Effect of sclerostin antibody treatment in a mouse model of severe osteogenesis imperfecta

Τhe story of sclerostin inhibition: the past, the present, and the future

Sclerostin inhibits osteoblast activity by hampering activation of the canonical Wnt signaling pathway and simultaneously stimulates osteoclastogenesis through upregulation of the receptor activator of NFκB ligand (RANKL). Thus, antibodies against sclerostin (Scl-Abs), besides promoting bone formation, suppress bone resorption and dissociate bone formation from resorption. This dual action results in remarkable increases of bone mineral density which are of a greater magnitude compared to the other antiosteoporotic treatments and are accompanied by decreases of fracture risk at all skeletal sites. The anabolic effect subsides after the first few months of treatment and a predominantly antiresorptive effect remains after this period, limiting its use to 12 months. Furthermore, these effects are largely reversible upon discontinuation; therefore, subsequent treatment with antiresorptives is indicated to maintain or further increase the bone gains achieved. Romosozumab is currently the only Scl-Ab approved for the treatment of severe postmenopausal osteoporosis. Indications for use in other populations, such as males, premenopausal women, and patients with glucocorticoid-induced osteoporosis, are pending. Additionally, the efficacy of Scl-Abs in other bone diseases, such as osteogenesis imperfecta, hypophosphatasia, X-linked hypophosphatemia, and bone loss associated with malignancies, is under thorough investigation. Cardiovascular safety concerns currently exclude patients at high cardiovascular risk from this treatment.

Bone Quality and Mineralization and Effects of Treatment in Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a rare congenital bone dysplasia characterized by high fracture rates and broad variations in clinical manifestations ranging from mild to increasingly severe and perinatal lethal forms. The underlying mutations affect either the synthesis or processing of the type I procollagen molecule itself or proteins that are involved in the formation and mineralization of the collagen matrix. Consequently, the collagen forming cells, the osteoblasts, become broadly dysfunctional in OI. Strikingly, hypermineralized bone matrix seems to be a frequent feature in OI, despite the variability in clinical severity and mutations in the so far studied different forms of human OI. While the causes of the increased mineral content of the bone matrix are not fully understood yet, there is evidence that the descendants of the osteoblasts, the osteocytes, which play a critical role not only in bone remodeling, but also in mineralization and sensing of mechanical loads, are also highly dysregulated and might be of major importance in the pathogenesis of OI. In this review article, we firstly summarize findings of cellular abnormalities in osteoblasts and osteocytes, alterations of the organic matrix, as well as of the microstructural organization of bone. Secondly, we focus on the hypermineralization of the bone matrix in OI as observed in several different forms of human OI as well as in animal models, its measurement and potential mechanical implications and its effect on the bone mineral density measured by dual X-ray absorptiometry. Thirdly, we give an overview of established medication treatments of OI and new approaches with a focus of their known or possible effects on the bone material, particularly on bone matrix mineralization.

Osteoclast indices in osteogenesis imperfecta: systematic review and meta-analysis

Osteogenesis imperfecta (OI) is a rare bone fragility disorder caused by mutations in genes encoding collagen type I or that affect its processing. Alterations in osteoclasts were suggested to contribute to OI pathophysiology. We aimed to systematically identify studies reporting measures of osteoclast formation and function in patients and mouse models of OI, to quantify OI-induced changes. The systematic search of Medline, Ovid, and Web of Science identified 798 unique studies. After screening, we included 23 studies for meta-analysis, reporting osteoclast parameters in 310 patients with OI of 9 different types and 16 studies reporting osteoclast parameters in 406 animals of 11 different OI mouse models. The standardized mean difference with 95% confidence interval (CI) was used as the effect size, and random-effects meta-analysis was performed. In patients with OI, collagen degradation markers were significantly higher compared with age-matched controls, with an effect size of 1.23 (CI: 0.36, 2.10]. Collagen degradation markers were the most elevated in the 3- to 7-year-old age group and in patients with more severe forms of OI. Bone histomorphometry demonstrated the trends for higher osteoclast numbers (1.16; CI: -0.22, 2.55) and osteoclast surface (0.43; CI: -0.63, 1.49), and significantly higher eroded surface (3.24; CI: 0.51, 5.96) compared with age-matched controls. In OI mice, meta-analysis demonstrated significant increases in collagen degradation markers (1.59; CI: 1.07, 2.11), in osteoclast numbers (0.94; CI: 0.50, 1.39), osteoclast surface (0.73; CI: 0.22, 1.23), and eroded surface (1.31; CI: 0.54, 2.08). The largest differences were in OI mice with the mutations in Col1a1 and Col1a2 genes. There were no differences between males and females in clinical or animal studies. Quantitative estimates of changes in osteoclast indices and their variance for patients with OI are important for planning future studies. We confirmed that similar changes are observed in mice with OI, supporting their translational utility.

Bone matrix properties in adults with osteogenesis imperfecta are not adversely affected by setrusumab-a sclerostin neutralizing antibody

Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by low bone mass and frequent fractures. Children with OI are commonly treated with bisphosphonates to reduce fracture rate, but treatment options for adults are limited. In the Phase 2b ASTEROID trial, setrusumab (a sclerostin neutralizing antibody, SclAb) improved bone density and strength in adults with type I, III, and IV OI. Here, we investigate bone matrix material properties in tetracycline-labeled trans iliac biopsies from 3 groups: (1) control: individuals with no metabolic bone disease, (2) OI: individuals with OI, (3) SclAb-OI: individuals with OI after 6 mo of setrusumab treatment (as part of the ASTEROID trial). In addition to bone histomorphometry, bone mineral and matrix properties were evaluated with nanoindentation, Raman spectroscopy, second harmonic generation imaging, quantitative backscatter electron imaging, and small-angle X-ray scattering. Spatial locations of fluorochrome labels were identified to differentiate inter-label bone of the same tissue age and intra-cortical bone. No difference in collagen orientation was found between the groups. The bone mineral density distribution and analysis of Raman spectra indicate that OI groups have greater mean mineralization, greater relative mineral content, and lower crystallinity than the control group, which was not altered by SclAb treatment. Finally, a lower modulus and hardness were measured in the inter-label bone of the OI-SclAb group compared to the OI group. Previous studies suggest that even though bone from OI has a higher mineral content, the extracellular matrix (ECM) has comparable mechanical properties. Therefore, fragility in OI may stem from contributions from other yet unexplored aspects of bone organization at higher length scales. We conclude that SclAb treatment leads to increased bone mass while not adversely affecting bone matrix properties in individuals with OI.

Combining anabolic loading and raloxifene improves bone quantity and some quality measures in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) increases fracture risk due to changes in bone quantity and quality caused by mutations in collagen and its processing proteins. Current therapeutics improve bone quantity, but do not treat the underlying quality deficiencies. Male and female G610C+/- mice, a murine model of OI, were treated with a combination of raloxifene and in vivo axial tibial compressive loading starting at 10 weeks of age and continuing for 6 weeks to improve bone quantity and quality. Bone geometry and mechanical properties were measured to determine whole bone and tissue-level material properties. A colocalized Raman/nanoindentation system was used to measure chemical composition and nanomechanical properties in newly formed bone compared to old bone to determine if bone formed during the treatment regimen differed in quality compared to bone formed prior to treatment. Lastly, lacunar geometry and osteocyte apoptosis were assessed. OI mice were able to build bone in response to the loading, but this response was less robust than in control mice. Raloxifene improved some bone material properties in female but not male OI mice. Raloxifene did not alter nanomechanical properties, but loading did. Lacunar geometry was largely unchanged with raloxifene and loading. However, osteocyte apoptosis was increased with loading in raloxifene treated female mice. Overall, combination treatment with raloxifene and loading resulted in positive but subtle changes to bone quality.

Sclerostin inhibition in rare bone diseases: Molecular understanding and therapeutic perspectives

Sclerostin emerges as a novel target for bone anabolic therapy in bone diseases. Osteogenesis imperfecta (OI) and X-linked hypophosphatemia (XLH) are rare bone diseases in which therapeutic potential of sclerostin inhibition cannot be ignored. In OI, genetic/pharmacologic sclerostin inhibition promoted bone formation of mice, but responses varied by genotype and age. Serum sclerostin levels were higher in young OI-I patients, while lower in adult OI-I/III/IV. It's worth investigating whether therapeutic response of OI to sclerostin inhibition could be clinically predicted by genotype and age. In XLH, preclinical/clinical data suggested factors other than identified FGF23 contributing to XLH. Higher levels of circulating sclerostin were detected in XLH. Sclerostin inhibition promoted bone formation in Hyp mice, while restored phosphate homeostasis in age-/gender-dependent manner. The role of sclerostin in regulating phosphate metabolism deserves investigation. Sclerostin/FGF23 levels of XLH patients with/without response to FGF23-antibody warrants study to develop precise sclerostin/FGF23 inhibition strategy or synergistic/additive strategy. Notably, OI patients were associated with cardiovascular abnormalities, so were XLH patients receiving conventional therapy. Targeting sclerostin loop3 promoted bone formation without cardiovascular risks. Further, blockade of sclerostin loop3-LRP4 interaction while preserving sclerostin loop2-ApoER2 interaction could be a potential precise sclerostin inhibition strategy for OI and XLH with cardiovascular safety. The Translational Potential of this Article. Preclinical data on the molecular understanding of sclerostin inhibition in OI and therapeutic efficacy in mouse models of different genotypes, as well as clinical data on serum sclerostin levels in patients with different phenotypes of OI, were reviewed and discussed. Translationally, it would facilitate to develop clinical prediction strategies (e.g. based on genotype and age, not just phenotype) for OI patients responsive to sclerostin inhibition. Both preclinical and clinical data suggested sclerostin as another factor contributing to XLH, in addition to the identified FGF23. The molecular understanding and therapeutic effects of sclerostin inhibition on both promoting bone anabolism and improving phosphate homostasis in Hyp mice were reviewed and discussed. Translationaly, it would facilitate the development of precise sclerostin/FGF23 inhibition strategy or synergistic/additive strategy for the treatment of XLH. Cardiovascular risk could not be ruled out during sclerostin inhibition treatment, especially for OI and XLH patients with cardiovascular diseases history and cardiovascular abnormalities. Studies on the role of sclerostin in inhiting bone formation and protecting cardiovascular system were reviewed and discussed. Translationaly, blockade of sclerostin loop3-LRP4 interaction while preserving sclerostin loop2-ApoER2 interaction could be a potential precise sclerostin inhibition strategy for OI and XLH with cardiovascular safety.

