A transgenic mouse model of OI type V supports a neomorphic mechanism of the IFITM5 mutation

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.

Does the c.-14C>T Mutation in the IFITM5 Gene Provide Identical Phenotypes for Osteogenesis Imperfecta Type V? Data from Russia and a Literature Review

Osteogenesis imperfecta (OI) is a large group of genetically heterogeneous diseases resulting from decreased bone density and an abnormal microarchitecture, which are clinically manifested by abnormal bone fractures. A distinctive clinical feature of this group of diseases is the presence of spontaneous fractures and skeletal deformities. However, the clinical manifestations of different types of OI are characterized by marked polymorphism with variable severity of skeletal and extra-skeletal features. Previous studies have shown that a mutation (c.-14C>T) in the IFITM5 gene is responsible for autosomal dominant OI type V. However, the mutation has a variable expression pattern and marked clinical heterogeneity. In this study, a clinical and genetic analysis of 12 cases with molecularly confirmed OI type V from 12 unrelated families was performed. Significant clinical heterogeneity of the disease with the same molecular defect was detected. In six subjects (50%), there were no classic signs of OI type V (formation of a hyperplastic bone callus, calcification of the interosseous membrane and dislocation of the radial head). In all cases, the mutation occurred de novo.

Curative Cell and Gene Therapy for Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) describes a series of genetic bone fragility disorders that can have a substantive impact on patient quality of life. The multidisciplinary approach to management of children and adults with OI primarily involves the administration of antiresorptive medication, allied health (physiotherapy and occupational therapy), and orthopedic surgery. However, advances in gene editing technology and gene therapy vectors bring with them the promise of gene-targeted interventions to provide an enduring or perhaps permanent cure for OI. This review describes emergent technologies for cell- and gene-targeted therapies, major hurdles to their implementation, and the prospects of their future success with a focus on bone disorders. © 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).

Understanding Musculoskeletal Disorders Through Next-Generation Sequencing

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An insight into musculoskeletal disorders through advancements in next-generation sequencing (NGS) promises to maximize benefits and improve outcomes through improved genetic diagnosis.

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The primary use of whole exome sequencing (WES) for musculoskeletal disorders is to identify functionally relevant variants.

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The current evidence has shown the superiority of NGS over conventional genotyping for identifying novel and rare genetic variants in patients with musculoskeletal disorders, due to its high throughput and low cost.

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Genes identified in patients with scoliosis, osteoporosis, osteoarthritis, and osteogenesis imperfecta using NGS technologies are listed for further reference.

The Osteogenesis Imperfecta Type V Mutant BRIL/IFITM5 Promotes Transcriptional Activation of MEF2, NFATc, and NR4A in Osteoblasts

BRIL (bone restricted ifitm-like; also known as IFITM5) is a transmembrane protein expressed in osteoblasts. Although its role in skeletal development and homeostasis is unknown, mutations in BRIL result in rare dominant forms of osteogenesis imperfecta. The pathogenic mechanism has been proposed to be a gain-of or neomorphic function. To understand the function of BRIL and its OI type V mutant (MALEP BRIL) and whether they could activate signaling pathways in osteoblasts, we performed a luciferase reporter assay screen based on the activity of 26 transcription factors. When overexpressed in MC3T3-E1 and MLO-A5 cells, the MALEP BRIL activated the reporters dependent on MEF2, NFATc, and NR4A significantly more. Additional co-transfection experiments with MEF2C and NFATc1 and a number of their modulators (HDAC4, calcineurin, RCAN, FK506) confirmed the additive or synergistic activation of the pathways by MALEP, and suggested a coordinated regulation involving calcineurin. Endogenous levels of Nr4a members, as well as Ptgs2, were upregulated by MALEP BRIL. Y2H and co-immunoprecipitation indicated that BRIL interacted with CAML, but its contribution as the most upstream stimulator of the Ca²⁺-calcineurin-MEF2/NFATc cascade was not confirmed convincingly. Altogether the data presented provide the first ever readout to monitor for BRIL activity and suggest a potential gain-of-function causative effect for MALEP BRIL in OI type V, leading to perturbed signaling events and gene expression.

