Autosomal dominant craniometaphyseal dysplasia is caused by mutations in the transmembrane protein ANK

Skeletal abnormalities caused by a Connexin43R239Q mutation in a mouse model for autosomal recessive craniometaphyseal dysplasia

Craniometaphyseal dysplasia (CMD), a rare craniotubular disorder, occurs in an autosomal dominant (AD) or autosomal recessive (AR) form. CMD is characterized by hyperostosis of craniofacial bones and flaring metaphyses of long bones. Many patients with CMD suffer from neurological symptoms. To date, the pathogenesis of CMD is not fully understood. Treatment is limited to decompression surgery. Here, we report a knock in (KI) mouse model for AR CMD carrying a R239Q mutation in CX43. Cx43KI/KI mice replicate many features of AR CMD in craniofacial and long bones. In contrast to Cx43+/+ littermates, Cx43KI/KI mice exhibit periosteal bone deposition and increased osteoclast (OC) numbers in the endosteum of long bones, leading to an expanded bone marrow cavity and increased cortical bone thickness. Although formation of Cx43+/+ and Cx43KI/KI resting OCs are comparable, on bone chips the actively resorbing Cx43KI/KI OCs resorb less bone. Cortical bones of Cx43KI/KI mice have an increase in degenerating osteocytes and empty lacunae. Osteocyte dendrite formation is decreased with reduced expression levels of Fgf23, Sost, Tnf-α, IL-1β, Esr1, Esr2, and a lower Rankl/Opg ratio. Female Cx43KI/KI mice display a more severe phenotype. Sexual dimorphism in bone becomes more evident as mice age. Our data show that the CX43R239Q mutation results in mislocalization of CX43 protein and impairment of gap junction and hemichannel activity. Different from CX43 ablation mouse models, the CX43R239Q mutation leads to the AR CMD-like phenotype in Cx43KI/KI mice not only by loss-of-function but also via a not yet revealed dominant function.

The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

Do pseudoxanthoma elasticum patients have higher prevalence of kidney stones on computed tomography compared to hospital controls?

BACKGROUND

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disease characterized by diminished inorganic plasma pyrophosphate (PPi), a strong calcification inhibitor. In addition to more typical calcification of skin, retina and arterial wall a diminished plasma PPi could lead to other ectopic calcification, such as formation of kidney stones.

OBJECTIVE

To compare the prevalence of kidney stones between PXE patients and hospital controls on computed tomography (CT).

METHOD

Low-dose CT images of PXE patients and controls were assessed by one radiologist, who was blinded for the diagnosis PXE. The number of kidney stones, and the size of the largest stone was recorded. Odds ratios (ORs) for having kidney stone were calculated using multivariable adjusted logistic regression.

RESULTS

Our study comprised 273 PXE patients and 125 controls. The mean age of PXE patients was 51.5 ± 15.9 years compared to 54.9 ± 14.2 in the control group (p = 0.04) and PXE patients more often were women (63 vs. 50%, p = 0.013). The prevalence of kidney stones on CT was similar: 6.9% in PXE patients, compared to 5.6% in controls (p = 0.6). In the multivariate analysis adjusting for age and sex, there was no significantly higher odds for PXE patients on having stones, compared to controls: OR 1.48 (95% CI 0.62-3.96).

CONCLUSION

There is no significant difference in the prevalence of incidental kidney stones on CT in PXE patients versus controls.

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.

Special manifestations and treatment of rare cases of snoring with special facial features and hearing loss in children

This current case report describes two rare cases of children with both hearing loss and snoring. Case 1, a 17-month-old male patient, and case 2, an 11-year-old male patient, both presented with nasal obstruction, snoring and hearing loss. Physical examinations showed obvious enlargement of the head circumference and special facial features. The two children underwent otolaryngology examinations, endoscopy, hearing tests, laboratory examinations for bone metabolism markers, cranial computed tomography, X-rays and genome-wide exon sequencing. The first case was diagnosed with craniometaphyseal dysplasia, which was relieved after giving a low-calcium diet. The second case was diagnosed with osteopathia striata with cranial sclerosis by gene sequencing. Snoring improved after medication and the speech and quality of life improved with a hearing aid. Paediatric otolaryngological physicians need to have a deeper understanding of congenital diseases involving the bones. Only by genetic testing to determine the pathogenesis can those children be given the correct treatment, which is of great importance for improving their prognosis.

Role of the extracellular ATP/pyrophosphate metabolism cycle in vascular calcification

Conventionally, ATP is considered to be the principal energy source in cells. However, over the last few years, a novel role for ATP as a potent extracellular signaling molecule and the principal source of extracellular pyrophosphate, the main endogenous inhibitor of vascular calcification, has emerged. A large body of evidence suggests that two principal mechanisms are involved in the initiation and progression of ectopic calcification: high phosphate concentration and pyrophosphate deficiency. Pathologic calcification of cardiovascular structures, or vascular calcification, is a feature of several genetic diseases and a common complication of chronic kidney disease, diabetes, and aging. Previous studies have shown that the loss of function of several enzymes and transporters involved in extracellular ATP/pyrophosphate metabolism is associated with vascular calcification. Therefore, pyrophosphate homeostasis should be further studied to facilitate the design of novel therapeutic approaches for ectopic calcification of cardiovascular structures, including strategies to increase pyrophosphate concentrations by targeting the ATP/pyrophosphate metabolism cycle.

Vascular Calcification: Key Roles of Phosphate and Pyrophosphate

Cardiovascular complications due to accelerated arterial stiffening and atherosclerosis are the leading cause of morbimortality in Western society. Both pathologies are frequently associated with vascular calcification. Pathologic calcification of cardiovascular structures, or vascular calcification, is associated with several diseases (for example, genetic diseases, diabetes, and chronic kidney disease) and is a common consequence of aging. Calcium phosphate deposition, mainly in the form of hydroxyapatite, is the hallmark of vascular calcification and can occur in the medial layer of arteries (medial calcification), in the atheroma plaque (intimal calcification), and cardiac valves (heart valve calcification). Although various mechanisms have been proposed for the pathogenesis of vascular calcification, our understanding of the pathogenesis of calcification is far from complete. However, in recent years, some risk factors have been identified, including high serum phosphorus concentration (hyperphosphatemia) and defective synthesis of pyrophosphate (pyrophosphate deficiency). The balance between phosphate and pyrophosphate, strictly controlled by several genes, plays a key role in vascular calcification. This review summarizes the current knowledge concerning phosphate and pyrophosphate homeostasis, focusing on the role of extracellular pyrophosphate metabolism in aortic smooth muscle cells and macrophages.

