CRTAP Is Required for Prolyl 3- Hydroxylation and Mutations Cause Recessive Osteogenesis Imperfecta

The life cycle of type IV collagen

Type IV collagen is a large triple helical molecule that forms a covalently cross-linked network within basement membranes (BMs). Type IV collagen networks play key roles in mechanically supporting tissues, shaping organs, filtering blood, and cell signaling. To ensure tissue health and function, all aspects of the type IV collagen life cycle must be carried out accurately. However, the large triple helical structure and complex life-cycle of type IV collagen, poses many challenges to cells and tissues. Type IV collagen predominantly forms heterotrimers and to ensure proper construction, expression of the distinct α-chains that comprise a heterotrimer needs tight regulation. The α-chains must also be accurately modified by several enzymes, some of which are specific to collagens, to build and stabilize the triple helical trimer. In addition, type IV collagen is exceptionally long (400 nm) and thus the packaging and trafficking of the triple helical trimer from the ER to the Golgi must be modified to accommodate the large type IV collagen molecule. During ER-to-Golgi trafficking, as well as during secretion and transport in the extracellular space, type IV collagen also associates with specific chaperone molecules that maintain the structure and solubility of collagen IV. Type IV collagen trimers are then delivered to BMs from local and distant sources where they are integrated into BMs by interactions with cell surface receptors and many diverse BM resident proteins. Within BMs type IV collagen self-associates into a network and is crosslinked by BM resident enzymes. Finally, homeostatic type IV collagen levels in BMs are maintained by poorly understood mechanisms involving proteolysis and endocytosis. Here, we provide an overview of the life cycle of collagen IV, highlighting unique mechanisms and poorly understood aspects of type IV collagen regulation.

Multiplex imaging of murine bone marrow using Phenocycler 2.0™

Bone marrow (BM) is a tissue that is of great importance to several areas of basic and translational research, including hematology, oncology, bone biology, and immunology. It is unique in that it is gelatinous in nature but housed in a hard casing of bone. Traditionally, flow cytometry and immunofluorescence (IF) techniques have been employed to study the composition of cellular interactions and elements of the BM. However, it has been challenging to study the BM in an unperturbed state using multiple fluorescent probes at a time to fully appreciate the diverse cell populations and their interactions and relative positioning with each other. This protocol addresses how Phenocycler 2.0TM, which uses co-detection by indexing (CODEX) in conjunction with HALO 4.0TM image analysis software, can overcome the obstacles faced by traditional techniques used to study the BM in an unperturbed state.

Registry-Based Frequency of Molecularly Confirmed Osteogenesis Imperfecta in a Swiss Cohort of Individuals With Connective Tissue Disorders

Patient registries play a crucial role in advancing our understanding of rare diseases, enabling the collection of comprehensive clinical and molecular data that inform diagnosis, treatment, and management strategies and advance our understanding of rare diseases. We showcase the first Swiss registry of 796 patients with suspected or confirmed connective tissue disorders (CTD) who were referred to our center over a period of 26 years between 1995 and 2022. The registry contains information on the natural history, anthropometrics, biochemical, histological, and genetic analyses. 61.3% of patients were referred by other hospitals or genetic specialists, with the primary reasons for referral being suspicion of Ehlers-Danlos syndrome (EDS) (53.6%) and osteogenesis imperfecta (OI) (28.1%). Molecular confirmation of these diagnoses was obtained in 60 cases of EDS and 98 cases of OI through genetic testing. In-depth analyses of 173 OI patients revealed that the majority of OI cases were caused by mutations in COL1A1 or COL1A2. Rarer variants were identified in genes involved in collagen synthesis and bone regulation. Genotype-phenotype correlations were observed in a small subset of patients, with a high prevalence of glycine substitutions in COL1A1 and COL1A2 variants associated with severe phenotypes. This registry offers insights into the molecular underpinnings of EDS and OI and underscores the importance of genetic testing for accurate diagnosis and management.

CRTAP-Null Osteoblasts Have Increased Proliferation, Protein Secretion, and Skeletal Morphogenesis Gene Expression with Downregulation of Cellular Adhesion

Type VII osteogenesis imperfecta (OI), caused by recessive CRTAP mutations, is predominantly lethal in the first year of life. Due to its early lethality, little is known about bone dysplasia mechanism. RNA-seq analysis of differentiated osteoblasts of siblings with a non-lethal homozygous CRTAP-null variant showed an enrichment of gene ontology terms involved in DNA replication and cell cycle compared to control. BrdU incorporation confirmed a ≈2-fold increase in proliferation in non-lethal proband osteoblasts in comparison to control cells. In addition, the expression of cyclin dependent kinase inhibitor 2A (CDKN2A), encoding a protein involved in cell cycle inhibition, was significantly reduced (>50%) in CRTAP-null osteoblasts, while cyclin B1 (CCNB1), encoding a promoter of the cell cycle, was enhanced. Ossification and bone and cartilage development gene ontology pathways were enriched among upregulated genes throughout osteoblast differentiation, as was protein secretion. Ingenuity pathway analysis indicated an upregulation of BMP2 signaling, supported by increase in both BMP2 and MSX2, an early BMP2-responsive gene, by qPCR. Throughout differentiation, CRTAP-null osteoblasts showed a decrease in transcripts related to cell adhesion and extracellular matrix organization pathways. We propose that increased proliferation and osteogenesis of type VII OI osteoblasts may be stimulated through upregulation of BMP2 signaling, altering bone homeostasis, and leading to weaker bones.

The Spectra of Pathogenic Variants and Phenotypes in a Chinese Cohort of 298 Families with Osteogenesis Imperfecta

Background: Osteogenesis imperfecta (OI) is marked by clinical and genetic heterogeneity, and the genotype-phenotype correlation remains not very clear. We conducted a clinical and genetic study in a Chinese OI cohort to determine the spectra of phenotypes and pathogenic variants. Methods: In this study, 298 Chinese families were recruited from 2019 to 2024. Clinical phenotypes including fractures, short stature, skeletal deformities, blue sclera, dentinogenesis imperfecta, and hearing loss were recorded and analyzed. Next-generation sequencing combined with PCR-based techniques was used to detect candidate pathogenic variants. Variant pathogenicity was evaluated via conservation analysis, bioinformatics analysis, and functional studies at the cellular level. In this OI cohort, the spectra of pathogenic variants, clinical phenotypes, and genotype-phenotype correlations were analyzed. Results: Our OI cohort included 71 type I (23.83%), 122 type III (40.94%), 90 type IV (30.20%), and 15 type V (5.03%) probands. The cohort consisted of 196 children (65.77%) and 102 adults (34.23%). For the first time, phenotypic differences between different age groups were confirmed. In total, we identified 231 variants, including 47 novel pathogenic variants. Notable variants include two atypical splicing variants, one small deletion, two small duplications, one gross deletion, and one gross duplication. New genotype-phenotype correlations were observed: patients with SERPINF1 variants had the highest fracture frequency, followed by those with WNT1 variants, compared to patients with other gene variants. Conclusions: We performed the clinical and genetic analysis in a large Chinese OI cohort. The expanded spectra of genetic variants and clinical phenotypes were constructed by identifying 47 novel pathogenic variants and summarizing the skeletal and extra-skeletal manifestations. The current paper will provide important evidence for the precise diagnosis of the disease.

