Osteopontin and the dento-osseous pathobiology of X-linked hypophosphatemia

X-linked hypophosphatemia: The value of feedback focus groups to assess patient and caregiver needs

X-linked hypophosphatemia (XLH) is a rare, multi-systemic, invalidating disease requiring a multi-disciplinary approach. No specific action in XLH, neither for the patients' specific needs nor for the methodology for the evaluation of these were found. Thus, to identify the needs of XLH patients and their caregivers, we organised focus groups in our reference centre with a view to build educational sessions. Focus groups including either XLH children, XLH adults, or caregivers ran in parallel. Each group was led by a person trained in therapeutic education (nurse, paediatric nephrologist) with another healthcare provider specialised in XLH (rheumatologist, nephrologist). One additional person with knowledge of XLH (clinical research associate, paediatric resident) took minutes. The duration of each session was 1.5h; XLH patients/caregivers were asked to answer age-adapted "open questions" on their daily life and quality of life. At the end, a global restitution was made. The needs identified were later grouped and analysed, which allowed us to build the educational sessions. The XLH children group included 5 children, the XLH adults group included 10 adults, and the caregivers group included 6 parents or partners. Major needs were identified: knowledge of XLH, treatment, dental care and adapted physical activity, with additional questions on socio-professional adaptations and financial support in adults. Partner patients were also identified to co-build the support programme. The study allowed us to identify the needs of XLH patients and their caregivers using the focus group method and then, using these needs, to build educational sessions and a therapeutic education programme for XLH patients.

Spheroids as a 3D in vitro model to study bone and bone mineralization

Traumas, fractures, and diseases can severely influence bone tissue. Insight into bone mineralization is essential for the development of therapies and new strategies to enhance bone regeneration. 3D cell culture systems, in particular cellular spheroids, have gained a lot of interest as they can recapitulate crucial aspects of the in vivo tissue microenvironment, such as the extensive cell-cell and cell-extracellular matrix (ECM) interactions found in tissue. The potential of combining spheroids and various classes of biomaterials opens also new opportunities for research within bone tissue engineering. Characterizing cellular organization, ECM structure, and ECM mineralization is a fundamental step for understanding the biological processes involved in bone tissue formation in a spheroid-based model system. Still, many experimental techniques used in this field of research are optimized for use with monolayer cell cultures. There is thus a need to develop new and improving existing experimental techniques, for applications in 3D cell culture systems. In this review, bone composition and spheroids properties are described. This is followed by an insight into the techniques that are currently used in bone spheroids research and how these can be used to study bone mineralization. We discuss the application of staining techniques used with optical and confocal fluorescence microscopy, molecular biology techniques, second harmonic imaging microscopy, Raman spectroscopy and microscopy, as well as electron microscopy-based techniques, to evaluate osteogenic differentiation, collagen production and mineral deposition. Challenges in the applications of these methods in bone regeneration and bone tissue engineering are described. STATEMENT OF SIGNIFICANCE: 3D cell cultures have gained a lot of interest in the last decades as a possible technique that can be used to recreate in vitro in vivo biological process. The importance of 3D environment during bone mineralization led scientists to use this cell culture to study this biological process, to obtain a better understanding of the events involved. New and improved techniques are also required for a proper analysis of this cell model and the process under investigation. This review summarizes the state of the art of the techniques used to study bone mineralization and how 3D cell cultures, in particular spheroids, are tested and analysed to obtain better resolved results related to this complex biological process.

The Role of Osteopontin in Psoriasis-A Scoping Review

Psoriasis is a chronic systemic disease with an immunological basis and a complex pathophysiology. The chronic inflammatory status of psoriasis is associated with several comorbidities, such as metabolic syndrome, obesity, and cardiovascular disease. The development of psoriasis is influenced by osteopontin, a glycoprotein that influences physiological and pathological reactions by modulating Th1 and Th17 cellular responses, stimulating keratinocyte proliferation, regulating cellular apoptosis, and promoting angiogenesis. The recent identification of immune pathways involved in psoriasis development has facilitated the development of biological treatments; however, a better understanding of the intricate relationship between underlying inflammatory processes, psoriasis development, and accompanying comorbidities is needed for improved disease management.

[The tooth: A marker of developmental abnormalities]

Tooth formation results from specific epithelial-mesenchymal interactions, which summarize a number of developmental processes. Tooth anomalies may thus reflect subclinical diseases of the kidney, bone and more broadly of the mineral metabolism, skin or nervous system. Odontogenesis starts from the 3rd week of intrauterine life by the odontogenic orientation of epithelial cells by a first PITX2 signal. The second phase is the acquisition of the number, shape, and position of teeth. It depends on multiple transcription and growth factors (BMP, FGF, SHH, WNT). These ecto-mesenchymal interactions guide cell migration, proliferation, apoptosis and differentiation ending in the formation of the specific dental mineralized tissues. Thus, any alteration will have consequences on the tooth structure or shape. Resulting manifestations will have to be considered in the patient phenotype and the multidisciplinary care, but also may contribute to identify the altered genetic circuity.

X-Linked Hypophosphatemia: Does Targeted Therapy Modify Dental Impairment?

X-linked hypophosphatemia is a rare, hereditary disorder that significant influences teeth and alveolar bone. The first clinical sign leading to the diagnosis of X-linked hypophosphatemia is often dental impairment with dental abscesses and dentin mineralization defects. Genetic analysis helped find the responsible gene and therefore opened up new ways of therapeutically managing X-linked hypophosphatemia. The human monoclonal antibody Burosumab represents a milestone in the targeted therapy of this hereditary disease by directly addressing its pathophysiology. Targeted therapy has been shown to improve skeletal impairment, pain, and phosphate metabolism. However, the influence of this new therapy on dental impairment has only been addressed in a few recent studies with varying results. Therefore, in this review, we aim to summarize the dental phenotype and analyze the different treatment modalities with a focus on dental impairment.

Dental impact of anti-fibroblast growth factor 23 therapy in X-linked hypophosphatemia

Elevated fibroblast growth factor 23 (FGF23) in X-linked hypophosphatemia (XLH) results in rickets and phosphate wasting, manifesting by severe bone and dental abnormalities. Burosumab, a FGF23-neutralizing antibody, an alternative to conventional treatment (phosphorus and active vitamin D analogs), showed significant improvement in the long bone phenotype. Here, we examined whether FGF23 antibody (FGF23-mAb) also improved the dentoalveolar features associated with XLH. Four-week-old male Hyp mice were injected weekly with 4 or 16 mg·kg-1 of FGF23-mAb for 2 months and compared to wild-type (WT) and vehicle (PBS) treated Hyp mice (n = 3-7 mice). Micro-CT analyses showed that both doses of FGF23-mAb restored dentin/cementum volume and corrected the enlarged pulp volume in Hyp mice, the higher concentration resulting in a rescue similar to WT levels. FGF23-mAb treatment also improved alveolar bone volume fraction and mineral density compared to vehicle-treated ones. Histology revealed improved mineralization of the dentoalveolar tissues, with a decreased amount of osteoid, predentin and cementoid. Better periodontal ligament attachment was also observed, evidenced by restoration of the acellular cementum. These preclinical data were consistent with the retrospective analysis of two patients with XLH showing that burosumab treatment improved oral features. Taken together, our data show that the dentoalveolar tissues are greatly improved by FGF23-mAb treatment, heralding its benefit in clinics for dental abnormalities.

Bone Material Properties in Bone Diseases Affecting Children

PURPOSE OF REVIEW

Metabolic and genetic bone disorders affect not only bone mass but often also the bone material, including degree of mineralization, matrix organization, and lacunar porosity. The quality of juvenile bone is moreover highly influenced by skeletal growth. This review aims to provide a compact summary of the present knowledge on the complex interplay between bone modeling and remodeling during skeletal growth and to alert the reader to the complexity of bone tissue characteristics in children with bone disorders.

RECENT FINDINGS

We describe cellular events together with the characteristics of the different tissues and organic matrix organization (cartilage, woven and lamellar bone) occurring during linear growth. Subsequently, we present typical alterations thereof in disorders leading to over-mineralized bone matrix compared to those associated with low or normal mineral content based on bone biopsy studies. Growth spurts or growth retardation might amplify or mask disease-related alterations in bone material, which makes the interpretation of bone tissue findings in children complex and challenging.