A Novel IFITM5 Variant Associated with Phenotype of Osteoporosis with Calvarial Doughnut Lesions: A Case Report

Osteogenesis imperfecta (OI) and other decreased bone density disorders comprise a heterogeneous group of heritable diseases with skeletal fragility. Recently, it was discovered that mutations in SGMS2, encoding sphingomyelin synthetase 2, result in aberrant sphingomyelin metabolism and lead to a novel form of OI termed osteoporosis with calvarial doughnut lesions (OP-CDL) with moderate to severe skeletal fragility and variable cranial hyperostotic lesions. This study describes a Japanese family with the skeletal phenotype of OP-CDL. The affected individuals have moderately severe, childhood-onset skeletal fragility with multiple long-bone fractures, scoliosis and bone deformities. In addition, they exhibit multiple CDLs or calvarial bumps with central radiolucency and peripheral radiopacity. However, SGMS2 sequencing was normal. Instead, whole-exome sequencing identified a novel IFITM5 missense mutation c.143A>G (p.N48S) (classified as a VUS by ACMG). IFITM5 encodes an osteoblast-restricted protein BRIL and a recurrent c.-14C>T mutation in its 5' UTR region results in OI type V, a distinctive subtype of OI associated with hyperplastic callus formation and ossification of the interosseous membranes. The patients described here have a phenotype clearly different from OI type V and with hyperostotic cranial lesions, feature previously unreported in association with IFITM5. Our findings expand the genetic spectrum of OP-CDL, indicate diverse phenotypic consequences of pathogenic IFITM5 variants, and imply an important role for BRIL in cranial skeletogenesis.

The Polygenic and Monogenic Basis of Paediatric Fractures

PURPOSE OF REVIEW

Fractures are frequently encountered in paediatric practice. Although recurrent fractures in children usually unveil a monogenic syndrome, paediatric fracture risk could be shaped by the individual genetic background influencing the acquisition of bone mineral density, and therefore, the skeletal fragility as shown in adults. Here, we examine paediatric fractures from the perspective of monogenic and complex trait genetics.

RECENT FINDINGS

Large-scale genome-wide studies in children have identified ~44 genetic loci associated with fracture or bone traits whereas ~35 monogenic diseases characterized by paediatric fractures have been described. Genetic variation can predispose to paediatric fractures through monogenic risk variants with a large effect and polygenic risk involving many variants of small effects. Studying genetic factors influencing peak bone attainment might help in identifying individuals at higher risk of developing early-onset osteoporosis and discovering drug targets to be used as bone restorative pharmacotherapies to prevent, or even reverse, bone loss later in life.

A novel variant of the IFITM5 gene within the 5'-UTR causes neonatal transverse clavicular fracture: Expanding the genetic spectrum

Background

Osteogenesis imperfecta (OI) type V is a rare heritable bone disorder caused by pathogenic variants of IFITM5. Only two mutated alleles in IFITM5 have been identified worldwide, the role of which in OI pathology is not fully understood.

Methods

A neonatal case of suspected OI, clinically manifested as a rare clavicle transection fracture with delayed early fracture healing, was studied. Subtle variants of OI‐associated genes were analyzed by whole exome sequencing and confirmed by Sanger sequencing.

Results

A de novo heterozygous pathogenic variant of IFITM5 within the 5′‐UTR (c.‐9C > A) was discovered in the proband. Bioinformatics analysis using a combination of various algorithms predicted that the variant would generate a new in‐frame start codon 9 bp upstream of the original and express a mutant IFITM5 protein with three additional amino acids (Met‐Glu‐Pro). After transfection into a eukaryocyte in vitro, the mutant IFITM5 construct produced a longer transcription product than that of wild‐type IFITM5.

Conclusion

This study identified a novel pathogenic variant of IFITM5, which not only manifested the molecular characteristics of IFITM5, but also provided new evidence for the study of the molecular mechanisms of IFITM5 association with OI.