New Perspectives of Therapies in Osteogenesis Imperfecta-A Literature Review

(1) Background: Osteogenesis imperfecta (OI) is a rare skeletal dysplasia characterized as a heterogeneous disorder group with well-defined phenotypic and genetic features that share uncommon bone fragility. The current treatment options, medical and orthopedic, are limited and not efficient enough to improve the low bone density, bone fragility, growth, and mobility of the affected individuals, creating the need for alternative therapeutic agents. (2) Methods: We searched the medical database to find papers regarding treatments for OI other than conventional ones. We included 45 publications. (3) Results: In reviewing the literature, eight new potential therapies for OI were identified, proving promising results in cells and animal models or in human practice, but further research is still needed. Bone marrow transplantation is a promising therapy in mice, adults, and children, decreasing the fracture rate with a beneficial effect on structural bone proprieties. Anti-RANKL antibodies generated controversial results related to the therapy schedule, from no change in the fracture rate to improvement in the bone mineral density resorption markers and bone formation, but with adverse effects related to hypercalcemia. Sclerostin inhibitors in murine models demonstrated an increase in the bone formation rate and trabecular cortical bone mass, and a few human studies showed an increase in biomarkers and BMD and the downregulation of resorption markers. Recombinant human parathormone and TGF-β generated good results in human studies by increasing BMD, depending on the type of OI. Gene therapy, 4-phenylbutiric acid, and inhibition of eIF2α phosphatase enzymes have only been studied in cell cultures and animal models, with promising results. (4) Conclusions: This paper focuses on eight potential therapies for OI, but there is not yet enough data for a new, generally accepted treatment. Most of them showed promising results, but further research is needed, especially in the pediatric field.

Sclerostin, Osteocytes, and Wnt Signaling in Pediatric Renal Osteodystrophy

The pathophysiology of chronic kidney disease-mineral and bone disorder (CKD-MBD) is not well understood. Specific factors secreted by osteocytes are elevated in the serum of adults and pediatric patients with CKD-MBD, including FGF-23 and sclerostin, a known inhibitor of the Wnt signaling pathway. The molecular mechanisms that promote bone disease during the progression of CKD are incompletely understood. In this study, we performed a cross-sectional analysis of 87 pediatric patients with pre-dialysis CKD and post-dialysis (CKD 5D). We assessed the associations between serum and bone sclerostin levels and biomarkers of bone turnover and bone histomorphometry. We report that serum sclerostin levels were elevated in both early and late CKD. Higher circulating and bone sclerostin levels were associated with histomorphometric parameters of bone turnover and mineralization. Immunofluorescence analyses of bone biopsies evaluated osteocyte staining of antibodies towards the canonical Wnt target, β-catenin, in the phosphorylated (inhibited) or unphosphorylated (active) forms. Bone sclerostin was found to be colocalized with phosphorylated β-catenin, which suggests that Wnt signaling was inhibited. In patients with low serum sclerostin levels, increased unphosphorylated "active" β-catenin staining was observed in osteocytes. These data provide new mechanistic insight into the pathogenesis of CKD-MBD and suggest that sclerostin may offer a potential biomarker or therapeutic target in pediatric renal osteodystrophy.

Effect of sclerostin inactivation in a mouse model of severe dominant osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a rare bone disease that is associated with fractures and low bone mass. Sclerostin inhibition is being evaluated as a potential approach to increase bone mass in OI. We had previously found that in Col1a1^Jrt/+ mice, a model of severe OI, treatment with an anti-sclerostin antibody had a minor effect on the skeletal phenotype. In the present study, we assessed the effect of genetic sclerostin inactivation in the Col1a1^Jrt/+ mouse. We crossed Col1a1^Jrt/+ mice with Sost knockout mice to generate Sost-deficient Col1a1^Jrt/+ mice and assessed differences between Col1a1^Jrt/+ mice with homozygous Sost deficiency and Col1a1^Jrt/+ mice with heterozygous Sost deficiency. We found that Col1a1^Jrt/+ mice with homozygous Sost deficiency had higher body mass, femur length, trabecular bone volume, cortical thickness and periosteal diameter as well as increased biomechanical parameters of bone strength. Differences between genotypes were larger at the age of 14 weeks than at 8 weeks of age. Transcriptome analysis of RNA extracted from the tibial diaphysis revealed only 5 differentially regulated genes. Thus, genetic inactivation of Sost increased bone mass and strength in the Col1a1^Jrt/+ mouse. It appears from these observations that the degree of Sost suppression that is required for eliciting a beneficial response can vary with the genetic cause of OI.

Role of Wnt signaling and sclerostin in bone and as therapeutic targets in skeletal disorders

Wnt signaling and its bone tissue-specific inhibitor sclerostin are key regulators of bone homeostasis. The therapeutic potential of anti-sclerostin antibodies (Scl-Abs), for bone mass recovery and fragility fracture prevention in low bone mass phenotypes, has been supported by animal studies. The Scl-Ab romosozumab is currently used for osteoporosis treatment.

INTRODUCTION

Wnt signaling is a key regulator of skeletal development and homeostasis; germinal mutations affecting genes encoding components, inhibitors, and enhancers of the Wnt pathways were shown to be responsible for the development of rare congenital metabolic bone disorders. Sclerostin is a bone tissue-specific inhibitor of the Wnt/β-catenin pathway, secreted by osteocytes, negatively regulating osteogenic differentiation and bone formation, and promoting osteoclastogenesis and bone resorption.

PURPOSE AND METHODS

Here, we reviewed current knowledge on the role of sclerostin and Wnt pathways in bone metabolism and skeletal disorders, and on the state of the art of therapy with sclerostin-neutralizing antibodies in low-bone-mass diseases.

RESULTS

Various in vivo studies on animal models of human low-bone-mass diseases showed that targeting sclerostin to recover bone mass, restore bone strength, and prevent fragility fracture was safe and effective in osteoporosis, osteogenesis imperfecta, and osteoporosis pseudoglioma. Currently, only treatment with romosozumab, a humanized monoclonal anti-sclerostin antibody, has been approved in human clinical practice for the treatment of osteoporosis, showing a valuable capability to increase BMD at various skeletal sites and reduce the occurrence of new vertebral, non-vertebral, and hip fragility fractures in treated male and female osteoporotic patients.

CONCLUSIONS

Preclinical studies demonstrated safety and efficacy of therapy with anti-sclerostin monoclonal antibodies in the preservation/restoration of bone mass and prevention of fragility fractures in low-bone-mass clinical phenotypes, other than osteoporosis, to be validated by clinical studies for their approved translation into prevalent clinical practice.

The Effect of Apple-Derived Nanovesicles on the Osteoblastogenesis of Osteoblastic MC3T3-E1 Cells

Osteoporosis is characterized by low bone mass and elevated structural deterioration of the bone tissue, resulting in bone weakness with an increased risk of fracture. Considering biological activities of various phytochemicals extracted from apples, we herein demonstrated the potential antiosteoporotic effects of apple-derived nanovesicles (apple NVs) using osteoblastic MC3T3-E1 cells. Apple NVs significantly stimulated the growth of MC3T3-E1 cells. The cellular alkaline phosphatase (ALP) activity was significantly upregulated in the 5 μg/mL apple NVs-treated group. In addition, the concentrarion of mineralized nodules was significantly increased in the apple NVs-treated groups. Furthermore, apple NVs increased the expression of the genes and proteins associated with osteoblast growth and differentiation, such as Runx2, ALP, OPN, and BMP2/4, which further activated ERK- and JNK-related mitogen-activated protein kinase signaling. These results demonstrate that apple NVs have a potential to prevent osteoporosis by promoting osteoblastogenesis in osteoblastic MC3T3-E1 cells through regulating the BMP2/Smad1 pathways.

Tendon properties in a mouse model of severe osteogenesis imperfecta

PURPOSE/AIM OF THE STUDY

Osteogenesis imperfecta is a heritable bone disorder that is usually caused by mutations in collagen type I encoding genes. The impact of such mutations on tendons, a structure with high collagen type I content, remains largely unexplored. We hypothesized that tendon properties are abnormal in the context of a mutation affecting collagen type I. The main purpose of the study was to assess the anatomical, mechanical, and material tendon properties of Col1a1^Jrt/+ mice, a model of severe dominant OI.

MATERIALS AND METHODS

The Flexor Digitorum Longus (FDL) tendon of Col1a1^Jrt/+ mice and wild-type littermates (WT) was assessed with in vitro mechanical testing.