Contributions of increased osteopontin and hypophosphatemia to dentoalveolar defects in osteomalacic Hyp mice

X-linked hypophosphatemia (XLH) is an inherited disorder caused by inactivating mutations in the PHEX gene leading to renal phosphate wasting, rickets and osteomalacia. XLH is also associated with dentoalveolar mineralization defects in tooth enamel, dentin and cementum, and in alveolar bone, which lead to an increased prevalence of dental abscesses, periodontal disease and tooth loss. Genetic mouse experiments, and deficiencies in XLH patient therapies where treatments do not fully ameliorate mineralization defects, suggest that other pathogenic mechanisms may exist in XLH. The mineralization-inhibiting, secreted extracellular matrix phosphoprotein osteopontin (OPN, gene Spp1) is a substrate for the PHEX enzyme whereby extensive and inactivating degradation of inhibitory OPN by PHEX facilitates mineralization. Conversely, excess OPN accumulation in skeletal and dental tissues - for example in XLH where inactivating mutations in the PHEX gene limit degradation of inhibitory OPN, or as occurs in Fgf23-null mice - contributes to mineralization defects. We hypothesized that Spp1/OPN ablation in Hyp mice (a mouse model for XLH) would reduce dentoalveolar mineralization defects. Immunostaining revealed increased OPN in Hyp vs. wild-type (WT) alveolar bone, particularly in osteocyte lacunocanalicular networks where Hyp mice have characteristic hypomineralized peri-osteocytic lesions (POLs). Micro-computed tomography and histology showed that ablation of Spp1 in Hyp mice (Hyp;Spp1-/-) on a normal diet did not ameliorate bulk defects in enamel, dentin, or alveolar bone. On a high-phosphate diet, both Hyp and Hyp;Spp1-/- mice showed improved mineralization of enamel, dentin, and alveolar bone. Silver staining indicated Spp1 ablation did not improve alveolar or mandibular bone osteocyte POLs in Hyp mice; however, they were normalized by a high-phosphate diet in both Hyp and Hyp;Spp1-/- mice, although inducing increased OPN. Collectively, these data indicate that despite changes in OPN content in the dentoalveolar mineralized tissues, there exist other compensatory mineralization mechanisms that arise from knockout of Spp1/OPN in the Hyp background.

X-linked hypophosphatemia caused by a deep intronic variant in PHEX identified by PCR-based RNA analysis of urine-derived cells

X-linked hypophosphatemia (XLH) is caused by dominant inactivating mutations in the phosphate regulating endopeptidase homology, X-linked (PHEX), resulting in elevated fibroblast growth factor 23 (FGF23), hypophosphatemia, rickets and osteomalacia. PHEX variants are identified in approximately 85 % of individuals with XLH, which leaves a substantial proportion of patients with negative DNA-based genetic testing. Here we describe a 16-year-old male who had typical features of XLH on clinical and radiological examination. Genomic DNA sequencing of a hypophosphatemia gene panel did not reveal a pathogenic variant. We therefore obtained a urine sample, established cell cultures and obtained PHEX cDNA from urine-derived cells. Sequencing of exon-spanning PCR products demonstrated the presence of an 84 bp pseudoexon in PHEX intron 21 due to a deep intronic variant (c.2147+1197A>G), which created a new splice donor site in intron 21. The corresponding PHEX protein would lack 33 amino acids on the C-terminus and instead include an unrelated sequence of 17 amino acids. The patient and his affected mother both had this variant. This report highlights that individuals with the typical clinical characteristics of XLH and negative genomic DNA sequence analysis can have deep intronic PHEX variants that are detectable by PCR-based RNA diagnostics.

Large scale analysis of osteocyte lacunae in klotho hypomorphic mice using high-resolution micro-computed tomography

BACKGROUND

Osteocytes are the most abundant cell type in adult bone, and the morphological characteristics of osteocytes and their lacunae appear to influence bone mass and fragility. Although conventional computed tomography (CT) has contributed greatly to advances in bone morphometry, capturing details of the entire hierarchical assembly, e.g., osteocyte lacuna parameters, has been limited by the analytical performance of CT (> 1 µm resolution).

METHODS

We used high-resolution (700 nm) micro-CT to evaluate and compare the osteocyte lacuna parameters over a large scale, i.e., in a maximum of about 45,700 lacunae (average), in tibial metaphyseal cortical bones of wild-type (WT) and αKlotho-hypomorphic (kl/kl) mice, the latter a model that exhibits osteopenia and aberrant osteocytes.

RESULTS

Of osteocyte lacuna parameters, lacunar surface per lacunar volume were significantly lower and lacuna diameter were significantly larger in kl/kl mice compared to WT mice. By analysis of individual osteocyte lacunae, we found that lacunar sphericity in kl/kl mice was higher than that in WT mice, and the diameters in the major and the minor axes were respectively lower and higher in kl/kl mice, especially at the proximal site of the region of interest.

CONCLUSION

We successfully assessed osteocyte lacuna parameters on the largest scale in mice reported to date and found that the shape of osteocyte lacunae of kl/kl mice are significantly different from those of WT mice. Although the mechanisms underlying the lacunar shape differences observed are not yet clear, changes in lacunar geometry are known to affect the transitions of strains to the osteocyte microenvironment and likely local osteocyte response(s). Thus, the fact that the differences are limited to the mesial region near the primary spongiosa suggests the likelihood of site-specific anomalies in mechanosensitive effects in kl/kl osteocytes with consequent site-specific effects bone metabolism and function.

Dentoalveolar Alterations in an Adenine-Induced Chronic Kidney Disease Mouse Model

Chronic kidney disease (CKD) is characterized by kidney damage and loss of renal function. CKD mineral and bone disorder (CKD-MBD) describes the dysregulation of mineral homeostasis, including hyperphosphatemia and elevated parathyroid hormone (PTH) secretion, skeletal abnormalities, and vascular calcification. CKD-MBD impacts the oral cavity, with effects including salivary gland dysfunction, enamel hypoplasia and damage, increased dentin formation, decreased pulp volume, pulp calcifications, and altered jaw bones, contributing to clinical manifestations of periodontal disease and tooth loss. Underlying mechanisms are not fully understood, and CKD mouse models commonly require invasive procedures with high rates of infection and mortality. We aimed to characterize the dentoalveolar effects of an adenine diet (AD)-induced CKD (AD-CKD) mouse model. Eight-week-old C57BL/6J mice were provided either a normal phosphorus diet control (CTR) or adenine and high-phosphorus diet CKD to induce kidney failure. Mice were euthanized at 15 weeks old, and mandibles were collected for micro-computed tomography and histology. CKD mice exhibited kidney failure, hyperphosphatemia, and hyperparathyroidism in association with porous cortical bone in femurs. CKD mice showed a 30% decrease in molar enamel volume compared to CTR mice. Enamel wear was associated with reduced ductal components, ectopic calcifications, and altered osteopontin (OPN) deposition in submandibular salivary glands of CKD mice. Molar cusps in CKD mice were flattened, exposing dentin. Molar dentin/cementum volume increased 7% in CKD mice and pulp volume decreased. Histology revealed excessive reactionary dentin and altered pulp-dentin extracellular matrix proteins, including increased OPN. Mandibular bone volume fraction decreased 12% and bone mineral density decreased 9% in CKD versus CTR mice. Alveolar bone in CKD mice exhibited increased tissue-nonspecific alkaline phosphatase localization, OPN deposition, and greater osteoclast numbers. AD-CKD recapitulated key aspects reported in CKD patients and revealed new insights into CKD-associated oral defects. This model has potential for studying mechanisms of dentoalveolar defects or therapeutic interventions. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Mineral tessellation in mouse enthesis fibrocartilage, Achilles tendon, and Hyp calcifying enthesopathy: A shared 3D mineralization pattern

The hallmark of enthesis architecture is the 3D compositional and structural gradient encompassing four tissue zones - tendon/ligament, uncalcified fibrocartilage, calcified fibrocartilage and bone. This functional gradient accommodates the large stiffness differential between calcified bone and uncalcified tendon/ligament. Here we analyze in 3D the organization of the mouse Achilles enthesis and mineralizing Achilles tendon in comparison to lamellar bone. We use correlative, multiscale high-resolution volume imaging methods including μCT with submicrometer resolution and FIB-SEM tomography (both with deep learning-based image segmentation), and TEM and SEM imaging, to describe ultrastructural features of physiologic, age-related and aberrant mineral patterning. We applied these approaches to murine wildtype (WT) Achilles enthesis tissues to describe in normal calcifying fibrocartilage a crossfibrillar mineral tessellation pattern similar to that observed in lamellar bone, but with greater variance in mineral tesselle morphology and size. We also examined Achilles enthesis structure in Hyp mice, a murine model for the inherited osteomalacic disease X-linked hypophosphatemia (XLH) with calcifying enthesopathy. In Achilles enthesis fibrocartilage of Hyp mice, we show defective crossfibrillar mineral tessellation similar to that which occurs in Hyp lamellar bone. At the cellular level in fibrocartilage, unlike in bone where enlarged osteocyte mineral lacunae are found as peri-osteocytic lesions, mineral lacunar volumes for fibrochondrocytes did not differ between WT and Hyp mice. While both WT and Hyp aged mice demonstrate Achilles tendon midsubstance ectopic mineralization, a consistently defective mineralization pattern was observed in Hyp mice. Strong immunostaining for osteopontin was observed at all mineralization sites examined in both WT and Hyp mice. Taken together, this new 3D ultrastructural information describes details of common mineralization trajectories for enthesis, tendon and bone, which in Hyp/XLH are defective.