Comprehensive analysis of the effects of P4ha1 and P4ha2 deletion on post-translational modifications of fibrillar collagens in mouse skin

Introduction

Collagens, the most abundant proteins in mammals, play pivotal roles in maintaining tissue structure, functions, cell-to-cell communication, cellular migration, cellular behavior, and growth. Structures of collagens are highly complex due to the presence of dynamic post-translational modifications (PTMs), such as hydroxylations (on prolines and lysine residues) and O-glycosylation (on hydroxylysines) enzymatically catalyzed during biosynthesis in the endoplasmic reticulum. Collagen PTMs are essential for maintaining structural stability, elasticity, and different functions of collagens. The most prevalent modification in fibrillar collagens is prolyl 4-hydroxylation catalyzed by collagen prolyl 4-hydroxylases (C-P4Hs). Prolyl 4-hydroxylation on collagens plays a critical role in collagen biosynthesis, thermostability, and cell-collagen interactions. Collagens are large proteins. Different regions of collagen perform different functions, so the presence or absence of a PTM on a particular collagen site can affect its functioning. However, comprehensive site-specific identification of these PTMs on fibrillar collagen chains of mice skin has not been performed yet. Furthermore, the effects of prolyl 4-hydroxylase alpha 1 (P4HA1) and P4HA2 on 3-hydroxyproline, 5-hydroxylysine, and O-glycosylation sites of fibrillar collagen chains have not yet been explored.

Methodology

This study presents a comprehensive PTM analysis of fibrillar collagen chains extracted from the skin of different mutants of C-P4Hs (P4ha1 +/- ; P4ha2-/-, P4ha1 +/+ ; P4ha2-/-, P4ha1 +/- ; P4ha2 +/- , P4ha1 +/+ ; P4ha2 +/- ) and wild-type mice. In this study, proteomics-based comprehensive PTM site identification by MS2 level ions from raw mass spectrometry data was performed, and MS1-level quantification was performed for PTM occupancy percentage analysis.

Results and discussion

A total of 421 site-specific PTMs were identified on fibrillar collagen chains (COL1A1, COL1A2, and COL3A1) extracted from wild-type mice skin. A total of 23 P4HA1-specific and seven P4HA2-specific 4-hydroxyproline sites on fibrillar collagen chains were identified. Moreover, it was found that the P4ha1 and P4ha2 deletion can affect the 3-hydroxyproline occupancy percentages in mice skin. Interestingly, increased levels of lysyl 5-hydroxylation were detected upon partial deletion of P4ha1 and full deletion of P4ha2. These findings show that the effects of deletion of prolyl 4-hydroxylases are not limited to less 4-hydroxylation on some specific proline sites, but it can also modulate the prolyl 3-hydroxylation, lysyl 5-hydroxylation, and O-glycosylation occupancy percentages in the fibrillar collagen chains in a site-specific manner.

Gene module reconstruction identifies cellular differentiation processes and the regulatory logic of specialized secretion in zebrafish

During differentiation, cells become structurally and functionally specialized, but comprehensive views of the underlying remodeling processes are elusive. Here, we leverage single-cell RNA sequencing (scRNA-seq) developmental trajectories to reconstruct differentiation using two secretory tissues as models-the zebrafish notochord and hatching gland. First, we integrated expression and functional similarities to identify gene modules, revealing dozens of modules representing known and newly associated differentiation processes and their dynamics. Second, we focused on the unfolded protein response (UPR) transducer module to study how general versus cell-type-specific secretory functions are regulated. Profiling loss- and gain-of-function embryos identified that the UPR transcription factors creb3l1, creb3l2, and xbp1 are master regulators of a general secretion program. creb3l1/creb3l2 additionally activate an extracellular matrix secretion program, while xbp1 partners with bhlha15 to activate a gland-like secretion program. Our study presents module identification via multi-source integration for reconstructing differentiation (MIMIR) and illustrates how transcription factors confer general and specialized cellular functions.

A new Col1a1 conditional knock-in mouse model to study osteogenesis imperfecta

Osteogenesis imperfecta (OI) constitutes a family of bone fragility disorders characterized by both genetic and clinical heterogeneity. Several different mouse models reproduce the classic features of OI, and the most commonly studied carry either a spontaneous or genetically induced pathogenic variant in the Col1a1 or Col1a2 gene. When OI is caused by primary alterations of type I collagen, it represents a systemic connective tissue disease that, in addition to the skeleton, also affects several extra-skeletal tissues and organs, such as skin, teeth, lung, heart, and others, where the altered type I collagen is also expressed. Currently, existing mouse models harbor a disease-causing genetic variant in all tissues and do not allow assessing the primary vs secondary consequences of the mutation on a specific organ/system. Here, we describe the generation of the first conditional knock-in allele for Col1a1 that can express a severe OI-causing glycine substitution (p.Gly1146Arg) in the triple helical region of α1(I) but only after Cre-driven recombination in the tissue of choice. We called this new dominant allele Col1a1G1146R-Floxed/+ and introduced it into the murine model. We describe its validation by crossing mice carrying this allele with EIIA-Cre expressing mice and showing that offspring with the recombined allele reproduce the classic features of a severe form of OI. The new mouse model will be useful to study the tissue-specific impact of this severe mutation on organs, such as the lung, the heart, and others.

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.

Clinical spectrum of rare bone fragility disorders and response to bisphosphonate treatment: a retrospective study

Osteogenesis Imperfecta (OI) is a clinically and genetically heterogeneous group of diseases characterized by brittle bones. Though genetic mutations in COL1A1 and COL1A2 account for approximately 85-90% of OI cases, there are now more than twenty genes described, responsible for rare forms of OI. Treatment is based on the use of bisphosphonates and though it is well established that they increase lumbar spine (LS) bone mineral density (BMD), the clinical impact on fracture reduction is still debated.In this study, we investigated the clinical characteristics of 38 patients with a bone fragility disorder that had variants in non-COL1A1/COL1A2 genes in order to study genotype-phenotype correlations, as the natural history of these rare forms is still not well known. We then studied the usefulness of bisphosphonate treatment by evaluating the effects on LS BMD, annual non-vertebral fracture rate, bone turnover markers and height. This study enabled us to better define the natural history of patients with non-COL1 pathogenic variants. Patients with CRTAP and TMEM38B variants consistently had a prenatal presentation with a short (<3rd p) and bowed femur. Importantly, this prenatal involvement does not predict the postnatal severity of the disease. Regarding treatment by bisphosphonates, all patients showed a significant increase in LS BMD while treated and this increase was dependent on the dose received. The increase in LS BMD also translated in a reduction of fracture rate during treatment. Finally, our study showed that the earlier bisphosphonates are initiated, the greater the fracture rate is reduced.

A Dyadic Nosology for Osteogenesis Imperfecta and Bone Fragility Syndromes 2024

In 2023 following extensive consultation with key stakeholders, the expert Nosology Working Group of the International Skeletal Dysplasia Society (ISDS) published the new Dyadic Nosology for Genetic Disorders of the Skeleton. Some 770 entities were delineated associated with 552 genes. From these entities, over 40 genes resulting in distinct forms of Osteogenesis Imperfecta (OI) and Bone Fragility and/or Familial Osteoporosis were identified. To assist clinicians and lay stake holders and bring the considerable body of knowledge of the matrix biology and genomics to people with OI as well as to clinicians and scientists, a dyadic nosology has been recommended. This combines a genomic co-descriptor with a phenotypic naming based on the widely used Sillence nosology for the OI syndromes and the many other syndromes characterized in part by bone fragility.This review recapitulates and explains the evolution from the simple Congenita and Tarda subclassification of OI in the 1970 nosology, which was replaced by the Sillence types I-IV nosology which was again replaced in 2009 with 5 clinical groups, type 1 to 5. Qualitative and quantitative defects in type I collagen polypeptides were postulated to account for the genetic heterogeneity in OI for nearly 30 years, when OI type 5, a non-collagen disorder was recognized. Advances in matrix biology and genomics since that time have confirmed a surprising complexity both in transcriptional as well as post-translational mechanisms of collagens as well as in the many mechanisms of calcified tissue homeostasis and integrity.

