Deficiency of cartilage-associated protein in recessive lethal osteogenesis imperfecta

Genotype-phenotype correlations in 294 pediatric patients with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is an inherited disorder characterized by bone fragility with extraskeletal manifestations mostly due to COL1A1 and COL1A2 variants. Currently, 23 genes have been implicated in the pathogenesis of OI; however, literature on genotype-phenotype correlation and incidence of non-skeletal clinical features are limited. This study aims to identify genotype-phenotype correlations in patients with OI, allowing clinicians to better inform families of prognosis, optimize patient care, and facilitate evidence-based clinical decision-making. We retrospectively reviewed 294 patients with OI to collect demographic data, clinical characteristics, and genotypic information. Patients were stratified by COL1A1/1A2 vs non-COL1A1/1A2 variants to evaluate differences in phenotype. The majority of OI was due to variants in COL1A1/1A2 (91%), with the remaining 9% due to non-COL1A1/1A2 variants. Most patients in the COL1A1/2 group were White compared to the non-COL1A1/2 group (78% vs 50%; p = 0.004). COL1A/1A2 patients had higher incidence of blue sclerae (83% vs 58%, p = 0.002), dentinogenesis imperfecta (49% vs 15%, p < 0.001), and family history of OI (34% vs 12%, p = 0.03). Those in the non-COL1A1/1A2 group have higher rates of scoliosis compared to those in the COL1A1/1A2 group (62% vs 40%, p = 0.04), as well as higher rates of expressive language disorder/delay (15% vs 0.4% in non-COL1A1/1A2 and COL1A1/1A2 patients, respectively; p < 0.001). Identifying the underlying molecular etiology early is imperative for optimal clinical care, allowing for appropriate risk counseling, identification of affected relatives, and improved anticipatory care and management. These data support that rare subtypes of OI occur more frequently in non-White individuals and demonstrated genetic associations with incidence of blue sclera, dentinogenesis imperfecta, scoliosis, and expressive language disorders.

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.

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.

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.

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.

Dickkopf-1 (DKK1) blockade mitigates osteogenesis imperfecta (OI) related bone disease

BACKGROUND

The current treatment of osteogenesis imperfecta (OI) is imperfect. Our study thus delves into the potential of using Dickkopf-1 antisense (DKK1-AS) to treat OI.

METHODS

We analysed serum DKK1 levels and their correlation with lumbar spine and hip T-scores in OI patients. Comparative analyses were conducted involving bone marrow stromal cells (BMSCs) and bone tissues from wild-type mice, untreated OI mice, and OI mice treated with DKK1-ASor DKK1-sense (DKK1-S).

RESULTS

Significant inverse correlations were noted between serum DKK1 levels and lumbar spine (correlation coefficient = - 0.679, p = 0.043) as well as hip T-scores (correlation coefficient = - 0.689, p = 0.042) in OI patients. DKK1-AS improved bone mineral density (p = 0.002), trabecular bone volume/total volume fraction (p < 0.001), trabecular separation (p = 0.010), trabecular thickness (p = 0.001), trabecular number (p < 0.001), and cortical thickness (p < 0.001) in OI mice. DKK1-AS enhanced the transcription of collagen 1α1, osteocalcin, runx2, and osterix in BMSC from OI mice (all p < 0.001), resulting in a higher von Kossa-stained matrix area (p < 0.001) in ex vivo osteogenesis assays. DKK1-AS also reduced osteoclast numbers (p < 0.001), increased β-catenin and T-cell factor 4 immunostaining reactivity (both p < 0.001), enhanced mineral apposition rate and bone formation rate per bone surface (both p < 0.001), and decreased osteoclast area (p < 0.001) in OI mice. DKK1-AS upregulated osteoprotegerin and downregulated nuclear factor-kappa B ligand transcription (both p < 0.001). Bone tissues from OI mice treated with DKK1-AS exhibited significantly higher breaking force compared to untreated OI mice (p < 0.001).

CONCLUSIONS

Our study elucidates that DKK1-AS has the capability to enhance bone mechanical properties, restore the transcription of osteogenic genes, promote osteogenesis, and inhibit osteoclastogenesis in OI mice.

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.

Case Report: A novel de novo variant of COL1A1 in fetal genetic osteogenesis imperfecta

Objective

Osteogenesis imperfecta (OI) is a rare genetic disorder. Clinical severity is heterogeneous. The purpose of this study was to investigate the genetic characteristics of a fetus with OI by whole exome sequencing (WES) and identify the cause of the disease.

