A Novel ANO5 Mutation Causing Gnathodiaphyseal Dysplasia With High Bone Turnover Osteosclerosis

Ano5Cys360Tyr mutation leads to bone dysfunction of gnathodiaphyseal dysplasia via disturbing Akt signaling

Background

Gnathodiaphyseal dysplasia (GDD) is a rare autosomal dominant genetic disease characterized by osteosclerosis of the tubular bones and cemento-osseous lesions of the mandibles. Anoctamin 5 (ANO5) is the pathogenic gene, however, the specific molecular mechanism of GDD remains unclear. Herein, a knockin (Ano5 KI/KI ) mouse model expressing the human mutation p.Cys360Tyr was used to investigate the role of Akt signaling in enhanced osteogenesis and decreased osteoclastogenesis in GDD.

Methods

Bone marrow-derived macrophages (BMMs) and mouse calvarial osteoblasts (mCOBs) were isolated from homozygous Ano5 KI/KI mice and treated with SC79, a specific Akt activator. The differentiation and F-actin ring formation of osteoclasts were examined by TRAP and phalloidin staining, respectively. Osteoblast differentiation and mineralization were examined by ALP and alizarin red staining. The expression of bone remodeling-related factors was measured by qRT-PCR.

Results

Akt activation promoted the generation of TRAP-positive multinucleated osteoclasts and the formation of actin rings in Ano5 KI/KI BMMs cultures, accompanied by increased expression of Nfatc1, Trap, Dc-stamp, Mmp9, Ctsk, and Atp6v0d2. Additionally, Ano5 Cys360Tyr mutation down-regulated the Akt phosphorylation level in osteoblast. ALP activity and matrix mineralization capacity in Ano5 KI/KI osteoblast cultures were inhibited after SC79 stimulation, with reduced expression of Runx2, Opn, Col1a1, and Ocn.

Conclusion

Akt activation by SC79 stimulation can obviously rescue abnormal increased osteogenesis and decreased osteoclastogenesis in Ano5 KI/KI mouse model, which demonstrated that disturbed Akt signaling pathway may play a pivotal role in the pathogenesis of GDD, and an Akt activator is probable a therapeutic target for GDD.

Multi-gene panel sequencing in highly consanguineous families and patients with congenital forms of skeletal dysplasias

Skeletal dysplasias (SKDs) are a heterogeneous group of more than 750 genetic disorders characterized by abnormal development, growth, and maintenance of bones or cartilage in the human skeleton. SKDs are often caused by variants in early patterning genes and in many cases part of multiple malformation syndromes and occur in combination with non-skeletal phenotypes. The aim of this study was to investigate the underlying genetic cause of congenital SKDs in highly consanguineous Pakistani families, as well as in sporadic and familial SKD cases from India using multigene panel sequencing analysis. Therefore, we performed panel sequencing of 386 bone-related genes in 7 highly consanguineous families from Pakistan and 27 cases from India affected with SKDs. In the highly consanguineous families, we were able to identify the underlying genetic cause in five out of seven families, resulting in a diagnostic yield of 71%. Whereas, in the sporadic and familial SKD cases, we identified 12 causative variants, corresponding to a diagnostic yield of 44%. The genetic heterogeneity in our cohorts was very high and we were able to detect various types of variants, including missense, nonsense, and frameshift variants, across multiple genes known to cause different types of SKDs. In conclusion, panel sequencing proved to be a highly effective way to decipher the genetic basis of SKDs in highly consanguineous families as well as sporadic and or familial cases from South Asia. Furthermore, our findings expand the allelic spectrum of skeletal dysplasias.

Genetic disruption of Ano5 leads to impaired osteoclastogenesis for gnathodiaphyseal dysplasia

OBJECTIVES

Gnathodiaphyseal dysplasia (GDD; OMIM#166260) is a rare skeletal genetic disorder characterized by sclerosis of tubular bones and cemento-osseous lesions in mandibles. TMEM16E/ANO5 gene mutations have been identified in patients with GDD. Here, Ano5 knockout (Ano5-/-) mice with enhanced osteoblastogenesis were used to investigate whether Ano5 disruption affects osteoclastogenesis.

SUBJECTS AND METHODS

The maturation of osteoclasts, formation of F-actin ring and bone resorption were detected by immunohistochemistry, TRAP, phalloidin staining and Coming Osteo assays. The expression of osteoclast-related factors was measured by qRT-PCR. Early signaling pathways were verified by western blot.

RESULTS

Ano5-/- mice exhibited inhibitory formation of multinucleated osteoclasts with a reduction of TRAP activity. The expression of Nfatc1, c-Fos, Trap, Ctsk, Mmp9, Rank and Dc-stamp was significantly decreased in bone tissues and bone marrow-derived macrophages (BMMs) of Ano5-/- mice. Ano5-/- osteoclasts manifested disrupted actin ring and less mineral resorption. RANKL-induced early signaling pathways were suppressed in Ano5-/- osteoclasts and Ano5 knockdown RAW264.7 cells. Moreover, the inhibitory effects of NF-κB signalling pathway on osteoclastogenesis were partially attenuated with NF-κB signalling activator.

CONCLUSIONS

Ano5 deficiency impairs osteoclastogenesis, which leads to enhanced osteogenic phenotypes mediated by bone homeostasis dysregulation.

Pediatric Gnathic Bony and Mesenchymal Tumors

Evaluation of bone pathology within the head and neck region, particularly the gnathic bonesis is complex, demonstrating unique pathologic processes. In part, this variation is due to odontogenesis and the embryological cells that may be involved, which can contribute to disease development and histologic variability. As with any boney pathosis, the key is to have clinical correlation, particularly with radiographic imaging prior to establishing a definitive diagnosis. This review will cover those entities that have a predilection for the pediatric population, and while it is not all inclusive, it should serve as a foundation for the pathologist who is evaluating bony lesions involving the craniofacial skeleton.