Sclerostin antibody improves alveolar bone quality in the Hyp mouse model of X-Linked Hypophosphatemia (XLH)

X-linked hypophosphatemia (XLH) is a rare disease of elevated fibroblast growth factor 23 (FGF23) production that leads to hypophosphatemia and poor mineralization of bone and teeth. The clinical manifestations of XLH include a high prevalence of dental abscesses, likely driven by poorly formed structures of the dentoalveolar complex, including the alveolar bone, cementum, dentin, and periodontal ligament. Our previous studies have demonstrated that sclerostin antibody (Scl-Ab) treatment improves phosphate homeostasis, and increases bone mass, strength and mineralization in the Hyp mouse model of XLH. In the current study, we investigated whether Scl-Ab impacts the dentoalveolar structures of Hyp mice. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or Scl-Ab twice weekly beginning at 12 weeks of age and euthanized at 20 weeks of age. Scl-Ab increased alveolar bone mass in both male and female mice and alveolar tissue mineral density in the male mice. The positive effects of Scl-Ab were consistent with an increase in the fraction of active (non-phosphorylated) β-catenin stained alveolar osteocytes. Scl-Ab had no effect on mineralized tissues of the tooth - dentin, enamel, acellular and cellular cementum. There was a non-significant trend toward increased periodontal ligament (PDL) attachment fraction within the Hyp mice. Additional PDL fibral structural parameters were not affected by Scl-Ab. The current study demonstrates that Scl-Ab can improve alveolar bone in the Hyp mouse model of XLH.

One half-century of advances in the evaluation and management of disorders of bone and mineral metabolism in children and adolescents

The past 50 years of research in pediatric bone and mineral metabolism have led to remarkable progress in the identification and characterization of disorders that affect the developing skeleton. Progress has been facilitated through advances in both technology and biology and this paper provides a brief description of some but not all of the key findings, including identification of the calcium sensing receptor and the polypeptides parathyroid hormone and parathyroid hormone-related protein as well as their shared receptor and signal generating pathways; the elucidation of vitamin D metabolism and actions; discovery of fibroblast growth factor 23 (FGF23), the sodium-phosphate co-transporters and the other components that regulate phosphate metabolism. Moreover, the past half-century of research has led to the delineation of the molecular bases for genetic forms of hypoparathyroidism, pseudohypoparathyroidism, and primary hyperparathyroidism as well as the determination of the genetic causes of osteogenesis imperfecta, osteopetrosis, hypophosphatasia, and other disorders of mineral/bone homeostasis. During the next decade we expect that many of these fundamental discoveries will lead to the development of innovative treatments that will improve the lives of children with these disorders.

Dental manifestations and treatment of hypophosphatemic rickets: A case report and review of literature

BACKGROUND

The treatment and management of patients suffering from hypophosphatemic rickets (HR) remain a major challenge for dental practitioners and affected patients.

OBJECTIVES

To report a case of HR presenting with specific dental findings and to review the dental manifestations and treatment of HR patients.

METHODS

Case: A 32-year-old male presented with multiple dental abscesses and short stature. A thorough history was taken followed by clinical oral examination, and relevant radiological investigation was done. Literature research: In 2020, electronic literature searches were carried out in PubMed and complemented by a careful assessment of the reference lists of the identified relevant papers. Articles and reports fulfilled the inclusion criteria: indexed reviews, case series and case reports in English and restricted to human studies were considered.

RESULTS

The intraoral examination revealed multiple dental abscesses and general periodontal disease; the radiographic examination showed poorly defined lamina dura, large pulp chambers and periapical lesions. Based on the contents of the 43 articles identified in the search, the current knowledge of dental manifestations, treatment and management of HR was summarized.

CONCLUSIONS

As HR is a multisystem disease, multidisciplinary care is needed. By summarizing current evidences, we proposed an evidence-based dental management and provided recommendations on diagnosis and treatment of the disease. It is of profound clinical significance to acquire knowledge of the dental manifestations and provide optimal treatment options for patients.

Between a rock and a hard place: Regulation of mineralization in the periodontium

The periodontium supports and attaches teeth via mineralized and nonmineralized tissues. It consists of two, unique mineralized tissues, cementum and alveolar bone. In between these tissues, lies an unmineralized, fibrous periodontal ligament (PDL), which distributes occlusal forces, nourishes and invests teeth, and harbors progenitor cells for dentoalveolar repair. Many unanswered questions remain regarding periodontal biology. This review will focus on recent research providing insights into one enduring mystery: the precise regulation of the hard-soft tissue borders in the periodontium which define the interfaces of the cementum-PDL-alveolar bone structure. We will focus on advances in understanding the molecular mechanisms that maintain the unmineralized PDL "between a rock and a hard place" by regulating the mineralization of cementum and alveolar bone.

Bone Matrix Mineralization and Response to Burosumab in Adult Patients With X-Linked Hypophosphatemia: Results From the Phase 3, Single-Arm International Trial

X-linked hypophosphatemia (XLH) is characterized by excess fibroblast growth factor 23 (FGF23) secretion, renal phosphate wasting, and low 1,25(OH)₂ D₃ . Adult patients present with osteomalacia, hypomineralized periosteocytic lesions, bone fragility, and pain. Burosumab is a fully human monoclonal FGF23 antibody approved for XLH treatment. UX023-CL304 was an open-label, phase 3 study investigating the effects of burosumab on osteomalacia in adults with XLH, who remained untreated at least 2 years prior enrollment. Here, we present the effect of burosumab on bone material properties. We analyzed transiliac bone biopsy samples from 11 individuals before and after 48 weeks of subcutaneous burosumab treatment (1.0 mg/kg administered every 4 weeks). We used quantitative backscattered electron imaging (qBEI) and Fourier transform infrared imaging (FTIRI) to assess bone mineralization density distribution (BMDD), mineralized bone volume, properties of the organic matrix, and size of periosteocytic lesions. The outcomes were compared with reference values from healthy adults and with four XLH patients either untreated or treated by conventional therapy. Prior to burosumab, the average mineralization in cancellous bone was lower than in healthy reference. CaLow, the fraction of lowly mineralized matrix, and CaHigh, the fraction of highly mineralized matrix, were both elevated resulting in a broad heterogeneity in mineralization (CaWidth). Burosumab resulted in a decrease of CaHigh toward normal range, whereas CaLow and CaWidth remained elevated. The mineralized bone volume was notably increased (+35.9%). The size of the periosteocytic lesions was variable but lower than in untreated XLH patients. FTIRI indicated decreased enzymatic collagen crosslink ratio heterogeneity. In summary, matrix mineralization in XLH is very heterogeneous. Highly mineralized regions represent old bone packets, probably protected from osteoclastic resorption by osteoid seams. The concomitant decrease of highly mineralized matrix, persistence of lowly mineralized matrix, and increase in mineralized bone volume after burosumab suggest a boost in mineralization of preexisting unmineralized or very lowly mineralized matrix, providing a potential explanation for previously observed improved osteomalacia. © 2022 American Society for Bone and Mineral Research (ASBMR).