Tlr2/4 ‐Mediated Hyperinflammation Promotes Cherubism‐Like Jawbone Expansion in Sh3bp2 ( P416R ) Knockin Mice

Cherubism (CBM), characterized by expansile jawbones with multilocular fibrocystic lesions, is caused by gain‐of‐function mutations in SH3 domain‐binding protein 2 (SH3BP2; mouse orthologue Sh3bp2). Loss of jawbone and dental integrity significantly decrease the quality of life for affected children. Treatment for CBM is limited to multiple surgeries to correct facial deformities. Despite significant advances made with CBM knockin (KI) mouse models (Sh3bp2^KI/KI), the activation mechanisms of CBM lesions remain unknown because mutant mice do not spontaneously develop expansile jawbones. We hypothesize that bony inflammation of an unknown cause triggers jawbone expansion in CBM. To introduce jawbone inflammation in a spatiotemporally controlled manner, we exposed pulp of the first right mandibular molar of 6‐week‐old Sh3bp2^+/+, Sh3bp2^KI/+, and Sh3bp2^KI/KI mice. Bacterial invasion from the exposed pulp into root canals led to apical periodontitis in wild‐type and mutant mice. The pathogen‐associated molecular patterns (PAMPs)‐induced inflammation of alveolar bone resulted in jawbone expansion in Sh3bp2^KI/+ and Sh3bp2^KI/KI mice. CBM‐like lesions developed exacerbated inflammation with increased neutrophil, macrophage, and osteoclast numbers. These lesions displayed excessive neutrophil extracellular traps (NETs) compared to Sh3bp2^+/+ mice. Expression levels of IL‐1β, IL‐6, and TNF‐α were increased in periapical lesions of Sh3bp2^+/+, Sh3bp2^KI/+, and Sh3bp2^KI/KI mice and also in plasma and the left untreated mandibles (with no pulp exposure) of Sh3bp2^KI/KI mice, suggesting a systemic upregulation. Ablation of Tlr2/4 signaling or depletion of neutrophils by Ly6G antibodies ameliorated jawbone expansion induced by PAMPs in Sh3bp2^KI/KI mice. In summary, successful induction of CBM‐like lesions in jaws of CBM mice is important for studying initiating mechanisms of CBM and for testing potential therapies. Our findings further emphasize a critical role of host immunity in the development of apical periodontitis and the importance of maintaining oral health in CBM patients. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

A three-year clinical investigation of a Chinese child with craniometaphyseal dysplasia caused by a mutated ANKH gene

BACKGROUND

Craniometaphyseal dysplasia (CMD) is a rare genetic disorder. Autosomal dominant CMD (AD-CMD) is caused by mutations in the ANKH gene. Affected individuals typically have distinctive facial features including progressive thickening of the craniofacial bones. Treatment for AD-CMD primarily consists of surgical intervention to release compression of the cranial nerves and the brain stem/spinal cord. To alleviate progression of the clinical course and improve the quality of life in children waiting to undergo the necessary surgery, we investigated clinical changes in a diagnosed patient with AD-CMD over three years.

CASE SUMMARY

A 17-mo-old boy presented with progressive nasal obstruction, snoring and hearing loss symptoms. Physical examination showed enlargement of the head circumference and clinical features such as wide nasal bridge, paranasal bossing, widely spaced eyes with an increased bizygomatic width, and a prominent mandible. The patient underwent otolaryngological examination, endoscopy, hearing test, laboratory examination of phosphorus and bone metabolism, cranial and femoral computed tomography, X-ray and next-generation sequencing. The patient was diagnosed with AD-CMD due to p.Phe377 deletion (c.1129_1131del) on exon 9 of the ANKH gene. After adherence to a prescribed low-calcium diet, the boy’s alkaline phosphatase (ALP) levels continuously decreased to within the normal range. However, after 14 mo of dietary intervention, his parents altered his diet to an intermittent low-calcium diet to include milk and eggs. The patient’s ALP was slightly higher than normal after the dietary change but remained close to the normal range. His serum osteocalcin changed to within normal levels after dietary regulation for 33 mo. His serum combined beta C-terminal telopeptide of type I collagen also continuously decreased after the nutritional intervention, although still slightly higher than normal levels. Despite fluctuating blood test results, the boy’s nasal symptoms were markedly relieved and steadily improved after dietary intervention. No significant changes were found in the craniofacial bones by cranial radiography. Close monitoring of clinical features is still ongoing. Calcitriol treatment is currently under consideration and a surgical procedure is planned as necessary in the future.

CONCLUSION

We herein report the first Chinese case of AD-CMD with heterozygous mutation of p.Phe377 deletion (c.1129_1131del) on the ANKH gene. Biochemical alterations were significantly improved after dietary intervention indicating that a low-calcium diet may be applied in pediatric AD-CMD patients with ANKH mutations to help alleviate phenotypic manifestations and improve the quality of life before surgical intervention. Further large scale studies are needed to replicate these findings and to establish the appropriate timing for nutritional and surgical interventions

Restriction of Dietary Phosphate Ameliorates Skeletal Abnormalities in a Mouse Model for Craniometaphyseal Dysplasia

Craniometaphyseal dysplasia (CMD), a rare genetic bone disorder, is characterized by lifelong progressive thickening of craniofacial bones and metaphyseal flaring of long bones. The autosomal dominant form of CMD is caused by mutations in the progressive ankylosis gene ANKH (mouse ortholog Ank), encoding a pyrophosphate (PPi) transporter. We previously reported reduced formation and function of osteoblasts and osteoclasts in a knockin (KI) mouse model for CMD (Ank^KI/KI) and in CMD patients. We also showed rapid protein degradation of mutant ANK/ANKH. Mutant ANK protein displays reduced PPi transport, which may alter the inorganic phosphate (Pi) and PPi ratio, an important regulatory mechanism for bone mineralization. Here we investigate whether reducing dietary Pi intake can ameliorate the CMD-like skeletal phenotype by comparing male and female Ank^+/+ and Ank^KI/KI mice exposed to a low (0.3%) and normal (0.7%) Pi diet for 13 weeks from birth. Serum Pi and calcium (Ca) levels were not significantly changed by diet, whereas PTH and 25-hydroxy vitamin D (25-OHD) were decreased by low Pi diet but only in male Ank^+/+ mice. Importantly, the 0.3% Pi diet significantly ameliorated mandibular hyperostosis in both sexes of Ank^KI/KI mice. A tendency of decreased femoral trabeculation was observed in male and female Ank^+/+ mice as well as in male Ank^KI/KI mice fed with the 0.3% Pi diet. In contrast, in female Ank^KI/KI mice the 0.3% Pi diet resulted in increased metaphyseal trabeculation. This was also the only group that showed increased bone formation rate. Low Pi diet led to increased osteoclast numbers and increased bone resorption in all mice. We conclude that lowering but not depleting dietary Pi delays the development of craniofacial hyperostosis in CMD mice without severely compromising serum levels of Pi, Ca, PTH, and 25-OHD. These findings may have implications for better clinical care of patients with CMD. © 2020 American Society for Bone and Mineral Research.

New insights into endogenous mechanisms of protection against arterial calcification

Cardiovascular complications due to accelerated atherosclerosis and arterial stiffening are the leading cause of morbidity and mortality in the Western society. Both pathologies are frequently associated with vascular calcification. Deposits of calcium phosphate salts, mainly in form of hydroxyapatite, is the hallmark of vascular calcification. Calcification is frequently observed in atherosclerotic lesions (intimal calcification) associated with vascular smooth muscle cells (VSMCs) and macrophages. By contrast, medial calcification, occurring in the elastic region of the arteries, is almost exclusively associated with VSMCs, and is common in arteriosclerosis related to aging, diabetes, and chronic kidney disease. In extracellular fluids, a range of endogenous low- and high-molecular weight calcification inhibitors are present, including osteopontin, matrix-Gla proteins and Fetuin A. Moreover, pyrophosphate deficiency plays a key role in vascular calcification. Pyrophosphate is produced by extracellular hydrolysis of ATP and is degraded to phosphate by tissue non-specific alkaline phosphatase. Loss of function in the enzymes and transporters involved in the extracellular pyrophosphate metabolism leads to excessive deposition of calcium-phosphate salts. This review summarizes the current knowledge about endogenous mechanisms of protection against calcification in the aortic wall, focusing on the role of extracellular pyrophosphate metabolism in vascular smooth muscle cells and macrophages.