Skeletal and Non-skeletal Phenotypes in Children with Osteogenesis Imperfecta

Although fractures are the defining characteristic of osteogenesis imperfecta (OI), the disorder affects many tissues. Here we discuss three facets of the OI phenotype, skeletal growth and development, skeletal muscle weakness and the dental and craniofacial characteristics. Short stature is almost universal in the more severe forms of OI and is probably caused by a combination of direct effects of the underlying genetic defect on growth plates and indirect effects of fractures, bone deformities and scoliosis. Recent studies have developed OI type-specific growth curves, which allow determining whether a given child with OI grows as expected for OI type. Impaired muscle function is an important OI-related phenotype in severe OI. Muscles may be directly affected in OI by collagen type I abnormalities in muscle connective tissue and in the muscle-tendon unit. Indirect effects like bone deformities and lack of physical activity may also contribute to low muscle mass and function. Dental and craniofacial abnormalities are also very common in severe OI and include abnormal tooth structure (dentinogenesis imperfecta), malocclusion, and deformities in the bones of the face and the skull. It is hoped that future treatment approaches will address these OI-related phenotypes.

Extra-Skeletal Manifestations in Osteogenesis Imperfecta Mouse Models

Osteogenesis imperfecta (OI) is a rare heritable connective tissue disorder of skeletal fragility with an incidence of roughly 1:15,000. Approximately 85% of the pathogenic variants responsible for OI are in the type I collagen genes, COL1A1 and COL1A2, with the remaining pathogenic OI variants spanning at least 20 additional genetic loci that often involve type I collagen post-translational modification, folding, and intracellular transport as well as matrix incorporation and mineralization. In addition to being the most abundant collagen in the body, type I collagen is an important structural and extracellular matrix signaling molecule in multiple organ systems and tissues. Thus, OI disease-causing variants result not only in skeletal fragility, decreased bone mineral density (BMD), kyphoscoliosis, and short stature, but can also result in hearing loss, dentinogenesis imperfecta, blue gray sclera, cardiopulmonary abnormalities, and muscle weakness. The extensive genetic and clinical heterogeneity in OI has necessitated the generation of multiple mouse models, the growing awareness of non-skeletal organ and tissue involvement, and OI being more broadly recognized as a type I collagenopathy.This has driven the investigation of mutation-specific skeletal and extra-skeletal manifestations and broadened the search of potential mechanistic therapeutic strategies. The purpose of this review is to outline several of the extra-skeletal manifestations that have recently been characterized through the use of genetically and phenotypically heterogeneous mouse models of osteogenesis imperfecta, demonstrating the significant potential impact of OI disease-causing variants as a collagenopathy (affecting multiple organ systems and tissues), and its implications to overall health.

Update on the Genetics of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous heritable skeletal dysplasia characterized by bone fragility and deformity, growth deficiency, and other secondary connective tissue defects. OI is now understood as a collagen-related disorder caused by defects of genes whose protein products interact with collagen for folding, post-translational modification, processing and trafficking, affecting bone mineralization and osteoblast differentiation. This review provides the latest updates on genetics of OI, including new developments in both dominant and rare OI forms, as well as the signaling pathways involved in OI pathophysiology. There is a special emphasis on discoveries of recessive mutations in TENT5A, MESD, KDELR2 and CCDC134 whose causality of OI types XIX, XX, XXI and XXI, respectively, is now established and expends the complexity of mechanisms underlying OI to overlap LRP5/6 and MAPK/ERK pathways. We also review in detail new discoveries connecting the known OI types to each other, which may underlie an eventual understanding of a final common pathway in OI cellular and bone biology.

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.

The structural basis for the collagen processing by human P3H1/CRTAP/PPIB ternary complex

Collagen posttranslational processing is crucial for its proper assembly and function. Disruption of collagen processing leads to tissue development and structure disorders like osteogenesis imperfecta (OI). OI-related collagen processing machinery includes prolyl 3-hydroxylase 1 (P3H1), peptidyl-prolyl cis-trans isomerase B (PPIB), and cartilage-associated protein (CRTAP), with their structural organization and mechanism unclear. We determine cryo-EM structures of the P3H1/CRTAP/PPIB complex. The active sites of P3H1 and PPIB form a face-to-face bifunctional reaction center, indicating a coupled modification mechanism. The structure of the P3H1/CRTAP/PPIB/collagen peptide complex reveals multiple binding sites, suggesting a substrate interacting zone. Unexpectedly, a dual-ternary complex is observed, and the balance between ternary and dual-ternary states can be altered by mutations in the P3H1/PPIB active site and the addition of PPIB inhibitors. These findings provide insights into the structural basis of collagen processing by P3H1/CRTAP/PPIB and the molecular pathology of collagen-related disorders.

Dentinogenesis Imperfecta in a 1-Year-Old Female Labrador Retriever Dog: A Case Report and Literature Review

Dentinogenesis imperfecta is a rare, autosomal dominant, hereditary disorder that occurs in humans and animals. In humans, known causative genetic mutations have been elucidated; however, veterinary literature on the topic is limited. This case report describes a 1-year-old female Labrador Retriever who presented for evaluation of generalized discoloration of the permanent dentition with historical discoloration of the deciduous dentition. Radiographic and histopathological findings will be discussed, as well as an in-depth review of the current human and veterinary literature pertaining to the pathogenesis and treatment options for dentinogenesis imperfecta.

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.

Genetic Analysis, Phenotypic Spectrum and Functional Study of Rare Osteogenesis Imperfecta Caused by CRTAP Variants

OBJECTIVE

Deficiency of cartilage-associated protein (CRTAP) can cause extremely rare autosomal recessive osteogenesis imperfecta (OI) type VII. We investigated the pathogenic mechanisms of CRTAP variants through functional studies on bones of patients with OI.

METHODS

Two nonconsanguineous families with CRTAP mutations were included and their phenotypes and genotypes were evaluated. Bone specimens were obtained from 1 patient with OI and a normal control during orthopedic surgery. The impacts of the novel variant on the CRTAP transcript were confirmed. The expression levels of CRTAP mRNA and CRTAP protein were analyzed. The quantification of prolyl 3-hydroxylation in the α1 chain of type I collagen was evaluated.

RESULTS

Patients with OI type VII had early-onset recurrent fractures, severe osteoporosis, and bone deformities. The c.621 + 1G > A and c.1153-3C > G mutations were identified in CRTAP in the patients with OI. The c.621 + 1G > A variant was a novel mutation that could impair mRNA transcription, leading to a truncated CRTAP protein. In a patient with c.621 + 1G > A and c.1153-3C > G mutations in CRTAP, the mRNA and protein levels of CRTAP in osteoblasts were significantly decreased and the osteoid volume and osteoblast numbers were markedly reduced compared with those in the normal control individual. This was simultaneously accompanied by significantly reduced prolyl 3-hydroxylation at Pro986 in the α1 chain of type I collagen and invisible active bone formation in bone.

CONCLUSION

The novel c.621 + 1G > A mutation in CRTAP expands the genotypic spectrum of type VII OI. Biallelic mutations of c.621 + 1G > A and c.1153-3C > G in CRTAP can lead to reduced CRTAP mRNA and deficient CRTAP protein in osteoblasts, which reduces 3-hydroxylation in Pro986 of the α1 chain of type I collagen and impairs bone formation, thus contributing to severe OI type VII.

ColPTMScape: An open access knowledge base for tissue-specific collagen PTM maps

Collagen is a key component of the extracellular matrix (ECM). In the remodeling of ECM, a remarkable variation in collagen post-translational modifications (PTMs) occurs. This makes collagen a potential target for understanding extracellular matrix remodeling during pathological conditions. Over the years, scientists have gathered a huge amount of data about collagen PTM during extracellular matrix remodeling. To make such information easily accessible in a consolidated space, we have developed ColPTMScape (https://colptmscape.iitmandi.ac.in/), a dedicated knowledge base for collagen PTMs. The identified site-specific PTMs, quantitated PTM sites, and PTM maps of collagen chains are deliverables to the scientific community, especially to matrix biologists. Through this knowledge base, users can easily gain information related to the difference in the collagen PTMs across different tissues in different organisms.