Skeletal and extraskeletal disorders of biomineralization

The physiological process of biomineralization is complex and deviation from it leads to a variety of diseases. Progress in the past 10 years has enhanced understanding of the genetic, molecular and cellular pathophysiology underlying these disorders; sometimes, this knowledge has both facilitated restoration of health and clarified the very nature of biomineralization as it occurs in humans. In this Review, we consider the principal regulators of mineralization and crystallization, and how dysregulation of these processes can lead to human disease. The knowledge acquired to date and gaps still to be filled are highlighted. The disorders of mineralization discussed comprise a broad spectrum of conditions that encompass bone disorders associated with alterations of mineral quantity and quality, as well as disorders of extraskeletal mineralization (hyperphosphataemic familial tumoural calcinosis). Included are disorders of alkaline phosphatase (hypophosphatasia) and phosphate homeostasis (X-linked hypophosphataemic rickets, fluorosis, rickets and osteomalacia). Furthermore, crystallopathies are covered as well as arterial and renal calcification. This Review discusses the current knowledge of biomineralization derived from basic and clinical research and points to future studies that will lead to new therapeutic approaches for biomineralization disorders.

PHEXL222P Mutation Increases Phex Expression in a New ENU Mouse Model for XLH Disease

PhexL222P mouse is a new ENU mouse model for XLH disease due to Leu to Pro amino acid modification at position 222. PhexL222P mouse is characterized by growth retardation, hypophosphatemia, hypocalcemia, reduced body bone length, and increased epiphyseal growth plate thickness and femur diameter despite the increase in PHEXL222P expression. Actually, PhexL222P mice show an increase in Fgf23, Dmp1, and Mepe and Slc34a1 (Na-Pi IIa cotransporter) mRNA expression similar to those observed in Hyp mice. Femoral osteocalcin and sclerostin and Slc34a1 do not show any significant variation in PhexL222P mice. Molecular dynamics simulations support the experimental data. P222 might locally break the E217-Q224 β-sheet, which in turn might disrupt inter-β-sheet interactions. We can thus expect local protein misfolding, which might be responsible for the experimentally observed PHEXL222P loss of function. This model could be a valuable addition to the existing XLH model for further comprehension of the disease occurrence and testing of new therapies.

Interdisciplinary management of FGF23-related phosphate wasting syndromes: a Consensus Statement on the evaluation, diagnosis and care of patients with X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most frequent cause of hypophosphataemia-associated rickets of genetic origin and is associated with high levels of the phosphaturic hormone fibroblast growth factor 23 (FGF23). In addition to rickets and osteomalacia, patients with XLH have a heavy disease burden with enthesopathies, osteoarthritis, pseudofractures and dental complications, all of which contribute to reduced quality of life. This Consensus Statement presents the outcomes of a working group of the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases, and provides robust clinical evidence on management in XLH, with an emphasis on patients' experiences and needs. During growth, conventional treatment with phosphate supplements and active vitamin D metabolites (such as calcitriol) improves growth, ameliorates leg deformities and dental manifestations, and reduces pain. The continuation of conventional treatment in symptom-free adults is still debated. A novel therapeutic approach is the monoclonal anti-FGF23 antibody burosumab. Although promising, further studies are required to clarify its long-term efficacy, particularly in adults. Given the diversity of symptoms and complications, an interdisciplinary approach to management is of paramount importance. The focus of treatment should be not only on the physical manifestations and challenges associated with XLH and other FGF23-mediated hypophosphataemia syndromes, but also on the major psychological and social impact of the disease.

Potential influences on optimizing long-term musculoskeletal health in children and adolescents with X-linked hypophosphatemia (XLH)

In recent years, much progress has been made in understanding the mechanisms of bone growth and development over a lifespan, including the crosstalk between muscle and bone, to achieve optimal structure and function. While there have been significant advances in understanding how to help improve and maintain bone health in normal individuals, there is limited knowledge on whether these mechanisms apply or are compromised in pathological states. X-linked hypophosphatemia (XLH) (ORPHA:89936) is a rare, heritable, renal phosphate-wasting disorder. The resultant chronic hypophosphatemia leads to progressive deterioration in musculoskeletal function, including impaired growth, rickets, and limb deformities in children, as well as lifelong osteomalacia with reduced bone quality and impaired muscle structure and function. The clinical manifestations of the disease vary both in presentation and severity in affected individuals, and many of the consequences of childhood defects persist into adulthood, causing significant morbidity that impacts physical function and quality of life. Intervention to restore phosphate levels early in life during the critical stages of skeletal development in children with XLH could optimize growth and may prevent or reduce bone deformities in childhood. A healthier bone structure, together with improved muscle function, can lead to physical activity enhancing musculoskeletal health throughout life. In adults, continued management may help to maintain the positive effects acquired from childhood treatment, thereby slowing or halting disease progression. In this review, we summarize the opinions from members of a working group with expertise in pediatrics, epidemiology, and bone, joint and muscle biology, on potential outcomes for people with XLH, who have been optimally treated from an early age and continue treatment throughout life.

Dental health of pediatric patients with X-linked hypophosphatemia (XLH) after three years of burosumab therapy

An inactivating PHEX gene mutation with the resultant accumulation of several mineralization-inhibiting proteins (e.g., FGF23) causes skeletal and dental morbidity in X-linked hypophosphatemia (XLH). This prospective case-control study explored the effect of burosumab, an anti-FGF23 antibody, on dental health of children with XLH. Ten children (age 4.3-15 years) with XLH underwent burosumab treatment per protocol. Assessment of their dental status at treatment initiation and after 1 and 3 years of treatment included clinical, laboratory and radiographic evaluation of rickets and dentition. Orthopantomographic examinations of ten healthy sex- and age-matched controls were selected for comparison. Coronal and pulp dimensions of a selected permanent mandibular molar were measured with Planmeca Romexis^® software. One year of treatment led to improvement of height z-score (p=0.019) and healing of the rickets (p<0.001) in the XLH patients, and those achievements were maintained after three years of treatment. Dental morphology of XLH patients, distinguished by increased pulp-coronal ratios compared to controls (p=0.002), remained larger after the first year of treatment (p<0.001) and did not attain the decrease expected with age after three years of treatment. Five patients had a history of recurrent dental abscesses, with three having undergone at least one episode during the year before burosumab initiation. One patient sustained recurrent abscesses throughout three years of treatment. The persistence of the unique dental morphology of XLH patients undergoing burosumab therapy, as evidenced by excessively larger pulp dimensions, supports the role of other PHEX gene-related local mineralization inhibitors, such as osteopontin, in the pathogenesis of dental morbidity.

Mineral tessellation in bone and the stenciling principle for extracellular matrix mineralization

We review here the Stenciling Principle for extracellular matrix mineralization that describes a double-negative process (inhibition of inhibitors) that promotes mineralization in bone and other mineralized tissues, whereas the default condition of inhibition alone prevents mineralization elsewhere in soft connective tissues. The stenciling principle acts across multiple levels from the macroscale (skeleton/dentition vs soft connective tissues), to the microscale (for example, entheses, and the tooth attachment complex where the soft periodontal ligament is situated between mineralized tooth cementum and mineralized alveolar bone), and to the mesoscale (mineral tessellation). It relates to both small-molecule (e.g. pyrophosphate) and protein (e.g. osteopontin) inhibitors of mineralization, and promoters (enzymes, e.g. TNAP, PHEX) that degrade the inhibitors to permit and regulate mineralization. In this process, an organizational motif for bone mineral arises that we call crossfibrillar mineral tessellation where mineral formations - called tesselles - geometrically approximate prolate ellipsoids and traverse multiple collagen fibrils (laterally). Tesselle growth is directed by the structural anisotropy of collagen, being spatially restrained in the shorter transverse tesselle dimensions (averaging 1.6 × 0.8 × 0.8 μm, aspect ratio 2, length range 1.5-2.5 μm). Temporo-spatially, the tesselles abut in 3D (close ellipsoid packing) to fill the volume of lamellar bone extracellular matrix. Poorly mineralized interfacial gaps between adjacent tesselles remain discernable even in mature lamellar bone. Tessellation of a same, small basic unit to form larger structural assemblies results in numerous 3D interfaces, allows dissipation of critical stresses, and enables fail-safe cyclic deformations. Incomplete tessellation in osteomalacia/odontomalacia may explain why soft osteomalacic bones buckle and deform under loading.