High Bone Mass Disorders: New Insights From Connecting the Clinic and the Bench

Monogenic high bone mass (HBM) disorders are characterized by an increased amount of bone in general, or at specific sites in the skeleton. Here, we describe 59 HBM disorders with 50 known disease-causing genes from the literature, and we provide an overview of the signaling pathways and mechanisms involved in the pathogenesis of these disorders. Based on this, we classify the known HBM genes into HBM (sub)groups according to uniform Gene Ontology (GO) terminology. This classification system may aid in hypothesis generation, for both wet lab experimental design and clinical genetic screening strategies. We discuss how functional genomics can shape discovery of novel HBM genes and/or mechanisms in the future, through implementation of omics assessments in existing and future model systems. Finally, we address strategies to improve gene identification in unsolved HBM cases and highlight the importance for cross-laboratory collaborations encompassing multidisciplinary efforts to transfer knowledge generated at the bench to the clinic. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Integration of metabolomics and transcriptomics provides insights into enhanced osteogenesis in Ano5Cys360Tyr knock-in mouse model

INTRODUCTION

Gnathodiaphyseal dysplasia (GDD; OMIM#166260) is a rare autosomal dominant disorder characterized by diaphyseal sclerosis of tubular bones and cemento-osseous lesions in mandibles. GDD is caused by point mutations in the ANO5 gene. However, the mechanisms underlying GDD have not been disclosed. We previously generated the first knock-in mouse model for GDD expressing a human mutation (p.Cys360Tyr) in ANO5 and homozygous Ano5 knock-in (Ano5^KI/KI ) mice exhibited representative traits of human GDD especially including enhanced osteogenesis.

METHODS

Metabolomics and transcriptomics analyses were conducted for wildtype (Ano5^+/+ ) and Ano5^KI/KI mature mouse calvarial osteoblasts (mCOBs) grown in osteogenic cultures for 14 days to identify differential intracellular metabolites and genes involved in GDD. Subsequently, related differential genes were validated by qRT-PCR. Cell proliferation was confirmed by CCK8 assay and calcium content in mineral nodules was detected using SEM-EDS.

RESULTS

Metabolomics identified 42 differential metabolites that are primarily involved in amino acid and pyrimidine metabolism, and endocrine and other factor-regulated calcium reabsorption. Concomitantly, transcriptomic analysis revealed 407 differentially expressed genes in Ano5^KI/KI osteoblasts compared with wildtype. Gene ontology and pathway analysis indicated that Ano5^Cys360Tyr mutation considerably promoted cell cycle progression and perturbed calcium signaling pathway, which were confirmed by validated experiments. qRT-PCR and CCK-8 assays manifested that proliferation of Ano5^KI/KI mCOBs was enhanced and the expression of cell cycle regulating genes (Mki67, Ccnb1, and Ccna2) was increased. In addition, SEM-EDS demonstrated that Ano5^KI/KI mCOBs developed higher calcium contents in mineral nodules than Ano5^+/+ mCOBs, while some calcium-related genes (Cacna1, Slc8a1, and Cyp27b1) were significantly up-regulated. Furthermore, osteocalcin which has been proved to be an osteoblast-derived metabolic hormone was upregulated in Ano5^KI/KI osteoblast cultures.

DISCUSSION

Our data demonstrated that the Ano5^Cys360Tyr mutation could affect the metabolism of osteoblasts, leading to unwonted calcium homeostasis and cellular proliferation that can contribute to the underlying pathogenesis of GDD disorders.

Ano5 modulates calcium signaling during bone homeostasis in gnathodiaphyseal dysplasia

ANO5 encodes transmembrane protein 16E (TMEM16E), an intracellular calcium-activated chloride channel in the endoplasmic reticulum. Mutations in ANO5 are associated with gnathodiaphyseal dysplasia (GDD), a skeletal disorder causing the jaw deformity and long bone fractures. However, the coordinated mechanism by which ANO5 mediates bone homeostasis in GDD remains poorly defined. Here, we show that ablation of Ano5 reduced intracellular calcium transients, leading to defects in osteogenesis and osteoclastogenesis and thus bone dysplasia. We found a causative de novo ANO5 frameshift insertion mutation (p.L370_A371insDYWRLNSTCL) in a GDD family with osteopenia, accompanied by a decrease in TMEM16E expression and impaired RANKL-induced intracellular calcium ([Ca²⁺]_i) oscillations in osteoclasts. Moreover, using Ano5 knockout (KO) mice, we found that they exhibited low bone volume, abnormal calcium deposits, and defective osteoblast and osteoclast differentiation. We also showed that Ano5 deletion in mice significantly diminished [Ca²⁺]_i oscillations in both osteoblasts and osteoclasts, which resulted in reduced WNT/β-Catenin and RANKL-NFATc1 signaling, respectively. Osteoanabolic treatment of parathyroid hormone was effective in enhancing bone strength in Ano5 KO mice. Consequently, these data demonstrate that Ano5 positively modulates bone homeostasis via calcium signaling in GDD.

Gnathodiaphyseal dysplasia with a novel genetic variant in a large family from Iran

Background

Gnathodiaphyseal dysplasia (GDD) is an ultrarare autosomal dominant bone dysplasia characterized by cementoosseous lesions of the jawbones, bone fragility, frequent bone fractures at the young age, bowing of tubular bones, and diaphyseal sclerosis of long bones associated with generalized osteopenia. GDD is caused by point mutations in anoctamin‐5 (ANO5) on chromosome 11p14.3. For the past few years, next generation sequencing (NGS) technology has facilitated the discovery of causative variants in genetically heterogeneous diseases.

Methods

In this study, exome sequencing (ES) was performed using the DNA sample of the proband. Family histories and clinical information were collected through comprehensive medical examination and genetic counseling.

Results

ES results identified a heterozygous variant, NM_213599.3:c.1078T>C(p.Cys360Arg) in the ANO5 gene. Sanger sequencing was performed to confirm the detected pathogenic variant in DNA samples of the entire family (except deceased individuals), which segregated with the disease within the family. Finally, in silico analysis was applied to test the pathogenicity of the variant using various online software.

Conclusion

In summary, our investigation identified a novel pathogenic variant in the ANO5, responsible for gnathodiaphyseal dysplasia in a large Iranian family. Therefore, based on the present study, this variant can be helpful for diagnosis and effective management of GDD patients.

Gnathodiaphyseal dysplasia (GDD) is an ultrarare autosomal dominant bone dysplasia characterized by cementoosseous lesions of the jawbones, bone fragility, frequent bone fractures at the young age. In this study, Exome Sequencing results identified a heterozygous mutation, NM_213599.3:c.1078T>C(p.Cys360Arg) in the ANO5 confirmed by Sanger sequencing.