The Genetic Architecture of High Bone Mass

The phenotypic trait of high bone mass (HBM) is an excellent example of the nexus between common and rare disease genetics. HBM may arise from carriage of many 'high bone mineral density [BMD]'-associated alleles, and certainly the genetic architecture of individuals with HBM is enriched with high BMD variants identified through genome-wide association studies of BMD. HBM may also arise as a monogenic skeletal disorder, due to abnormalities in bone formation, bone resorption, and/or bone turnover. Individuals with monogenic disorders of HBM usually, though not invariably, have other skeletal abnormalities (such as mandible enlargement) and thus are best regarded as having a skeletal dysplasia rather than just isolated high BMD. A binary etiological division of HBM into polygenic vs. monogenic, however, would be excessively simplistic: the phenotype of individuals carrying rare variants of large effect can still be modified by their common variant polygenic background, and by the environment. HBM disorders-whether predominantly polygenic or monogenic in origin-are not only interesting clinically and genetically: they provide insights into bone processes that can be exploited therapeutically, with benefits both for individuals with these rare bone disorders and importantly for the many people affected by the commonest bone disease worldwide-i.e., osteoporosis. In this review we detail the genetic architecture of HBM; we provide a conceptual framework for considering HBM in the clinical context; and we discuss monogenic and polygenic causes of HBM with particular emphasis on anabolic causes of HBM.

Mandibular dysmorphology due to abnormal embryonic osteogenesis in FGFR2-related craniosynostosis mice

One diagnostic feature of craniosynostosis syndromes is mandibular dysgenesis. Using three mouse models of Apert, Crouzon and Pfeiffer craniosynostosis syndromes, we investigated how embryonic development of the mandible is affected by fibroblast growth factor receptor 2 (Fgfr2) mutations. Quantitative analysis of skeletal form at birth revealed differences in mandibular morphology between mice carrying Fgfr2 mutations and their littermates that do not carry the mutations. Murine embryos with the mutations associated with Apert syndrome in humans (Fgfr2^+/S252W and Fgfr2^+/P253R) showed an increase in the size of the osteogenic anlagen and Meckel's cartilage (MC). Changes in the microarchitecture and mineralization of the developing mandible were visualized using histological staining. The mechanism for mandibular dysgenesis in the Apert Fgfr2^+/S252W mouse resulting in the most severe phenotypic effects was further analyzed in detail and found to occur to a lesser degree in the other craniosynostosis mouse models. Laser capture microdissection and RNA-seq analysis revealed transcriptomic changes in mandibular bone at embryonic day 16.5 (E16.5), highlighting increased expression of genes related to osteoclast differentiation and dysregulated genes active in bone mineralization. Increased osteoclastic activity was corroborated by TRAP assay and in situ hybridization of Csf1r and Itgb3. Upregulated expression of Enpp1 and Ank was validated in the mandible of Fgfr2^+/S252W embryos, and found to result in elevated inorganic pyrophosphate concentration. Increased proliferation of osteoblasts in the mandible and chondrocytes forming MC was identified in Fgfr2^+/S252W embryos at E12.5. These findings provide evidence that FGFR2 gain-of-function mutations differentially affect cartilage formation and intramembranous ossification of dermal bone, contributing to mandibular dysmorphogenesis in craniosynostosis syndromes.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: FGFR2 gain-of-function mutations differentially affect cartilage formation and intramembranous ossification of dermal bone, resulting in abnormal embryonic osteogenesis of the mandible.

Craniometaphyseal dysplasia: Report of 2 cases with an emphasis on panoramic imaging features

Craniometaphyseal dysplasia (CMD) is a rare hereditary disorder characterized by hyperostosis of the craniofacial bones and flared metaphyses of the long bones. Although some reports have described the dentomaxillofacial characteristics of CMD, including increased density of the jaw, malocclusion, and delayed eruption of the permanent teeth, only a few studies have reported the distinct imaging features of CMD on panoramic radiography. This report presents 2 cases of confirmed CMD patients with an emphasis on panoramic imaging features. The patients' images revealed hyperostosis and sclerosis of the maxilla and mandibular alveolar bone, but there was no change in the mandibular basal bone. In both cases, the mandibular condyle heads exhibited a short clubbed shape with hyperplasia of the coronoid process. For patients without clear otorhinolaryngological symptoms, common radiologic features of CMD could be visualized by routinely-taken panoramic radiographs, and further medical examinations and treatment can be recommended.

Rapid degradation of progressive ankylosis protein (ANKH) in craniometaphyseal dysplasia

Mutations in the progressive ankylosis protein (NP_473368, human ANKH) cause craniometaphyseal dysplasia (CMD), characterized by progressive thickening of craniofacial bones and widened metaphyses in long bones. The pathogenesis of CMD remains largely unknown, and treatment for CMD is limited to surgical intervention. We have reported that knock-in mice (AnkKI/KI) carrying a F377del mutation in ANK (NM_020332, mouse ANK) replicate many features of CMD. Interestingly, ablation of the Ank gene in AnkKO/KO mice also leads to several CMD-like phenotypes. Mutations causing CMD led to decreased steady-state levels of ANK/ANKH protein due to rapid degradation. While wild type (wt) ANK was mostly associated with plasma membranes, endoplasmic reticulum (ER), Golgi apparatus and lysosomes, CMD-linked mutant ANK was aberrantly localized in cytoplasm. Inhibitors of proteasomal degradation significantly restored levels of overexpressed mutant ANK, whereas endogenous CMD-mutant ANK/ANKH levels were more strongly increased by inhibitors of lysosomal degradation. However, these inhibitors do not correct the mislocalization of mutant ANK. Co-expressing wt and CMD-mutant ANK in cells showed that CMD-mutant ANK does not negatively affect wt ANK expression and localization, and vice versa. In conclusion, our finding that CMD mutant ANK/ANKH protein is short-lived and mislocalized in cells may be part of the CMD pathogenesis.

Sclerosing bone dysplasias

The group of sclerosing bone dysplasia's is a clinically and genetically heterogeneous group of rare bone disorders which, according to the latest Nosology and classification of genetic skeletal disorders (2015), can be subdivided in three subgroups; the neonatal osteosclerotic dysplasias, the osteopetroses and related disorders and the other sclerosing bone disorders. Here, we give an overview of the most important radiographic and clinical symptoms, the underlying genetic defect and potential treatment options of the different sclerosing dysplasias included in these subgroups.

Human Genetics of Sclerosing Bone Disorders

PURPOSE OF REVIEW

The group of sclerosing bone disorders encompasses a variety of disorders all marked by increased bone mass. In this review, we give an overview of the genetic causes of this heterogeneous group of disorders and briefly touch upon the value of these findings for the development of novel therapeutic agents.

RECENT FINDINGS

Advances in the next-generation sequencing technologies are accelerating the molecular dissection of the pathogenic mechanisms underlying skeletal dysplasias. Throughout the years, the genetic cause of these disorders has been extensively studied which resulted in the identification of a variety of disease-causing genes and pathways that are involved in bone formation by osteoblasts, bone resorption by osteoclasts, or both processes. Due to this rapidly increasing knowledge, the insights into the regulatory mechanisms of bone metabolism are continuously improving resulting in the identification of novel therapeutic targets for disorders with reduced bone mass and increased bone fragility.

Structural biology of solute carrier (SLC) membrane transport proteins

The human solute carriers (SLCs) comprise over 400 different transporters, organized into 65 families ( http://slc.bioparadigms.org/ ) based on their sequence homology and transport function. SLCs are responsible for transporting extraordinarily diverse solutes across biological membranes, including inorganic ions, amino acids, lipids, sugars, neurotransmitters and drugs. Most of these membrane proteins function as coupled symporters (co-transporters) utilizing downhill ion (H⁺ or Na⁺) gradients as the driving force for the transport of substrate against its concentration gradient into cells. Other members work as antiporters (exchangers) that typically contain a single substrate-binding site with an alternating access mode of transport, while a few members exhibit channel-like properties. Dysfunction of SLCs is correlated with numerous human diseases and therefore they are potential therapeutic drug targets. In this review, we identified all of the SLC crystal structures that have been determined, most of which are from prokaryotic species. We further sorted all the SLC structures into four main groups with different protein folds and further discuss the well-characterized MFS (major facilitator superfamily) and LeuT (leucine transporter) folds. This review provides a systematic analysis of the structure, molecular basis of substrate recognition and mechanism of action in different SLC family members.