The role of biomineralization in disorders of skeletal development and tooth formation

The major mineralized tissues are bone and teeth, which share several mechanisms governing their development and mineralization. This crossover includes the hormones that regulate circulating calcium and phosphate concentrations, and the genes that regulate the differentiation and transdifferentiation of cells. In developing endochondral bone and in developing teeth, parathyroid hormone-related protein (PTHrP) acts in chondrocytes to delay terminal differentiation, thereby increasing the pool of precursor cells. Chondrocytes and (in specific circumstances) pre-odontoblasts can also transdifferentiate into osteoblasts. Moreover, bone and teeth share outcomes when affected by systemic disorders of mineral homeostasis or of the extracellular matrix, and by adverse effects of treatments such as bisphosphonates and fluoride. Unlike bone, teeth have more permanent effects from systemic disorders because they are not remodelled after they are formed. This Review discusses the normal processes of bone and tooth development, followed by disorders that have effects on both bone and teeth, versus disorders that have effects in one without affecting the other. The takeaway message is that bone specialists should know when to screen for dental disorders, just as dental specialists should recognize when a tooth disorder should raise suspicions about a possible underlying bone disorder.

Burosumab for Pediatric X-Linked Hypophosphatemia

PURPOSE OF REVIEW

X-Linked hypophosphatemia (XLH) is the most common genetic cause of rickets. This review describes advances in the management of XLH using burosumab which was FDA approved for treating children with XLH in 2018.

RECENT FINDINGS

Elevated FGF23 in XLH leads to systemic hypophosphatemia and several musculoskeletal manifestations, including rachitic bone deformities, impaired growth, dental abscesses, insufficiency fractures, osteoarthritis, and enthesopathy, with lifelong consequences for physical function and quality of life. Burosumab treatment has demonstrated clinical improvement of rickets and growth in children, including during a randomized controlled trial compared with conventional therapy. Burosumab also improved pseudofracture healing in adults. Burosumab led to greater improvement in rickets and growth than conventional therapy. However, many questions remain regarding the impact of burosumab on several outcomes, including final height, nephrocalcinosis, dental disease, enthesopathy, and surgical interventions.

Impact of Tenofovir Disoproxil-Induced Fanconi Syndrome on Bone Material Quality: A Case Report

Tenofovir is a nucleotide analog reverse‐transcriptase inhibitor (NtARTI) used for treatment of chronic hepatitis B and human immunodeficiency virus (HIV). Fanconi syndrome (FS) is a condition affecting the proximal tubules of the kidney, leading to increased passage and impaired reabsorption of various small molecules such as glucose, phosphate, bicarbonate, and amino acids. Tenofovir disoproxil fumarate (TDF) is one of two pro‐drugs of tenofovir associated with a greater nephrotoxicity and renal complications such as FS with subsequent osteomalacia, acute kidney injury, and reduction of glomerular filtration rate (GFR) compared with tenofovir alafenamide (TAF). We present the case of a 33‐year‐old white woman treated with TDF because of chronic hepatitis B infection suffering four atraumatic fractures over the period of 2 years. The patient was taken off the TDF regimen 3 months before presentation. Initial blood and urine samples suggested the presence of TDF‐induced osteomalacia, which was confirmed by transiliac bone biopsy and histomorphometry. Moreover, bone mineral density distribution (BMDD) by quantitative backscattered electron imaging (qBEI) analysis showed that approximately 56% of the bone surface was normally mineralized and 44% showed a reduced mineralization consistent with the presence of osteomalacia. The patient made a significant recovery upon cessation of the causative agent. This case report emphasizes the use of bone biopsy, histomorphometry and qBEI in confirming the diagnosis of drug‐induced Fanconi syndrome and associated osteomalacia. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Effects of Active Vitamin D or FGF23 Antibody on Hyp Mice Dentoalveolar Tissues

Mutations in the PHEX gene lead to X-linked hypophosphatemia (XLH), a form of inherited rickets featuring elevated fibroblast growth factor 23 (FGF23), reduced 1,25-dihydroxyvitamin D (1,25D), and hypophosphatemia. Hyp mutant mice replicate the XLH phenotype, including dentin, alveolar bone, and cementum defects. We aimed to compare effects of 1,25D versus FGF23-neutralizing antibody (FGF23Ab) monotherapies on Hyp mouse dentoalveolar mineralization. Male Hyp mice, either injected subcutaneously with daily 1,25D or thrice weekly with FGF23 blocking antibody from 2 to 35 d postnatal, were compared to wild-type (WT) controls and untreated Hyp mice. Mandibles were analyzed by high-resolution micro-computed tomography (micro-CT), histology, and immunohistochemistry. Both interventions maintained normocalcemia, increased serum phosphate levels, and improved dentoalveolar mineralization in treated versus untreated Hyp mice. 1,25D increased crown dentin volume and thickness and root dentin/cementum volume, whereas FGF23Ab effects were limited to crown dentin volume. 1,25D increased bone volume fraction, bone mineral density, and tissue mineral density in Hyp mice, whereas FGF23Ab failed to significantly affect these alveolar bone parameters. Neither treatment fully attenuated dentin and bone defects to WT levels, and pulp volumes remained elevated regardless of treatment. Both treatments reduced predentin thickness and improved periodontal ligament organization, while 1,25D promoted a more profound improvement in acellular cementum thickness. Altered cell densities and lacunocanalicular properties of alveolar and mandibular bone osteocytes and cementocytes in Hyp mice were partially corrected by either treatment. Neither treatment normalized the altered distributions of bone sialoprotein and osteopontin in Hyp mouse alveolar bone. Moderate improvements from both 1,25D and FGF23Ab treatment regimens support further studies and collection of oral health data from subjects receiving a newly approved anti-FGF23 therapy. The inability of either treatment to fully correct Hyp mouse dentin and bone prompts further experiments into underlying pathological mechanisms to identify new therapeutic approaches.

Comparison of the effectiveness of Ankaferd Blood Stopper® and Emdogain in periodontal regeneration

OBJECTIVES

The present study was performed to compare the effectiveness of Ankaferd Blood Stopper^® (ABS) with enamel matrix derivatives (EMD) for treating fenestration defects in rats.

MATERIALS AND METHODS

Forty-eight male Wistar rats were randomly divided into six groups (each n = 8). Fenestration defects were created in all rats, to which ABS, EMD, or saline (S) was then applied. The rats were grouped and sacrificed at one of two different time points, as follows: ABS-10-group, ABS-treatment/sacrifice on day 10; EMD-10-group, EMD-treatment/sacrifice on day 10; S-10-group, S-treatment/sacrifice on day 10; ABS-38-group, ABS-treatment/sacrifice on day 38; EMD-38-group, EMD-treatment/sacrifice on day 38; and S-38-group, S-treatment/sacrifice on day 38. Then, histomorphometric analysis including measurements of new bone area (NBA) and new bone ratio (NBR), and immunohistochemical analysis including the determination of osteopontin (OPN) and type-III-collagen (C-III) expression were performed.

RESULTS

The NBA and NBR were significantly higher in the ABS-10-group and EMD-10-group compared to the S-10-group (p < .05), and in the EMD-38-group compared to the S-38-group (p < .05). The levels of C-III and OPN immunoreactivity were significantly higher in the ABS-10-group compared to the S-10-group (p < .017).

CONCLUSIONS

The results of this study suggested that ABS can promote early periodontal regeneration, although its efficacy seems to decrease over time.

Pulp chamber features, prevalence of abscesses, disease severity, and PHEX mutation in X-linked hypophosphatemic rickets

INTRODUCTION

Rickets, growth failure, and recurrent periapical abscesses with fistulae are main signs in patients with X-linked hypophosphatemic rickets (XLH). Prevalence of abscesses, pulp chamber features, biochemical findings, disease severity, and PHEX gene mutation were examined.

MATERIALS AND METHODS

Pulp chambers size, shape, and morphology were assessed by orthopantomography in XLH patients (n = 24, age 5.8 ± 1.6 years) and in sex and age-matched healthy controls (n = 23, age 6.2 ± 1.4 years). XLH patients received conventional treatment (3.5 ± 1.9 years). Pulp chamber features were assessed in teeth of primary dentition and in the permanent left mandibular first molar and compared with those of controls. Rickets severity score was assessed at wrist, knee, and ankle.

RESULTS

The mean pulp chamber area/tooth area ratio, mean pulp chamber height/pulp chamber width ratio, and prominence of pulp horns into the tooth crown in primary and secondary molars were significantly higher in patients than in controls and in patients suffered abscesses than in patients without abscesses. Sixteen patients (67%) had a history of abscesses; incisors were affected more than canines and molars. Severity of rickets and mean serum parathyroid hormone (PTH) levels were significantly higher, and mean serum 1,25-dihydroxyvitamin D [1,25(OH)₂D] levels significantly lower in patients suffered abscesses than in patients without abscesses. PHEX gene mutations were not correlated with dental phenotype and disease severity.