Introduction of a Cys360Tyr Mutation in ANO5 Creates a Mouse Model for Gnathodiaphyseal Dysplasia

Gnathodiaphyseal dysplasia (GDD) is a rare autosomal dominant genetic disease characterized by the osteosclerosis of tubular bones and the formation of cemento-osseous lesions in mandibles. Although genetic mutations for GDD have been identified in the ANO5/TMEM16E gene, the cellular and molecular mechanisms behind the pathogenesis of GDD remain unclear. Here, we generated the first knock-in mouse model for GDD with the expression of human mutation p.Cys360Tyr in ANO5. Homozygous Ano5 knock-in mice (Ano5^KI/KI ) replicated GDD-like skeletal features, including massive jawbones, bowing tibia, bone fragility, sclerosis, and cortical thickening of the femoral and tibial diaphysis. Serum alkaline phosphatase (ALP) levels were elevated in Ano5^KI/KI mice as in GDD patients with p.Cys360Tyr mutation. Calvaria-derived Ano5^KI/KI osteoblast cultures showed increased osteoblastogenesis, including hypermineralized bone matrix and enhanced bone formation-related factors expression. Interestingly, Ano5^KI/KI bone marrow-derived macrophage cultures showed decreased osteoclastogenesis, and Ano5^KI/KI osteoclasts exhibited disrupted actin ring formation, which may be associated with some signaling pathways. In conclusion, this new mouse model may facilitate elucidation of the pathogenesis of GDD and shed more light on its treatment. © 2021 American Society for Bone and Mineral Research (ASBMR).

Osteogenesis imperfecta in children

Osteogenesis imperfecta (OI) is a disease characterised by altered bone tissue material properties together with abnormal micro and macro-architecture and thus bone fragility, increased bone turnover and hyperosteocytosis. Increasingly appreciated are the soft tissue changes, sarcopenia in particular. Approaches to treatment are now multidisciplinary, with bisphosphonates having been the primary pharmacological intervention over the last 20 years. Whilst meta-analyses suggest that anti-fracture efficacy across the life course is equivocal, there is good evidence that for children bisphosphonates reduce fracture risk, increase vertebral size and improve vertebral shape, as well as improving motor function and mobility. The genetics of OI continues to provide insights into the molecular pathogenesis of the disease, although the pathophysiology is less clear. The complexity of the multi-scale interactions of bone tissue with cellular function are gradually being disentangled, but the fundamental question of why increased tissue brittleness should be associated with so many other changes is unclear; ER stress, pro-inflammatory cytokines, accelerated senesence and altered matrix component release might all contribute, but a unifying hypothesis remains elusive. New approaches to therapy are focussed on increasing bone mass, following the paradigm established by the treatment of postmenopausal osteoporosis. For adults, this brings the prospect of restoring previously lost bone - for children, particularly at the severe end of the spectrum, the possibility of further reducing fracture frequency and possibly altering growth and long term function are attractive. The alternatives that might affect tissue brittleness are autophagy enhancement (through the removal of abnormal type I collagen aggregates) and stem cell transplantation - both still at the preclinical stage of assessment. Preclinical assessment is not supportive of targeting inflammatory pathways, although understanding why TGFb signalling is increased, and whether that presents a treatment target in OI, remains to be established.

Relevant genetic variants are common in women with pregnancy and lactation-associated osteoporosis (PLO) and predispose to more severe clinical manifestations

Pregnancy and lactation-associated osteoporosis (PLO) is a rare skeletal disorder characterized by early-onset osteoporosis typically manifestating with vertebral compression fractures or transient osteoporosis of the hip. We hypothesized that genetic variants may play a role in the development of PLO. This study aimed to analyze the presence of genetic variants and a potential association with the clinical presentation in PLO. 42 women with PLO were included from 2013 to 2019 in a multicenter study in Germany. All cases underwent comprehensive genetic analysis based on a custom-designed gene panel including genes relevant for skeletal disorders. The skeletal status was assessed using dual-energy X-ray absorptiometry (DXA). Subgroups were further analyzed by serum bone turnover markers (n = 31) and high-resolution peripheral computed tomography (HR-pQCT; n = 23). We detected relevant genetic variants in 21 women (50%), with LRP5, WNT1 and COL1A1/A2 being the most commonly involved genes. The mean number of vertebral compression fractures was 3.3 ± 3.4 per case with a significantly higher occurrence in the subgroup with genetic variants (4.8 ± 3.7 vs. 1.8 ± 2.3, p = 0.02). Among the total cohort, DXA Z-scores were significantly lower at the lumbar spine compared to the femoral neck (p = 0.002). HR-pQCT revealed a pronounced reduction of trabecular and cortical thickness, while trabecular number was within the reference range. Eighteen women (43%) received a bone-specific therapy (primarily teriparatide). Overall, a steep increase in bone mass (+37.7%) was observed after 3 years. In conclusion, pregnancy and lactation represent skeletal risk factors, which may unmask hereditary bone disorders leading to PLO. These cases were affected more severely. Nevertheless, a timely diagnosis and adequate treatment can ensure a substantial recovery potential even without specific therapy. Patients with genetically induced low bone turnover (e.g.; LRP5, WNT1) may especially benefit from osteo-anabolic medication.

Response of the ENPP1-Deficient Skeletal Phenotype to Oral Phosphate Supplementation and/or Enzyme Replacement Therapy: Comparative Studies in Humans and Mice