Biopsy-proven multiple sclerosis in an adult patient with atypical craniometaphyseal dysplasia

Craniometaphyseal dysplasia (CMD) is a rare condition characterised by progressive, diffuse hyperostosis of cranial and long bones, with compression of cranial nerves, linked to mutations in ANKH or GJA1 genes. Here we describe an adult case with clinical features of CMD, who developed cerebral expansive lesion of undetermined nature. Brain biopsy revealed active demyelinating lesions, consistent with multiple sclerosis. The genetic screening of target genes for CMD (ANKH and GJA1) resulted negative in this patient. The peculiar clinical association and the negativity of genetic analyses allow to hypothesise that other genetic causes, not already known, are responsible for the combination of these pathological conditions. Future studies aim to identify the genetic causes of CMD, which will be important to further understand the pathogenetic mechanism of this rare and invalidating disease.

Role of pyrophosphate in vascular calcification in chronic kidney disease

Vascular calcification is a pathology characterized by the deposition of calcium-phosphate in cardiovascular structures, mainly in the form of hydroxyapatite crystals, resulting in ectopic calcification. It is correlated with increased risk of cardiovascular disease and myocardial infarction in diabetic patients and in those with chronic kidney disease (CKD). Vascular smooth muscle cells are sensitive to changes in inorganic phosphate (Pi) levels. They are able to adapt and modify some of their functions and promote changes which trigger calcification. Pi is regulated by parathyroid hormone and 1,25-dihydroxyvitamin D. Changes in the transport of Pi are the primary factor responsible for the regulation of Pi homeostasis and the calcification process. Synthesis of calcification inhibitors is the main mechanism by which cells are able to prevent vascular calcification. Extracellular pyrophosphate (PPi) is a potent endogenous inhibitor of calcium-phosphate deposition both in vivo and in vitro. Patients with CKD show lower levels of PPi and increased activity of the enzyme alkaline phosphatase. Numerous enzymes implicated in the metabolism of PPi have been associated with vascular calcifications. PPi is synthesized from extracellular ATP by nucleotide pyrophosphatase/phosphodiesterase from extracellular ATP hydrolysis. PPi is hydrolyzed into Pi by tissue-nonspecific alkaline phosphatase. ATP can be hydrolyzed to Pi via the ectonucleoside triphosphate diphosphohydrolase family. All these enzymes must be in balance, thereby preventing calcifications. However, diseases like CKD or diabetes induce alterations in their levels. Administration of PPi could open up new treatment options for these patients.

An atypical case of craniometaphyseal dysplasia. Case report and surgical treatment

Introduction

Craniometaphyseal dysplasia is a rare hereditary bone disease presenting metaphyseal widening of the tubular bones, sclerosis of craniofacial bones and bony overgrowth of the facial and skull bones. Craniometaphyseal dysplasia occurs in an autosomal dominant (AD) and an autosomal recessive (AR) form.

Case report

We present a 32-year-old patient arrived at our unit in May 2009. His main discomfort was a major limitation of the mouth opening, in the context of a craniofacial deformity. Relying on patient's medical history and the performed diagnostic tests, the diagnosis of craniometaphyseal dysplasia was made.

Conclusion

After careful evaluation of the clinical case, in accordance with the requirements of the patient, we opted for a surgical treatment aimed at correction of functional limitation of temporomandibular joint and aesthetic improvement of the facial bones. The stability of the clinical results led us to suggest and to undertake the surgical path, also due to the lack of safe and consolidated non-surgical treatments for the specific case.

Craniometaphyseal Dysplasia Mutations in ANKH Negatively Affect Human Induced Pluripotent Stem Cell Differentiation into Osteoclasts

We identified osteoclast defects in craniometaphyseal dysplasia (CMD) using an easy-to-use protocol for differentiating osteoclasts from human induced pluripotent stem cells (hiPSCs). CMD is a rare genetic bone disorder, characterized by life-long progressive thickening of craniofacial bones and abnormal shape of long bones. hiPSCs from CMD patients with an in-frame deletion of Phe377 or Ser375 in ANKH are more refractory to in vitro osteoclast differentiation than control hiPSCs. To exclude differentiation effects due to genetic variability, we generated isogenic hiPSCs, which have identical genetic background except for the ANKH mutation. Isogenic hiPSCs with ANKH mutations formed fewer osteoclasts, resorbed less bone, expressed lower levels of osteoclast marker genes, and showed decreased protein levels of ANKH and vacuolar proton pump v-ATP6v0d2. This proof-of-concept study demonstrates that efficient and reproducible differentiation of isogenic hiPSCs into osteoclasts is possible and a promising tool for investigating mechanisms of CMD or other osteoclast-related disorders.

MECHANISMS IN ENDOCRINOLOGY: Genetics of human bone formation

Throughout life, bone is continuously remodelled to be able to fulfil its multiple functions. The importance of strictly regulating the bone remodelling process, which is defined by the sequential actions of osteoclasts and osteoblasts, is shown by a variety of disorders with abnormalities in bone mass and strength. The best known and most common example of such a disorder is osteoporosis, which is marked by a decreased bone mass and strength that consequently results in an increased fracture risk. As osteoporosis is a serious health problem, a large number of studies focus on elucidating the aetiology of the disease as well as on the identification of novel therapeutic targets for the treatment of osteoporotic patients. These studies have demonstrated that a large amount of variation in bone mass and strength is often influenced by genetic variation in genes encoding important regulators of bone homeostasis. Throughout the years, studies into the genetic causes of osteoporosis as well as several rare monogenic disorders with abnormal high or low bone mass and strength have largely increased the knowledge on regulatory pathways important for bone resorption and formation. This review gives an overview of genes and pathways that are important for the regulation of bone formation and that are identified through their involvement in monogenic and complex disorders with abnormal bone mass. Furthermore, novel bone-forming strategies for the treatment of osteoporosis that resulted from these discoveries, such as antibodies against sclerostin, are discussed as well.

Craniometaphyseal Dysplasia: A review and novel oral manifestation

Craniometaphyseal Dysplasia (CMD) is a sclerosing osseous dysplasia characterised by hyperostosis of craniofacial and long bones, resulting in distortion and cranial nerve palsies. We present a case report on the management of a 63 year old female with Craniometaphyseal Dysplasia. This report describes an additional clinical manifestation of hypercementosis, which although well recognised in other sclerosing osseous dysplasias, is not reported in the literature for Craniometaphyseal Dysplasia. We discuss established in vivo studies in mice which link the genetic mutations found in Craniometaphyseal Dysplasia to hypercementosis, and how this report describes the same manifestation in humans. This novel finding can aid the clinician in the management of patients with Craniometaphyseal Dysplasia, and complications that can arise in dentoalveolar surgery.

Can acetazolamide be used to treat diseases involving increased bone mineral density?

Sclerosing bone dysplasias are a series of clinically and genetically heterogeneous diseases characterized by functional failure of the osteoclasts in bone resorption, leading to an excessive amount of bone mineral density (BMD) which could have serious clinical consequences. We treated three children affected with seriously high levels of BMD with acetazolamide, with the intention of inducing metabolic acidosis, thus increasing bone resorption and reducing BMD. All our patients tolerated and followed the treatment well and the clinical response was satisfactory in all cases.