Methods

In this study, a fetus with osteogenic dysplasia was referred to our hospital. DNA was extracted from the aborted fetal tissue and peripheral blood of the parents. To identify the pathogenic genes, we conducted the trio-WES using DNA. A de novo variant in the COL1A1 gene is suspected to be the cause of the OI phenotype. We used Sanger sequencing for validation and various bioinformatics methods (such as SIFT, PolyPhen2, Mutation Taster, conservative analysis, SWISS Model, glycosylation site prediction, and I-Mutant 2.0) for analysis.

Results

Both WES and Sanger sequencing identified a novel de novo variant of COL1A1 (c. 1309G>A, p. Gly437Ser) in a fetus with OI. Bioinformatic analysis showed that the affected residue, p. Gly437, was highly conserved in multiple species and predicted that the variant was deleterious and may have an impact on protein function. This variant is present in highly conserved glycine residues of Gly-X-Y sequence repeats of the triple helical region of the collagen type I α chain, which may be the cause of OI.

Conclusion

This study revealed that the c.1309G>A (p. Gly437Ser) variant in the COL1A1 gene may be the genetic cause of fetal OI in this case. The discovery of this variant enriched the variation spectrum of OI. WES improves the accurate diagnosis of fetal OI, and doctors can provide patients with appropriate genetic counseling.

Analysis of miRNAs in Osteogenesis imperfecta Caused by Mutations in COL1A1 and COL1A2: Insights into Molecular Mechanisms and Potential Therapeutic Targets

Osteogenesis imperfecta (OI) is a group of connective tissue disorders leading to abnormal bone formation, mainly due to mutations in genes encoding collagen type I (Col I). Osteogenesis is regulated by a number of molecules, including microRNAs (miRNAs), indicating their potential as targets for OI therapy. The goal of this study was to identify and analyze the expression profiles of miRNAs involved in bone extracellular matrix (ECM) regulation in patients diagnosed with OI type I caused by mutations in COL1A1 or COL1A2. Primary skin fibroblast cultures were used for DNA purification and sequence analysis, followed by analysis of miRNA expression. Sequencing analysis revealed mutations of the COL1A1 or COL1A2 genes in all OI patients, including four previously unreported. Amongst the 40 miRNAs analyzed, 9 were identified exclusively in OI cells and 26 in both OI patients and the controls. In the latter case, the expression of six miRNAs (hsa-miR-10b-5p, hsa-miR-19a-3p, hsa-miR-19b-3p, has-miR-204-5p, has-miR-216a-5p, and hsa-miR-449a) increased, while four (hsa-miR-129-5p, hsa-miR-199b-5p, hsa-miR-664a-5p, and hsa-miR-30a-5p) decreased significantly in OI cells in comparison to their expression in the control cells. The identified mutations and miRNA expression profiles shed light on the intricate processes governing bone formation and ECM regulation, paving the way for further research and potential therapeutic advancements in OI and other genetic diseases related to bone abnormality management.

Modeling collagen fibril self-assembly from extracellular medium in embryonic tendon

Collagen is a key structural component of multicellular organisms and is arranged in a highly organized manner. In structural tissues such as tendons, collagen forms bundles of parallel fibers between cells, which appear within a 24-h window between embryonic day 13.5 (E13.5) and E14.5 during mouse embryonic development. Current models assume that the organized structure of collagen requires direct cellular control, whereby cells actively lay down collagen fibrils from cell surfaces. However, such models appear incompatible with the time and length scales of fibril formation. We propose a phase-transition model to account for the rapid development of ordered fibrils in embryonic tendon, reducing reliance on active cellular processes. We develop phase-field crystal simulations of collagen fibrillogenesis in domains derived from electron micrographs of inter-cellular spaces in embryonic tendon and compare results qualitatively and quantitatively to observed patterns of fibril formation. To test the prediction of this phase-transition model that free protomeric collagen should exist in the inter-cellular spaces before the formation of observable fibrils, we use laser-capture microdissection, coupled with mass spectrometry, which demonstrates steadily increasing free collagen in inter-cellular spaces up to E13.5, followed by a rapid reduction of free collagen that coincides with the appearance of less-soluble collagen fibrils. The model and measurements together provide evidence for extracellular self-assembly of collagen fibrils in embryonic mouse tendon, supporting an additional mechanism for rapid collagen fibril formation during embryonic development.

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.