Inactivating mutations in human ecto-nucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) may result in early-onset osteoporosis (EOOP) in haploinsufficiency and autosomal recessive hypophosphatemic rickets (ARHR2) in homozygous deficiency. ARHR2 patients are frequently treated with phosphate supplementation to ameliorate the rachitic phenotype, but elevating plasma phosphorus concentrations in ARHR2 patients may increase the risk of ectopic calcification without increasing bone mass. To assess the risks and efficacy of conventional ARHR2 therapy, we performed comprehensive evaluations of ARHR2 patients at two academic medical centers and compared their skeletal and renal phenotypes with ENPP1-deficient Enpp1^asj/asj mice on an acceleration diet containing high phosphate treated with recombinant murine Enpp1-Fc. ARHR2 patients treated with conventional therapy demonstrated improvements in rickets, but all adults and one adolescent analyzed continued to exhibit low bone mineral density (BMD). In addition, conventional therapy was associated with the development of medullary nephrocalcinosis in half of the treated patients. Similar to Enpp1^asj/asj mice on normal chow and to patients with mono- and biallelic ENPP1 mutations, 5-week-old Enpp1^asj/asj mice on the high-phosphate diet exhibited lower trabecular bone mass, reduced cortical bone mass, and greater bone fragility. Treating the Enpp1^asj/asj mice with recombinant Enpp1-Fc protein between weeks 2 and 5 normalized trabecular bone mass, normalized or improved bone biomechanical properties, and prevented the development of nephrocalcinosis and renal failure. The data suggest that conventional ARHR2 therapy does not address low BMD inherent in ENPP1 deficiency, and that ENPP1 enzyme replacement may be effective for correcting low bone mass in ARHR2 patients without increasing the risk of nephrocalcinosis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Surgical Treatment of Pathological Tibial Shaft Fracture in Adult Patient With Gnathodiaphyseal Dysplasia: A Case Report

CASE

Gnathodiaphyseal dysplasia is a generalized skeletal syndrome characterized by frequent bone fractures in childhood, sclerosis and bowing of tubular bones, and cemento-osseous lesions of the jawbones. We present the case of a 53-year-old man with gnathodiaphyseal dysplasia who presented with pathological fracture of the tibial shaft. Internal fixation with the Ender nail was successfully performed with 2 years of follow-up.

CONCLUSION

This is the first report describing the treatment of fracture in an adult patient with gnathodiaphyseal dysplasia. Internal fixation with the Ender nail was effective for the tubular bone with deformity. Callus formation was observed without delay.

Mice Carrying a Ubiquitous R235W Mutation of Wnt1 Display a Bone-Specific Phenotype

Since a key function of Wnt1 in brain development was established early on through the generation of non-viable Wnt1-deficient mice, it was initially surprising that WNT1 mutations were found to cause either early-onset osteoporosis (EOOP) or osteogenesis imperfecta type XV (OI-XV). The deduced function of Wnt1 as an osteoanabolic factor has been confirmed in various mouse models with bone-specific inactivation or overexpression, but mice carrying disease-causing Wnt1 mutations have not yet been described. Triggered by the clinical analysis of EOOP patients carrying a heterozygous WNT1 mutation (p.R235W), we introduced this mutation into the murine Wnt1 gene to address the question of whether this would cause a skeletal phenotype. We observed that Wnt1^+/R235W and Wnt1^R235W/R235W mice were born at the expected Mendelian ratio and that they did not display postnatal lethality or obvious nonskeletal phenotypes. At 12 weeks of age, the homozygous presence of the Wnt1 mutation was associated with reduced trabecular and cortical bone mass, explained by a lower bone formation rate compared with wild-type littermates. At 52 weeks of age, we also observed a moderate bone mass reduction in heterozygous Wnt1^+/R235W mice, thereby underscoring their value as a model of WNT1-dependent EOOP. Importantly, when we treated wild-type and Wnt1^+/R235W mice by daily injection of parathyroid hormone (PTH), we detected the same osteoanabolic influence in both groups, together with an increased cortical thickness in the mutant mice. Our data demonstrate the pathogenicity of the WNT1-R235W mutation, confirm that controlling skeletal integrity is the primary physiological function of Wnt1, and suggest that osteoanabolic treatment with teriparatide should be applicable for individuals with WNT1-dependent EOOP. © 2020 American Society for Bone and Mineral Research.

Adult Osteosclerotic Metaphyseal Dysplasia With Progressive Osteonecrosis of the Jaws and Abnormal Bone Resorption Pattern Due to a LRRK1 Splice Site Mutation

Osteosclerotic metaphyseal dysplasia (OSMD) is a rare autosomal recessive sclerosing skeletal dysplasia. We report on a 34-year-old patient with sandwich vertebrae, platyspondyly, osteosclerosis of the tubular bones, pathologic fractures, and anemia. In the third decade, he developed osteonecrosis of the jaws, which was progressive in spite of repeated surgical treatment over a period of 11 years. An iliac crest bone biopsy revealed the presence of hypermineralized cartilage remnants, large multinucleated osteoclasts with abnormal morphology, and inadequate bone resorption typical for osteoclast-rich osteopetrosis. After exclusion of mutations in TCIRG1 and CLCN7 we performed trio-based exome sequencing. The novel homozygous splice-site mutation c.261G>A in the gene LRRK1 was found and co-segregated with the phenotype in the family. cDNA sequencing showed nearly complete skipping of exon 3 leading to a frameshift (p.Ala34Profs*33). Osteoclasts differentiated from the patient's peripheral blood monocytes were extremely large. Instead of resorption pits these cells were only capable of superficial erosion. Phosphorylation of L-plastin at position Ser5 was strongly reduced in patient-derived osteoclasts showing a loss of function of the mutated LRRK1 kinase protein. Our analysis indicates a strong overlap of LRRK1-related OSMD with other forms of intermediate osteopetrosis, but an exceptional abnormality of osteoclast resorption. Like in other osteoclast pathologies an increased risk for progressive osteonecrosis of the jaws should be considered in OSMD, an intermediate form of osteopetrosis. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Large osteocyte lacunae in iliac crest infantile bone are not associated with impaired mineral distribution or signs of osteocytic osteolysis

The enlargement of osteocyte lacunae via osteocytic osteolysis was previously detected in situations of increased calcium demand (e.g., lactation, vitamin D deficiency). However, it is unclear whether similar processes occur also in the growing infantile skeleton and how this is linked to the mineral distribution within the bone matrix. Human iliac crest biopsies of 30 subjects (0-6 months, n = 14; 2-8 years, n = 6 and 18-25 years, n = 10) were acquired. Bone microarchitecture was assessed by micro-CT, while cellular bone histomorphometry was performed on undecalcified histological sections. Quantitative backscattered electron imaging (qBEI) was conducted to determine the bone mineral density distribution (BMDD) as well as osteocyte lacunar size and density. We additionally evaluated cathepsin K positive osteocytes using immunohistochemistry. Infantile bone was characterized by various signs of ongoing bone development such as higher bone (re)modeling, lower cortical and trabecular thickness compared to young adults. Importantly, a significantly higher osteocyte lacunar density and increased lacunar area were detected. Large osteocyte lacunae were associated with a more heterogeneous bone mineral density distribution of the trabecular bone matrix due to the presence of hypermineralized cartilage remnants, whereas the mean mineralization (i.e., CaMean) was not different in infantile bone. Absence of cathepsin K expression in osteocyte lacunae indicated nonexistent osteocytic osteolysis. Taken together, we demonstrated that the overall mineralization distribution in infantile bone is not altered compared to young adults besides high trabecular mineralization heterogeneity. Our study also provides important reference values for bone microstructure, BMDD and osteocyte characteristics in infants, children and young adults. Infantile bone displays large osteocyte lacunae indicating a developmental phenomenon rather than osteocytic osteolysis. Larger osteocytes may have superior mechanosensory abilities to enable bone adaption during growth.