Dietary phosphate supplement does not rescue skeletal phenotype in a mouse model for craniometaphyseal dysplasia

Background

Mutations in the human progressive ankylosis gene (ANKH; Mus musculus ortholog Ank) have been identified as cause for craniometaphyseal dysplasia (CMD), characterized by progressive thickening of craniofacial bones and flared metaphyses of long bones. We previously reported a knock-in (KI) mouse model (Ank^KI/KI) for CMD and showed transiently lower serum phosphate (Pi) as well as significantly higher mRNA levels of fibroblast growth factor 23 (Fgf23) in Ank^KI/KI mice. FGF23 is secreted by bone and acts in kidney to promote Pi wasting which leads to lower serum Pi levels. Here, we examined whether increasing the Pi level can partially rescue the CMD-like skeletal phenotype by feeding Ank^+/+ and Ank^KI/KI mice with high Pi (1.7 %) diet from birth for 6 weeks. We studied the Pi metabolism in Ank^KI/KI mice and CMD patients by examining the Pi regulators FGF23 and parathyroid hormone (PTH).

Results

High Pi diet did not correct CMD-like features, including massive jawbone, increased endosteal and periosteal perimeters and extensive trabeculation of femurs in Ank^KI/KI mice shown by computed microtomography (μCT). This unexpected negative result is, however, consistent with normal serum/plasma levels of the intact/active form of FGF23 and PTH in Ank^KI/KI mice and in CMD patients. In addition, FGF23 protein expression was unexpectedly normal in Ank^KI/KI femoral cortical bone as shown by immunohistochemistry despite increased mRNA levels for Fgf23. Renal expression of genes involved in the FGF23 bone-kidney axis, including mFgfr1, mKlotho, mNpt2a, mCyp24a1 and m1αOHase, were comparable between Ank^+/+ and Ank^KI/KI mice as shown by quantitative real-time PCR. Different from normal FGF23 and PTH, serum 25-hydroxyvitamin D was significantly lower in Ank^KI/KI mice and vitamin D insufficiency was found in four out of seven CMD patients.

Conclusions

Our data suggests that FGF23 signaling and Pi metabolism are not significantly affected in CMD and transiently low Pi level is not a major contributor to CMD.

How rare bone diseases have informed our knowledge of complex diseases

Rare bone diseases, generally defined as monogenic traits with either autosomal recessive or dominant patterns of inheritance, have provided a rich database of genes and associated pathways over the past 2-3 decades. The molecular genetic dissection of these bone diseases has yielded some major surprises in terms of the causal genes and/or involved pathways. The discovery of genes/pathways involved in diseases such as osteopetrosis, osteosclerosis, osteogenesis imperfecta and many other rare bone diseases have all accelerated our understanding of complex traits. Importantly these discoveries have provided either direct validation for a specific gene embedded in a group of genes within an interval identified through a complex trait genome-wide association study (GWAS) or based upon the pathway associated with a monogenic trait gene, provided a means to prioritize a large number of genes for functional validation studies. In some instances GWAS studies have yielded candidate genes that fall within linkage intervals associated with monogenic traits and resulted in the identification of causal mutations in those rare diseases. Driving all of this discovery is a complement of technologies such as genome sequencing, bioinformatics and advanced statistical analysis methods that have accelerated genetic dissection and greatly reduced the cost. Thus, rare bone disorders in partnership with GWAS have brought us to the brink of a new era of personalized genomic medicine in which the prevention and management of complex diseases will be driven by the molecular understanding of each individuals contributing genetic risks for disease.

Craniometaphyseal dysplasia in a 14-month old: a case report and review of imaging differential diagnosis

We report a 14-month-old male with craniometaphyseal dysplasia (CMD). The patient presented with a history of diminishing vision and hearing loss. Cranial computed tomography scan showed diffuse calvarial and skull base hyperostosis with excessive bone narrowing the internal auditory canals and skull base foramina. A subsequent skeletal survey revealed other skeletal abnormalities, which led to the diagnosis of CMD. This was later confirmed by ANKH mutation. CMD is a rare genetic disorder that belongs to the group of craniotubular bone dysplasias. It is important to recognize this condition from other causes of craniotubular bone dysplasias to institute early treatment and explain prognosis.

Disruption of biomineralization pathways in spinal tissues of a mouse model of diffuse idiopathic skeletal hyperostosis

Equilibrative nucleoside transporter 1 (ENT1) mediates passage of adenosine across the plasma membrane. We reported previously that mice lacking ENT1 (ENT1(-/-)) exhibit progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis (DISH) in humans. Here, we investigated mechanisms underlying aberrant mineralization in ENT1(-/-) mice. Micro-CT revealed ectopic mineralization of spinal tissues in both male and female ENT1(-/-) mice, involving the annulus fibrosus of the intervertebral discs (IVDs) of older mice. IVDs were isolated from wild-type and ENT1(-/-) mice at 2months of age (prior to disc mineralization), 4, and 6months of age (disc mineralization present) and processed for real-time PCR, cell isolation, or histology. Relative to the expression of ENTs in other tissues, ENT1 was the primary nucleoside transporter expressed in wild-type IVDs and mediated the functional uptake of [(3)H]2-chloroadenosine by annulus fibrosus cells. No differences in candidate gene expression were detected in IVDs from ENT1(-/-) and wild-type mice at 2 or 4months of age. However, at 6months of age, expression of genes that inhibit biomineralization Mgp, Enpp1, Ank, and Spp1 were reduced in IVDs from ENT1(-/-) mice. To assess whether changes detected in ENT1(-/-) mice were cell autonomous, annulus fibrosus cell cultures were established. Compared to wild-type cells, cells isolated from ENT1(-/-) IVDs at 2 or 6months of age demonstrated greater activity of alkaline phosphatase, a promoter of biomineralization. Cells from 2-month-old ENT1(-/-) mice also showed greater mineralization than wild-type. Interestingly, altered localization of alkaline phosphatase activity was detected in the inner annulus fibrosus of ENT1(-/-) mice in vivo. Alkaline phosphatase activity, together with the marked reduction in mineralization inhibitors, is consistent with the mineralization of IVDs seen in ENT1(-/-) mice at older ages. These findings establish that both cell-autonomous and systemic mechanisms contribute to ectopic mineralization in ENT1(-/-) mice.

Matrix stiffness promotes cartilage endplate chondrocyte calcification in disc degeneration via miR-20a targeting ANKH expression

The mechanical environment is crucial for intervertebral disc degeneration (IDD). However, the mechanisms underlying the regulation of cartilage endplate (CEP) calcification by altered matrix stiffness remain unclear. In this study, we found that matrix stiffness of CEP was positively correlated with the degree of IDD, and stiff matrix, which mimicked the severe degeneration of CEP, promoted inorganic phosphate-induced calcification in CEP chondrocytes. Co-expression analysis of the miRNA and mRNA profiles showed that increasing stiffness resulted in up-regulation of miR-20a and down-regulation of decreased ankylosis protein homolog (ANKH) during inorganic phosphate-induced calcification in CEP chondrocytes. Through a dual luciferase reporter assay, we confirmed that miR-20a directly targets 3'-untranslated regions of ANKH. The inhibition of miR-20a attenuated the calcium deposition and calcification-related gene expression, whereas the overexpression of miR-20a enhanced calcification in CEP chondrocytes on stiff matrix. The rescue of ANKH expression restored the decreased pyrophosphate efflux and inhibited calcification. In clinical samples, the levels of ANKH expression were inversely associated with the degeneration degree of CEP. Thus, our findings demonstrate that the miR-20a/ANKH axis mediates the stiff matrix- promoted CEP calcification, suggesting that miR-20a and ANKH are potential targets in restraining the progression of IDD.