RESULTS

The results showed that width and thickness of FDL tendons were about 40% larger in WT (p < 0.01) than in Col1a1^Jrt/+ mice, whereas the cross-sectional area was 138% larger (p < 0.001). The stiffness, peak- and yield-force were between 160% and 194% higher in WT vs. Col1a1^Jrt/+ mice. The material properties did not show significant differences between mouse strains with differences <15% between WT and Col1a1^Jrt/+ (p > 0.05). Analysis of the Achilles tendon collagen showed no difference between mice strains for the content but collagen solubility in acetic acid was 66% higher in WT than in Col1a1^Jrt/+ (p < 0.001).

CONCLUSIONS

This study shows that the FDL tendon of Col1a1^Jrt/+ mice has reduced mechanical properties but apparently normal material properties. It remains unclear whether the tendon phenotype of Col1a1^Jrt/+ mice is secondary to muscle weakness or a direct effect of the Col1a1 mutation or a combination of both.

Murine Animal Models in Osteogenesis Imperfecta: The Quest for Improving the Quality of Life

Osteogenesis imperfecta is a rare genetic disorder characterized by bone fragility, due to alterations in the type I collagen molecule. It is a very heterogeneous disease, both genetically and phenotypically, with a high variability of clinical phenotypes, ranging from mild to severe forms, the most extreme cases being perinatal lethal. There is no curative treatment for OI, and so great efforts are being made in order to develop effective therapies. In these attempts, the in vivo preclinical studies are of paramount importance; therefore, serious analysis is required to choose the right murine OI model able to emulate as closely as possible the disease of the target OI population. In this review, we summarize the features of OI murine models that have been used for preclinical studies until today, together with recently developed new murine models. The bone parameters that are usually evaluated in order to determine the relevance of new developing therapies are exposed, and finally, current and innovative therapeutic strategies attempts considered in murine OI models, along with their mechanism of action, are reviewed. This review aims to summarize the in vivo studies developed in murine models available in the field of OI to date, in order to help the scientific community choose the most accurate OI murine model when developing new therapeutic strategies capable of improving the quality of life.

An Update on Animal Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous disorder characterized by bone fragility, multiple fractures, bone deformity, and short stature. In recent years, the application of next generation sequencing has triggered the discovery of many new genetic causes for OI. Until now, more than 25 genetic causes of OI and closely related disorders have been identified. However, the mechanisms of many genes on skeletal fragility in OI are not entirely clear. Animal models of OI could help to understand the cellular, signaling, and metabolic mechanisms contributing to the disease, and how targeting these pathways can provide therapeutic targets. To date, a lot of animal models, mainly mice and zebrafish, have been described with defects in 19 OI-associated genes. In this review, we summarize the known genetic causes and animal models that recapitulate OI with a main focus on engineered mouse and zebrafish models. Additionally, we briefly discuss domestic animals with naturally occurring OI phenotypes. Knowledge of the specific molecular basis of OI will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged <50 years. EOOP may be secondary to a chronic illness, long-term medication, nutritional deficiencies, etc. If no such cause is identified, EOOP is regarded primary and may then be related to rare variants in genes playing a pivotal role in bone homeostasis. If the cause remains unknown, EOOP is considered idiopathic. The scope of this review is to guide through clinical and genetic diagnostics of EOOP, summarize the present knowledge on rare monogenic forms of EOOP, and describe how analysis of bone biopsy samples can lead to a better understanding of the disease pathogenesis. The diagnostic pathway of EOOP is often complicated and extensive assessments may be needed to reliably exclude secondary causes. Due to the genetic heterogeneity and overlapping features in the various genetic forms of EOOP and other bone fragility disorders, the genetic diagnosis usually requires the use of next-generation sequencing to investigate several genes simultaneously. Recent discoveries have elucidated the complexity of disease pathogenesis both regarding genetic architecture and bone tissue-level pathology. Two rare monogenic forms of EOOP are due to defects in genes partaking in the canonical WNT pathway: LRP5 and WNT1. Variants in the genes encoding plastin-3 (PLS3) and sphingomyelin synthase 2 (SGMS2) have also been found in children and young adults with skeletal fragility. The molecular mechanisms leading from gene defects to clinical manifestations are often not fully understood. Detailed analysis of patient-derived transiliac bone biopsies gives valuable information to understand disease pathogenesis, distinguishes EOOP from other bone fragility disorders, and guides in patient management, but is not widely available in clinical settings. Despite the great advances in this field, EOOP remains an insufficiently explored entity and further research is needed to optimize diagnostic and therapeutic approaches. © 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).

Dissecting the phenotypic variability of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous family of collagen type I-related diseases characterized by bone fragility. OI is most commonly caused by single-nucleotide substitutions that replace glycine residues or exon splicing defects in the COL1A1 and COL1A2 genes that encode the α1(I) and α2(I) collagen chains. Mutant collagen is partially retained intracellularly, impairing cell homeostasis. Upon secretion, it assembles in disorganized fibrils, altering mineralization. OI is characterized by a wide range of clinical outcomes, even in the presence of identical sequence variants. Given the heterotrimeric nature of collagen I, its amino acid composition and the peculiarity of its folding, several causes may underlie the phenotypic variability of OI. A deep analysis of entries regarding glycine and splice site collagen substitution of the largest publicly available patient database reveals a higher risk of lethal phenotype for carriers of variants in α1(I) than in α2(I) chain. However, splice site variants are predominantly associated with lethal phenotype when they occur in COL1A2. In addition, lethality is increased when mutations occur in regions of importance for extracellular matrix interactions. Both extracellular and intracellular determinants of OI clinical severity are discussed in light of the findings from in vitro and in vivo OI models. Combined with meticulous tracking of clinical cases via a publicly available database, the available OI animal models have proven to be a unique tool to shed light on new modulators of phenotype determination for this rare heterogeneous disease.

Drug discovery of sclerostin inhibitors

Sclerostin, a protein secreted from osteocytes, negatively regulates the WNT signaling pathway by binding to the LRP5/6 co-receptors and further inhibits bone formation and promotes bone resorption. Sclerostin contributes to musculoskeletal system-related diseases, making it a promising therapeutic target for the treatment of WNT-related bone diseases. Additionally, emerging evidence indicates that sclerostin contributes to the development of cancers, obesity, and diabetes, suggesting that it may be a promising therapeutic target for these diseases. Notably, cardiovascular diseases are related to the protective role of sclerostin. In this review, we summarize three distinct types of inhibitors targeting sclerostin, monoclonal antibodies, aptamers, and small-molecule inhibitors, from which monoclonal antibodies have been developed. As the first-in-class sclerostin inhibitor approved by the U.S. FDA, the monoclonal antibody romosozumab has demonstrated excellent effectiveness in the treatment of postmenopausal osteoporosis; however, it conferred high cardiovascular risk in clinical trials. Furthermore, romosozumab could only be administered by injection, which may cause compliance issues for patients who prefer oral therapy. Considering these above safety and compliance concerns, we therefore present relevant discussion and offer perspectives on the development of next-generation sclerostin inhibitors by following several ways, such as concomitant medication, artificial intelligence-based strategy, druggable modification, and bispecific inhibitors strategy.

Combining sclerostin neutralization with tissue engineering: An improved strategy for craniofacial bone repair

Scaffolds associated with different types of mesenchymal stromal stem cells (MSC) are extensively studied for the development of novel therapies for large bone defects. Moreover, monoclonal antibodies have been recently introduced for the treatment of cancer-associated bone loss and other skeletal pathologies. In particular, antibodies against sclerostin, a key player in bone remodeling regulation, have demonstrated a real benefit for treating osteoporosis but their contribution to bone tissue-engineering remains uncharted. Here, we show that combining implantation of dense collagen hydrogels hosting wild-type (WT) murine dental pulp stem cells (mDPSC) with weekly systemic injections of a sclerostin antibody (Scl-Ab) leads to increased bone regeneration within critical size calvarial defects performed in WT mice. Furthermore, we show that bone formation is equivalent in calvarial defects in WT mice implanted with Sost knock-out (KO) mDPSC and in Sost KO mice, suggesting that the implantation of sclerostin-deficient MSC similarly promotes new bone formation than complete sclerostin deficiency. Altogether, our data demonstrate that an antibody-based therapy can potentialize tissue-engineering strategies for large craniofacial bone defects and urges the need to conduct research for antibody-enabled local inhibition of sclerostin. STATEMENT OF SIGNIFICANCE: The use of monoclonal antibodies is nowadays broadly spread for the treatment of several conditions including skeletal bone diseases. However, their use to potentialize tissue engineering constructs for bone repair remains unmet. Here, we demonstrate that the neutralization of sclerostin, through either a systemic inhibition by a monoclonal antibody or the implantation of sclerostin-deficient mesenchymal stromal stem cells (MSC) directly within the defect, improves the outcome of a tissue engineering approach, combining dense collagen hydrogels and MSC derived from the dental pulp, for the treatment of large craniofacial bone defects.

The Role of Sclerostin in Bone Diseases

Sclerostin has been identified as an important regulator of bone homeostasis through inhibition of the canonical Wnt-signaling pathway, and it is involved in the pathogenesis of many different skeletal diseases. Many studies have been published in the last few years regarding sclerostin's origin, regulation, and mechanism of action. The ongoing research emphasizes the potential therapeutic implications of sclerostin in many pathological conditions with or without skeletal involvement. Antisclerostin antibodies have recently been approved for the treatment of osteoporosis, and several animal studies and clinical trials are currently under way to evaluate the effectiveness of antisclerostin antibodies in the treatment of other than osteoporosis skeletal disorders and cancer with promising results. Understanding the exact role of sclerostin may lead to new therapeutic approaches for the treatment of skeletal disorders.