Suitability and limitations of mesenchymal stem cells to elucidate human bone illness

Functional impairment of mesenchymal stem cells (MSCs), osteoblast progenitor cells, has been proposed to be a pathological mechanism contributing to bone disorders, such as osteoporosis (the most common bone disease) and other rare inherited skeletal dysplasias. Pathological bone loss can be caused not only by an enhanced bone resorption activity but also by hampered osteogenic differentiation of MSCs. The majority of the current treatment options counteract bone loss, and therefore bone fragility by blocking bone resorption. These so-called antiresorptive treatments, in spite of being effective at reducing fracture risk, cannot be administered for extended periods due to security concerns. Therefore, there is a real need to develop osteoanabolic therapies to promote bone formation. Human MSCs emerge as a suitable tool to study the etiology of bone disorders at the cellular level as well as to be used for cell therapy purposes for bone diseases. This review will focus on the most relevant findings using human MSCs as an in vitro cell model to unravel pathological bone mechanisms and the application and outcomes of human MSCs in cell therapy clinical trials for bone disease.

Bone biology: insights from osteogenesis imperfecta and related rare fragility syndromes

The limited accessibility of bone and its mineralized nature have restricted deep investigation of its biology. Recent breakthroughs in identification of mutant proteins affecting bone tissue homeostasis in rare skeletal diseases have revealed novel pathways involved in skeletal development and maintenance. The characterization of new dominant, recessive and X-linked forms of the rare brittle bone disease osteogenesis imperfecta (OI) and other OI-related bone fragility disorders was a key player in this advance. The development of in vitro models for these diseases along with the generation and characterization of murine and zebrafish models contributed to dissecting previously unknown pathways. Here, we describe the most recent advances in the understanding of processes involved in abnormal bone mineralization, collagen processing and osteoblast function, as illustrated by the characterization of new causative genes for OI and OI-related fragility syndromes. The coordinated role of the integral membrane protein BRIL and of the secreted protein PEDF in modulating bone mineralization as well as the function and cross-talk of the collagen-specific chaperones HSP47 and FKBP65 in collagen processing and secretion are discussed. We address the significance of WNT ligand, the importance of maintaining endoplasmic reticulum membrane potential and of regulating intramembrane proteolysis in osteoblast homeostasis. Moreover, we also examine the relevance of the cytoskeletal protein plastin-3 and of the nucleotidyltransferase FAM46A. Thanks to these advances, new targets for the development of novel therapies for currently incurable rare bone diseases have been and, likely, will be identified, supporting the important role of basic science for translational approaches.

Increased autophagy in EphrinB2-deficient osteocytes is associated with elevated secondary mineralization and brittle bone

Mineralized bone forms when collagen-containing osteoid accrues mineral crystals. This is initiated rapidly (primary mineralization), and continues slowly (secondary mineralization) until bone is remodeled. The interconnected osteocyte network within the bone matrix differentiates from bone-forming osteoblasts; although osteoblast differentiation requires EphrinB2, osteocytes retain its expression. Here we report brittle bones in mice with osteocyte-targeted EphrinB2 deletion. This is not caused by low bone mass, but by defective bone material. While osteoid mineralization is initiated at normal rate, mineral accrual is accelerated, indicating that EphrinB2 in osteocytes limits mineral accumulation. No known regulators of mineralization are modified in the brittle cortical bone but a cluster of autophagy-associated genes are dysregulated. EphrinB2-deficient osteocytes displayed more autophagosomes in vivo and in vitro, and EphrinB2-Fc treatment suppresses autophagy in a RhoA-ROCK dependent manner. We conclude that secondary mineralization involves EphrinB2-RhoA-limited autophagy in osteocytes, and disruption leads to a bone fragility independent of bone mass.

Osteoblasts mediate bone formation, and their differentiation requires expression of EphrinB2. Here, the authors show that EphrinB2 is also expressed by osteocytes, and that its genetic ablation in mice is associated with altered autophagy, elevated mineralization and brittle bone.