Altered Sox9 and FGF signaling gene expression in Aga2 OI mice negatively affects linear growth

Osteogenesis imperfecta (OI), or brittle bone disease, is a disorder characterized by bone fragility and increased fracture incidence. All forms of OI also feature short stature, implying an effect on endochondral ossification. Using the Aga2+/- mouse, which has a mutation in type I collagen, we show an affected growth plate primarily due to a shortened proliferative zone. We used single-cell RNA-Seq analysis of tibial and femoral growth plate tissues to understand transcriptional consequences on growth plate cell types. We show that perichondrial cells, which express abundant type I procollagen, and growth plate chondrocytes, which were found to express low amounts of type I procollagen, had ER stress and dysregulation of the same unfolded protein response pathway as previously demonstrated in osteoblasts. Aga2+/- proliferating chondrocytes showed increased FGF and MAPK signaling, findings consistent with accelerated differentiation. There was also increased Sox9 expression throughout the growth plate, which is expected to accelerate early chondrocyte differentiation but reduce late hypertrophic differentiation. These data reveal that mutant type I collagen expression in OI has an impact on the cartilage growth plate. These effects on endochondral ossification indicate that OI is a biologically complex phenotype going beyond its known impacts on bone to negatively affect linear growth.

Characterization of three adults and an adolescent with Osteogenesis Imperfecta type VI and a novel founder SERPINF1 variant

Osteogenesis imperfecta (OI) type VI is an extremely rare form of OI caused by biallelic variants in the SERPINF1 gene, which codes for the pigment-epithelium derived factor (PEDF). We report on four patients (three adults and one adolescent) with a severe deforming form of OI. All patients presented no abnormalities at birth, frequent long bone and vertebrae fractures (mainly during childhood), marked short stature, severe bone deformities, chronic mild to moderate pain, and severe limitation of mobility, with three being completely wheelchair bound. Blue sclera and dentinogenesis imperfecta were absent, although some patients presented tooth, ophthalmological, and/or cardiac features. Radiographic findings included, among others, thin diaphysis and popcorn calcifications, both of which are non-specific to this type of OI. The novel homozygous variants c.816_819del (p.Met272Ilefs*8) and c.283+2T > G in SERPINF1 were identified in three and one patient, respectively. The three patients carrying the frameshift variant were born in nearby regions suggesting a founder effect. Describing the long-term outcomes of four patients with OI type VI, this cohort adds relevant data on the clinical features and prognosis of this type of OI.

Current Status of Next-Generation Sequencing in Bone Genetic Diseases

The development of next-generation sequencing (NGS) has dramatically increased the speed and volume of genetic analysis. Furthermore, the range of applications of NGS is rapidly expanding to include genome, epigenome (such as DNA methylation), metagenome, and transcriptome analyses (such as RNA sequencing and single-cell RNA sequencing). NGS enables genetic research by offering various sequencing methods as well as combinations of methods. Bone tissue is the most important unit supporting the body and is a reservoir of calcium and phosphate ions, which are important for physical activity. Many genetic diseases affect bone tissues, possibly because metabolic mechanisms in bone tissue are complex. For instance, the presence of specialized immune cells called osteoclasts in the bone tissue, which absorb bone tissue and interact with osteoblasts in complex ways to support normal vital functions. Moreover, the many cell types in bones exhibit cell-specific proteins for their respective activities. Mutations in the genes encoding these proteins cause a variety of genetic disorders. The relationship between age-related bone tissue fragility (also called frailty) and genetic factors has recently attracted attention. Herein, we discuss the use of genomic, epigenomic, transcriptomic, and metagenomic analyses in bone genetic disorders.

Pathogenic mechanisms of osteogenesis imperfecta, evidence for classification

Osteogenesis imperfecta (OI) is a connective tissue disorder affecting the skeleton and other organs, which has multiple genetic patterns, numerous causative genes, and complex pathogenic mechanisms. The previous classifications lack structure and scientific basis and have poor applicability. In this paper, we summarize and sort out the pathogenic mechanisms of OI, and analyze the molecular pathogenic mechanisms of OI from the perspectives of type I collagen defects(synthesis defects, processing defects, post-translational modification defects, folding and cross-linking defects), bone mineralization disorders, osteoblast differentiation and functional defects respectively, and also generalize several new untyped OI-causing genes and their pathogenic mechanisms, intending to provide the evidence of classification and a scientific basis for the precise diagnosis and treatment of OI.

Absence of TRIC-B from type XIV Osteogenesis Imperfecta osteoblasts alters cell adhesion and mitochondrial function - A multi-omics study

Osteogenesis Imperfecta (OI) is a heritable collagen-related bone dysplasia characterized by bone fractures, growth deficiency and skeletal deformity. Type XIV OI is a recessive OI form caused by null mutations in TMEM38B, which encodes the ER membrane intracellular cation channel TRIC-B. Previously, we showed that absence of TMEM38B alters calcium flux in the ER of OI patient osteoblasts and fibroblasts, which further disrupts collagen synthesis and secretion. How the absence of TMEM38B affects osteoblast function is still poorly understood. Here we further investigated the role of TMEM38B in human osteoblast differentiation and mineralization. TMEM38B-null osteoblasts showed altered expression of osteoblast marker genes and decreased mineralization. RNA-Seq analysis revealed that cell-cell adhesion was one of the most downregulated pathways in TMEM38B-null osteoblasts, with further validation by real-time PCR and Western blot. Gap and tight junction proteins were also decreased by TRIC-B absence, both in patient osteoblasts and in calvarial osteoblasts of Tmem38b-null mice. Disrupted cell adhesion decreased mutant cell proliferation and cell cycle progression. An important novel finding was that TMEM38B-null osteoblasts had elongated mitochondria with altered fusion and fission markers, MFN2 and DRP1. In addition, TMEM38B-null osteoblasts exhibited a significant increase in superoxide production in mitochondria, further supporting mitochondrial dysfunction. Together these results emphasize the novel role of TMEM38B/TRIC-B in osteoblast differentiation, affecting cell-cell adhesion processes, gap and tight junction, proliferation, cell cycle, and mitochondrial function.

Cell differentiation and matrix organization are differentially affected during bone formation in osteogenesis imperfecta zebrafish models with different genetic defects impacting collagen type I structure

Osteogenesis imperfecta (OI) is a family of rare heritable skeletal disorders associated with dominant mutations in the collagen type I encoding genes and recessive defects in proteins involved in collagen type I synthesis and processing and in osteoblast differentiation and activity. Historically, it was believed that the OI bone phenotype was only caused by abnormal collagen type I fibrils in the extracellular matrix, but more recently it became clear that the altered bone cell homeostasis, due to mutant collagen retention, plays a relevant role in modulating disease severity in most of the OI forms and it is correlated to impaired bone cell differentiation. Despite in vitro evidence, in vivo data are missing. To better understand the physiopathology of OI, we used two zebrafish models: Chihuahua (Chi/+), carrying a dominant p.G736D substitution in the α1 chain of collagen type I, and the recessive p3h1-/-, lacking prolyl 3-hydroxylase (P3h1) enzyme. Both models share the delay of collagen type I folding, resulting in its overmodification and partial intracellular retention. The regeneration of the bony caudal fin of Chi/+ and p3h1-/- was employed to investigate the impact of abnormal collagen synthesis on bone cell differentiation. Reduced regenerative ability was evident in both models, but it was associated to impaired osteoblast differentiation and osteoblastogenesis/adipogenesis switch only in Chi/+. On the contrary, reduced osteoclast number and activity were found in both models during regeneration. The dominant OI model showed a more detrimental effect in the extracellular matrix organization. Interestingly, the chemical chaperone 4-phenylbutyrate (4-PBA), known to reduce cellular stress and increase collagen secretion, improved bone formation only in p3h1-/- by favoring caudal fin growth without affecting bone cell markers expression. Taken together, our in vivo data proved the negative impact of structurally abnormal collagen type I on bone formation but revealed a gene mutation-specific effect on bone cell differentiation and matrix organization in OI. These, together with the distinct ability to respond to the chaperone treatment, underline the need for precision medicine approaches to properly treat the disease.