Inhibition of TGF-β Increases Bone Volume and Strength in a Mouse Model of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI), is a genetic disorder of bone fragility caused by mutations in collagen I or proteins involved in collagen processing. Previous studies in mice and human OI bones have shown that excessive activation of TGF-β signaling plays an important role in dominant and recessive OI disease progression. Inhibition of TGF-β signaling with a murine pan-specific TGF-β neutralizing antibody (1D11) was shown to significantly increase trabecular bone volume and long bone strength in mouse models of OI. To investigate the frequency of dosing and dose options of TGF-β neutralizing antibody therapy, we assessed the effect of 1D11 on disease progression in a dominant OI mouse model (col1a2 gene mutation at G610C). In comparison with OI mice treated with a control antibody, we attempted to define mechanistic effects of 1D11 measured via μCT, biomechanical, dynamic histomorphometry, and serum biomarkers of bone turnover. In addition, osteoblast and osteoclast numbers in histological bone sections were assessed to better understand the mechanism of action of the 1D11 antibody in OI. Here we show that 1D11 treatment resulted in both dose and frequency dependency, increases in trabecular bone volume fraction and ultimate force in lumbar bone, and ultimate force, bending strength, yield force, and yield strength in the femur (p ≤ 0.05). Suppression of serum biomarkers of osteoblast differentiation, osteocalcin, resorption, CTx-1, and bone formation were observed after 1D11 treatment of OI mice. Immunohistochemical analysis showed dose and frequency dependent decreases in runt-related transcription factor, and increase in alkaline phosphatase in lumbar bone sections. In addition, a significant decrease in TRACP and the number of osteoclasts to bone surface area was observed with 1D11 treatment. Our results show that inhibition of the TGF-β pathway corrects the high-turnover aspects of bone disease and improves biomechanical properties of OI mice. These results highlight the potential for a novel treatment for osteogenesis imperfecta. © 2021 Sanofi-Genzyme. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Gender-Related Impact of Sclerostin Antibody on Bone in the Osteogenesis Imperfecta Mouse

Osteogenesis imperfecta (OI), which is most often due to a collagen type 1 gene mutation, is characterized by low bone density and bone fragility. In OI patients, gender-related differences were reported, but data in the literature are not convergent. We previously observed that sclerostin antibody (Scl-Ab), which stimulates osteoblast Wnt pathway via sclerostin inactivation, improved spine and long-bone parameters and biomechanical strength in female oim/oim mice, a validated model of human type 3 OI. Here, we wanted to highlight the effect of Scl-Ab on male oim/oim bones in order to identify a possible distinct therapeutic effect from that observed in females. According to the same protocol as our previous study with female mice, male wild-type (Wt) and oim/oim mice received vehicle or Scl-Ab from 5 to 14 weeks of age. Clinimetric and quantitative bone parameters were studied using X-rays, peripheral quantitative computed tomography, microradiography, and dynamic histomorphometry and compared to those of females. Contrary to Wt mice, male oim/oim had significantly lower weight, snout-sacrum length, and bone mineral content than females at 5 weeks. No significant difference in these clinimetric parameters was observed at 14 weeks, whereas male oim showed significantly more long-bone fractures than females. Scl-Ab improved bone mineral density and bone volume/total volume ratio (BV/TV) of vertebral body in Wt and oim/oim, without significant difference between male and female at 14 weeks. Male vehicle oim/oim had a significantly lower cortical thickness (Ct.Th) and BV/TV of tibial diaphysis than female and showed a higher number of fractures at 14 weeks. Scl-Ab increased midshaft periosteal apposition rate in such a way that tibial Ct.Th of male oim/oim was not significantly different from the female one at 14 weeks. The number of fractures was lower in male than female oim/oim after 14 weeks of Scl-Ab treatment, but this difference was not significant. Nevertheless, Scl-Ab-treated oim/oim male and female mice remained smaller than the Wt ones. In conclusion, our results highlighted differences between male and female oim/oim at 4 and 14 weeks of age, as well as some male-specific response of cortical bone to Scl-Ab. These gender-related particularities of oim/oim should be considered when testing experimental treatments.

Wnt signaling activation: targets and therapeutic opportunities for stem cell therapy and regenerative medicine

Wnt proteins are secreted morphogens that play critical roles in embryonic development, stem cell proliferation, self-renewal, tissue regeneration and remodeling in adults. While aberrant Wnt signaling contributes to diseases such as cancer, activation of Wnt/β-catenin signaling is a target of interest in stem cell therapy and regenerative medicine. Recent high throughput screenings from chemical and biological libraries, combined with improved gene expression reporter assays of Wnt/β-catenin activation together with rational drug design, led to the development of a myriad of Wnt activators, with different mechanisms of actions. Among them, Wnt mimics, antibodies targeting Wnt inhibitors, glycogen-synthase-3β inhibitors, and indirubins and other natural product derivatives are emerging modalities to treat bone, neurodegenerative, eye, and metabolic disorders, as well as prevent ageing. Nevertheless, the creation of Wnt-based therapies has been hampered by challenges in developing potent and selective Wnt activators without off-target effects, such as oncogenesis. On the other hand, to avoid these risks, their use to promote ex vivo expansion during tissue engineering is a promising application.

Calvaria Bone Transcriptome in Mouse Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a bone fragility disorder that is usually caused by mutations affecting collagen type I. We compared the calvaria bone tissue transcriptome of male 10-week-old heterozygous Jrt (Col1a1 mutation) and homozygous oim mice (Col1a2 mutation) to their respective littermate results. We found that Jrt and oim mice shared 185 differentially expressed genes (upregulated: 106 genes; downregulated: 79 genes). A total of seven genes were upregulated by a factor of two or more in both mouse models (Cyp2e1, Slc13a5, Cgref1, Smpd3, Ifitm5, Cthrc1 and Rerg). One gene (Gypa, coding for a blood group antigen) was downregulated by a factor of two or more in both OI mouse models. Overrepresentation analyses revealed that genes involved in ‘ossification’ were significantly overrepresented among upregulated genes in both Jrt and oim mice, whereas hematopoietic genes were downregulated. Several genes involved in Wnt signaling and transforming growth factor beta signaling were upregulated in oim mice, but less so in Jrt mice. Thus, this study identified a set of genes that are dysregulated across various OI mouse models and are likely to play an important role in the pathophysiology of this disorder.

Impact of Intrinsic Muscle Weakness on Muscle-Bone Crosstalk in Osteogenesis Imperfecta

Bone and muscle are highly synergistic tissues that communicate extensively via mechanotransduction and biochemical signaling. Osteogenesis imperfecta (OI) is a heritable connective tissue disorder of severe bone fragility and recently recognized skeletal muscle weakness. The presence of impaired bone and muscle in OI leads to a continuous cycle of altered muscle-bone crosstalk with weak muscles further compromising bone and vice versa. Currently, there is no cure for OI and understanding the pathogenesis of the skeletal muscle weakness in relation to the bone pathogenesis of OI in light of the critical role of muscle-bone crosstalk is essential to developing and identifying novel therapeutic targets and strategies for OI. This review will highlight how impaired skeletal muscle function contributes to the pathophysiology of OI and how this phenomenon further perpetuates bone fragility.

Effects of treatment with a bone-targeted prostaglandin E2 receptor 4 agonist C3 (Mes-1007) in a mouse model of severe osteogenesis imperfecta

OBJECTIVE

Osteogenesis imperfecta (OI) is a heritable bone fragility disorder that is usually caused by mutations affecting collagen type I synthesis in osteoblasts. Bisphosphonates are widely used to decrease fracture rate but are only partially effective. Bone anabolic compounds, such as prostaglandin E2 receptor 4 (EP4) agonists may be an alternative treatment approach. Here we assessed the effect of Mes-1007, a novel bone-targeted EP4 agonist in Jrt mice, a model of severe OI.

STUDY DESIGN

Experimental study.

RESULTS

Male 8-week old wild type (WT) and OI mice were randomly assigned to 4 weeks of three intraperitoneal injections per week with Mes-1007 (25 mg per kg body mass), phosphate-buffered saline, zoledronate (5 μg per kg), or a combination treatment of zoledronate and Mes-1007. Treatment with Mes-1007 alone did not lead to higher trabecular bone volume per tissue volume (BV/TV) in the distal femur or lumbar vertebra 4 in either WT or OI mice. Treatment with zoledronate alone was associated with a significant increase in distal femur and vertebra BV/TV in both genotypes. In zoledronate-treated WT and OI mice, Mes-1007 increased bone formation rate in vertebral trabecular bone and had an additive effect on BV/TV. Vertebral BV/TV in OI mice that received zoledronate or Mes-1007/zoledronate combination treatment was similar to untreated WT mice (p = 0.25). At the femoral midshaft, Mes-1007/zoledronate combination treatment increased cortical thickness in both genotypes and led to higher periosteal diameter in OI mice. Three-point bending tests of femurs showed that Mes-1007/zoledronate combination treatment increased the stiffness, load at yield and maximal load in WT but not in OI mice.

CONCLUSION

Dosing Mes-1007 in combination with zoledronate improved the bone properties in a manner that is consistent with a mechanism of action of EP4 agonists on bone and additive to effects of anti-resorptives typified by zoledronate.

