.

OBJECTIVE

To investigate the association of maternal and neonatal vitamin D concentrations with the dental development of 10-y-old children, in a population-based prospective cohort study among 3,770 mothers and children in the Netherlands.

METHODS

Maternal venous blood samples were collected in the second trimester (median 20.4 weeks of gestation; range: 18.5-23.2 wk) whereas umbilical cord blood samples were collected immediately after delivery (median 40.1 weeks of gestation; range 35.9-42.3 wk). Dental development was defined using the Demirjian method. Multivariate regression models were built to analyze the studied associations.

RESULTS

High concentrations of 25-hydroxyvitamin D [25(OH)D] during midpregnancy (β: -0.04; 95% CI: -0.08, -0.01) and at birth (β: -0.06; 95% CI: -0.10, -0.02) were associated with a lower dental age in children. The children of mothers with severe vitamin D deficiency [25(OH)D <25.0 nmol/L] during midpregnancy exhibited a higher dental age (β: 0.14; 95% CI: 0.03, 0.24) and higher developmental stages of the mandibular first premolar (β: 0.32; 95% CI: 0.04, 0.60) compared with the children of mothers with optimal values of 25(OH)D (≥75.0 nmol/L). Children with vitamin D deficiency [25(OH)D 25.0-49.9 nmol/L] at birth exhibited a higher dental age (β: 0.11; 95% CI: 0.01, 0.20), higher developmental stages of the mandibular second premolar (β: 0.27; 95% CI: 0.02, 0.51), and higher developmental stages of the mandibular second molar (β: 0.24; 95% CI: 0.00, 0.48) compared with children with sufficient-to-optimal values of 25(OH)D (≥50.0 nmol/L) at birth.

CONCLUSION

Higher maternal and neonatal 25(OH)D concentrations are associated with decelerated dental development in childhood. The lower the vitamin D level during midpregnancy or at birth, the higher the dental age of children, and the higher the developmental stages of the mandibular teeth.

Altered Ca2+ signaling in enamelopathies

Biomineralization requires the controlled movement of ions across cell barriers to reach the sites of crystal growth. Mineral precipitation occurs in aqueous phases as fluids become supersaturated with specific ionic compositions. In the biological world, biomineralization is dominated by the presence of calcium (Ca²⁺) in crystal lattices. Ca²⁺ channels are intrinsic modulators of this process, facilitating the availability of Ca²⁺ within cells in a tightly regulated manner in time and space. Unequivocally, the most mineralized tissue produced by vertebrates, past and present, is dental enamel. With some of the longest carbonated hydroxyapatite (Hap) crystals known, dental enamel formation is fully coordinated by specialized epithelial cells of ectodermal origin known as ameloblasts. These cells form enamel in two main developmental stages: a) secretory; and b) maturation. The secretory stage is marked by volumetric growth of the tissue with limited mineralization, and the opposite is found in the maturation stage, as enamel crystals expand in width concomitant with increased ion transport. Disruptions in the formation and/or mineralization stages result, in most cases, in permanent alterations in the crystal assembly. This introduces weaknesses in the material properties affecting enamel's hardness and durability, thus limiting its efficacy as a biting, chewing tool and increasing the possibility of pathology. Here, we briefly review enamel development and discuss key properties of ameloblasts and their Ca²⁺-handling machinery, and how alterations in this toolkit result in enamelopathies.

Expression of Steroid Receptors in Ameloblasts during Amelogenesis in Rat Incisors

Endocrine disrupting chemicals (EDCs) play a part in the modern burst of diseases and interfere with the steroid hormone axis. Bisphenol A (BPA), one of the most active and widely used EDCs, affects ameloblast functions, leading to an enamel hypomineralization pattern similar to that of Molar Incisor Hypomineralization (MIH). In order to explore the molecular pathways stimulated by BPA during amelogenesis, we thoroughly investigated the receptors known to directly or indirectly mediate the effects of BPA. The expression patterns of high affinity BPA receptors (ERRγ, GPR30), of ketosteroid receptors (ERs, AR, PGR, GR, MR), of the retinoid receptor RXRα, and PPARγ were established using RT-qPCR analysis of RNAs extracted from microdissected enamel organ of adult rats. Their expression was dependent on the stage of ameloblast differentiation, except that of ERβ and PPARγ which remained undetectable. An additional large scale microarray analysis revealed three main groups of receptors according to their level of expression in maturation-stage ameloblasts. The expression level of RXRα was the highest, similar to the vitamin D receptor (VDR), whereas the others were 13 to 612-fold lower, with AR and GR being intermediate. Immunofluorescent analysis of VDR, ERα and AR confirmed their presence mainly in maturation- stage ameloblasts. These data provide further evidence that ameloblasts express a specific combination of hormonal receptors depending on their developmental stage. This study represents the first step toward understanding dental endocrinology as well as some of the effects of EDCs on the pathophysiology of amelogenesis.