Outcomes of Pediatric Osteogenesis Imperfecta Patients Requiring Port-a-Cath Placement for Long-Term Vascular Access

Intravenous (iv) bisphosphonates are widely used to treat the skeletal manifestations of osteogenesis imperfecta (OI). Obtaining peripheral iv access in pediatric patients with OI is often difficult and traumatic. Although this may be mitigated with surgically placed iv ports (port-a-caths), surgeons may be hesitant to perform this procedure on these children because of the lack of safety data. This study aims to gain better insight into the safety and efficacy of port-a-cath use in this population and identify risk factors for port-a-cath complications. In the present study, we conducted a retrospective cohort analysis of patient characteristics and the incidence of port-a-cath-related complications in children with OI. Fifty-three port-a-caths were placed in 29 children (21 males and 8 females). Of the 29 patients, most are OI type III (n = 18), followed by type I (n = 4), type IV (n = 4), and type V (n = 3). At the time of initial port-a-cath placement, the median age was 52 months (10-191 months), and the median weight was 7.9 kg (5.1-41.1 kg). Most patients (n = 20) weighed less than 10 kg during initial placement. Weight correlated significantly with OI type (p = 0.048), sex (p = 0.03), and vessel used (p = 0.02). Median initial port-a-cath longevity was 43 months (1-113 months), and we found no significant difference in port-a-cath longevity between sexes, OI types, or vessels used. Most patients (n = 19) required multiple port-a-cath placements. There is a significant difference (p = 0.02) between the number of placements and OI type, with type IV having more than type III. Port-a-cath removal was almost always due to mechanical complications (n = 19) but also for infection (n = 1) and malposition (n = 1). Eight patients still had their initial port-a-caths in place at the conclusion of this study. These findings indicate that complications associated with port-a-cath placement are mild and can be used to safely deliver iv bisphosphonates to pediatric OI patients. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

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.

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.

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.

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.

The UIP/IPF fibroblastic focus is a collagen biosynthesis factory embedded in a distinct extracellular matrix

Usual interstitial pneumonia (UIP) is a histological pattern characteristic of idiopathic pulmonary fibrosis (IPF). The UIP pattern is patchy with histologically normal lung adjacent to dense fibrotic tissue. At this interface, fibroblastic foci (FF) are present and are sites where myofibroblasts and extracellular matrix (ECM) accumulate. Utilizing laser capture microdissection-coupled mass spectrometry, we interrogated the FF, adjacent mature scar, and adjacent alveoli in 6 fibrotic (UIP/IPF) specimens plus 6 nonfibrotic alveolar specimens as controls. The data were subjected to qualitative and quantitative analysis and histologically validated. We found that the fibrotic alveoli protein signature is defined by immune deregulation as the strongest category. The fibrotic mature scar classified as end-stage fibrosis whereas the FF contained an overabundance of a distinctive ECM compared with the nonfibrotic control. Furthermore, FF were positive for both TGFB1 and TGFB3, whereas the aberrant basaloid cell lining of FF was predominantly positive for TGFB2. In conclusion, spatial proteomics demonstrated distinct protein compositions in the histologically defined regions of UIP/IPF tissue. These data revealed that FF are the main site of collagen biosynthesis and that the adjacent alveoli are abnormal. This essential information will inform future mechanistic studies on fibrosis progression.

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.

Does the Skeletal Phenotype of Osteogenesis Imperfecta Differ for Patients With Non-COL1A1/2 Mutations? A Retrospective Study in 113 Patients

BACKGROUND

Osteogenesis imperfecta (OI) is a heritable disease characterized by bone fragility and other extra skeletal manifestations. Most patients with OI have mutations in the COL1A1 or COL1A2 genes. However, a significant minority of patients with clinical OI have non-COL1A1/2 mutations, which have become easier to detect with the use of genetic panels. Traditional understanding of OI pathogenesis was expanded because of these new mutations, and their phenotypic-genotypic relationship is largely unknown. We hypothesized that patients with non-COL1A1/2 mutations have different skeletal clinical presentations from those with OI caused by COL1A1/2 mutations.

METHODS

Patients were categorized into 4 groups according to our modified functional classification, namely, quantitative COL1A1/2 haploinsufficiency (group 1), qualitative COL1A1/2 dominant negative mutations (group 2), mutations indirectly affecting type I collagen synthesis, processing and posttranslational modification (group 3) and mutations altering osteoblast differentiation and function (group 4). Both group 3 and 4 were classified as non-COL1A1/2 mutation group.