CONCLUSION

Enlarged pulp chambers with altered shape and morphology affected the majority of XLH patients predisposing to recurrent periapical abscesses with fistulae. Dental phenotype was associated with severity of rickets, high serum PTH, and low serum 1,25(OH)₂D levels.

Congenital Conditions of Hypophosphatemia in Children

Great strides over the past few decades have increased our understanding of the pathophysiology of hypophosphatemic disorders. Phosphate is critically important to a variety of physiologic processes, including skeletal growth, development and mineralization, as well as DNA, RNA, phospholipids, and signaling pathways. Consequently, hypophosphatemic disorders have effects on multiple systems, and may cause a variety of nonspecific signs and symptoms. The acute effects of hypophosphatemia include neuromuscular symptoms and compromise. However, the dominant effects of chronic hypophosphatemia are the effects on musculoskeletal function including rickets, osteomalacia and impaired growth during childhood. While the most common causes of chronic hypophosphatemia in children are congenital, some acquired conditions also result in hypophosphatemia during childhood through a variety of mechanisms. Improved understanding of the pathophysiology of these congenital conditions has led to novel therapeutic approaches. This article will review the pathophysiologic causes of congenital hypophosphatemia, their clinical consequences and medical therapy.

The role of extracellular matrix phosphorylation on energy dissipation in bone

Protein phosphorylation, critical for cellular regulatory mechanisms, is implicated in various diseases. However, it remains unknown whether heterogeneity in phosphorylation of key structural proteins alters tissue integrity and organ function. Here, osteopontin phosphorylation level declined in hypo- and hyper- phosphatemia mouse models exhibiting skeletal deformities. Phosphorylation increased cohesion between osteopontin polymers, and adhesion of osteopontin to hydroxyapatite, enhancing energy dissipation. Fracture toughness, a measure of bone’s mechanical competence, increased with ex-vivo phosphorylation of wildtype mouse bones and declined with ex-vivo dephosphorylation. In osteopontin-deficient mice, global matrix phosphorylation level was not associated with toughness. Our findings suggest that phosphorylated osteopontin promotes fracture toughness in a dose-dependent manner through increased interfacial bond formation. In the absence of osteopontin, phosphorylation increases electrostatic repulsion, and likely protein alignment and interfilament distance leading to decreased fracture resistance. These mechanisms may be of importance in other connective tissues, and the key to unraveling cell–matrix interactions in diseases.

Mineralized tissues in hypophosphatemic rickets

Hypophosphatemic rickets is caused by renal phosphate wasting that is most commonly due to X-linked dominant mutations in PHEX. PHEX mutations cause hypophosphatemia indirectly, through the increased expression of fibroblast growth factor 23 (FGF23) by osteocytes. FGF23 decreases renal phosphate reabsorption and thereby increases phosphate excretion. The lack of phosphate leads to a mineralization defect at the level of growth plates (rickets), bone tissue (osteomalacia), and teeth, where the defect facilitates the formation of abscesses. The bone tissue immediately adjacent to osteocytes often remains unmineralized ("periosteocytic lesions"), highlighting the osteocyte defect in this disorder. Common clinical features of XLH include deformities of the lower extremities, short stature, enthesopathies, dental abscesses, as well as skull abnormalities such as craniosynostosis and Chiari I malformation. For the past four decades, XLH has been treated by oral phosphate supplementation and calcitriol, which improves rickets and osteomalacia and the dental manifestations, but often does not resolve all aspects of the mineralization defects. A newer treatment approach using inactivating FGF23 antibodies leads to more stable control of serum inorganic phosphorus levels and seems to heal rickets more reliably. However, the long-term benefits of FGF23 antibody treatment remain to be elucidated.

Genetic Ablation of Osteopontin in Osteomalacic Hyp Mice Partially Rescues the Deficient Mineralization Without Correcting Hypophosphatemia

PHEX is predominantly expressed by bone and tooth-forming cells, and its inactivating mutations in X-linked hypophosphatemia (XLH) lead to renal phosphate wasting and severe hypomineralization of bones and teeth. Also present in XLH are hallmark hypomineralized periosteocytic lesions (POLs, halos) that persist despite stable correction of serum phosphate (P_i ) that improves bulk bone mineralization. In XLH, mineralization-inhibiting osteopontin (OPN, a substrate for PHEX) accumulates in the extracellular matrix of bone. To investigate how OPN functions in Hyp mice (a model for XLH), double-null (Hyp;Opn^-/- ) mice were generated. Undecalcified histomorphometry performed on lumbar vertebrae revealed that Hyp;Opn^-/- mice had significantly reduced osteoid area/bone area (OV/BV) and osteoid thickness of trabecular bone as compared to Hyp mice, despite being as hypophosphatemic as Hyp littermate controls. However, tibias examined by synchrotron radiation micro-CT showed that mineral lacunar volumes remained abnormally enlarged in these double-null mice. When Hyp;Opn^-/- mice were fed a high-P_i diet, serum P_i concentration increased, and OV/BV and osteoid thickness normalized, yet mineral lacunar area remained abnormally enlarged. Enpp1 and Ankh gene expression were increased in double-null mice fed a high-P_i diet, potentially indicating a role for elevated inhibitory pyrophosphate (PP_i ) in the absence of OPN. To further investigate the persistence of POLs in Hyp mice despite stable correction of serum P_i , immunohistochemistry for OPN on Hyp mice fed a high-P_i diet showed elevated OPN in the osteocyte pericellular lacunar matrix as compared to Hyp mice fed a control diet. This suggests that POLs persisting in Hyp mice despite correction of serum P_i may be attributable to the well-known upregulation of mineralization-inhibiting OPN by P_i , and its accumulation in the osteocyte pericellular matrix. This study shows that OPN contributes to osteomalacia in Hyp mice, and that genetic ablation of OPN in Hyp mice improves the mineralization phenotype independent of systemic P_i -regulating factors. © 2020 American Society for Bone and Mineral Research.

Development of Enthesopathies and Joint Structural Damage in a Murine Model of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia, caused by inactivating mutations in the Phosphate-regulating endopeptidase homolog X-linked (PHEX) gene. With aging, adult patients develop paradoxical heterotopic calcifications of tendons and ligaments at their insertion sites (enthesophytes), and joint alterations. Understanding the progression of this structural damage that severely affects patients’ quality of life will help to improve the management of XLH. Here, we characterized the occurrence of enthesophytes and joint alterations through a 12 month in vivo micro-CT follow-up in the Hyp mouse, a murine model of XLH (n = 5 mice per group). Similar to adult patients with XLH, Hyp mice developed calcaneal enthesophytes, hip joint alterations, erosions of the sacroiliac joints and periarticular calcifications. These lesions were already present at month 3 and gradually worsened over time. In sharp contrast, no abnormalities were observed in control mice at early time points. Histological analyses confirmed the presence of bone erosions, calcifications and expansion of mineralizing enthesis fibrocartilage in Hyp mice and their absence in controls and suggested that new bone formation is driven by altered mechanical strain. Interestingly, despite a strong deformation of the curvature, none of the Hyp mice displayed enthesophyte at the spine. Peripheral enthesophytes and joint alterations develop at the early stages of the disease and gradually worsen overtime. Overall, our findings highlight the relevance of this preclinical model to test new therapies aiming to prevent bone and joint complications in XLH.

Compositional and microhardness findings in tooth affected by X-linked hypophosphatemic rickets

Background

This study aimed to evaluate the X-linked hypophosphatemic rickets (XLHR)-related compositional and microhardness tooth aspects.

Material and Methods

One affected and one non-affected teeth by XLHR were sectioned transversely, and each section was separated for Micro-Raman spectroscopy, Knoop microhardness and scanning electron microscopy with energy dispersive x-ray microanalysis (SEM-EDS). The outcomes of these analyses were assessed.

Results

Outcomes of Raman analysis of inorganic/organic components (~958/~1250+~1450 cm−1) and carbonate/phosphate (~1070/~958 cm−1) ratios showed areas of altered enamel and dentin (interglobular dentin, calcospherites, and mantle dentin) with an increase of inorganic content in the rickets tooth. Microhardness reduction was observed in the affected tooth, with a more evident drop in regions of mantle dentin, interglobular dentin, and calcospherites. SEM-EDS analysis showed demonstrated the absence of calcium and phosphorus in interglobular spaces.