The Progress of CRISPR/Cas9-Mediated Gene Editing in Generating Mouse/Zebrafish Models of Human Skeletal Diseases

Genetic factors play a substantial role in the etiology of skeletal diseases, which involve 1) defects in skeletal development, including intramembranous ossification and endochondral ossification; 2) defects in skeletal metabolism, including late bone growth and bone remodeling; 3) defects in early developmental processes related to skeletal diseases, such as neural crest cell (NCC) and cilia functions; 4) disturbance of the cellular signaling pathways which potentially affect bone growth. Efficient and high-throughput genetic methods have enabled the exploration and verification of disease-causing genes and variants. Animal models including mouse and zebrafish have been extensively used in functional mechanism studies of causal genes and variants. The conventional approaches of generating mutant animal models include spontaneous mutagenesis, random integration, and targeted integration via mouse embryonic stem cells. These approaches are costly and time-consuming. Recent development and application of gene-editing tools, especially the CRISPR/Cas9 system, has significantly accelerated the process of gene-editing in diverse organisms. Here we review both mice and zebrafish models of human skeletal diseases generated by CRISPR/Cas9 system, and their contributions to deciphering the underpins of disease mechanisms.

IFITM5 pathogenic variant causes osteogenesis imperfecta V with various phenotype severity in Ukrainian and Vietnamese patients

BACKGROUND

Osteogenesis imperfecta (OI) covers a spectrum of bone fragility disorders. OI is classified into five types; however, the genetic causes of OI might hide in pathogenic variants of 20 different genes. Often clinical OI types mimic each other. This sometimes makes it impossible to identify the OI type clinically, which can be a risk for patients. Up to 90% of OI types I-IV are caused by pathogenic variants in the COL1A1/2 genes. OI type V is caused by the c.-14C > T pathogenic variant in the 5'UTR of the IFITM5 gene and is characterized by hyperplastic callus formation and the ossification of interosseous membranes.

RESULTS

In the current study, we performed IFITM5 5'UTR region mutational analysis using Sanger sequencing on 90 patients who were negative for COL1A1/2 pathogenic variants. We also investigated the phenotypes of five patients with genetically confirmed OI type V. The proportion of OI type V patients in our cohort of all OI patients was 1.48%. In one family, there was a history of OI in at least three generations. Phenotype severity differed from mild to extremely severe among patients, but all patients harbored the same typical pathogenic variant. One patient had no visible symptoms of OI type V and was suspected to have had OI type IV previously. We also identified a case of extremely severe hyperplastic callus in a 15-year-old male, who has hearing loss and brittleness of teeth.

CONCLUSIONS

OI type V is underlined with some unique clinical features; however, not all patients develop them. The phenotype spectrum might be even broader than previously suspected, including typical OI features: teeth brittleness, bluish sclera, hearing loss, long bones deformities, and joint laxity. We suggest that all patients negative for COL1A1/2 pathogenic variants be tested for the presence of an IFITM5 pathogenic variant, even if they are not expressing typical OI type V symptoms. Further studies on the pathological nature and hyperplastic callus formation mechanisms of OI type V are necessary.

Expanding the phenotypic spectrum of osteogenesis imperfecta type V including heterotopic ossification of muscle origins and attachments

Background

Osteogenesis imperfecta (OI) is a clinical and genetic heterogeneous group of connective tissue disorders, characterized by bone fragility and a propensity to fracture.

Methods

In this report we describe the clinical phenotype of two patients, a 28‐year‐old woman and her mother (54 years old), both with a history of short stature and multiple fractures.

Results

Exome sequencing revealed the recurring IFITM5:c.‐14 C>T variant causing OI type V. Both patients had several fractures during childhood. CT‐scan and scintigraphy showed ossification of the origin and attachment of muscles and hypertrophic callus formation.

Conclusion

Ossification of the origin and attachment of muscles seems to be part of the phenotype in patients with OI type V.

Tissue Morphology and Antigenicity in Mouse and Rat Tibia: Comparing 12 Different Decalcification Conditions

Conventional bone decalcification is a time-consuming process and is therefore unsuitable for clinical applications and time-limited research projects. Consequently, we compared the effect of four different decalcification solutions applied at three different temperatures, and assessed the rate of decalcification and the implications on tissue morphology and antigenicity of mouse and rat tibiae. Bones were decalcified with 10% ethylenediaminetetraacetic acid (EDTA), 10% formic acid, 5% hydrochloric acid, and 5% nitric acid at 4C, 25C, and 37C. Decalcification in both species was fastest in nitric acid at 37C and slowest in EDTA at 4C. Histological and immunohistochemical staining confirmed that the conventional protocols of EDTA at 4C and 25C remain the best option regarding the quality of tissue preservation. Whereas formic acid at 4C is a good alternative saving about 90% of the decalcification time, hydrochloric and nitric acids should be avoided particularly in case of rat tibia. By contrast, due to their smaller size, mouse tibiae had shorter decalcification times and tolerated higher temperatures and exposure to acids much better. In conclusion, this study demonstrated that depending on the specific research question and sample size, alternative decalcification methods could be used to decrease the time of decalcification while maintaining histological accuracy.