Deep intronic mutation in CRTAP results in unstable isoforms of the protein to induce type I collagen aggregation in a lethal type of osteogenesis imperfecta type VII

Genetic mutations are involved in Mendelian disorders. Unbuffered intronic mutations in gene variants can generate aberrant splice sites in mutant transcripts, resulting in mutant isoforms of proteins with modulated expression, stability, and function in diseased cells. Here, we identify a deep intronic variant, c.794_1403A>G, in CRTAP by genome sequencing of a male fetus with osteogenesis imperfecta (OI) type VII. The mutation introduces cryptic splice sites in intron-3 of CRTAP, resulting in two mature mutant transcripts with cryptic exons. While transcript-1 translates to a truncated isoform (277 amino acids) with thirteen C-terminal non-wild-type amino acids, transcript-2 translates to a wild-type protein sequence, except that this isoform contains an in-frame fusion of non-wild-type twenty-five amino acids in a tetratricopeptide repeat sequence. Both mutant isoforms of CRTAP are unstable due to the presence of a unique 'GWxxI' degron, which finally leads to loss of proline hydroxylation and aggregation of type I collagen. Although type I collagen aggregates undergo autophagy, the overall proteotoxicity resulted in death of the proband cells by senescence. In summary, we present a genetic disease pathomechanism by linking a novel deep intronic mutation in CRTAP to unstable mutant isoforms of the protein in lethal OI type VII.

A Rab33b missense mouse model for Smith-McCort dysplasia shows bone resorption defects and altered protein glycosylation

Smith McCort (SMC) dysplasia is a rare, autosomal recessive, osteochondrodysplasia that can be caused by pathogenic variants in either RAB33B or DYM genes. These genes codes for proteins that are located at the Golgi apparatus and have a role in intracellular vesicle trafficking. We generated mice that carry a Rab33b disease-causing variant, c.136A>C (p.Lys46Gln), which is identical to that of members from a consanguineous family diagnosed with SMC. In male mice at 4 months of age, the Rab33b variant caused a mild increase in trabecular bone thickness in the spine and femur and in femoral mid-shaft cortical thickness with a concomitant reduction of the femoral medullary area, suggesting a bone resorption defect. In spite of the increase in trabecular and cortical thickness, bone histomorphometry showed a 4-fold increase in osteoclast parameters in homozygous Rab33b mice suggesting a putative impairment in osteoclast function, while dynamic parameters of bone formation were similar in mutant versus control mice. Femur biomechanical tests showed an increased in yield load and a progressive elevation, from WT to heterozygote to homozygous mutants, of bone intrinsic properties. These findings suggest an overall impact on bone material properties which may be caused by disturbed protein glycosylation in cells contributing to skeletal formation, supported by the altered and variable pattern of lectin staining in murine and human tissue cultured cells and in liver and bone murine tissues. The mouse model only reproduced some of the features of the human disease and was sex-specific, manifesting in male but not female mice. Our data reveal a potential novel role of RAB33B in osteoclast function and protein glycosylation and their dysregulation in SMC and lay the foundation for future studies.

Extracellular Targets to Reduce Excessive Scarring in Response to Tissue Injury

Excessive scar formation is a hallmark of localized and systemic fibrotic disorders. Despite extensive studies to define valid anti-fibrotic targets and develop effective therapeutics, progressive fibrosis remains a significant medical problem. Regardless of the injury type or location of wounded tissue, excessive production and accumulation of collagen-rich extracellular matrix is the common denominator of all fibrotic disorders. A long-standing dogma was that anti-fibrotic approaches should focus on overall intracellular processes that drive fibrotic scarring. Because of the poor outcomes of these approaches, scientific efforts now focus on regulating the extracellular components of fibrotic tissues. Crucial extracellular players include cellular receptors of matrix components, macromolecules that form the matrix architecture, auxiliary proteins that facilitate the formation of stiff scar tissue, matricellular proteins, and extracellular vesicles that modulate matrix homeostasis. This review summarizes studies targeting the extracellular aspects of fibrotic tissue synthesis, presents the rationale for these studies, and discusses the progress and limitations of current extracellular approaches to limit fibrotic healing.

Classification of osteogenesis imperfecta: Importance for prophylaxis and genetic counseling

Osteogenesis imperfecta (OI) is a genetically heterogeneous monogenic disease characterized by decreased bone mass, bone fragility, and recurrent fractures. The phenotypic spectrum varies considerably ranging from prenatal fractures with lethal outcomes to mild forms with few fractures and normal stature. The basic mechanism is a collagen-related defect, not only in synthesis but also in folding, processing, bone mineralization, or osteoblast function. In recent years, great progress has been made in identifying new genes and molecular mechanisms underlying OI. In this context, the classification of OI has been revised several times and different types are used. The Sillence classification, based on clinical and radiological characteristics, is currently used as a grading of clinical severity. Based on the metabolic pathway, the functional classification allows identifying regulatory elements and targeting specific therapeutic approaches. Genetic classification has the advantage of identifying the inheritance pattern, an essential element for genetic counseling and prophylaxis. Although genotype-phenotype correlations may sometimes be challenging, genetic diagnosis allows a personalized management strategy, accurate family planning, and pregnancy management decisions including options for mode of delivery, or early antenatal OI treatment. Future research on molecular pathways and pathogenic variants involved could lead to the development of genotype-based therapeutic approaches. This narrative review summarizes our current understanding of genes, molecular mechanisms involved in OI, classifications, and their utility in prophylaxis.

Perturbed post-translational modification (PTM) network atlas of collagen I during stent-induced neointima formation

Myocardial infarction (MI) leading to heart failure contributes to almost 85% of deaths associated with CVDs. MI results from plaque formation in the coronary artery which leads to a lack of oxygen and nutrients in the myocardium. To date, stenting is a widely used gold-standard technique to maintain the proper blood flow through coronary circulation in the myocardium. Bare metal stents (BMS) and drug-eluting stents (DES) are majorly used in implantation. However, BMS and DES both can induce neointima formation by depositing excessive collagens in the coronary arteries leading to restenosis. Identification and quantitative analysis of site-specific post-translational modifications (PTMs) of deposited COL1A1 from neointima ECM are not known. Applying our in-house workflow, we re-analyzed a previously published mass-spectrometry data set to comprehensively map site-specific prolyl-hydroxylation, lysyl hydroxylation, and O-glycosylation sites in COL1A1 from neointima ECM. Furthermore, we quantitated the occupancy level of 9 3-hydroxyproline (3-HyP) sites, 2 hydroxylysine sites, and glycosylation microheterogeneity on 6 lysine sites of COL1A1. Although the total level of COL1A1 was decreased in DES-induced neointima, the occupancy levels of 2 3-HyP sites (P⁸⁷², and P⁸⁸¹) and 2 HyK (K⁴³⁵ and K⁷⁶⁸) sites of COL1A1 were significantly (p < 0.05) elevated in DES-induced neointima compared to BMS-induced neointima. We also found O-glycosylation to be significantly elevated on 3 lysine sites (K⁵⁷³, K³³⁹, and K and K⁸⁴⁹) of COL1A1 in DES-induced neointima compared to BMS-induced neointima. Taken together, our first comprehensive PTM analysis of COL1A1 reflected significant site-specific alterations that may play a very important role in the ECM remodeling during stent-induced neointima formation in MI patients. SIGNIFICANCE: The knowledge about site-specific post-translational modifications (PTMs) of collagen 1 deposited in the neointima ECM during the post-stenting restenosis process is absent. Here for the first time, we report the altered levels of COL1A1 PTMs during metal stent and drug-eluting stent-induced neointima formation. Our study showcases a novel ECM remodeling through site-specific collagen PTMs during stent-induced restenosis.