Pyrophosphate: a key inhibitor of mineralisation

Inorganic pyrophosphate has long been known as a by-product of many intracellular biosynthetic reactions, and was first identified as a key endogenous inhibitor of biomineralisation in the 1960s. The major source of pyrophosphate appears to be extracellular ATP, which is released from cells in a controlled manner. Once released, ATP can be rapidly hydrolysed by ecto-nucleotide pyrophosphatase/phosphodiesterases to produce pyrophosphate. The main action of pyrophosphate is to directly inhibit hydroxyapatite formation thereby acting as a physiological 'water-softener'. Evidence suggests pyrophosphate may also act as a signalling molecule to influence gene expression and regulate its own production and breakdown. This review will summarise our current understanding of pyrophosphate metabolism and how it regulates bone mineralisation and prevents harmful soft tissue calcification.

Craniometaphyseal dysplasia with obvious biochemical abnormality and rickets-like features

BACKGROUND

Craniometaphyseal dysplasia (CMD) is a rare genetic disorder that is characterized by progressive sclerosis of the craniofacial bones and metaphyseal widening of long bones, and biochemical indexes were mostly normal. To further the understanding of the disease from a biochemical perspective, we reported a CMD case with obviously abnormal biochemical indexes.

CASE REPORT

A 1-year-old boy was referred to our clinic. Biochemical test showed obviously increased alkaline phosphatase (ALP) and parathyroid hormone (PTH), mild hypocalcemia and hypophosphatemia. Moreover, significant elevated receptor activator of nuclear factor kappa-B ligand (RANKL) level, but normal β-C-terminal telopeptide of type I collagen (β-CTX) concentration were revealed. He was initially suspected of rickets, because the radiological examination also showed broadened epiphysis in his long bones. Supplementation with calcium and calcitriol alleviated biochemical abnormality. However, the patient gradually developed osteosclerosis which was inconformity with rickets. Considering that he was also presented with facial paralysis and nasal obstruction symptom, the diagnosis of craniometaphyseal dysplasia was suspected, and then was confirmed by the mutation analysis of ANKH of the proband and his family, which showed a de novo heterozygous mutation (C1124-1126delCCT) on exon 9.

CONCLUSIONS

Our study revealed that obvious biochemical abnormality and rickets-like features might present as uncommon characteristics in CMD patients, and the calcium and calcitriol supplementation could alleviate biochemical abnormalities. Furthermore, although early osteoclast differentiation factor was excited in CMD patient, activity of osteoclast was still inert.

Vascular Calcification Revisited: A New Perspective for Phosphate Transport

Elevated serum phosphorus has emerged as a key risk factor for pathologic calcification of cardiovascular structures, or vascular calcification (VC). To prevent the formation of calciumphosphate deposits (CPD), the body uses adenosine-5’-triphosphate (ATP) to synthesize inhibitors of calcification, including proteins and inhibitors of low molecular weight. Extracellular pyrophosphate (PPi) is a potent inhibitor of VC, which is produced during extracellular hydrolysis of ATP. Loss of function in the enzymes and transporters that are involved in the cycle of extracellular ATP, including Pi transporters, leads to excessive deposition of calcium-phosphate salts. Treatment of hyperphosphatemia with Pi-binders and Injection of exogenous PPi are the effective treatments to prevent CPD in the aortic wall. The role of sodium phosphate cotransporters in ectopic calcification is contradictory and not well defined, but their important role in the control of intracellular Pi levels and the synthesis of ATP make them an important target to study.

The Use of Patient-Specific Induced Pluripotent Stem Cells (iPSCs) to Identify Osteoclast Defects in Rare Genetic Bone Disorders

More than 500 rare genetic bone disorders have been described, but for many of them only limited treatment options are available. Challenges for studying these bone diseases come from a lack of suitable animal models and unavailability of skeletal tissues for studies. Effectors for skeletal abnormalities of bone disorders may be abnormal bone formation directed by osteoblasts or anomalous bone resorption by osteoclasts, or both. Patient-specific induced pluripotent stem cells (iPSCs) can be generated from somatic cells of various tissue sources and in theory can be differentiated into any desired cell type. However, successful differentiation of hiPSCs into functional bone cells is still a challenge. Our group focuses on the use of human iPSCs (hiPSCs) to identify osteoclast defects in craniometaphyseal dysplasia. In this review, we describe the impact of stem cell technology on research for better treatment of such disorders, the generation of hiPSCs from patients with rare genetic bone disorders and current protocols for differentiating hiPSCs into osteoclasts.

Ultrasonographic pilot study of mental foramen size, with and without postoperative neurosensory dysfunction

OBJECTIVES

To evaluate the ability of a portable ultrasound system to visualize the mental foramen in patients with unilateral neurosensory dysfunction after third molar removal.

STUDY DESIGN

In 20 patients, an ultrasound scanner (13 MHz linear-array transducer) was used to measure the mental foramen. The anteroposterior diameter was calculated on both sides, and differences between injured and contralateral sides were tested. Intrarater and interrater reliability were calculated using Intraclass Correlation Coefficient (ICC).

RESULTS

We found a relatively high reliability on interindividual observations (ICC ≥0.89) and intraindividual observations (ICC ≥0.76). The average diameter on the injured sides was significantly smaller than the control sides. Subjective sensibility was not significantly correlated to the dimension of the mental foramen.

CONCLUSION

The results indicate a reduced size of the mental foramen, as determined by ultrasonography, in patients with permanent neurosensory dysfunction. These findings have to be verified in a larger number of patients.

Inflammation: a culprit for vascular calcification in atherosclerosis and diabetes

It is today acknowledged that aging is associated with a low-grade chronic inflammatory status, and that inflammation exacerbates age-related diseases such as osteoporosis, Alzheimer's disease, atherosclerosis and type 2 diabetes mellitus (T2DM). Vascular calcification is a complication that also occurs during aging, in particular in association with atherosclerosis and T2DM. Recent studies provided compelling evidence that vascular calcification is associated with inflammatory status and is enhanced by inflammatory cytokines. In the present review, we propose on one hand to highlight the most important and recent findings on the cellular and molecular mechanisms of vascular inflammation in atherosclerosis and T2DM. On the other hand, we will present the effects of inflammatory mediators on the trans-differentiation of vascular smooth muscle cell and on the deposition of crystals. Since vascular calcification significantly impacts morbidity and mortality in affected individuals, a better understanding of its induction and development will pave the way to develop new therapeutic strategies.

Molecular, phenotypic aspects and therapeutic horizons of rare genetic bone disorders

A rare disease afflicts less than 200,000 individuals, according to the National Organization for Rare Diseases (NORD) of the United States. Over 6,000 rare disorders affect approximately 1 in 10 Americans. Rare genetic bone disorders remain the major causes of disability in US patients. These rare bone disorders also represent a therapeutic challenge for clinicians, due to lack of understanding of underlying mechanisms. This systematic review explored current literature on therapeutic directions for the following rare genetic bone disorders: fibrous dysplasia, Gorham-Stout syndrome, fibrodysplasia ossificans progressiva, melorheostosis, multiple hereditary exostosis, osteogenesis imperfecta, craniometaphyseal dysplasia, achondroplasia, and hypophosphatasia. The disease mechanisms of Gorham-Stout disease, melorheostosis, and multiple hereditary exostosis are not fully elucidated. Inhibitors of the ACVR1/ALK2 pathway may serve as possible therapeutic intervention for FOP. The use of bisphosphonates and IL-6 inhibitors has been explored to be useful in the treatment of fibrous dysplasia, but more research is warranted. Cell therapy, bisphosphonate polytherapy, and human growth hormone may avert the pathology in osteogenesis imperfecta, but further studies are needed. There are still no current effective treatments for these bone disorders; however, significant promising advances in therapeutic modalities were developed that will limit patient suffering and treat their skeletal disabilities.