Expanding the Clinical Spectrum of Osteogenesis Imperfecta Type V: 13 Additional Patients and Review

Osteogenesis imperfecta (OI) is an inherited connective tissue disorder characterized by bone fragility and is characterized by clinical and genetic heterogeneity. Previous studies showed that the same mutation (c.-14C> T) of the IFITM5 gene is responsible for autosomal dominant OI type V. However, the mutation has a variable expressivity. Clinical heterogeneity has been recognized in OI type V. In this study, we investigated 13 individuals with molecularly confirmed OI type V from seven Chinese families and explored the genotype-phenotype relationship. Increased callus formation is not observed in all individuals, and several novel clinical features were described: joint contractures (three individuals) and unexplained hip arthritis (six individuals). Significant clinical variability was observed even within families. Specific facial features were observed in six individuals from two families consistent with the facial features associated with OI type V reported so far in the literature. Interestingly, we report the process of hypertrophic callus formation in detail for the first time, and in five individuals with hyperplastic callus, increased erythrocyte sedimentation rate (ESR) and levels of C-reactive protein (C-RP) were measured, suggestive of inflammatory activation.

Oro-dental and cranio-facial characteristics of osteogenesis imperfecta type V

Osteogenesis imperfecta (OI) type V is an ultrarare heritable bone disorder caused by the heterozygous c.-14C > T mutation in IFITM5. The oro-dental and craniofacial phenotype has not been described in detail, which we therefore undertook to evaluate in a multicenter study (Brittle Bone Disease Consortium). Fourteen individuals with OI type V (age 3-50 years; 10 females, 4 males) underwent dental and craniofacial assessment. None of the individuals had dentinogenesis imperfecta. Six of the 9 study participants (66%) for whom panoramic radiographs were obtained had at least one missing tooth (range 1-9). Class II molar occlusion was present in 8 (57%) of the 14 study participants. The facial profile was retrusive and lower face height was decreased in 8 (57%) individuals. Cephalometry, performed in three study participants, revealed a severely retrusive maxilla and mandible, and moderately to severly retroclined incisors in a 14-year old girl, a protrusive maxilla and a retrusive mandible in a 14-year old boy. Cone beam computed tomograpy scans were obtained from two study participants and demonstrated intervertebral disc calcification at the C2-C3 level in one individual. Our study observed that OI type V is associated with missing permanent teeth, especially permanent premolar, but not with dentinogenesis imperfecta. The pattern of craniofacial abnormalities in OI type V thus differs from that in other severe OI types, such as OI type III and IV, and could be described as a bimaxillary retrusive malocclusion with reduced lower face height and multiple missing teeth.

Crispr-Cas9 engineered osteogenesis imperfecta type V leads to severe skeletal deformities and perinatal lethality in mice

Osteogenesis imperfecta (OI) type V is caused by an autosomal dominant mutation in the IFITM5 gene, also known as BRIL. The c.-14C>T mutation in the 5'UTR of BRIL creates a novel translational start site adding 5 residues (MALEP) in frame with the natural coding of BRIL. A neomorphic function has been proposed for the MALEP-BRIL but the mechanisms at play are still unknown. In order to further understand the effects of MALEP-BRIL in vivo, we generated a knockin (KI) mouse model having the exact genetic -14C>T replica of patients with OI type V. Live KI descendants were never obtained from 2 male mosaic founders. Skeletal staining with alizarin red/alcian blue and μCT imaging of KI embryos revealed striking skeletal anomalies such as hypomineralized skull, short and bent long bones, and frail and wavy ribs. Histology and histochemical labeling revealed that midshaft of long bones was filled with hypertrophic chondrocytes, lacked a defined primary ossification center with the absence of defined cortices. Gene expression monitoring at E15.5 and E17.5 showed no change in Osx but decreased Bril itself as well as other differentiated osteoblast markers (Ibsp, Bglap, Sost). However, upregulation of Ptgs2 and Nr4a3 suggested that a pro-inflammatory reaction was activated. Primary osteoblasts from KI calvaria showed delayed differentiation and mineralization, with decreased abundance of BRIL. However, the upregulation AdipoQ and Fabp4 in young cultures indicated a possible switch in fate towards adipogenesis. Altogether our data suggest that the low level expression of MALEP-BRIL in Osx+ mesenchymal progenitors blunted their further differentiation into mature osteoblasts, which may have resulted in part from an inflammatory response.