Molar incisor hypomineralization in HIV-infected children and adolescents

The objective was to determine the prevalence of molar incisor hypomineralization (MIH) among individuals between 7 and 15 years old infected or noninfected with human immunodeficiency virus (HIV). The study was conducted with 33 HIV-infected individuals (study group; SG) and 66 non-HIV-infected schoolchildren (control group; CG), paired by gender and age. Data collection was based on medical records (SG), a questionnaire for caregivers and oral examination for diagnosis of MIH (European Academy of Pediatric Dentistry criteria) and caries (DMFT index and ICDAS). Data were analyzed with Mann-Whitney, chi-square, and Fisher's exact tests and logistic regression. In SG, MIH (45.5%) and caries (87.9%) had higher prevalence. MIH was associated with use of protease inhibitors in SG (OR: 2.14; 95% CI: 1.21 to 3.77) and incubator need in CG (OR: 2.80; 95% CI: 1.71 to 9.10). HIV-infected patients had a higher prevalence of MIH and dental caries in the permanent dentition.

Dental enamel development defects in children and adolescents with HIV infection: case-control study

To determine the frequency of Enamel Development Defects (DDE) and associated factors in children and adolescents infected with HIV. A case-control study was conducted in HIV-infected patients (n = 52), aged 3 to 15, and a control group formed by preschool and schoolchildren (n = 104) matched according to gender, age and household income. Data on medical history, neonatal and maternal conditions were obtained. For diagnosis of enamel defects was used modified DDE Index. DDE frequency was 61.5% in the case group and 58.7% in the control group (p = 0.569). Infection of the genitourinary tract and maternal hemorrhage were factors associated with DDE in the case and control groups, respectively. An association was observed between the use of antiretroviral regimens with protease inhibitors or efavirenz and DDE in the permanent dentition. Children and adolescents HIV-infected showed a DDE frequency similar to healthy patients, but factors associated with this condition were different between the groups.

Store-operated Ca2+ Entry Modulates the Expression of Enamel Genes

Dental enamel formation is an intricate process tightly regulated by ameloblast cells. The correct spatiotemporal patterning of enamel matrix protein (EMP) expression is fundamental to orchestrate the formation of enamel crystals, which depend on a robust supply of Ca2+. In the extracellular milieu, Ca2+ -EMP interactions occur at different levels. Despite its recognized role in enamel development, the molecular machinery involved in Ca2+ homeostasis in ameloblasts remains poorly understood. A common mechanism for Ca2+ influx is store-operated Ca2+ entry (SOCE). We evaluated the possibility that Ca2+ influx in enamel cells might be mediated by SOCE and the Ca2+ release-activated Ca2+ (CRAC) channel, the prototypical SOCE channel. Using ameloblast-like LS8 cells, we demonstrate that these cells express Ca2+ -handling molecules and mediate Ca2+ influx through SOCE. As a rise in the cytosolic Ca2+ concentration is a versatile signal that can modulate gene expression, we assessed whether SOCE in enamel cells had any effect on the expression of EMPs. Our results demonstrate that stimulating LS8 cells or murine primary enamel organ cells with thapsigargin to activate SOCE leads to increased expression of Amelx, Ambn, Enam, Mmp20. This effect is reversed when cells are treated with a CRAC channel inhibitor. These data indicate that Ca2+ influx in LS8 cells and enamel organ cells is mediated by CRAC channels and that Ca2+ signals enhance the expression of EMPs. Ca2+ plays an important role not only in mineralizing dental enamel but also in regulating the expression of EMPs.