Gnathodiaphyseal dysplasia is not recapitulated in a respective mouse model carrying a mutation of the Ano5 gene

Mutations in the gene ANO5, encoding for the transmembrane protein Anoctamin 5 (Ano5), have been identified to cause gnathodiaphyseal dysplasia (GDD) in humans, a skeletal disorder characterized by sclerosis of tubular bones, increased fracture risk and fibro-osseous lesions of the jawbones. To better understand the pathomechanism of GDD we have generated via Crispr/CAS9 gene editing a mouse model harboring the murine equivalent (Ano5 p.T491F) of a GDD-causing ANO5 mutation identified in a previously reported patient. Skeletal phenotyping by contact radiography, μCT and undecalcified histomorphometry was performed in male mice, heterozygous and homozygous for the mutation, at the ages of 12 and 24 weeks. These mice did not display alterations of skeletal microarchitecture or mandible morphology. The results were confirmed in female mice and animals derived from a second, independent clone. Finally, no skeletal phenotype was observed in mice lacking ~40% of their Ano5 gene due to a frameshift mutation. Therefore, our results indicate that Ano5 is dispensable for bone homeostasis in mice, at least under unchallenged conditions, and that these animals may not present the most adequate model to study the physiological role of Anoctamin 5.

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We present the first mouse model with an Ano5 mutation causing GDD in humans.

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The Ano5 p.T491F mutation does not influence skeletal structure in mice.

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There are no indications of effects on the mandible or extra-skeletal organs.

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The results were consistent in both genders and independent clones.

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Ano5 is dispensable for bone homeostasis in mice under unchallenged conditions.

TMEM16E/ANO5 mutations related to bone dysplasia or muscular dystrophy cause opposite effects on lipid scrambling

Mutations in the human TMEM16E/ANO5 gene are causative for gnathodiaphyseal dysplasia (GDD), a rare bone malformation and fragility disorder, and for two types of muscular dystrophy (MD). Previous studies have demonstrated that TMEM16E/ANO5 is a Ca²⁺ -activated phospholipid scramblase and that the mutation c.1538C>T (p.Thr513Ile) causing GDD leads to a gain-of-function phenotype. Here, using established HEK293-based functional assays, we investigated the effects of MD-related and further GDD-related amino acid exchanges on TMEM16E/ANO5 function in the same expression system. These experiments also revealed that the gradual changes in HEK293 cell morphology observed upon expression of TMEM16E/ANO5^GDD mutants are a consequence of aberrant protein activity. Our results collectively demonstrate that, on the level of protein function, MD mutations are associated to loss-of-function and GDD mutations to gain-of-function phenotypes, confirming conjectures made on the basis of inheritance modes.

Skeletal deterioration in COL2A1-related spondyloepiphyseal dysplasia occurs prior to osteoarthritis

OBJECTIVE

Spondyloepiphyseal dysplasia, a combination of progressive arthropathy with variable signs of skeletal dysplasia, can be a result of mutations in the collagen, type II, alpha 1 (COL2A1) gene. However, the bone involvement (e.g., density, microstructure) in this disorder has hitherto not been studied.

DESIGN

A 50-year-old female patient and her 8-year-old son with flattening of vertebral bodies and early-onset osteoarthritis were genetically tested using a custom designed gene bone panel including 386 genes. Bone microstructure and turnover were assessed using high-resolution peripheral quantitative computed tomography (HR-pQCT) and serum bone turnover markers, respectively. Furthermore, the bone and cartilage phenotype of male mice heterozygous for the loss-of-function mutation of Col2a1 (Col2a1^+/d) was analyzed compared to wildtype littermates using μ-CT and histomorphometry.

RESULTS

We identified a dominant COL2A1 mutation (c.620G > A p.(Gly207Glu)) indicating spondyloepiphyseal dysplasia in the female patient and her son, both being severely affected by skeletal deterioration. Although there was no osteoarthritis detectable at first visit, the son was affected by trabecular osteopenia, which progressed over time. In an iliac crest biopsy obtained from the mother, osteoclast indices were remarkably increased. Col2a1^+/d mice developed a moderate skeletal phenotype expressed by reduced cortical and trabecular parameters at 4 weeks. Importantly, no articular defects could be observed in the knee joints at 4 weeks, while osteoarthritis was only detectable in 12-week-old mice.

CONCLUSIONS

Our results indicate that collagen type II deficiency in spondyloepiphyseal dysplasia leads to skeletal deterioration with early-onset in humans and mice that occurs prior to the development of osteoarthritis.

Juvenile Paget's disease with compound heterozygous mutations in TNFRSF11B presenting with recurrent clavicular fractures and a mild skeletal phenotype

Juvenile Paget's disease (JPD) is a rare recessively-inherited bone dysplasia. The great majority of cases described to date have had homozygous mutations in TNFRSF11B, the gene encoding osteoprotegerin. We describe a boy who presented with recurrent clavicular fractures following minor trauma (8 fractures from age 2 to 11). He was of normal height and despite mild lateral bowing of the thighs and anterior bowing of the shins he remained physically active. Abnormal modelling was noted in ribs and humeri on clavicular radiographs, and a skeletal survey at the age of 7 showed generalised diaphyseal expansion of the long bones with thickening of the periosteal and endosteal surfaces of the cortices. On biochemical evaluation, serum alkaline phosphatase was noted to be persistently elevated. The diagnosis of JPD was confirmed by the finding of compound heterozygous mutations in TNFRSF11B: a maternally-inherited A>G missense mutation at position 1 of the first amino acid codon (previously reported) and a paternally-inherited splice acceptor site mutation in intron 3 at a highly conserved position (not previously reported). Bioinformatics analysis suggested both mutations were disease-causing. Compound heterozygote mutations in TNFRSF11B causing JPD have been previously reported only once - in a boy who also had a relatively mild skeletal phenotype. The milder features may lead to delay in diagnosis and diagnostic confusion with other entities, but the extraskeletal features of JPD may nonetheless develop.