Conclusions

In conclusion, compositional and structural deficiencies were observed in deciduous tooth affected by XLHR. Also, it was observed the absence of hydroxyapatite in the interglobular dentin by using Raman spectroscopy analysis.

Key words:Dentin, dentin permeability, X-linked hypophosphatemic rickets, tooth, tooth calcification, Raman spectroscopy.

Biological stenciling of mineralization in the skeleton: Local enzymatic removal of inhibitors in the extracellular matrix

Biomineralization is remarkably diverse and provides myriad functions across many organismal systems. Biomineralization processes typically produce hardened, hierarchically organized structures usually having nanostructured mineral assemblies that are formed through inorganic-organic (usually protein) interactions. Calcium‑carbonate biomineral predominates in structures of small invertebrate organisms abundant in marine environments, particularly in shells (remarkably it is also found in the inner ear otoconia of vertebrates), whereas calcium-phosphate biomineral predominates in the skeletons and dentitions of both marine and terrestrial vertebrates, including humans. Reconciliation of the interplay between organic moieties and inorganic crystals in bones and teeth is a cornerstone of biomineralization research. Key molecular determinants of skeletal and dental mineralization have been identified in health and disease, and in pathologic ectopic calcification, ranging from small molecules such as pyrophosphate, to small membrane-bounded matrix vesicles shed from cells, and to noncollagenous extracellular matrix proteins such as osteopontin and their derived bioactive peptides. Beyond partly knowing the regulatory role of the direct actions of inhibitors on vertebrate mineralization, more recently the importance of their enzymatic removal from the extracellular matrix has become increasingly understood. Great progress has been made in deciphering the relationship between mineralization inhibitors and the enzymes that degrade them, and how adverse changes in this physiologic pathway (such as gene mutations causing disease) result in mineralization defects. Two examples of this are rare skeletal diseases having osteomalacia/odontomalacia (soft bones and teeth) - namely hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH) - where inactivating mutations occur in the gene for the enzymes tissue-nonspecific alkaline phosphatase (TNAP, TNSALP, ALPL) and phosphate-regulating endopeptidase homolog X-linked (PHEX), respectively. Here, we review and provide a concept for how existing and new information now comes together to describe the dual nature of regulation of mineralization - through systemic mineral ion homeostasis involving circulating factors, coupled with molecular determinants operating at the local level in the extracellular matrix. For the local mineralization events in the extracellular matrix, we present a focused concept in skeletal mineralization biology called the Stenciling Principle - a principle (building upon seminal work by Neuman and Fleisch) describing how the action of enzymes to remove tissue-resident inhibitors defines with precision the location and progression of mineralization.

Insights into dental mineralization from three heritable mineralization disorders

Teeth are comprised of three unique mineralized tissues, enamel, dentin, and cementum, that are susceptible to developmental defects similar to those affecting bone. X-linked hypophosphatemia (XLH), caused by PHEX mutations, leads to increased fibroblast growth factor 23 (FGF23)-driven hypophosphatemia and local extracellular matrix disturbances. Hypophosphatasia (HPP), caused by ALPL mutations, results in increased levels of inorganic pyrophosphate (PP_i), a mineralization inhibitor. Generalized arterial calcification in infancy (GACI), caused by ENPP1 mutations, results in vascular calcification due to decreased PP_i, later compounded by FGF23-driven hypophosphatemia. In this perspective, we compare and contrast dental defects in primary teeth associated with XLH, HPP, and GACI, briefly reviewing genetic and biochemical features of these disorders and findings of clinical and preclinical studies to date, including some of our own recent observations. The distinct dental defects associated with the three heritable mineralization disorders reflect unique processes of the respective dental hard tissues, revealing insights into their development and clues about pathological mechanisms underlying such disorders.

Genetic and pharmacologic modulation of cementogenesis via pyrophosphate regulators

Pyrophosphate (PP_i) serves as a potent and physiologically important regulator of mineralization, with systemic and local concentrations determined by several key regulators, including: tissue-nonspecific alkaline phosphatase (ALPL gene; TNAP protein), the progressive ankylosis protein (ANKH; ANK), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1; ENPP1). Results to date have indicated important roles for PP_i in cementum formation, and we addressed several gaps in knowledge by employing genetically edited mouse models where PP_i metabolism was disrupted and pharmacologically modulating PP_i in a PP_i-deficient mouse model. We demonstrate that acellular cementum growth is inversely proportional to PP_i levels, with reduced cementum in Alpl KO (increased PP_i levels) mice and excess cementum in Ank KO mice (decreased PP_i levels). Moreover, simultaneous ablation of Alpl and Ank results in reestablishment of functional cementum in dKO mice. Additional reduction of PP_i by dual deletion of Ank and Enpp1 does not further increase cementogenesis, and PDL space is maintained in part through bone modeling/remodeling by osteoclasts. Our results provide insights into cementum formation and expand our knowledge of how PP_i regulates cementum. We also demonstrate for the first time that pharmacologic manipulation of PP_i through an ENPP1-Fc fusion protein can regulate cementum growth, supporting therapeutic interventions targeting PP_i metabolism.

Alterations of bone material properties in adult patients with X-linked hypophosphatemia (XLH)

X-linked hypophosphatemia (XLH) caused by PHEX mutations results in elevated serum FGF23 levels, renal phosphate wasting and low 1,25-dihydroxyvitamin D. The glycophosphoprotein osteopontin, a potent inhibitor of mineralization normally degraded by PHEX, accumulates within the bone matrix. Conventional therapy consisting of supplementation with phosphate and vitamin D analogs is burdensome and the effects on bone material poorly characterized. We analyzed transiliac bone biopsies from four adult patients, two of them severely affected due to no diagnosis and no treatment until adulthood. We used light microscopy, qBEI and FTIRI to study histology, histomorphometry, bone mineralization density distribution, properties of the organic matrix and size of hypomineralized periosteocytic lesions. Non-treatment resulted in severe osteomalacia, twice the amount of mineralized trabecular volume, multiple osteon-like perforations, continuity of lamellae from mineralized to unmineralized areas and distinctive patches of woven bone. Periosteocytic lesions were larger than in treated patients. The latter had nearly normal osteoid thicknesses, although surface was still elevated. The median calcium content of the matrix was always within normal range, although the percentage of lowly mineralized bone areas was highly increased in non-treated patients, resulting in a marked heterogeneity in mineralization. Divalent collagen cross-links were evident independently of the mineral content of the matrix. Broad osteoid seams lacked measurable pyridinoline, a mature trivalent cross-link and exhibited considerable acidic lipid content, typically found in matrix vesicles. Based on our results, we propose a model that possibly integrates the relationship between the observed mineralization disturbances, FGF23 secretion and the known osteopontin accumulation in XLH.

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.

Decreased Compressional Sound Velocity Is an Indicator for Compromised Bone Stiffness in X-Linked Hypophosphatemic Rickets (XLH)

Objectives: To assess the diagnostic potential of bidirectional axial transmission (BDAT) ultrasound, and high-resolution peripheral quantitative computed tomography (HR-pQCT) in X-linked hypophosphatemia (XLH, OMIM #307800), a rare genetic disorder of phosphate metabolism caused by mutations in the PHEX gene. Methods: BDAT bone ultrasound was performed at the non-dominant distal radius (33% relative to distal head) and the central left tibia (50%) in eight XLH patients aged between 4.2 and 20.8 years and compared to twenty-nine healthy controls aged between 5.8 and 22.4 years. In eighteen controls, only radius measurements were performed. Four patients and four controls opted to participate in HR-pQCT scanning of the ultradistal radius and tibia. Results: Bone ultrasound was feasible in patients and controls as young as 4 years of age. The velocity of the first arriving signal (ν_FAS) in BDAT ultrasound was significantly lower in XLH patients compared to healthy controls: In the radius, mean ν_FAS of XLH patients and controls was 3599 ± 106 and 3866 ± 142 m/s, respectively (-6.9%; p < 0.001). In the tibia, it was 3578 ± 129 and 3762 ± 124 m/s, respectively (-4.9%; p = 0.006). HR-pQCT showed a higher trabecular thickness in the tibia of XLH patients (+16.7%; p = 0.021). Conclusions: Quantitative bone ultrasound revealed significant differences in cortical bone quality of young XLH patients as compared to controls. Regular monitoring of XLH patients by a radiation-free technology such as BDAT might provide valuable information on bone quality and contribute to the optimization of treatment. Further studies are needed to establish this affordable and time efficient method in the XLH patients.