A Founder Intronic Variant in P3H1 Likely Results in Aberrant Splicing and Protein Truncation in Patients of Karen Descent with Osteogenesis Imperfecta Type VIII

One of the most important steps in post-translational modifications of collagen type I chains is the hydroxylation of carbon-3 of proline residues by prolyl-3-hydroxylase-1 (P3H1). Genetic variants in P3H1 have been reported to cause autosomal recessive osteogenesis imperfecta (OI) type VIII. Clinical and radiographic examinations, whole-exome sequencing (WES), and bioinformatic analysis were performed in 11 Thai children of Karen descent affected by multiple bone fractures. Clinical and radiographic findings in these patients fit OI type VIII. Phenotypic variability is evident. WES identified an intronic homozygous variant (chr1:43212857A > G; NM_022356.4:c.2055 + 86A > G) in P3H1 in all patients, with parents in each patient being heterozygous for the variant. This variant is predicted to generate a new "CAG" splice acceptor sequence, resulting in the incorporation of an extra exon that leads to a frameshift in the final exon and subsequent non-functional P3H1 isoform a. Alternative splicing of P3H1 resulting in the absence of functional P3H1 caused OI type VIII in 11 Thai children of Karen descent. This variant appears to be specific to the Karen population. Our study emphasizes the significance of considering intronic variants.

Mutant MESD links cellular stress to type I collagen aggregation in osteogenesis imperfecta type XX

Aberrant forms of endoplasmic reticulum (ER)-resident chaperones are implicated in loss of protein quality control in rare diseases. Here we report a novel mutation (p.Asp233Asn) in the ER retention signal of MESD by whole exome sequencing of an individual diagnosed with osteogenesis imperfecta (OI) type XX. While MESDD233N has similar stability and chaperone activity as wild-type MESD, its mislocalization to cytoplasm leads to imbalance of ER proteostasis, resulting in improper folding and aggregation of proteins, including LRP5 and type I collagen. Aggregated LRP5 loses its plasma membrane localization to disrupt the expression of WNT-responsive genes, such as BMP2, BMP4, in proband fibroblasts. We show that MESD is a direct chaperone of pro-α1(I) [COL1A1], and absence of MESDD233N in ER results in cytosolic type I collagen aggregates that remain mostly not secreted. While cytosolic type I collagen aggregates block the intercellular nanotubes, decreased extracellular type I collagen also results in loss of interaction of ITGB1 with type I collagen and weaker attachment of fibroblasts to matrix. Although proband fibroblasts show increased autophagy to degrade the aggregated type I collagen, an overall cellular stress overwhelms the proband fibroblasts. In summary, we present an essential chaperone function of MESD for LRP5 and type I collagen and demonstrating how the D233N mutation in MESD correlates with impaired WNT signaling and proteostasis in OI.

Distinct type I collagen alterations cause intrinsic lung and respiratory defects of variable severity in mouse models of osteogenesis imperfecta

Type I collagen alterations cause osteogenesis imperfecta (OI), a connective tissue disorder characterized by severe bone fragility. Patients with OI can suffer from significant pulmonary manifestations including severe respiratory distress in the neonatal period and a progressive decline in respiratory function in adulthood. We and others have shown intrinsic lung defects in some mouse models of OI. In this large study, we performed histological, histomorphometric, microcomputed tomography and invasive studies on oim/+, Col1a2+/G610C , CrtapKO and oim/oim mice, mimicking mild to moderate to severe OI, with the overall goal of determining the extent of their pulmonary and respiratory mechanics defects and whether these defects correlate with the skeletal disease severity and affect each sex equally. Although with variable severity, OI lung histology consistently showed alveolar simplification with enlarged acinar airspace and reduced alveolar surface. Numerous respiratory mechanics parameters, including respiratory system resistance and elastance, tissue damping, inspiratory capacity, total lung capacity, and others, were significantly and similarly impacted in CrtapKO and oim/oim but not in oim/+ or Col1a2+/G610C compared to control mice. Our data indicate that the impact of type I collagen alterations and OI on lung morphology and function positively correlate with the severity of the extracellular matrix deficiency. Moreover, the respiratory defects were more pronounced in male compared to female mice. It will be important to determine whether our observations in mice translate to OI patients and to dissect the respective contribution of intrinsic lung defects vs. extrinsic skeletal defects to impaired lung function in OI. KEY POINTS: Different type I collagen alterations in mouse models of osteogenesis imperfecta (OI) cause similar abnormal lung histology, with alveolar simplification and reduced alveolar surface, reminiscent of emphysema. Several respiratory mechanics parameters are altered in mouse models of OI. The impact of type I collagen alterations and OI on lung morphology and function positively correlate with the severity of the extracellular matrix deficiency. Respiratory defects were more pronounced in male compared to female mice. It will be important to determine whether our observations in mice translate to OI patients and to dissect the respective contribution of intrinsic lung defects vs. extrinsic skeletal defects to impaired lung function in OI.

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.

Genome-Wide Association Study Reveals SNPs and Candidate Genes Related to Growth and Body Shape in Bighead Carp (Hypophthalmichthys nobilis)

Growth is an economically important trait in bighead carp and other aquaculture species that affects production efficiency. Interestingly, the head of the bighead carp has a high market value in China; therefore, it is important to study the genetic bases of both growth and body shape traits. A genome-wide association study was performed based on 2b-RAD sequencing of 776 individuals to identify SNPs associated with growth and body shape traits, including body weight, body length, body height, and deheaded body length. In total, 26 significant and 19 suggestive SNPs were identified, and more than half of these significant SNPs were clustered in LG16. Two LGs (LG16 and LG21) contained QTLs associated with body weight. Fourteen SNPs of LG16 and two LG21 SNPs were found to be associated with body length. For body height, 12 significantly associated SNPs were identified in LG16. Additionally, 12 SNPs of LG16 and 3 SNPs of LG21 were found to be associated with deheaded body length. Forty-three genes were significantly or suggestively associated with body shape/growth traits based on GWAS results, 18 of which were candidate genes for all BW, BL, BH, and DBL traits. One of these genes, fndc5b, was selected for further analyses. Association analysis revealed that one SNP (g.245 C > T) in the introns of fndc5b was significantly associated with growth-related traits in growth-extreme samples. The mRNA levels of fndc5b in the brains of the lightweight group were significantly higher than those of the heavy-weight group. This study helps to reveal the genetic structure of growth and body development in fish and provides candidate genes for future molecular marker-assisted selection for fast growth and better body conformation in bighead carp.

Craniofacial and dental phenotype of two girls with osteogenesis imperfecta due to mutations in CRTAP

Mutations in CRTAP lead to an extremely rare form of recessive osteogenesis imperfecta (OI). CRTAP deficient mice have a brachycephalic skull, fusion of facial bones, midface retrusion and class III dental malocclusion, but in humans, the craniofacial and dental phenotype has not been reported in detail. Here, we describe craniofacial and dental findings in two 11-year-old girls with biallelic CRTAP mutations. Patient 1 has a homozygous c.472-1021C>G variant in CRTAP intron 1 and a moderately severe OI phenotype. The variant is known to create a cryptic splice site, leading to a frameshift and nonsense-mediated RNA decay. Patient 1 started intravenous bisphosphonate treatment at 2 years of age. At age 11 years, height Z-score was +0.6. She had a short and wide face, concave profile and class III malocclusion, with a prognathic mandible and an antero-posterior crossbite. A panoramic radiograph showed a poor angulation of the second upper right premolar, and no dentinogenesis imperfecta or dental agenesis. Cone-beam computed tomography confirmed these findings and did not reveal any other abnormalities. Patient 2 has a homozygous CRTAP deletion of two amino acids (c.804_809del, p.Glu269_Val270del) and a severe OI phenotype. As previously established, the variant leads to instability of CRTAP protein. Intravenous bisphosphonate treatment was started at the age of 15 months. At 11 years of age her height Z-score was -9.7. She had a long and narrow face and convex profile, maxillary retrusion leading to a class III malocclusion, an edge-to-edge overjet and lateral open bite. Panoramic radiographs showed no dental abnormalities. Cone-beam computed tomography showed occipital bossing, platybasia and wormian bones. In these two girls with CRTAP mutations, the severity of the skeletal phenotype was mirrored in the severity of the craniofacial phenotype. Class III malocclusion and antero-posterior crossbite were a common trait, while dental agenesis or dentinogenesis imperfecta were not detected.