The osteogenic cell surface marker BRIL/IFITM5 is dispensable for bone development and homeostasis in mice

BRIL (bone-restricted IFITM-like), is a short transmembrane protein expressed almost exclusively in osteoblasts. Although much is known about its bone-restricted gene expression pattern and protein biochemical and topological features, little information is available for BRIL physiological function. Two autosomal dominant forms of osteogenesis imperfecta (OI) are caused by distinct, but recurrent mutations in the BRIL gene. Yet, the underlying mechanisms by which those mutations lead to OI are still poorly understood. A previous report indicated that BRIL knockout (KO) mice had bone deformities, shortened long bones, and reproductive problems. Here we generated and systematically analyzed the skeletal phenotype of a new global Bril KO/LacZ knockin mouse model. KO mice reproduced and thrived normally up to 12 month of age. The skeletal phenotype of KO and WT littermates was assessed at embryonic (E13.5 to E18.5) and postnatal (2 days, 3 weeks, 3 months and 8 months) time-points. Embryos from E13.5 through to E18.5 showed significant X-Gal staining in all skeletal elements without any apparent patterning anomalies. Although bone deformities were never observed at any postnatal ages, minor and transient differences were noted in terms of bone length and static uCT parameters, but not systematically across all ages and genders. These changes, however, were not accompanied by significant alteration in bone material properties as assessed by a 3-point bending test. In addition, no changes were detected in circulating serum markers of bone turnover (P1NP, CTX-I, and osteocalcin). Gene expression monitoring also revealed no major impact of the loss of BRIL. Further, when mice were challenged with a surgically-induced fracture in tibia, bones repaired equally well in the KO mice as compared to WT. Finally, we showed that BRIL C-terminus is not a bona fide binding site for calcium. In conclusion, our in depth analysis suggest that skeletal patterning, bone mass accrual and remodeling in mice proceeded independent of BRIL.

Genetic causes and mechanisms of Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) is a genetic disorder characterized by various clinical features including bone deformities, low bone mass, brittle bones, and connective tissue manifestations. The predominant cause of OI is due to mutations in the two genes that encode type I collagen. However, recent advances in sequencing technology has led to the discovery of novel genes that are implicated in recessive and dominant OI. These include genes that regulate the post-translational modification, secretion and processing of type I collagen as well as those required for osteoblast differentiation and bone mineralization. As such, OI has become a spectrum of genetic disorders informing about the determinants of both bone quantity and quality. Here we summarize the known genetic causes of OI, animal models that recapitulate the human disease and mechanisms that underlie disease pathogenesis. Additionally, we discuss the effects of disrupted collagen networks on extracellular matrix signaling and its impact on disease progression.

Osteogenesis imperfecta: new genes reveal novel mechanisms in bone dysplasia

Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by fragile bones and short stature and known for its clinical and genetic heterogeneity which is now understood as a collagen-related disorder. During the last decade, research has made remarkable progress in identifying new OI-causing genes and beginning to understand the intertwined molecular and biochemical mechanisms of their gene products. Most cases of OI have dominant inheritance. Each new gene for recessive OI, and a recently identified gene for X-linked OI, has shed new light on its (often previously unsuspected) function in bone biology. Here, we summarize the literature that has contributed to our current understanding of the pathogenesis of OI.