Sclerosing bone dysplasias: leads toward novel osteoporosis treatments

Sclerosing bone dysplasias are a group of rare, monogenic disorders characterized by increased bone density resulting from the disturbance in the fragile equilibrium between bone formation and resorption. Over the last decade, major contributions have been made toward better understanding of the pathogenesis of these conditions. These studies provided us with important insights into the bone biology and yielded the identification of numerous drug targets for the prevention and treatment of osteoporosis. Here, we review this heterogeneous group of disorders focusing on their utility in the development of novel osteoporosis therapies.

The role of ANK interactions with MYBBP1a and SPHK1 in catabolic events of articular chondrocytes

OBJECTIVE

To determine the role of progressive ankylosis protein (ANK)/Myb-binding protein 1a (MYBBP1a) and sphingosine kinase 1 (SPHK1) interactions in catabolic events of articular chondrocytes.

METHOD

ANK/MYBBP1a and SPHK1 interactions were identified using yeast two-hybrid screening and co-immunoprecipitation. To determine the role of these interactions in catabolic events of articular chondrocytes, ank/ank and wild type (WT) mouse chondrocytes transfected with full-length or mutant ank expression vectors (EVs) or femoral heads were treated with interleukin-1beta (IL-1β) in the absence or presence of SPHK inhibitor. Catabolic marker mRNA levels were analyzed by real time PCR; proteoglycan loss using safranin O staining and MMP-13 immunostaining were determined in femoral head explants; NF-κB activity was determined by transfecting chondrocytes with an NF-κB-specific luciferase reporter and analyzing nuclear translocation of p65 by immunoblotting; MYBBP1a nuclear or cytoplasmic amounts were determined by immunohistochemistry and immunoblotting.

RESULTS

The ANK N-terminal region interacted with SPHK1, whereas a cytoplasmic C-terminal loop interacted with MYBBP1a. Lack of ANK/MYBBP1a and SPHK1 interactions in ank/ank chondrocytes resulted in increased MYBBP1a nuclear amounts and decreased SPHK1 activity, and consequently decreased NF-κB activity, catabolic marker mRNA levels, proteoglycan loss, and MMP-13 immunostaining in IL-1β-treated articular chondrocytes or femoral heads. Transfection with full-length ank EV reduced nuclear MYBBP1a amounts and fully restored SPHK and NF-κB activities in IL-1β-treated ank/ank chondrocytes, whereas transfection with P5L or F376del mutant ank reduced nuclear MYBBP1a or increased SPHK activity, respectively, and consequently either transfection only partially restored NF-κB activity.

CONCLUSION

ANK/MYBBP1a and SPHK1 interactions stimulate catabolic events in IL-1β-mediated cartilage degradation.

Dental Anomalies Associated with Craniometaphyseal Dysplasia

Craniometaphyseal dysplasia (CMD) is a rare genetic disorder encompassing hyperostosis of craniofacial bones and metaphyseal widening of tubular bones. Dental abnormalities are features of CMD that have been little discussed in the literature. We performed dentofacial examination of patients with CMD and evaluated consequences of orthodontic movement in a mouse model carrying a CMD knock-in (KI) mutation (Phe377del) in the Ank gene. All patients have a history of delayed eruption of permanent teeth. Analysis of data obtained by cone-beam computed tomography showed significant bucco-lingual expansion of jawbones, more pronounced in mandibles than in maxillae. There was no measurable increase in bone density compared with that in unaffected individuals. Orthodontic cephalometric analysis showed that patients with CMD tend to have a short anterior cranial base, short upper facial height, and short maxillary length. Microcomputed tomography (micro-CT) analysis in homozygous Ank (KI/KI) mice, a model for CMD, showed that molars can be moved by orthodontic force without ankylosis, however, at a slower rate compared with those in wild-type Ank (+/+) mice (p < .05). Histological analysis of molars in Ank (KI/KI) mice revealed decreased numbers of TRAP(+) osteoclasts on the bone surface of pressure sides. Based on these findings, recommendations for the dental treatment of patients with CMD are provided.

Induced pluripotent stem cell reprogramming by integration-free Sendai virus vectors from peripheral blood of patients with craniometaphyseal dysplasia

Studies of rare genetic bone disorders are often limited due to unavailability of tissue specimens and the lack of animal models fully replicating phenotypic features. Craniometaphyseal dysplasia (CMD) is a rare monogenic disorder characterized by hyperostosis of craniofacial bones concurrent with abnormal shape of long bones. Mutations for autosomal dominant CMD have been identified in the ANK gene (ANKH). Here we describe a simple and efficient method to reprogram adherent cells cultured from peripheral blood to human induced pluripotent stem cells (hiPSCs) from eight CMD patients and five healthy controls. Peripheral blood mononuclear cells (PBMCs) were separated from 5-7 mL of whole blood by Ficoll gradient, expanded in the presence of cytokines and transduced with Sendai virus (SeV) vectors encoding OCT3/4, SOX2, KLF4, and c-MYC. SeV vector, a cytoplasmic RNA vector, is lost from host cells after propagation for 10-13 passages. These hiPSCs express stem cell markers, have normal karyotypes, and are capable of forming embryoid bodies in vitro as well as teratomas in vivo. Further differentiation of these patient-specific iPSCs into osteoblasts and osteoclasts can provide a useful tool to study the effects CMD mutations on bone, and this approach can be applied for disease modeling of other rare genetic musculoskeletal disorders.

A novel autosomal recessive GJA1 missense mutation linked to Craniometaphyseal dysplasia

Craniometaphyseal dysplasia (CMD) is a rare sclerosing skeletal disorder with progressive hyperostosis of craniofacial bones. CMD can be inherited in an autosomal dominant (AD) trait or occur after de novo mutations in the pyrophosphate transporter ANKH. Although the autosomal recessive (AR) form of CMD had been mapped to 6q21-22 the mutation has been elusive. In this study, we performed whole-exome sequencing for one subject with AR CMD and identified a novel missense mutation (c.716G>A, p.Arg239Gln) in the C-terminus of the gap junction protein alpha-1 (GJA1) coding for connexin 43 (Cx43). We confirmed this mutation in 6 individuals from 3 additional families. The homozygous mutation cosegregated only with affected family members. Connexin 43 is a major component of gap junctions in osteoblasts, osteocytes, osteoclasts and chondrocytes. Gap junctions are responsible for the diffusion of low molecular weight molecules between cells. Mutations in Cx43 cause several dominant and recessive disorders involving developmental abnormalities of bone such as dominant and recessive oculodentodigital dysplasia (ODDD; MIM #164200, 257850) and isolated syndactyly type III (MIM #186100), the characteristic digital anomaly in ODDD. However, characteristic ocular and dental features of ODDD as well as syndactyly are absent in patients with the recessive Arg239Gln Cx43 mutation. Bone remodeling mechanisms disrupted by this novel Cx43 mutation remain to be elucidated.