Dspp-independent Effects of Transgenic Trps1 Overexpression on Dentin Formation

The Trps1 transcription factor is highly expressed in dental mesenchyme and preodontoblasts, while in mature, secretory odontoblasts, it is expressed at low levels. Previously, we have shown that high Trps1 levels in mature odontoblasts impair their function in vitro and in vivo. Col1a1-Trps1 transgenic (Trps1-Tg) mice demonstrate defective dentin secretion and mineralization, which are associated with significantly decreased Dspp expression due to direct repression of the Dspp gene by Trps1. Here, by crossing Trps1-Tg and Col1a1-Dspp transgenic (Dspp-Tg) mice, we generated Col1a1-Trps1;Col1a1-Dspp double transgenic (double-Tg) mice in which Dspp was restored in odontoblasts overexpressing Trps1. Comparative micro-computed tomography analyses revealed partial correction of the dentin volume and no improvement of dentin mineralization in double transgenic mice in comparison with Trps1-Tg and wild-type (WT) mice. In addition, dentin of double-Tg mice has an irregular mineralization pattern characteristic for dentin in hypophosphatemic rickets. Consistent with this phenotype, decreased levels of Phex, Vdr, and Fam20c proteins are detected in both Trps1-Tg and double-Tg odontoblasts in comparison with WT and Dspp-Tg odontoblasts. This suggests that the Dspp-independent dentin mineralization defects in Trps1-Tg mice are a result of downregulation of a group of proteins critical for mineral deposition within the dentin matrix. In summary, by demonstrating that Trps1 functions as a repressor of later stages of dentinogenesis, we provide functional significance of the dynamic Trps1 expression pattern during dentinogenesis.

MSX2 in ameloblast cell fate and activity

While many effectors have been identified in enamel matrix and cells via genetic studies, physiological networks underlying their expression levels and thus the natural spectrum of enamel thickness and degree of mineralization are now just emerging. Several transcription factors are candidates for enamel gene expression regulation and thus the control of enamel quality. Some of these factors, such as MSX2, are mainly confined to the dental epithelium. MSX2 homeoprotein controls several stages of the ameloblast life cycle. This chapter introduces MSX2 and its target genes in the ameloblast and provides an overview of knowledge regarding its effects in vivo in transgenic mouse models. Currently available in vitro data on the role of MSX2 as a transcription factor and its links to other players in ameloblast gene regulation are considered. MSX2 modulations are relevant to the interplay between developmental, hormonal and environmental pathways and in vivo investigations, notably in the rodent incisor, have provided insight into dental physiology. Indeed, in vivo models are particularly promising for investigating enamel formation and MSX2 function in ameloblast cell fate. MSX2 may be central to the temporal-spatial restriction of enamel protein production by the dental epithelium and thus regulation of enamel quality (thickness and mineralization level) under physiological and pathological conditions. Studies on MSX2 show that amelogenesis is not an isolated process but is part of the more general physiology of coordinated dental-bone complex growth.

Background factors of molar-incisor hypomineralization in a group of Finnish children

OBJECTIVE

Molar-Incisor Hypomineralization (MIH) is a common developmental enamel defect characterized by demarcated opacities in permanent molars and incisors. Its etiology still remains unclear. The aim of this retrospective cohort study was to assess if the socioeconomic environment of the child is associated with MIH.

MATERIALS AND METHODS

The study was located in two rural towns and three urban cities in Finland. A total of 818 children, between 7-13 years old, were examined for MIH using the evaluation criteria in line with those of the European Academy of Paediatric Dentistry, but excluding opacities smaller than 2 mm in diameter. The mothers filled in a questionnaire which included questions related to the family's way of living (e.g. area of residency, farming, day care attendance) and socioeconomic status (family income, number of mother's school years, level of maternal education).

RESULTS

The prevalence of MIH in the study population was 17.1%. Family income, urban residency and day care attendance were associated with MIH in the univariate analysis. In the multivariate analysis using binary logistic regression, only urban residency during a child's first 2 years of life remained associated with MIH. The prevalence of MIH in urban areas was 21.3% and in rural areas 11.5% (OR = 2.18, CI = 1.35-3.53, p = 0.001).

CONCLUSIONS

The prevalence of MIH was related to urban residency and could not be explained by any other factor included in the study.

Prevention of the disrupted enamel phenotype in Slc4a4-null mice using explant organ culture maintained in a living host kidney capsule