Mice lacking plastin-3 display a specific defect of cortical bone acquisition

Although inactivating mutations of PLS3, encoding the actin-bundling protein plastin-3, have been identified to cause X-linked osteoporosis, the cellular and molecular influence of PLS3 on bone remodeling is poorly defined. Moreover, although a previous study has demonstrated moderate osteopenia in 12 week-old Pls3-deficient mice based on μCT scanning, there is no reported analysis of such a model on the basis of undecalcified histology and bone-specific histomorphometry. To fill this knowledge gap we applied a deep phenotyping approach and studied Pls3-deficient mice at different ages. Surprisingly, we did not detect significant differences between wildtype and Pls3-deficient littermates with respect to trabecular bone mass, and the same was the case for all histomorphometric parameters determined at 12 weeks of age. Remarkably however, the cortical thickness in both, tibia and femur, was significantly reduced in Pls3-deficient mice in all age groups. We additionally studied the ex vivo behavior of Pls3-deficient primary osteoblasts, which displayed moderately impaired mineralization capacity. Of note, while most osteoblastogenesis markers were not differentially expressed between wildtype and Pls3-deficient cultures, the expression of Sfrp4 was significantly reduced in the latter, a potentially relevant finding, since Sfrp4 inactivation, in mice and humans, specifically causes cortical thinning. We finally addressed the question, if Pls3-deficiency would impair the osteoanabolic influence of parathyroid hormone (PTH). For this purpose we applied daily injection of PTH into wildtype and Pls3-deficient mice and found a similar response regardless of the genotype. Taken together, our data reveal that Pls3-deficiency in mice only recapitulates the cortical bone phenotype of individuals with X-linked osteoporosis by negatively affecting the early stage of cortical bone acquisition.

Genetic Disruption of Anoctamin 5 in Mice Replicates Human Gnathodiaphyseal Dysplasia (GDD)

Gnathodiaphyseal dysplasia (GDD; OMIM#166260) is a rare skeletal disorder which is mainly characterized by cemento-osseous lesions in mandibles, bone fragility, bowing and diaphyseal sclerosis of tubular bones. GDD is caused by point mutations in Anoctamin-5 (ANO5); however, the disease mechanisms remain unclear. Here we generated Ano5-knockout (KO) mice using a CRISPR/Cas 9 approach to study loss of function aspects of GDD mutations. Homozygous Ano5 knockout mice (Ano5^-/-) replicate some typical traits of human GDD including massive jawbones, bowing tibia, sclerosis and cortical thickening of femoral and tibial diaphyses. Serum alkaline phosphatase (ALP) levels were elevated in Ano5^-/- mice as in GDD patients. Calvaria-derived Ano5^-/- osteoblast cultures show increased osteoblastogenesis, which is consistent with our previous in vitro observations. Bone matrix is hypermineralized, and the expression of bone formation-related factors is enhanced in Ano5^-/- mice, suggesting that the osteogenic anomaly arises from a genetic disruption of Ano5. We believe this new mouse model will shed more light on the development of skeletal abnormalities in GDD on a cellular and molecular level.

Identification of a novel ANO5 missense mutation in a Chinese family with familial florid osseous dysplasia

Familial florid osseous dysplasia (FFOD) is an autosomal dominant disorder of connective tissue, characterized by lobulated cementum-like masses scattered throughout the jaws and the alveolar process. This study aimed to identify the genetic etiology of a three-generation Chinese family affected with FFOD. A novel missense mutation p.C356W in anoctamin 5 (ANO5) gene was successfully identified as the pathogenic mutation by whole-exome sequencing (WES). The p.C356W mutation is located in the first loop between the first and second transmembrane domain of ANO5 protein. Sequence alignment of ANO5 protein among many different species revealed that this position is highly conserved. The p.C356W mutation may damage the predicted protein stability of ANO5 by altering the structure of several extracellular loops of ANO5 and affecting the formation of the disulfide bond, thereby disrupting the correct folding of ANO5 protein. Thus, the amino acid at position 356 appears to play a key role in the protein structural stability and function of ANO5 protein. Our results may also provide new insights into the cause and diagnosis of FFOD and may have implications for genetic counseling and clinical management.

Role of anoctamin 5, a gene associated with gnathodiaphyseal dysplasia, in osteoblast and osteoclast differentiation

Anoctamin 5 (Ano5) mutations are responsible for gnathodiaphyseal dysplasia, a rare skeletal syndrome. Despite the close linkage of Ano5 to bone remodeling, the molecular mechanisms underlying the role of Ano5 in bone remodeling remain unknown. In this study, we investigated whether Ano5 regulates osteoblast or osteoclast differentiation to maintain normal bone remodeling. Downregulation of Ano5 expression did not affect osteoblast differentiation and mineralization, while ectopic expression of Ano5 significantly enhanced receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclast differentiation. Furthermore, Ano5-mediated Akt phosphorylation resulted in nuclear factor of activated T-cells c1 (NFATc1) activation, indicating that Ano5 regulates osteoclast differentiation through activation of the Akt-NFATc1 signaling pathway. Thus, our results suggest a possibility that Ano5 is involved in bone remodeling through regulating the function of osteoclasts rather than that of osteoblasts.

Novel ANO5 mutation c.1067G>T (p.C356F) identified by whole genome sequencing in a big family with atypical gnathodiaphyseal dysplasia

BACKGROUND

Gnathodiaphyseal dysplasia (GDD) is a rare skeletal disorder that has not been well studied.

METHODS

Sanger sequencing, whole-genome sequencing (WGS), and bioinformatics and structural modeling analyses were performed.