Loss of equilibrative nucleoside transporter 1 in mice leads to progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis in humans

Diffuse idiopathic skeletal hyperostosis (DISH) is a noninflammatory spondyloarthropathy, characterized by ectopic calcification of spinal tissues. Symptoms include spine pain and stiffness, and in severe cases dysphagia and spinal cord compression. The etiology of DISH is unknown and there are no specific treatments. Recent studies have suggested a role for purine metabolism in the regulation of biomineralization. Equilibrative nucleoside transporter 1 (ENT1) transfers hydrophilic nucleosides, such as adenosine, across the plasma membrane. In mice lacking ENT1, we observed the development of calcified lesions resembling DISH. By 12 months of age, ENT1(-/-) mice exhibited signs of spine stiffness, hind limb dysfunction, and paralysis. Micro-computed tomography (µCT) revealed ectopic mineralization of paraspinal tissues in the cervical-thoracic region at 2 months of age, which extended to the lumbar and caudal regions with advancing age. Energy-dispersive X-ray microanalysis of lesions revealed a high content of calcium and phosphorus with a ratio similar to that of cortical bone. At 12 months of age, histological examination of ENT1(-/-) mice revealed large, irregular accumulations of eosinophilic material in paraspinal ligaments and entheses, intervertebral discs, and sternocostal articulations. There was no evidence of mineralization in appendicular joints or blood vessels, indicating specificity for the axial skeleton. Plasma adenosine levels were significantly greater in ENT1(-/-) mice than in wild-type, consistent with loss of ENT1--a primary adenosine uptake pathway. There was a significant reduction in the expression of Enpp1, Ank, and Alpl in intervertebral discs from ENT1(-/-) mice compared to wild-type mice. Elevated plasma levels of inorganic pyrophosphate in ENT1(-/-) mice indicated generalized disruption of pyrophosphate homeostasis. This is the first report of a role for ENT1 in regulating the calcification of soft tissues. Moreover, ENT1(-/-) mice may be a useful model for investigating pathogenesis and evaluating therapeutics for the prevention of mineralization in DISH and related disorders.

Friend or foe: high bone mineral density on routine bone density scanning, a review of causes and management

A finding of high BMD on routine DXA scanning is not infrequent and most commonly reflects degenerative disease. However, BMD increases may also arise secondary to a range of underlying disorders affecting the skeleton. Although low BMD increases fracture risk, the converse may not hold for high BMD, since elevated BMD may occur in conditions where fracture risk is increased, unaffected or reduced. Here we outline a classification for the causes of raised BMD, based on identification of focal or generalized BMD changes, and discuss an approach to guide appropriate investigation by clinicians after careful interpretation of DXA scan findings within the context of the clinical history. We will also review the mild skeletal dysplasia associated with the currently unexplained high bone mass phenotype and discuss recent advances in osteoporosis therapies arising from improved understanding of rare inherited high BMD disorders.

Thickened skull, scoliosis and other skeletal findings in Unverricht-Lundborg disease link cystatin B function to bone metabolism

PURPOSE

Unverricht-Lundborg disease (EPM1) is a rare type of inherited progressive myoclonic epilepsy resulting from mutations in the cystatin B gene, CSTB, which encodes a cysteine cathepsin inhibitor. Cystatin B, cathepsin K, and altered osteoclast bone resorption activity are interconnected in vitro. This study evaluated the skeletal characteristics of patients with EPM1.

METHODS

Sixty-six genetically verified EPM1 patients and 50 healthy controls underwent head MRI. Skull dimensions and regional calvarial thickness was measured perpendicular to each calvarial bone from T1-weighted 3-dimensional images using multiple planar reconstruction tools. All clinical X-ray files of EPM1 patients were collected and reviewed by an experienced radiologist. A total of 337 X-ray studies were analyzed, and non-traumatic structural anomalies, dysplasias and deformities were registered.

RESULTS

EPM1 patients exhibited significant thickening in all measured cranial bones compared to healthy controls. The mean skull thickness was 10.0±2.0mm in EPM1 patients and 7.6±1.2mm in healthy controls (p<0.001). The difference was evident in all age groups and was not explained by former phenytoin use. Observed abnormalities in other skeletal structures in EPM1 patients included thoracic scoliosis (35% of EPM1 patients) and lumbar spine scoliosis (35%), large paranasal sinuses (27%), accessory ossicles of the foot, and arachnodactyly (18%).

CONCLUSIONS

Skull thickening and an increased prevalence of abnormal findings in skeletal radiographs of patients with EPM1 suggest that this condition is connected to defective cystatin B function. These findings further emphasize the role of cystatin B in bone metabolism in humans.

Dental abnormalities in a mouse model for craniometaphyseal dysplasia

Mice carrying a knock-in mutation (Phe377del) in the Ank gene replicate many skeletal characteristics of human craniometaphyseal dysplasia, including hyperostotic mandibles. Ank (KI/KI) mice have normal morphology of erupted molars and incisors but excessive cementum deposition with increased numbers of Ibsp- and Dmp1-positive cells on root surfaces. The cervical loops of adult Ank (KI/KI) lower incisors are at the level of the third molars, while they are close to the mandibular foramen in Ank (+/+) mice. Furthermore, Ank (KI/KI) incisors show decreased eruption rates, decreased proliferation of odontoblast precursors, and increased cell apoptosis in the stellate reticulum. However, their capability for continuous elongation is not compromised. Quantification of TRAP-positive cells in the apical ends of Ank (KI/KI) incisors revealed decreased osteoclast numbers and osteoclast surfaces. Bisphosphonate injections in Ank (+/+) mice replicate the Ank (KI/KI) incisor phenotype. These results and a comparison with the dental phenotype of Ank loss-of-function mouse models suggest that increased cementum thickness may be caused by decreased extracellular PPi levels and that the incisor phenotype is likely due to hyperostosis of mandibles, which distinguishes Ank (KI/KI) mice from the other Ank mouse models.

Extracellular pyrophosphate in the kidney: how does it get there and what does it do?

Pyrophosphate (PPi) is well known as a regulator of calcification, and the ANKH (ANK in mouse) protein has a role in the membrane transport of PPi. Earlier work concentrated on bones and joints, but ANKH is also likely to have important roles in the kidney, with newer studies focusing on vascular calcification in renal failure. Renal calcification can occur due to a naturally occurring ANK mouse mutation, yet other ANK mutations do not cause a renal phenotype. Despite evidence over 10 years of ANKH's involvement in PPi transport, efflux of PPi via ANKH has never been demonstrated. Rather than physically moving PPi, the ANKH protein may assist its membrane transport in other ways such as by hydrolysis and compartmentalisation. Protein complexes may account for effects of ANKH that are specific to particular tissues. In the kidney, recent localisation data may be helpful in suggesting physiological roles for ANKH, such as its co-localisation with aquaporin-2 and cilial proteins. Such diverse functions would reflect the ubiquitous nature of ANKH in tissues and its profound evolutionary conservation.

Genetics and mechanisms of crystal deposition in calcium pyrophosphate deposition disease

Calcium pyrophosphate deposition (CPPD) disease (common in older adults) can be asymptomatic, associated with osteoarthritis, or can present as acute/chronic inflammatory arthritis. Due to the phenotypic complexity of CPPD, the European League Against Rheumatism (EULAR) recently made recommendations on terminology, diagnosis, and management based on available research evidence and expert consensus. There are no disease-modifying treatments for CPPD disease, and therapy remains nonspecific with the use of anti-inflammatory and analgesic drugs. For years, it has been known that inorganic phosphate and pyrophosphate regulate the formation of CPP or hydroxyapatite crystals. The discovery of ANKH (human homologue of progressive ankylosis) mutations in familial CPPD disease confirmed the importance of phosphate/pyrophosphate homeostasis in CPPD, with ANKH being a regulator of inorganic pyrophosphate transport. Despite progress in our understanding of the function of ANKH, much remains to be investigated. This review summarizes the genetic basis of this disease and focuses on the challenges of research in this area.