Pharmacological and biological therapeutic strategies for osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility, low bone mass, and bone deformities. The majority of cases are caused by autosomal dominant pathogenic variants in the COL1A1 and COL1A2 genes that encode type I collagen, the major component of the bone matrix. The remaining cases are caused by autosomal recessively or dominantly inherited mutations in genes that are involved in the post-translational modification of type I collagen, act as type I collagen chaperones, or are members of the signaling pathways that regulate bone homeostasis. The main goals of treatment in OI are to decrease fracture incidence, relieve bone pain, and promote mobility and growth. This requires a multi-disciplinary approach, utilizing pharmacological interventions, physical therapy, orthopedic surgery, and monitoring nutrition with appropriate calcium and vitamin D supplementation. Bisphosphonate therapy, which has become the mainstay of treatment in OI, has proven beneficial in increasing bone mass, and to some extent reducing fracture risk. However, the response to treatment is not as robust as is seen in osteoporosis, and it seems less effective in certain types of OI, and in adult OI patients as compared to most pediatric cases. New pharmacological treatments are currently being developed, including anti-resorptive agents, anabolic treatment, and gene- and cell-therapy approaches. These therapies are under different stages of investigation from the bench-side, to pre-clinical and clinical trials. In this review, we will summarize the recent findings regarding the pharmacological and biological strategies for the treatment of patients with OI. © 2016 Wiley Periodicals, Inc.

Animal models of osteogenesis imperfecta: applications in clinical research

Osteogenesis imperfecta (OI), commonly known as brittle bone disease, is a genetic disease characterized by extreme bone fragility and consequent skeletal deformities. This connective tissue disorder is caused by mutations in the quality and quantity of the collagen that in turn affect the overall mechanical integrity of the bone, increasing its vulnerability to fracture. Animal models of the disease have played a critical role in the understanding of the pathology and causes of OI and in the investigation of a broad range of clinical therapies for the disease. Currently, at least 20 animal models have been officially recognized to represent the phenotype and biochemistry of the 17 different types of OI in humans. These include mice, dogs, and fish. Here, we describe each of the animal models and the type of OI they represent, and present their application in clinical research for treatments of OI, such as drug therapies (ie, bisphosphonates and sclerostin) and mechanical (ie, vibrational) loading. In the future, different dosages and lengths of treatment need to be further investigated on different animal models of OI using potentially promising treatments, such as cellular and chaperone therapies. A combination of therapies may also offer a viable treatment regime to improve bone quality and reduce fragility in animals before being introduced into clinical trials for OI patients.

IFITM5 mutations and osteogenesis imperfecta

Interferon-induced transmembrane protein 5 (IFITM5) is an osteoblast-specific membrane protein that has been shown to be a positive regulatory factor for mineralization in vitro. However, Ifitm5 knockout mice do not exhibit serious bone abnormalities, and thus the function of IFITM5 in vivo remains unclear. Recently, a single point mutation (c.-14C>T) in the 5' untranslated region of IFITM5 was identified in patients with osteogenesis imperfecta type V (OI-V). Furthermore, a single point mutation (c.119C>T) in the coding region of IFITM5 was identified in OI patients with more severe symptoms than patients with OI-V. Although IFITM5 is not directly involved in the formation of bone in vivo, the reason why IFITM5 mutations cause OI remains a major mystery. In this review, the current state of knowledge of OI pathological mechanisms due to IFITM5 mutations will be reviewed.

Clinical and Molecular Characterization of Osteogenesis Imperfecta Type V

Osteogenesis imperfecta type V (OI-V) has a wide clinical variability, with distinct clinical/radiological features, such as calcification of the interosseous membrane (CIM) between the radius-ulna and/or tibia-fibula, hyperplastic callus (HPC) formation, dislocation of the radial head (DRH), and absence of dentinogenesis imperfecta (DI). Recently, a single heterozygous mutation (c.-14C>T) in the 5'UTR of the IFITM5 gene was identified to be causative for OI-V. Here, we describe 7 individuals from 5 unrelated families that carry the c.-14C>T IFITM5 mutation. The clinical findings in these cases are: absence of DI in all patients, presence of blue sclera in 2 cases, and 4 patients with DRH. Radiographic findings revealed HPC in 3 cases. All patients presented CIM between the radius and ulna, while 4 patients presented additional CIM between the tibia and fibula. Spinal fractures by vertebral compression were observed in all individuals. The proportion of cases identified with this mutation represents 4% of OI cases at our institution. The clinical identification of OI-V is crucial, as this mutation has an autosomal dominant inheritance with variable expressivity.