RESULTS

A family with patients with fibro-osseous lesions of the jawbones were initially diagnosed with cherubism. Sequencing of SH3BP2, which is the causal gene of cherubism, revealed no pathogenic mutation. Through WGS, we identified a novel mutation c.1067G>T (p.C356F) in ANO5, and bioinformatics analyses and structural modeling showed that the mutation was deleterious. Because ANO5 is the gene responsible for GDD, we reappraised the clinical data of the patients, and the diagnosis was corrected to atypical GDD. A review of the literature showed that 67% of GDD cases confirmed by molecular testing were initially misdiagnosed.

CONCLUSIONS

The novel mutation c.1067G>T (p.C356F) in ANO5 is responsible for the atypical GDD observed in our patients. GDD should be included in the differential diagnosis for patients with fibro-osseous lesions.

Comparison of Bone Microarchitecture Between Adult Osteogenesis Imperfecta and Early-Onset Osteoporosis

Diagnosis and management of adult individuals with low bone mass and increased bone fragility before the age of 50 can be challenging. A number of these patients are diagnosed with mild osteogenesis imperfecta (OI) through detection of COL1A1 or COL1A2 mutations; however, a clinical differentiation from early-onset osteoporosis (EOOP) may be difficult. The purpose of this study was to determine the bone microstructural differences between mild OI and EOOP patients. 29 patients showed mutations in COL1A1 or COL1A2 and were classified as OI. Skeletal assessment included dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and bone turnover serum analyses. Bone microstructure of 21/29 OI patients was assessed and compared to 23 age- and sex-matched patients clinically classified EOOP but without mutations in the known disease genes as well as to 20 healthy controls. In the OI patients, we did not observe an age-dependent decrease in DXA Z-scores. HR-pQCT revealed a significant reduction in volumetric BMD and microstructural parameters in the distal radius and tibia in both the OI and EOOP cohorts compared to the healthy controls. When comparing the bone microstructure of OI patients with the EOOP cohort, significant differences were found in terms of bone geometry in the radius, while no significant changes were detected in all other HR-pQCT parameters at the radius and tibia. Taken together, adult mild OI patients demonstrate a predominantly high bone turnover trabecular bone loss syndrome that shows minor microstructural differences compared to EOOP without mutation detection.

Mutational analysis uncovers monogenic bone disorders in women with pregnancy-associated osteoporosis: three novel mutations in LRP5, COL1A1, and COL1A2

Pregnancy was found to be a skeletal risk factor promoting the initial onset of previously unrecognized monogenic bone disorders, thus explaining a proportion of cases with pregnancy-associated osteoporosis. Therapeutic measures should focus in particular on the normalization of the disturbed calcium homeostasis in order to enable the partial skeletal recovery.

INTRODUCTION

Pregnancy-associated osteoporosis (PAO) is a rare skeletal condition, which is characterized by a reduction in bone mineral density (BMD) in the course of pregnancy and lactation. Typical symptoms include vertebral compression fractures and transient osteoporosis of the hip. Since the etiology is not well understood, this prospective study was conducted in order to elucidate the relevance of pathogenic gene variants for the development of PAO.

METHODS

Seven consecutive cases with the diagnosis of PAO underwent a skeletal assessment (blood tests, DXA, HR-pQCT) and a comprehensive genetic analysis using a custom-designed gene panel.

RESULTS

All cases showed a reduced BMD (DXA T-score, lumbar spine - 3.2 ± 1.0; left femur - 2.2 ± 0.5; right femur - 1.9 ± 0.5), while the spine was affected more severely (p < 0.05). The trabecular and cortical thickness was overall reduced in HR-pQCT, while the trabecular number showed no alterations in most cases. The genetic analysis revealed three novel mutations in LRP5, COL1A1, and COL1A2.

CONCLUSION

Our data show that previously unrecognized monogenic bone disorders play an important role in PAO. Pregnancy should be considered a skeletal risk factor, which can promote the initial clinical onset of such skeletal disorders. The underlying increased calcium demand is essential in terms of prophylactic and therapeutic measures, which are especially required in individuals with a genetically determined low bone mass. The implementation of this knowledge in clinical practice can enable the partial recovery of the skeleton. Consistent genetic studies are needed to analyze the frequency of pathogenic variants in women with PAO.

Gain of function of TMEM16E/ANO5 scrambling activity caused by a mutation associated with gnathodiaphyseal dysplasia

Mutations in the human TMEM16E (ANO5) gene are associated both with the bone disease gnathodiaphyseal dysplasia (GDD; OMIM: 166260) and muscle dystrophies (OMIM: 611307, 613319). However, the physiological function of TMEM16E has remained unclear. We show here that human TMEM16E, when overexpressed in mammalian cell lines, displayed partial plasma membrane localization and gave rise to phospholipid scrambling (PLS) as well as non-selective ionic currents with slow time-dependent activation at highly depolarized membrane potentials. While the activity of wild-type TMEM16E depended on elevated cytosolic Ca2+ levels, a mutant form carrying the GDD-causing T513I substitution showed PLS and large time-dependent ion currents even at low cytosolic Ca2+ concentrations. Contrarily, mutation of the homologous position in the Ca2+-activated Cl- channel TMEM16B paralog hardly affected its function. In summary, these data provide the first direct demonstration of Ca2+-dependent PLS activity for TMEM16E and suggest a gain-of-function phenotype related to a GDD mutation.

Gnathodiaphyseal dysplasia: Severe atypical presentation with novel heterozygous mutation of the anoctamin gene (ANO5)

Gnathodiaphyseal dysplasia (GDD; OMIM #166260) is an ultra-rare autosomal dominant disorder caused by heterozygous mutation in the anoctamin 5 (ANO5) gene and features fibro-osseous lesions of the jawbones, bone fragility with recurrent fractures, and bowing/sclerosis of tubular bones. The physiologic role of ANO5 is unknown. We report a 5-year-old boy with a seemingly atypical and especially severe presentation of GDD and unique ANO5 mutation. Severe osteopenia was associated with prenatal femoral fractures, recurrent postnatal fractures, and progressive bilateral enlargement of his maxilla and mandible beginning at ~2months-of-age that interfered with feeding and speech and required four debulking operations. Histopathological analysis revealed benign fibro-osseous lesions resembling cemento-ossifying fibromas of the jaw without psammomatoid bodies. A novel, de novo, heterozygous, missense mutation was identified in exon 15 of ANO5 (c.1553G>A; p.Gly518Glu). Our findings broaden the phenotypic and molecular spectra of GDD. Fractures early in life with progressive facial swelling are key features. We assessed his response to a total of 7 pamidronate infusions commencing at age 15months. Additional reports must further elucidate the phenotype, explore any genotype-phenotype correlation, and evaluate treatments.