Bone Biomineral Properties Vary across Human Osteonal Bone

The biomineralization of bone remains a puzzle. During Haversian remodeling in the dense human cortical bone, osteoclasts excavate a tunnel that is then filled in by osteoblasts with layers of bone of varying fibril orientations, resulting in a lamellar motif. Such bone represents an excellent possibility to increase our understanding of bone as a material as well as bone biomineralization by studying spatio/temporal variations in the biomineral across an osteon. To this end, fluorescence computed tomography and diffraction scattering computed tomography with sub-micrometer resolution is applied to obtain position resolved fluorescence spectra and diffraction patterns in a 3D volume. The microstructural properties of the apatite biomineral are not homogeneous but depend critically on the time point at which it was laid down. This indicates that the nature of bone biomineral is highly dependent on the microenvironment during bone formation and remodeling.

FGF23 and Associated Disorders of Phosphate Wasting

Fibroblast growth factor 23 (FGF23), one of the endocrine fibroblast growth factors, is a principal regulator in the maintenance of serum phosphorus concentration. Binding to its cofactor αKlotho and a fibroblast growth factor receptor is essential for its activity. Its regulation and interaction with other factors in the bone-parathyroid-kidney axis is complex. FGF23 reduces serum phosphorus concentration through decreased reabsorption of phosphorus in the kidney and by decreasing 1,25 dihydroxyvitamin D (1,25(OH)2D) concentrations. Various FGF23-mediated disorders of renal phosphate wasting share similar clinical and biochemical features. The most common of these is X-linked hypophosphatemia (XLH). Additional disorders of FGF23 excess include autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Treatment is challenging, requiring careful monitoring and titration of dosages to optimize effectiveness and to balance side effects. Conventional therapy for XLH and other disorders of FGF23-mediated hypophosphatemia involves multiple daily doses of oral phosphate salts and active vitamin D analogs, such as calcitriol or alfacalcidol. Additional treatments may be used to help address side effects of conventional therapy such as thiazides to address hypercalciuria or nephrocalcinosis, and calcimimetics to manage hyperparathyroidism. The recent development and approval of an anti-FGF23 antibody, burosumab, for use in XLH provides a novel treatment option.

FGF23, Hypophosphatemia, and Emerging Treatments

FGF23 is an important hormonal regulator of phosphate homeostasis. Together with its co-receptor Klotho, it modulates phosphate reabsorption and both 1α-hydroxylation and 24-hydroxylation in the renal proximal tubules. The most common FGF23-mediated hypophosphatemia is X-linked hypophosphatemia (XLH), caused by mutations in the PHEX gene. FGF23-mediated forms of hypophosphatemia are characterized by phosphaturia and low or low-normal calcitriol concentrations, and unlike nutritional rickets, these cannot be cured with nutritional vitamin D supplementation. Autosomal dominant and autosomal recessive forms of FGF23-mediated hypophosphatemias show a similar pathophysiology, despite a variety of different underlying genetic causes. An excess of FGF23 activity has also been associated with a number of other conditions causing hypophosphatemia, including tumor-induced osteomalacia, fibrous dysplasia of the bone, and cutaneous skeletal hypophosphatemia syndrome. Historically phosphate supplementation and therapy using analogs of highly active vitamin D (eg, calcitriol, alfacalcidol, paricalcitol, eldecalcitol) have been used to manage conditions involving hypophosphatemia; however, recently a neutralizing antibody for FGF23 (burosumab) has emerged as a promising treatment agent for FGF23-mediated disorders. This review discusses the progression of clinical trials for burosumab for the treatment of XLH and its recent availability for clinical use. Burosumab may have potential for treating other conditions associated with FGF23 overactivity, but these are not yet supported by trial data. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Impaired mineral quality in dentin in X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a skeletal disorder arising from mutations in the PHEX gene, transmitted in most cases as an X-linked dominant trait. PHEX deficiency leads to renal phosphate wasting and hypophosphatemia, as well as impaired mineralization of bone and dentin, resulting in severe skeletal and dental complications. Dentin mineralization defects appear as characteristic, large interglobular spaces resulting from the lack of fusion of calculospherites in the circumpulpal region during the mineralization process. Here, we examined changes in the composition and structure of dentin using Raman spectroscopy on XLH human teeth, and using transmission electron microscopy on the dentin of Hyp mice (the murine model of XLH). The dentin of patients with XLH showed changes in the quality of the apatitic mineral, with greater carbonate substitution and lower crystallinity compared to the dentin of age-matched control teeth. In addition, ultrastructural analysis by transmission electron microscopy revealed a major disorganization of the peri- and intertubular structure of the dentin, with odontoblast processes residing within an unmineralized matrix sheath in the Hyp mouse. Taken together, these results indicate that like for bone and tooth cementum, there are impaired mineral quality and matrix changes in XLH dentin reflecting high sensitivity to systemic serum phosphate levels and possibly other local changes in the dentin matrix.

Phosphorylation of Extracellular Bone Matrix Proteins and Its Contribution to Bone Fragility

Phosphorylation of bone matrix proteins is of fundamental importance to all vertebrates including humans. However, it is currently unknown whether increase or decline of total protein phosphorylation levels, particularly in hypophosphatemia-related osteoporosis, osteomalacia, and rickets, contribute to bone fracture. To address this gap, we combined biochemical measurements with mechanical evaluation of bone to discern fracture characteristics associated with age-related development of skeletal fragility in relation to total phosphorylation levels of bone matrix proteins and one of the key representatives of bone matrix phosphoproteins, osteopontin (OPN). Here for the first time, we report that as people age the total phosphorylation level declines by approximately 20% for bone matrix proteins and approximately 30% for OPN in the ninth decade of human life. Moreover, our results suggest that the decline of total protein phosphorylation of extracellular matrix (ECM) contributes to bone fragility, but less pronouncedly than glycation. We theorize that the separation of two sources of OPN negative charges, acidic backbone amino acids and phosphorylation, would be nature's means of assuring that OPN functions in both energy dissipation and biomineralization. We propose that total phosphorylation decline could be an important contributor to the development of osteoporosis, increased fracture risk and skeletal fragility. Targeting the enzymes kinase FamC20 and bone alkaline phosphatase involved in the regulation of matrix proteins' phosphorylation could be a means for the development of suitable therapeutic treatments. © 2018 American Society for Bone and Mineral Research.

Pharmacological management of X-linked hypophosphataemia

The most common heritable disorder of renal phosphate wasting, X-linked hypophosphataemia (XLH), was discovered to be caused by inactivating mutations in the phosphate regulating gene with homology to endopeptidases on the X-chromosome (PHEX) gene in 1995. Although the exact molecular mechanisms by which PHEX mutations cause disturbed phosphate handling in XLH remain unknown, focus for novel therapies has more recently been based upon the finding that the bone-produced phosphaturic hormone fibroblast growth factor-23 is elevated in XLH patient plasma. Previous treatment strategies for XLH were based upon phosphate repletion plus active vitamin D analogues, which are difficult to manage, fail to address the primary pathogenesis of the disease, and can have deleterious side effects. A novel therapy for XLH directly targeting fibroblast growth factor-23 via a humanized monoclonal antibody (burosumab-twza/CRYSVITA, henceforth referred to just as burosumab) has emerged as an effective, and recently approved, pharmacological treatment for both children and adults. This review will provide an overview of the clinical manifestations of XLH, the molecular pathophysiology, and summarize its current treatment.

Hypocalcaemic and hypophosphatemic rickets

Rickets refers to deficient mineralization at the growth plate and is usually associated with abnormal serum calcium and/or phosphate. There are several subtypes of rickets, including hypophosphatemic rickets (vitamin-D-resistant rickets secondary to renal phosphate wasting), vitamin D-dependent rickets (defects of vitamin D metabolism) and nutritional rickets (caused by dietary deficiency of vitamin D, and/or calcium, and/or phosphate). Most rickets manifest as bone deformities, bone pain, and impaired growth velocity. Diagnosis of rickets is established through the medical history, physical examination, biochemical tests and radiographs. It is of crucial importance to determine the cause of rickets, including the molecular characterization in case of vitamin D resistant rickets, and initiate rapidly the appropriate therapy. In this review, we describe the different causes and therapies of genetic and nutritional rickets, supported by the recent progress in genetics and development of novel molecules such as anti-FGF23 antibody.