The Prognostic Significance and Potential Mechanism of Prolyl 3-Hydroxylase 1 in Hepatocellular Carcinoma

Background. Prolyl 3-hydroxylase 1 (P3H1) is essential for human collagen synthesis. Here, we investigated its relevance to multiple cancers, especially hepatocellular carcinoma (LIHC). Methods. We estimated the relationship of P3H1 with 33 cancers using publicly available databases. And immunohistochemistry was utilized to verify the P3H1 expression in liver, gastric, colon, pancreatic, and rectal cancer. Then, we attenuated P3H1 expression in BEL-7402 and HLF cells by lentivirus technology and assessed the effect of P3H1 on cell proliferation, migration, and invasion. Results. Bioinformatic analysis revealed a significantly higher expression of P3H1 in almost all tumors, which was consistent with the immunohistochemical findings in the liver, gastric, colon, pancreatic, and rectal cancers. P3H1 expression was associated with overall survival, progression-free interval, disease-specific survival, and disease-free interval in most cancers, particularly in LIHC. Besides, we also found that P3H1 expression was an independent prognostic factor for LIHC. And knockdown of P3H1 significantly reduced liver cancer cell proliferation, migration, and invasion in liver cancer cells. Interestingly, P3H1 expression levels showed a significant positive connection with Th2 infiltration through multiple immune infiltration algorithms. ICI treatment was less effective in LIHC patients with high P3H1 expression. Finally, we also identified an upstream regulatory mechanism of P3H1 in LIHC, namely, AL355488.1, HCG18, and THUMPD3-AS1/hsa-miR-29c-3p-P3H1 axis. Conclusion. We have systematically described for the first time that P3H1 is closely related to various tumors, particularly in LIHC, and interference with P3H1 may be a therapeutic target for patients with LIHC.

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.

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

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

Lysyl hydroxylase 2 mediated collagen post-translational modifications and functional outcomes

Lysyl hydroxylase 2 (LH2) is a member of LH family that catalyzes the hydroxylation of lysine (Lys) residues on collagen, and this particular isozyme has been implicated in various diseases. While its function as a telopeptidyl LH is generally accepted, several fundamental questions remain unanswered: 1. Does LH2 catalyze the hydroxylation of all telopeptidyl Lys residues of collagen? 2. Is LH2 involved in the helical Lys hydroxylation? 3. What are the functional consequences when LH2 is completely absent? To answer these questions, we generated LH2-null MC3T3 cells (LH2KO), and extensively characterized the type I collagen phenotypes in comparison with controls. Cross-link analysis demonstrated that the hydroxylysine-aldehyde (Hyl^ald)-derived cross-links were completely absent from LH2KO collagen with concomitant increases in the Lys^ald-derived cross-links. Mass spectrometric analysis revealed that, in LH2KO type I collagen, telopeptidyl Lys hydroxylation was completely abolished at all sites while helical Lys hydroxylation was slightly diminished in a site-specific manner. Moreover, di-glycosylated Hyl was diminished at the expense of mono-glycosylated Hyl. LH2KO collagen was highly soluble and digestible, fibril diameters were diminished, and mineralization impaired when compared to controls. Together, these data underscore the critical role of LH2-catalyzed collagen modifications in collagen stability, organization and mineralization in MC3T3 cells.

Cell surface accumulation of intracellular leucine proline-enriched proteoglycan 1 enhances odontogenic potential of human dental pulp stem cells

Primary dental pulp cells can be differentiated into odontoblast-like cells, which are responsible for dentin formation and mineralization. Successful differentiation of primary dental pulp cells can be verified using a few markers. However, odontoblast-specific cell surface markers have not been fully studied yet. LEucine PRoline-Enriched Proteoglycan 1 (LEPRE1) is a basement membrane-associated proteoglycan. LEPRE1 protein levels are increased during odontoblastic differentiation of human dental pulp cells. Intracellular and cell surface accumulation of this protein completely disappeared during dentin maturation and mineralization. Cell surface binding of an anti-LEPRE1 monoclonal antibody that could recognize an extracellular region was gradually increased in the odontoblastic stage. Overexpression and knock-down experiments showed that accumulation of intracellular LEPRE1 could lead to inefficient odontoblastic differentiation and that the movement of LEPRE1 from intracellular region to the cell surface was required for odontoblastic differentiation. Indeed, when LEPRE1 already located on the cell surface was blocked by the anti-LEPRE1 monoclonal antibody, odontoblastic differentiation of human dental pulp cells was inhibited. In this study, we looked at other aspects of LEPRE1 function as a cell surface molecule rather than its known intracellular hydroxylase activity. Our results indicate that this protein has potential as a specific cell surface marker in odontoblastic differentiation.

Comprehensive Mapping and Dynamics of Site-Specific Prolyl-Hydroxylation, Lysyl-Hydroxylation and Lysyl O-Glycosylation of Collagens Deposited in ECM During Zebrafish Heart Regeneration

Cardiac fibrosis-mediated heart failure (HF) is one of the major forms of end-stage cardiovascular diseases (CVDs). Cardiac fibrosis is an adaptive response of the myocardium upon any insult/injury. Excessive deposition of collagen molecules in the extracellular matrix (ECM) is the hallmark of fibrosis. This fibrotic response initially protects the myocardium from ventricular rupture. Although in mammals this fibrotic response progresses towards scar-tissue formation leading to HF, some fishes and urodeles have mastered the art of cardiac regeneration following injury-mediated fibrotic response. Zebrafish have a unique capability to regenerate the myocardium after post-amputation injury. Following post-amputation, the ECM of the zebrafish heart undergoes extensive remodeling and deposition of collagen. Being the most abundant protein of ECM, collagen plays important role in the assembly and cell-matrix interactions. However, the mechanism of ECM remodeling is not well understood. Collagen molecules undergo heavy post-translational modifications (PTMs) mainly hydroxylation of proline, lysine, and glycosylation of lysine during biosynthesis. The critical roles of these PTMs are emerging in several diseases, embryonic development, cell behavior regulation, and cell-matrix interactions. The site-specific identification of these collagen PTMs in zebrafish heart ECM is not known. As these highly modified peptides are not amenable to mass spectrometry (MS), the site-specific identification of these collagen PTMs is challenging. Here, we have implemented our in-house proteomics analytical pipeline to analyze two ECM proteomics datasets (PXD011627, PXD010092) of the zebrafish heart during regeneration (post-amputation). We report the first comprehensive site-specific collagen PTM map of zebrafish heart ECM. We have identified a total of 36 collagen chains (19 are reported for the first time here) harboring a total of 95 prolyl-3-hydroxylation, 108 hydroxylysine, 29 galactosyl-hydroxylysine, and 128 glucosylgalactosyl-hydroxylysine sites. Furthermore, we comprehensively map the three chains (COL1A1a, COL1A1b, and COL1A2) of collagen I, the most abundant protein in zebrafish heart ECM. We achieved more than 95% sequence coverage for all the three chains of collagen I. Our analysis also revealed the dynamics of prolyl-3-hydroxylation occupancy oscillations during heart regeneration at these sites. Moreover, quantitative site-specific analysis of lysine-O-glycosylation microheterogeneity during heart regeneration revealed a significant (p &lt; 0.05) elevation of site-specific (K1017) glucosylgalactosyl-hydroxylysine on the col1a1a chain. Taken together, these site-specific PTM maps and the dynamic changes of site-specific collagen PTMs in ECM during heart regeneration will open up new avenues to decode ECM remodeling and may lay the foundation to tinker the cardiac regeneration process with new approaches.