Impact of Genetic and Pharmacologic Inhibition of Myostatin in a Murine Model of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a genetic connective tissue disorder characterized by compromised skeletal integrity, altered microarchitecture, and bone fragility. Current OI treatment strategies focus on bone antiresorptives and surgical intervention with limited effectiveness, and thus identifying alternative therapeutic options remains critical. Muscle is an important stimulus for bone formation. Myostatin, a TGF-β superfamily myokine, acts through ActRIIB to negatively regulate muscle growth. Recent studies demonstrated the potential benefit of myostatin inhibition with the soluble ActRIIB fusion protein on skeletal properties, although various OI mouse models exhibited variable skeletal responses. The genetic and clinical heterogeneity associated with OI, the lack of specificity of the ActRIIB decoy molecule for myostatin alone, and adverse events in human clinical trials further the need to clarify myostatin's therapeutic potential and role in skeletal integrity. In this study, we determined musculoskeletal outcomes of genetic myostatin deficiency and postnatal pharmacological myostatin inhibition by a monoclonal anti-myostatin antibody (Regn647) in the G610C mouse, a model of mild-moderate type I/IV human OI. In the postnatal study, 5-week-old wild-type and +/G610C male and female littermates were treated with Regn647 or a control antibody for 11 weeks or for 7 weeks followed by a 4-week treatment holiday. Inhibition of myostatin, whether genetically or pharmacologically, increased muscle mass regardless of OI genotype, although to varying degrees. Genetic myostatin deficiency increased hindlimb muscle weights by 6.9% to 34.4%, whereas pharmacological inhibition increased them by 13.5% to 29.6%. Female +/mstn +/G610C (Dbl.Het) mice tended to have similar trabecular and cortical bone parameters as Wt showing reversal of +/G610C characteristics but with minimal effect of +/mstn occurring in male mice. Pharmacologic myostatin inhibition failed to improve skeletal bone properties of male or female +/G610C mice, although skeletal microarchitectural and biomechanical improvements were observed in male wild-type mice. Four-week treatment holiday did not alter skeletal outcomes. © 2020 American Society for Bone and Mineral Research (ASBMR).

Osteoporosis Treatment with Anti-Sclerostin Antibodies-Mechanisms of Action and Clinical Application

Osteoporosis is characterized by reduced bone mass and disruption of bone architecture, resulting in increased risk of fragility fractures and significant long-term disability. Although both anti-resorptive treatments and osteoanabolic drugs, such as parathyroid hormone analogues, are effective in fracture prevention, limitations exist due to lack of compliance or contraindications to these drugs. Thus, there is a need for novel potent therapies, especially for patients at high fracture risk. Romosozumab is a monoclonal antibody against sclerostin with a dual mode of action. It enhances bone formation and simultaneously suppresses bone resorption, resulting in a large anabolic window. In this opinion-based narrative review, we highlight the role of sclerostin as a critical regulator of bone mass and present human diseases of sclerostin deficiency as well as preclinical models of genetically modified sclerostin expression, which led to the development of anti-sclerostin antibodies. We review clinical studies of romosozumab in terms of bone mass accrual and anti-fracture activity in the setting of postmenopausal and male osteoporosis, present sequential treatment regimens, and discuss its safety profile and possible limitations in its use. Moreover, an outlook comprising future translational applications of anti-sclerostin antibodies in diseases other than osteoporosis is given, highlighting the clinical significance and future scopes of Wnt signaling in these settings.

New therapeutic options for bone diseases

In recent years, new treatment options for both common and rare bone diseases have become available. The sclerostin antibody romosozumab is the most recently approved drug for the therapy of postmenopausal osteoporosis. Its anabolic capacity makes it a promising treatment option for severe osteoporosis. Other sclerostin antibodies for the treatment of rare bone diseases such as osteogenesis imperfecta are currently being investigated. For rare bone diseases such as X‑linked hypophosphatemia (XLH) and hypophosphatasia (HPP), specific therapies are now also available, showing promising data in children and adults with a severe disease course. However, long-term data are needed to assess a sustained benefit for patients.

Signaling pathways affected by mutations causing osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous connective tissue disorder characterized by bone fragility and skeletal deformity. To maintain skeletal strength and integrity, bone undergoes constant remodeling of its extracellular matrix (ECM) tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. There are at least 20 recognized OI-forms caused by mutations in the two collagen type I-encoding genes or genes implicated in collagen folding, posttranslational modifications or secretion of collagen, osteoblast differentiation and function, or bone mineralization. The underlying disease mechanisms of non-classical forms of OI that are not caused by collagen type I mutations are not yet completely understood, but an altered ECM structure as well as disturbed intracellular homeostasis seem to be the main defects. The ECM orchestrates local cell behavior in part by regulating bioavailability of signaling molecules through sequestration, release and activation during the constant bone remodeling process. Here, we provide an overview of signaling pathways that are associated with known OI-causing genes and discuss the impact of these genes on signal transduction. These pathways include WNT-, RANK/RANKL-, TGFβ-, MAPK- and integrin-mediated signaling as well as the unfolded protein response.

Gene Expression Profile and Acute Gene Expression Response to Sclerostin Inhibition in Osteogenesis Imperfecta Bone

Sclerostin antibody (SclAb) therapy has been suggested as a novel therapeutic approach toward addressing the fragility phenotypic of osteogenesis imperfecta (OI). Observations of cellular and transcriptional responses to SclAb in OI have been limited to mouse models of the disorder, leaving a paucity of data on the human OI osteoblastic cellular response to the treatment. Here, we explore factors associated with response to SclAb therapy in vitro and in a novel xenograft model using OI bone tissue derived from pediatric patients. Bone isolates (approximately 2 mm3) from OI patients (OI type III, type III/IV, and type IV, n = 7; non-OI control, n = 5) were collected to media, randomly assigned to an untreated (UN), low-dose SclAb (TRL, 2.5 μg/mL), or high-dose SclAb (TRH, 25 μg/mL) group, and maintained in vitro at 37°C. Treatment occurred on days 2 and 4 and was removed on day 5 for TaqMan qPCR analysis of genes related to the Wnt pathway. A subset of bone was implanted s.c. into an athymic mouse, representing our xenograft model, and treated (25 mg/kg s.c. 2×/week for 2/4 weeks). Implanted OI bone was evaluated using μCT and histomorphometry. Expression of Wnt/Wnt-related targets varied among untreated OI bone isolates. When treated with SclAb, OI bone showed an upregulation in osteoblast and osteoblast progenitor markers, which was heterogeneous across tissue. Interestingly, the greatest magnitude of response generally corresponded to samples with low untreated expression of progenitor markers. Conversely, samples with high untreated expression of these markers showed a lower response to treatment. in vivo implanted OI bone showed a bone-forming response to SclAb via μCT, which was corroborated by histomorphometry. SclAb induced downstream Wnt targets WISP1 and TWIST1, and elicited a compensatory response in Wnt inhibitors SOST and DKK1 in OI bone with the greatest magnitude from OI cortical bone. Understanding patients' genetic, cellular, and morphological bone phenotypes may play an important role in predicting treatment response. This information may aid in clinical decision-making for pharmacological interventions designed to address fragility in OI. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Sclerostin-Antibody Treatment Decreases Fracture Rates in Axial Skeleton and Improves the Skeletal Phenotype in Growing oim/oim Mice

In osteogenesis imperfecta (OI), vertebrae brittleness causes thorax deformations and leads to cardiopulmonary failure. As sclerostin-neutralizing antibodies increase bone mass and strength in animal models of osteoporosis, their administration in two murine models of severe OI enhanced the strength of vertebrae in growing female Crtap^-/- mice but not in growing male Col1a1^Jrt/+ mice. However, these two studies ignored the impact of antibodies on spine growth, fracture rates, and compressive mechanical properties. Here, we conducted a randomized controlled trial in oim/oim mice, an established model of human severe OI type III due to a mutation in Col1a2. Five-week-old female WT and oim/oim mice received either PBS or sclerostin antibody (Scl-Ab) for 9 weeks. Analyses included radiography, histomorphometry, pQCT, microcomputed tomography, and biomechanical testing. Though it did not modify vertebral axial growth, Scl-Ab treatment markedly reduced the fracture prevalence in the pelvis and caudal vertebrae, enhanced osteoblast activity (L4), increased cervico-sacral spine BMD, and improved the lumbosacral spine bone cross-sectional area. Scl-Ab did not impact vertebral height and body size but enhanced the cortical thickness and trabecular bone volume significantly in the two Scl-Ab groups. At lumbar vertebrae and tibial metaphysis, the absolute increase in cortical and trabecular bone mass was higher in Scl-Ab WT than in Scl-Ab oim/oim. The effects on trabecular bone mass were mainly due to changes in trabecular number at vertebrae and in trabecular thickness at metaphyses. Additionally, Scl-Ab did not restore a standard trabecular network, but improved bone compressive ultimate load with more robust effects at vertebrae than at metaphysis. Overall, Scl-Ab treatment may be beneficial for reducing vertebral fractures and spine deformities in patients with severe OI.

Finite element analysis of bone strength in osteogenesis imperfecta

As a dedicated experimentalist, John Currey praised the high potential of finite element (FE) analysis but also recognized its critical limitations. The application of the FE methodology to bone tissue is reviewed in the light of his enthusiastic and colorful statements. In the past decades, FE analysis contributed substantially to the understanding of structure-function properties in the hierarchical organization of bone and to the simulation of bone adaptation. The systematic experimental validation of FE analysis of bone strength in anatomical locations at risk of fracture led to its application in clinical studies to evaluate efficacy of antiresorptive or anabolic treatment of bone fragility. Beyond the successful analyses of healthy or osteoporotic bone, FE analysis becomes increasingly involved in the investigation of other fragility-related bone diseases. The case of osteogenesis imperfecta (OI) is exposed, the multiscale alterations of the bone tissue and the effect of treatment summarized. A few FE analyses attempting to answer open questions in OI are then reported. An original study is finally presented that explored the structural properties of the Brtl/+ murine model of OI type IV subjected to sclerostin neutralizing antibody treatment using microFE analysis. The use of identical material properties in the four-point bending FE simulations of the femora reproduced not only the experimental values but also the statistical comparisons examining the effect of disease and treatment. Further efforts are needed to build upon the extraordinary legacy of John Currey and clarify the impact of different bone diseases on the hierarchical mechanical properties of bone.