Slc4a4-null mice are a model of proximal renal tubular acidosis (pRTA). Slc4a4 encodes the electrogenic sodium base transporter NBCe1 that is involved in transcellular base transport and pH regulation during amelogenesis. Patients with mutations in the SLC4A4 gene and Slc4a4-null mice present with dysplastic enamel, amongst other pathologies. Loss of NBCe1 function leads to local abnormalities in enamel matrix pH regulation. Loss of NBCe1 function also results in systemic acidemic blood pH. Whether local changes in enamel pH and/or a decrease in systemic pH are the cause of the abnormal enamel phenotype is currently unknown. In the present study we addressed this question by explanting fetal wild-type and Slc4a4-null mandibles into healthy host kidney capsules to study enamel formation in the absence of systemic acidemia. Mandibular E11.5 explants from NBCe1^−/− mice, maintained in host kidney capsules for 70 days, resulted in teeth with enamel and dentin with morphological and mineralization properties similar to cultured NBCe1^+/+ mandibles grown under identical conditions. Ameloblasts express a number of proteins involved in dynamic changes in H⁺/base transport during amelogenesis. Despite the capacity of ameloblasts to dynamically modulate the local pH of the enamel matrix, at least in the NBCe1^−/− mice, the systemic pH also appears to contribute to the enamel phenotype. Extrapolating these data to humans, our findings suggest that in patients with NBCe1 mutations, correction of the systemic metabolic acidosis at a sufficiently early time point may lead to amelioration of enamel abnormalities.

Dental complications of rickets in early childhood: case report on 2 young girls

Vitamin D is an essential hormone for calcium gut absorption. It is also involved in child growth, cancer prevention, immune system responses, and tooth formation. Due to inadequate vitamin D intake and/or decreased sunlight exposure, vitamin D deficiency has resurfaced in developed countries despite known inexpensive and effective preventive methods. Vitamin D deficiency is a common cause of rickets, a condition that affects bone development in children and that can have serious dental complications. Deficiency during pregnancy can cause enamel hypoplasia of primary teeth. Enamel regeneration is currently impossible; hypoplasia is therefore irreversible, and once affected, teeth are prone to fast caries development. Deficiency during early childhood can affect permanent teeth and ensuing caries can sometimes lead to tooth loss at a young age. Oral manifestations of rickets should be diagnosed early by both physicians and dentists to prevent severe dental complications. This case study presents 2 young girls with rickets in early childhood who suffered from subsequent serious tooth decay.

The rachitic tooth

Teeth are mineralized organs composed of three unique hard tissues, enamel, dentin, and cementum, and supported by the surrounding alveolar bone. Although odontogenesis differs from osteogenesis in several respects, tooth mineralization is susceptible to similar developmental failures as bone. Here we discuss conditions fitting under the umbrella of rickets, which traditionally referred to skeletal disease associated with vitamin D deficiency but has been more recently expanded to include newly identified factors involved in endocrine regulation of vitamin D, phosphate, and calcium, including phosphate-regulating endopeptidase homolog, X-linked, fibroblast growth factor 23, and dentin matrix protein 1. Systemic mineral metabolism intersects with local regulation of mineralization, and factors including tissue nonspecific alkaline phosphatase are necessary for proper mineralization, where rickets can result from loss of activity of tissue nonspecific alkaline phosphatase. Individuals suffering from rickets often bear the additional burden of a defective dentition, and transgenic mouse models have aided in understanding the nature and mechanisms involved in tooth defects, which may or may not parallel rachitic bone defects. This report reviews dental effects of the range of rachitic disorders, including discussion of etiologies of hereditary forms of rickets, a survey of resulting bone and tooth mineralization disorders, and a discussion of mechanisms, known and hypothesized, involved in the observed dental pathologies. Descriptions of human pathology are augmented by analysis of transgenic mouse models, and new interpretations are brought to bear on questions of how teeth are affected under conditions of rickets. In short, the rachitic tooth will be revealed.

Physiological insights from the vitamin D receptor knockout mouse

Identification of vitamin D as a potent antirachitic factor almost a century ago prompted investigations aimed at addressing its mechanism of action and key target tissues. Studies in vitamin D deficiency models and in kindreds with impaired hormone activation and function were critical in identifying key steps in the vitamin D signaling pathway. Studies in humans with vitamin D receptor (VDR) mutations provided a tremendous amount of information regarding the role of this receptor in calcium and skeletal homeostasis. The availability of mouse models of VDR ablation provided an important tool for detailed molecular analyses of the pathophysiologic basis for the skeletal, parathyroid and cutaneous phenotypes observed in mice and humans with impaired VDR function. These investigations revealed that a critical action of the liganded receptor is the promotion of intestinal calcium absorption. Bypassing this defect by dietary or transgenic rescue prevents the severe skeletal phenotype of the VDR ablated mice, as well as the development of hyperparathyroidism. In contrast, intestine specific ablation of the receptor results in marked skeletal pathology. Like their human counterparts, VDR knockout mice develop alopecia. Studies in these mice demonstrated that the actions of the VDR required for cyclical regeneration of the hair follicle and prevention of alopecia were shown independent of 1,25-dihydroxyvitamin D demonstrating that the unliganded receptor has an important role in the cutaneous homeostasis.