Mutational Screening of Skeletal Genes in 14 Chinese Children with Osteogenesis Imperfecta Using Targeted Sequencing

Background

As a heterogeneous hereditary connective tissue disorder, osteogenesis imperfecta (OI) is clinically characterized by increased fracture susceptibility. Analysis of genetic pathogenic variants in patients with OI provides a basis for genetic counseling and prenatal diagnosis.

Methods

In this study, 14 diagnosed OI patients from sporadic Chinese families were enrolled to be screened for potential mutations from these patients by next-generation sequencing technology.

Results

34 different variants were identified. 18 variants were from 4 OI-related genes including COL1A1, COL1A2, P3H1, and WNT1, and 10 variants are novel. Most OI patients (11 out of 14, 78%) harbor variants in type I collagen genes.

Conclusions

Our results support previously established estimates of the distribution and prevalence of OI mutations and highlight both phenotype and genetic heterogeneity among and within families. We report several novel variants of OI, which expands the clinical spectrum of OI. In summary, our data provides disease-causing genes information for genetic counseling towards OI patients and families and also provides a reference for clinicians in the diagnosis of OI, also in prenatal diagnosis of this disease.

Non-collagen pathogenic variants resulting in the osteogenesis imperfecta phenotype in children: a single-country observational cohort study

BACKGROUND/OBJECTIVES

In England, children (0-18 years) with severe, complex and atypical osteogenesis imperfecta (OI) are managed by four centres (Birmingham, Bristol, London, Sheffield) in a 'Highly Specialised Service' (HSS OI); affected children with a genetic origin for their disease that is not in COL1A1 or COL1A2 form the majority of the 'atypical' group, which has set criteria for entry into the service. We have used the data from the service to assess the range and frequency of non-collagen pathogenic variants resulting in OI in a single country.

METHODS

Children with atypical OI were identified through the HSS OI service database. All genetic testing for children with OI in the service were undertaken at the Sheffield Diagnostic Genetics Service. Variant data were extracted and matched to individual patients. This study was done as part of a service evaluation project registered with the Sheffield Children's Hospital Clinical Governance Department.

RESULTS

One hundred of 337 children in the HSS met the 'atypical' criteria. Eighty have had genetic testing undertaken; 72 had genetic changes detected, 67 in 13 genes known to be causative for OI. The most frequently affected genes were IFITM5 (22), P3H1 (12), SERPINF1 (8) and BMP1 (6).

CONCLUSION

Among children with more severe forms of OI (approximately one-third of all children with OI), around 20% have pathogenic variants in non-collagen genes. IFITM5 was the most commonly affected gene, followed by genes within the P3H1 complex. These data provide additional information regarding the likelihood of different genetic origins of the disease in children with OI, which may influence clinical care.

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).

Genotype-phenotype correlations and long-term efficacy of pamidronate therapy in patients with osteogenesis imperfecta

PURPOSE

Osteogenesis imperfecta (OI) is a rare bone fragility disorder caused by defects in type 1 collagen biosynthesis. This study investigated the genotype-phenotype correlations and the efficacy of pamidronate therapy in patients with OI in a single academic center.

METHODS

This study included 24 patients with OI. A clinical scoring system was used to evaluate disorder severity. COL1A1 and COL1A2 genes were analyzed in 13 patients using Sanger sequencing. Genotype-phenotype correlations and the efficacy of pamidronate therapy were analyzed through a retrospective medical chart review.

RESULTS

Of the 24 patients, 18 (75%) were classified as type I (12 with type Ia and 6 with type Ib), 2 as type III (8.4%), and 4 as type IV (16.7%). Type Ia patients showed relatively higher lumbar bone mineral density (BMD) standard deviation scores (SDS) and lower clinical scores than those with other types. Seven patients with qualitative mutations had lower lumbar BMD-SDS (P=0.015) and higher clinical scores (P=0.008) than 6 patients with quantitative mutations. The annual fracture frequency and lumbar BMD-SDS improved in patients with qualitative mutations after pamidronate treatment.

CONCLUSION

This study demonstrated that OI patients with qualitative mutations in COL1A1/2 had a more severe phenotype than those with quantitative mutations. Patients with qualitative mutations showed a significant reduction in fracture frequency and an increase in lumbar BMD-SDS after pamidronate treatment. Clinical score and genotype might be helpful for predicting phenotype and response to pamidronate therapy in OI patients.

On the association between Chiari malformation type 1, bone mineral density and bone related genes

Background

Chiari malformation type 1 (C1M) is a neurological disease characterized by herniation of the cerebellar tonsils below the foramen magnum. Cranial bone constriction is suspected to be its main cause. To date, genes related to bone development (e.g. DKK1 or COL1A2) have been associated with C1M, while some bone diseases (e.g. Paget) have been found to cosegregate with C1M. Nevertheless, the association between bone mineral density (BMD) and C1M has not been investigated, yet. Here, we systematically investigate the association between C1M and BMD, and between bone related genes and C1M.

Methods

We have recruited a small cohort of C1M patients (12 unrelated patients) in whom we have performed targeted sequencing of an in-house bone-related gene panel and BMD determination through non-invasive DXA.

Results

In the search for association between the bone related genes and C1M we have found variants in more than one C1M patient in WNT16, CRTAP, MYO7A and NOTCH2. These genes have been either associated with craniofacial development in different ways, or previously associated with C1M (MYO7A). Regarding the potential link between BMD and C1M, we have found three osteoporotic patients and one patient who had high BMD, very close to the HBM phenotype values, although most patients had normal BMD.

Conclusions

Variants in bone related genes have been repeatedly found in some C1M cases. The relationship of bone genes with C1M deserves further study, to get a clearer estimate of their contribution to its etiology. No direct correlation between BMD and C1M was observed.

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We used an in-house bone gene panel to investigate a small cohort of C1M patients.

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Variants in WNT16, CRTAP, MYO7A and NOTCH2 were found in more than one C1M patient.

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No clear relationship was found between C1M and BMD in this small C1M cohort.

Phenotypic Variation in Vietnamese Osteogenesis Imperfecta Patients Sharing a Recessive P3H1 Pathogenic Variant

Osteogenesis imperfecta (OI) is a syndromic disorder of bone fragility with high variation in its clinical presentation. Equally variable is molecular aetiology; recessive forms are caused by approximately 20 different genes, many of which are directly implicated in collagen type I biosynthesis. Biallelic variants in prolyl 3-hydroxylase 1 (P3H1) are known to cause severe OI by affecting the competence of the prolyl 3-hydroxylation—cartilage associated protein—peptidyl-prolyl cis-trans isomerase B (P3H1-CRTAP-CyPB) complex, which acts on the Pro986 residue of collagen type I α 1 (COL1A1) and Pro707 collagen type I α 2 (COL1A2) chains. The investigation of an OI cohort of 146 patients in Vietnam identified 14 families with P3H1 variants. The c.1170+5G>C variant was found to be very prevalent (12/14) and accounted for 10.3% of the Vietnamese OI cohort. New P3H1 variants were also identified in this population. Interestingly, the c.1170+5G>C variants were found in families with the severe clinical Sillence types 2 and 3 but also the milder types 1 and 4. This is the first time that OI type 1 is reported in patients with P3H1 variants expanding the clinical spectrum. Patients with a homozygous c.1170+5G>C variant shared severe progressively deforming OI type 3: bowed long bones, deformities of ribcage, long phalanges and hands, bluish sclera, brachycephaly, and early intrauterine fractures. Although it remains unclear if the c.1170+5G>C variant constitutes a founder mutation in the Vietnamese population, its prevalence makes it valuable for the molecular diagnosis of OI in patients of the Kinh ethnicity. Our study provides insight into the clinical and genetic variation of P3H1-related OI in the Vietnamese population.