Sclerostin Antibody Treatment Increases Bone Mass and Normalizes Circulating Phosphate Levels in Growing Hyp Mice

X-linked hypophosphatemia (XLH), caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX), is the most common form of vitamin D-resistant rickets. Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23) levels, impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily in osteocytes, suppresses bone formation by antagonizing Wnt signaling and is reported to be elevated in XLH patients. This study used the Hyp mouse model to investigate sclerostin's role in the pathophysiology of XLH by evaluating the use of a monoclonal antibody to sclerostin in a mouse model of XLH, the Hyp mouse. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or sclerostin antibody (Scl-Ab) twice weekly, beginning at 4 weeks of age and euthanized at 8 weeks of age. Scl-Ab treatment increased serum phosphate levels and suppressed circulating levels of intact FGF23 in treated wild-type and Hyp mice of both sexes. Cortical area, trabecular bone volume fraction (BV/TV), metaphyseal apparent density, and the peak load increased with Scl-Ab treatment in both sexes. This short-term treatment study suggests that Scl-Ab treatment can effectively improve some of the pathologies associated with XLH, including normalization of phosphate, and that sclerostin may play a role in regulating FGF23 and phosphate metabolism in XLH. © 2019 American Society for Bone and Mineral Research.

Combination therapy in the Col1a2G610C mouse model of Osteogenesis Imperfecta reveals an additive effect of enhancing LRP5 signaling and inhibiting TGFβ signaling on trabecular bone but not on cortical bone

Enhancing LRP5 signaling and inhibiting TGFβ signaling have each been reported to increase bone mass and improve bone strength in wild-type mice. Monotherapy targeting LRP5 signaling, or TGFβ signaling, also improved bone properties in mouse models of Osteogenesis Imperfecta (OI). We investigated whether additive or synergistic increases in bone properties would be attained if enhanced LRP5 signaling was combined with TGFβ inhibition. We crossed an Lrp5 high bone mass (HBM) allele (Lrp5^A214V) into the Col1a2^G610C/+ mouse model of OI. At 6-weeks-of-age we began treating mice with an antibody that inhibits TGFβ1, β2, and β3 (mAb 1D11), or with an isotype-matched control antibody (mAb 13C4). At 12-weeks-old, we observed that combining enhanced LRP5 signaling with inhibited TGFβ signaling produced an additive effect on femoral and vertebral trabecular bone volumes, but not on cortical bone volumes. Although enhanced LRP5 signaling increased femur strength in a 3-point bending assay in Col1a2^G610C/+ mice, femur strength did not improve further with TGFβ inhibition. Neither enhanced LRP5 signaling nor TGFβ inhibition, alone or in combination, improved femur 3-point-bending post-yield displacement in Col1a2^G610C/+ mice. These pre-clinical studies indicate combination therapies that target LRP5 and TGFβ signaling should increase trabecular bone mass in patients with OI more than targeting either signaling pathway alone. Whether additive increases in trabecular bone mass will occur in, and clinically benefit, patients with OI needs to be determined.

A xenograft model to evaluate the bone forming effects of sclerostin antibody in human bone derived from pediatric osteogenesis imperfecta patients

Osteogenesis imperfecta (OI) is a rare and severe skeletal dysplasia marked by low bone mass and poor bone quality which is especially burdensome during childhood. Since clinical trials for pediatric OI are difficult, there is a widespread reliance on genetically modified murine models to understand the skeletal effects of emerging therapeutics. However a common model does not yet exist to understand how patient-specific genotype may influence treatment efficacy. Recently, sclerostin antibody (SclAb) has been introduced as a novel putative anabolic therapy for diseases of low bone mass, but effects in pediatric patients remain unexplored. In this study, we aim to establish a direct xenograft approach using OI patient-derived bone isolates which retain patient-specific genetic defects and cells residing in their intrinsic extracellular environment to evaluate the bone-forming effects of SclAb as a bridge to clinical trials. OI and age matched non-OI patient bone typically discarded as surgical waste during corrective orthopaedic procedures were collected, trimmed and implanted subcutaneously (s.c.) on the dorsal surface of 4-6-week athymic mice. A subset of implanted mice were evaluated at short (1 week), intermediate (4 week), and long-term (12 week) durations to assess bone cell survival and presence of donor bone cells in order to determine an appropriate treatment duration. Remaining implanted mice were randomly assigned to a two or four-week SclAb-treated (25mg/kg s.c. 2QW) or untreated control group. Immunohistochemistry determined osteocyte and osteoblast donor/host relationship, TRAP staining quantified osteoclast activity, and TUNEL assay was used to understand rates of bone cell apoptosis at each implantation timepoint. Longitudinal changes of in vivo μCT outcomes and dynamic histomorphometry were used to assess treatment response and ex vivo μCT and dynamic histomorphometry of host femora served as a positive internal control to confirm a bone forming response to SclAb. Human-derived osteocytes and lining cells were present up to 12 weeks post-implantation with nominal cell apoptosis in the implant. Sclerostin expression remained donor-derived throughout the study. Osterix expression was primarily donor-derived in treated implants and shifted in favor of the host when implants remained untreated. μCT measures of BMD, TMD, BV/TV and BV increased with treatment but response was variable and impacted by bone implant morphology (trabecular, cortical) which was corroborated by histomorphometry. There was no statistical difference between treated and untreated osteoclast number in the implants. Host femora confirmed a systemic bone forming effect of SclAb. Findings support use of the xenograft model using solid bone isolates to explore the effects of novel bone-targeted therapies. These findings will impact our understanding of SclAb therapy in pediatric OI tissue through establishing the efficacy of this treatment in human cells prior to extension to the clinic.

Sclerostin Antibody-Induced Changes in Bone Mass Are Site Specific in Developing Crania

Sclerostin antibody (Scl-Ab) is an anabolic bone agent that has been shown to increase bone mass in clinical trials of adult diseases of low bone mass, such as osteoporosis and osteogenesis imperfecta (OI). Its use to decrease bone fragility in pediatric OI has shown efficacy in several growing mouse models, suggesting translational potential to pediatric disorders of low bone mass. However, the effects of pharmacologic inhibition of sclerostin during periods of rapid growth and development have not yet been described with respect to the cranium, where lifelong deficiency of functioning sclerostin leads to patterns of excessive bone growth, cranial compression, and facial palsy. In the present study, we undertook dimensional and volumetric measurements in the skulls of growing Brtl/+ OI mice treated with Scl-Ab to examine whether therapy-induced phenotypic changes were similar to those observed clinically in patients with sclerosteosis or Van Buchem disorder. Mice treated between 3 and 14 weeks of age with high doses of Scl-Ab show significant calvarial thickening capable of rescuing OI-induced deficiencies in skull thickness. Other changes in cranial morphology, such as lengths and distances between anatomic landmarks, intracranial volume, and suture interdigitation, showed minimal effects of Scl-Ab when compared with growth-induced differences over the treatment duration. Treatment-induced narrowing of foramina was limited to sites of vascular but not neural passage, suggesting patterns of local regulation. Together, these findings reveal a site specificity of Scl-Ab action in the calvaria with no measurable cranial nerve impingement or brainstem compression. This differentiation from the observed outcomes of lifelong sclerostin deficiency complements reports of Scl-Ab treatment efficacy at other skeletal sites with the prospect of minimal cranial secondary complications. © 2019 American Society for Bone and Mineral Research. © 2019 American Society for Bone and Mineral Research.

Biomechanical and Bone Material Properties of Schnurri-3 Null Mice

Schnurri‐3 (Shn3) is an essential regulator of postnatal skeletal remodeling. Shn3‐deficient mice (Shn3–/–) have high bone mass; however, their bone mechanical and material properties have not been investigated to date. We performed three‐point bending of femora, compression tests of L3 vertebrae. We also measured intrinsic material properties, including bone mineralization density distribution (BMDD) and osteocyte lacunae section (OLS) characteristics by quantitative backscatter electron imaging, as well as collagen cross‐linking by Fourier transform infrared microspectroscopy of femora from Shn3–/– and WT mice at different ages (6 weeks, 4 months, and 18 months). Moreover, computer modeling was performed for the interpretation of the BMDD outcomes. Femora and L3 vertebrae from Shn3–/– aged 6 weeks revealed increased ultimate force (2.2‐ and 3.2‐fold, p < .01, respectively). Mineralized bone volume at the distal femoral metaphysis was about twofold (at 6 weeks) to eightfold (at 4 and 18 months of age) in Shn3–/– (p < .001). Compared with WT, the average degree of trabecular bone mineralization was similar at 6 weeks, but increased at 4 and 18 months of age (+12.6% and +7.7%, p < .01, respectively) in Shn3–/–. The analysis of OLS characteristics revealed a higher OLS area for Shn3–/– versus WT at all ages (+16%, +23%, +21%, respectively, p < .01). The collagen cross‐link ratio was similar between groups. We conclude that femora and vertebrae from Shn3–/– had higher ultimate force in mechanical testing. Computer modeling demonstrated that in cases of highly increased bone volume, the average degree of bone matrix mineralization can be higher than in WT bone, which was actually measured in the older Shn3–/– groups. The area of 2D osteocyte lacunae sections was also increased in Shn3‐deficiency, which could only partly be explained by larger remnant areas of primary cortical bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Sclerostin antibody reduces long bone fractures in the oim/oim model of osteogenesis imperfecta