Early bone tissue aging in human auditory ossicles is accompanied by excessive hypermineralization, osteocyte death and micropetrosis

Within the mineralized bone, osteocytes form a multifunctional mechanosensitive network orchestrating bone remodelling. A preserved osteocyte population is a crucial determinant of bone quality. In human auditory ossicles, the early decrease in osteocyte numbers but maintained integrity remains an unexplained phenomenon that might serve for sound transmission from air to the labyrinth. Here we analysed the frequency, size and composition of osteocyte lacunae in the auditory ossicles of 22 individuals from early postnatal period to old age. Mineralization of the bone matrix was determined using backscattered electron imaging. No signs of bone remodelling were observed above the age of 1 year. We detected characteristics of early bone tissue aging, such as decrease in osteocytes, lower total lacunar density and lacunar area, as well as high matrix mineralization accompanied by distinct accumulation of micropetrotic lacunae and decreased indentation depths. The majority of these changes took place in the first months and years of life, while afterwards only minor reorganization was present. With osteocyte apoptosis potentially being a consequence of low mechanical stimuli, the early loss of osteocytes without initiation of bone remodelling indicates an adaptive response conserving the architecture of the auditory ossicles and ensuring stable sound transmission throughout life.

A novel COL1A2 C-propeptide cleavage site mutation causing high bone mass osteogenesis imperfecta with a regional distribution pattern

Osteogenesis imperfecta (OI) is typically characterized by low bone mass and increased bone fragility caused by heterozygous mutations in the type I procollagen genes (COL1A1/COL1A2). We report two cases of a 56-year-old woman and her 80-year-old mother who suffered from multiple vertebral and non-vertebral fractures with onset in early childhood. A full osteologic assessment including dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses pointed to a high bone mineral density (BMD) in the hip (DXA Z-score + 3.7 and + 3.9) but low to normal bone mass in the spine and preserved bone microstructure in the distal tibia. Serum markers of bone formation and bone resorption were elevated. Using whole exome sequencing, we identified a novel mutation in the COL1A2 gene causing a p. (Asp1120Gly) substitution at the protein level and affecting the type I procollagen C-propeptide cleavage site. In line with previously reported cases, our data independently prove the existence of an unusual phenotype of high bone mass OI caused by a mutation in the procollagen C-propeptide cleavage with a clinically persistent phenotype through adulthood.

Severe bone loss and multiple fractures in SCN8A-related epileptic encephalopathy

Mutations in the SCN8A gene encoding the neuronal voltage-gated sodium channel Nav1.6 are known to be associated with epileptic encephalopathy type 13. We identified a novel de novo SCN8A mutation (p.Phe360Ala, c.1078_1079delTTinsGC, Exon 9) in a 6-year-old girl with epileptic encephalopathy accompanied by severe juvenile osteoporosis and multiple skeletal fractures, similar to three previous case reports. Skeletal assessment using dual energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT) and serum analyses revealed a combined trabecular and cortical bone loss syndrome with elevated bone resorption. Likewise, when we analyzed the skeletal phenotype of 2week-old Scn8a-deficient mice we observed reduced trabecular and cortical bone mass, as well as increased osteoclast indices by histomorphometric quantification. Based on this cumulative evidence the patient was treated with neridronate (2mg/kg body weight administered every 3months), which fully prevented additional skeletal fractures for the next 25months. Taken together, our data provide evidence for a negative impact of SCN8A mutations on bone mass, which can be positively influenced by anti-resorptive treatment.

Molecular, biophysical, and pharmacological properties of calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) are a family of anionic transmembrane ion channels. They are mainly responsible for the movement of Cl^- and other anions across the biological membranes, and they are widely expressed in different tissues. Since the Cl^- flow into or out of the cell plays a crucial role in hyperpolarizing or depolarizing the cells, respectively, the impact of intracellular Ca²⁺ concentration on these channels is attracting a lot of attentions. After summarizing the molecular, biophysical, and pharmacological properties of CaCCs, the role of CaCCs in normal cellular functions will be discussed, and I will emphasize how dysregulation of CaCCs in pathological conditions can account for different diseases. A better understanding of CaCCs and a pivotal regulatory role of Ca²⁺ can shed more light on the therapeutic strategies for different neurological disorders that arise from chloride dysregulation, such as asthma, cystic fibrosis, and neuropathic pain.

Three novel ANO5 missense mutations in Caucasian and Chinese families and sporadic cases with gnathodiaphyseal dysplasia

Gnathodiaphyseal dysplasia (GDD; MIM#166260) is an autosomal dominant syndrome with characteristic cemento-osseous lesions of jawbones, bone fragility, and diaphyseal sclerosis of tubular bones. To date, only five mutations in the proposed calcium-activated chloride channel ANO5/TMEM16E gene have been identified. In this study, we describe two families and two singular patients with three new mutations. One Caucasian family with seven affected members exhibited frequent bone fractures and florid osseous dysplasia (p.Cys356Tyr), while one Chinese family with two affected members suffered from cementoma and purulent osteomyelitis (p.Cys360Tyr). In addition, two different novel mutations (p.Gly518Glu and p.Arg215Gly) were identified in sporadic patients without family history. In vitro studies overexpressing GDD mutations (p.Cys356Tyr and p.Cys360Tyr) showed significantly reduced ANO5 protein. It appears that all GDD mutations known so far locate in an extracellular domain following the first transmembrane domain or in the 4^th putative transmembrane domain. Both wild-type and mutant ANO5 protein localize to the endoplasmic reticulum. After Ano5 gene knock-down with shRNA in MC3T3-E1 osteoblast precursors we saw elevated expression of osteoblast-related genes such as Col1a1, osteocalcin, osterix and Runx2 as well as increased mineral nodule formation in differentiating cells. Our data suggest that ANO5 plays a role in osteoblast differentiation.