Osteogenesis imperfecta type III (OI) is a serious genetic condition with poor bone quality and a high fracture rate in children. In a previous study, it was shown that a monoclonal antibody neutralizing sclerostin (Scl-Ab) increases strength and vertebral bone mass while reducing the number of axial fractures in oim/oim, a mouse model of OI type III. Here, we analyze the impact of Scl-Ab on long bones in OI mice. After 9 weeks of treatment, Scl-Ab significantly reduced long bone fractures (3.6 ± 0.3 versus 2.1 ± 0.8 per mouse, p < 0.001). In addition, the cortical thickness of the tibial midshaft was increased (+42%, p < 0.001), as well as BMD (+28%, p < 0.001), ultimate load (+86%, p < 0.05), plastic energy (+184%; p < 0.05) and stiffness (+172%; p < 0.01) in OI Scl-Ab mice compared to OI vehicle controls. Similar effects of Scl-Ab were observed in Wild type (Wt) mice. The plastic energy, which reflects the fragility of the tissue, was lower in the OI than in the Wt and significantly improved with the Scl-Ab treatment. At the tissue level by nanoindentation, Scl-Ab slightly increased the elastic modulus in bones of both OI and Wt, while moderately increasing tissue hardness (+13% compared to the vehicle; p < 0.05) in Wt bones, but not in OI bones. Although it did not change the properties of the OI bone matrix material, Scl-Ab reduced the fracture rate of the long bones by improving its bone mass, density, geometry, and biomechanical strength. These results suggest that Scl-Ab can reduce long-bone fractures in patients with OI.

Differential changes in bone strength of two inbred mouse strains following administration of a sclerostin-neutralizing antibody during growth

Administration of sclerostin-neutralizing antibody (Scl-Ab) treatment has been shown to elicit an anabolic bone response in growing and adult mice. Prior work characterized the response of individual mouse strains but did not establish whether the impact of Scl-Ab on whole bone strength would vary across different inbred mouse strains. Herein, we tested the hypothesis that two inbred mouse strains (A/J and C57BL/6J (B6)) will show different whole bone strength outcomes following sclerostin-neutralizing antibody (Scl-Ab) treatment during growth (4.5–8.5 weeks of age). Treated B6 femurs showed a significantly greater stiffness (S) (68.8% vs. 46.0%) and maximum load (ML) (84.7% vs. 44.8%) compared to A/J. Although treated A/J and B6 femurs showed greater cortical area (Ct.Ar) similarly relative to their controls (37.7% in A/J and 41.1% in B6), the location of new bone deposition responsible for the greater mass differed between strains and may explain the greater whole bone strength observed in treated B6 mice. A/J femurs showed periosteal expansion and endocortical infilling, while B6 femurs showed periosteal expansion. Post-yield displacement (PYD) was smaller in treated A/J femurs (-61.2%, p < 0.001) resulting in greater brittleness compared to controls; an effect not present in B6 mice. Inter-strain differences in S, ML, and PYD led to divergent changes in work-to-fracture (Work). Work was 27.2% (p = 0.366) lower in treated A/J mice and 66.2% (p < 0.001) greater in treated B6 mice relative to controls. Our data confirmed the anabolic response to Scl-Ab shown by others, and provided evidence suggesting the mechanical benefits of Scl-Ab administration may be modulated by genetic background, with intrinsic growth patterns of these mice guiding the location of new bone deposition. Whether these differential outcomes will persist in adult and elderly mice remains to be determined.

Stimulatory Effects of Boron Containing Bioactive Glass on Osteogenesis and Angiogenesis of Polycaprolactone: In Vitro Study

Polycaprolactone (PCL) has attracted great attention for bone regeneration attributed to its cost-efficiency, high toughness, and good processability. However, the relatively low elastic modulus, hydrophobic nature, and insufficient bioactivity of pure PCL limited its wider application for bone regeneration. In the present study, the effects of the addition of boron containing bioactive glass (B-BG) materials on the mechanical properties and biological performance of PCL polymer were investigated with different B-BG contents (0, 10, 20, 30, and 40 wt.%), in order to evaluate the potential applications of B-BG/PCL composites for bone regeneration. The results showed that the B-BG/PCL composites possess better tensile strength, human neutral pH value, and fast degradation as compared to pure PCL polymers. Moreover, the incorporation of B-BG could enhance proliferation, osteogenic differentiation, and angiogenic factor expression for rat bone marrow stromal cells (rBMSCs) as compared to pure PCL polymers. Importantly, the B-BG also promoted the angiogenic differentiation for human umbilical vein endothelial cells (HUVECs). These enhanced effects had a concentration dependence of B-BG content, while 30 wt.% B-BG/PCL composites achieved the greatest stimulatory effect. Therefore the 30 wt.% B-BG/PCL composites have potential applications in bone reconstruction fields.

Osteogenesis Imperfecta: New Perspectives From Clinical and Translational Research

Osteogenesis imperfecta (OI) is a monogenic bone fragility disorder that usually is caused by mutations in one of the two genes coding for collagen type I alpha chains, COL1A1 or COL1A2. Mutations in at least 18 other genes can also lead to an OI phenotype. As genetic testing is more widely used, mutations in these genes are also more frequently discovered in individuals who have a propensity for fractures, but who do not have other typical clinical characteristics of OI. Intravenous bisphosphonate therapy is still the most widely used drug treatment approach. Preclinical studies in OI mouse models have shown encouraging effects when the antiresorptive effect of a bisphosphonate was combined with bone anabolic therapy using a sclerostin antibody. Other novel experimental treatment approaches include inhibition of transforming growth factor beta signaling with a neutralizing antibody and the inhibition of myostatin and activin A by a soluble activin receptor 2B. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research

Effect of Anti-TGF-β Treatment in a Mouse Model of Severe Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heritable bone fragility disorder that is usually caused by mutations affecting collagen type I encoding genes. Recent studies in mouse models of recessive OI, Crtap^-/- mice, and dominant OI, +/G610C mice, found that application of a transforming growth factor beta (TGF-β) neutralizing antibody 1D11 rescues the bone phenotype. In the present study, we investigated TGF-β signaling in a mouse model of severe dominant OI with a high incidence of spontaneous fractures, Col1a1^Jrt/+ mice, and the effect of TGF-β neutralizing antibody 1D11 on bone phenotype in 8-week-old mice. Col1a1^Jrt/+ mice had elevated TGF-β signaling in bone tissue. Treatment of Col1a1^Jrt/+ mice with 1D11 was associated with increased bone length but had no significant effect on bone mass or bone mechanical properties, and no significant treatment-associated differences in serum markers of bone formation (alkaline phosphatase activity) or resorption (tartrate-resistant acid phosphatase) were found. Our data thus indicate that the TGF-β neutralizing antibody 1D11 is not effective in a mouse model of dominant OI with a high incidence of spontaneous fractures. © 2018 American Society for Bone and Mineral Research.

Management of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is the term used to describe a group of rare inherited skeletal disorders characterized by a greatly increased risk of fragility fractures (1). Mutations in several genes can cause OI but the condition is most commonly caused by mutations of COLIA1 or COL1A2 resulting in the production of collagen which is abnormal or present in reduced amounts. Fractures in OI are particularly common during childhood but the elevated fracture risk continues throughout life. Bone mineral density (BMD) can be reduced in OI but the magnitude of increase in fracture risk is far greater than can be accounted for by low BMD, highlighting that a key mechanism of bone fragility is reduced bone quality due to defects of bone matrix and mineralization. A multidisciplinary approach is needed to optimize management of OI, with input from physicians, orthopedic surgeons, physiotherapists, occupational therapists, and other allied health professionals. Orthopedic surgery plays a key role both in the fixation of fractures and in the correction of limb deformities. Bisphosphonates have been widely used in the treatment of children and adults with OI. Although there is good evidence that they increase BMD, it is uncertain to what extent they reduce fracture risk. Clinical trials of bone anabolic drugs such as teriparatide and inhibitors of sclerostin have also been studied; although they increase BMD, studies of these agents have not been powered to look at fracture endpoints. Various other treatment modalities including denosumab, and cell therapy have been explored but haven't gained acceptance in routine clinical practice. There have been huge advances in understanding the pathogenesis of OI but these have not been accompanied by advances in treatment. This signals need for well-designed clinical trials with fracture endpoints in OI, both with existing agents and with the newer therapeutic agents that are now starting to emerge.

High Fidelity of Mouse Models Mimicking Human Genetic Skeletal Disorders

The 2019 International Skeletal Dysplasia Society nosology update lists 441 genes for which mutations result in rare human skeletal disorders. These genes code for enzymes (33%), scaffolding proteins (18%), signal transduction proteins (16%), transcription factors (14%), cilia proteins (8%), extracellular matrix proteins (5%), and membrane transporters (4%). Skeletal disorders include aggrecanopathies, channelopathies, ciliopathies, cohesinopathies, laminopathies, linkeropathies, lysosomal storage diseases, protein-folding and RNA splicing defects, and ribosomopathies. With the goal of evaluating the ability of mouse models to mimic these human genetic skeletal disorders, a PubMed literature search identified 260 genes for which mutant mice were examined for skeletal phenotypes. These mouse models included spontaneous and ENU-induced mutants, global and conditional gene knockouts, and transgenic mice with gene over-expression or specific base-pair substitutions. The human X-linked gene ARSE and small nuclear RNA U4ATAC, a component of the minor spliceosome, do not have mouse homologs. Mouse skeletal phenotypes mimicking human skeletal disorders were observed in 249 of the 260 genes (96%) for which comparisons are possible. A supplemental table in spreadsheet format provides PubMed weblinks to representative publications of mutant mouse skeletal phenotypes. Mutations in 11 mouse genes (Ccn6, Cyp2r1, Flna, Galns, Gna13, Lemd3, Manba, Mnx1, Nsd1, Plod1, Smarcal1) do not result in similar skeletal phenotypes observed with mutations of the homologous human genes. These discrepancies can result from failure of mouse models to mimic the exact human gene mutations. There are no obvious commonalities among these 11 genes. Body BMD and/or radiologic dysmorphology phenotypes were successfully identified for 28 genes by the International Mouse Phenotyping Consortium (IMPC). Forward genetics using ENU mouse mutagenesis successfully identified 37 nosology gene phenotypes. Since many human genetic disorders involve hypomorphic, gain-of-function, dominant-negative and intronic mutations, future studies will undoubtedly utilize CRISPR/Cas9 technology to examine transgenic mice having genes modified to exactly mimic variant human sequences. Mutant mice will increasingly be employed for drug development studies designed to treat human genetic skeletal disorders.

SIGNIFICANCE

Great progress is being made identifying mutant genes responsible for human rare genetic skeletal disorders and mouse models for genes affecting bone mass, architecture, mineralization and strength. This review organizes data for 441 human genetic bone disorders with regard to heredity, gene function, molecular pathways, and fidelity of relevant mouse models to mimic the human skeletal disorders. PubMed weblinks to citations of 249 successful mouse models are provided.

Osteogenesis imperfecta and therapeutics

Osteogenesis imperfecta, or brittle bone disease, is a congenital disease that primarily causes low bone mass and bone fractures but it can negatively affect other organs. It is usually inherited in an autosomal dominant fashion, although rarer recessive and X-chromosome-linked forms of the disease have been identified. In addition to type I collagen, mutations in a number of other genes, often involved in type I collagen synthesis or in the differentiation and function of osteoblasts, have been identified in the last several years. Seldom, the study of a rare disease has delivered such a wealth of new information that have helped our understanding of multiple processes involved in collagen synthesis and bone formation. In this short review I will describe the clinical features and the molecular genetics of the disease, but then focus on how OI dysregulates all aspects of extracellular matrix biology. I will conclude with a discussion about OI therapeutics.

Osteogenesis Imperfecta: Muscle-Bone Interactions when Bi-directionally Compromised

PURPOSE OF REVIEW

Osteogenesis imperfecta (OI) is a hereditary connective tissue disorder of skeletal fragility and more recently muscle weakness. This review highlights our current knowledge of the impact of compromised OI muscle function on muscle-bone interactions and skeletal strength in OI.

RECENT FINDINGS

The ramifications of inherent muscle weakness in OI muscle-bone interactions are just beginning to be elucidated. Studies in patients and in OI mouse models implicate altered mechanosensing, energy metabolism, mitochondrial dysfunction, and paracrine/endocrine crosstalk in the pathogenesis of OI. Compromised muscle-bone unit impacts mechanosensing and the ability of OI muscle and bone to respond to physiotherapeutic and pharmacologic treatment strategies. Muscle and bone are both compromised in OI, making it essential to understand the mechanisms responsible for both impaired muscle and bone functions and their interdependence, as this will expand and drive new physiotherapeutic and pharmacological approaches to treat OI and other musculoskeletal disorders.

Loss of RANKL in osteocytes dramatically increases cancellous bone mass in the osteogenesis imperfecta mouse (oim)

Osteogenesis imperfecta (OI) is characterized by osteopenia and bone fragility, and OI patients during growth often exhibit high bone turnover with the net result of low bone mass. Recent evidence shows that osteocytes significantly affect bone remodeling under physiological and pathological conditions through production of osteoclastogenic cytokines. The receptor activator of nuclear factor kappa-B ligand (RANKL) produced by osteocytes for example, is a critical mediator of bone loss caused by ovariectomy, low-calcium diet, unloading and glucocorticoid treatment. Because OI bone has increased density of osteocytes and these cells are embedded in matrix with abnormal type I collagen, we hypothesized that osteocyte-derived RANKL contributes to the OI bone phenotype. In this study, the conditional loss of RANKL in osteocytes in oim/oim mice (oim-RANKL-cKO) resulted in dramatically increased cancellous bone mass in both the femur and lumbar spine compared to oim/oim mice. Bone cortical thickness increased significantly only in spine but ultimate bone strength in the long bone and spine was minimally improved in oim-RANKL-cKO mice compared to oim/oim mice. Furthermore, unlike previous findings, we report that oim/oim mice do not exhibit high bone turnover suggesting that their low bone mass is likely due to defective bone formation and not increased bone resorption. The loss of osteocyte-derived RANKL further diminished parameters of formation in oim-RANKL-cKO. Our results indicate that osteocytes contribute significantly to the low bone mass observed in OI and the effect of loss of RANKL from these cells is similar to its systemic inhibition.

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Osteocyte-specific deletion of RANKL in oim mice greatly increases cancellous bone.

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Skeletal effects of osteocyte RANKL deletion on OI mimic its systemic inhibition.

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Oim mice do not have high bone turnover.

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Low bone mass in oim mice is primarily caused by decreased bone formation.

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This study supports a potentially important role for osteocytes in OI.

Anabolic Therapy for the Treatment of Osteoporosis in Childhood

PURPOSE OF REVIEW

Numerous forms of osteoporosis in childhood are characterized by low bone turnover (for example, osteoporosis due to neuromuscular disorders and glucocorticoid exposure). Anti-resorptive therapy, traditionally used to treat osteoporosis in the young, is associated with further reductions in bone turnover, raising concerns about the long-term safety and efficacy of such therapy. These observations have led to increasing interest in the role of anabolic therapy to treat pediatric osteoporosis.

RECENT FINDINGS

While growth hormone and androgens appears to be relatively weak anabolic modulators of bone mass, emerging therapies targeting bone formation pathways (anti-transforming growth factor beta antibody and anti-sclerostin antibody) hold considerable promise. Teriparatide remains an attractive option that merits formal study for patients post-epiphyseal fusion, although it must be considered that adult studies have shown its effect is blunted when administered following bisphosphonate therapy. Mechanical stimulation of bone through whole body vibration therapy appears to be much less effective than bisphosphonate therapy for treating osteoporosis in children. New anabolic therapies which target important pathways in skeletal metabolism merit further study in children, including their effects on fracture risk reduction and after treatment discontinuation.

Osteogenesis imperfecta - A clinical update

Osteogenesis imperfecta (OI) is the most common inherited form of bone fragility and includes a heterogenous group of genetic disorders which most commonly result from defects associated with type 1 collagen. 85%-90% of cases are inherited in an autosomal dominant manner and are caused by mutations in the COL1A1 and COL1A2 genes, leading to quantitative or qualitative defects in type 1 collagen. In the last decade, defects in several other proteins involved in the normal processing of type 1 collagen have been described. Recent advances in genetics have called for reconsideration of the classification of OI, however, most recent classifications align with the classic clinical classification by Sillence. The hallmark of the disease is bone fragility but other tissues are also affected. Intravenous bisphosphonates (BPs) are the most widely used intervention, having significant favorable effects regarding areal bone mineral density (BMD) and vertebral reshaping following fractures in growing children. BPs have a modest effect in long bone fracture incidence, their effects in adults with OI concerns only BMD, while there are reports of subtrochanteric fractures resembling atypical femoral fractures. Other therapies showing promising results include denosumab, teriparatide, sclerostin inhibition, combination therapy with antiresorptive and anabolic drugs and TGF-β inhibition. Gene targeting approaches are under evaluation. More research is needed to delineate the best therapeutic approach in this heterogeneous disease.

Sclerostin Antibody Treatment Stimulates Bone Formation to Normalize Bone Mass in Male Down Syndrome Mice

Down syndrome (DS), characterized by trisomy of human chromosome 21, is associated with a variety of endocrine disorders as well as profound skeletal abnormalities. The low bone mass phenotype in DS is defined by low bone turnover due to decreased osteoclast and osteoblast activity, decreasing the utility of antiresorptive agents in people with DS. Sclerostin antibody (SclAb) is a therapeutic candidate currently being evaluated as a bone anabolic agent. Scl, the product of the sclerostin gene (SOST), inhibits bone formation through its inhibition of Wnt signaling. SclAb increases bone mass by suppressing the action of the endogenous inhibitor of bone formation, Scl. To examine the effects of SclAb on the DS bone phenotype, 8‐week‐old male wild‐type (WT) andTs65Dn DS mice were treated with 4 weekly iv injections of 100 mg/kg SclAb. Dual‐energy X‐ray absorptiometry (DXA), microCT, and dynamic histomorphometry analyses revealed that SclAb had a significant anabolic effect on both age‐matched WT littermate controls and Ts65Dn DS mice that was osteoblast mediated, without significant changes in osteoclast parameters. SclAb treatment significantly increased both cortical and trabecular bone mass at multiple sites; SclAb treatment resulted in the normalization of Ts65Dn bone mineral density (BMD) to WT levels in the proximal tibia, distal femur, and whole body. Ex vivo bone marrow cultures demonstrated that SclAb increased the recruitment of the mesenchymal progenitors into the osteoblast lineage, as indicated by increased alkaline phosphatase–positive colonies, with no effect on osteoclast differentiation. Together, in the setting of a murine model of DS and decreased bone turnover, SclAb had a potent anabolic effect. SclAb stimulated bone formation and increased osteoblastogenesis without affecting osteoclastogenesis or bone resorption. These data suggest that SclAb is a promising new therapy to improve bone mass and reduce fracture risk in the face of the low bone mass and turnover prevalent in the DS population. © 2017 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.