RESULTS

Of 113 OI patients included, 51 had COL1A1/2 quantitative haploinsufficiency mutations (group 1), 39 had COL1A1/2 qualitative dominant negative mutations (group 2), and 23 patients had OI caused by mutations in 1 of 9 other noncollagen genes (groups 3/4). Patients with non-COL1A1/2 mutations (groups 3 and 4) have severe skeletal presentations. Specifically, OI patients with non-COL1A1/2 mutations experienced more perinatal fractures, vertebral compression fractures and had more long bone deformities. Although the occurrence of scoliosis was similar, the cobb angle was larger in the non-COL1A1/2 mutation group. Radial head dislocations, ossification of interosseous membrane, extraskeletal ossification, cervical kyphosis, and champagne glass deformity of the pelvis were more frequent in this group.

CONCLUSIONS

The clinical phenotype of OI in patients with non-COL1A1/2 is severe and has unique features. This information is useful for clinical diagnosis and prognosis.

LEVEL OF EVIDENCE

Level III.

Quantitative Proteomics Reveals Molecular Network Driving Stromal Cell Differentiation: Implications for Corneal Wound Healing

The differentiation of keratocytes to fibroblasts and myofibroblasts is an essential requisite during corneal wound closure. The aim of this study is to uncover factors involved in differentiation-dependent alteration in the protein profile of human corneal stromal cells using quantitative proteomics. Human corneal fibroblasts were cultured and differentiated into keratocytes in serum-free media and myofibroblasts through treatment with TGF-β. The protein cell lysates from the donors were tryptic and were digested and labeled using a 3-plex iTRAQ kit. The labeled peptides were subjected to LCMS analysis. Biological functional analysis revealed a set of crucial proteins involved in the differentiation of human corneal stromal cells which were found to be significantly enriched. The selected proteins were further validated by immunohistochemistry. Quantitative proteomics identified key differentially expressed proteins which are involved in cellular signaling pathways. Proteins involved in integrin signaling (Ras-RAP1b, TLN and FN) and SLIT-ROBO pathways (PFN1, CAPR1, PSMA5) as well as extracellular matrix proteins (SERPINH1, SPARC, ITGβ1, CRTAP) showed enhanced expression in corneal fibroblasts and myofibroblasts compared to keratocytes, indicating their possible role in wound healing. Corneal stromal cell differentiation is associated with the activation of diverse molecular pathways critical for the repair of fibroblasts and myofibroblasts. Identified proteins such as profilin 1 and talin could play a tentative role in corneal healing and serve as a potential target to treat corneal fibrosis.

Osteogenesis Imperfecta: Mechanisms and Signaling Pathways Connecting Classical and Rare OI Types

Osteogenesis imperfecta (OI) is a phenotypically and genetically heterogeneous skeletal dysplasia characterized by bone fragility, growth deficiency, and skeletal deformity. Previously known to be caused by defects in type I collagen, the major protein of extracellular matrix, it is now also understood to be a collagen-related disorder caused by defects in collagen folding, posttranslational modification and processing, bone mineralization, and osteoblast differentiation, with inheritance of OI types spanning autosomal dominant and recessive as well as X-linked recessive. This review provides the latest updates on OI, encompassing both classical OI and rare forms, their mechanism, and the signaling pathways involved in their pathophysiology. There is a special emphasis on mutations in type I procollagen C-propeptide structure and processing, the later causing OI with strikingly high bone mass. Types V and VI OI, while notably different, are shown to be interrelated by the interferon-induced transmembrane protein 5 p.S40L mutation that reveals the connection between the bone-restricted interferon-induced transmembrane protein-like protein and pigment epithelium-derived factor pathways. The function of regulated intramembrane proteolysis has been extended beyond cholesterol metabolism to bone formation by defects in regulated membrane proteolysis components site-2 protease and old astrocyte specifically induced-substance. Several recently proposed candidate genes for new types of OI are also presented. Discoveries of new OI genes add complexity to already-challenging OI management; current and potential approaches are summarized.

Osteogenesis Imperfecta: Current and Prospective Therapies

Osteogenesis Imperfecta (OI) is a group of connective tissue disorders with a broad range of phenotypes characterized primarily by bone fragility. The prevalence of OI ranges from about 1:15,000 to 1:20,000 births. Five types of the disease are commonly distinguished, ranging from a mild (type I) to a lethal one (type II). Types III and IV are severe forms allowing survival after the neonatal period, while type V is characterized by a mild to moderate phenotype with calcification of interosseous membranes. In most cases, there is a reduction in the production of normal type I collagen (col I) or the synthesis of abnormal collagen as a result of mutations in col I genes. Moreover, mutations in genes involved in col I synthesis and processing as well as in osteoblast differentiation have been reported. The currently available treatments try to prevent fractures, control symptoms and increase bone mass. Commonly used medications in OI treatment are bisphosphonates, Denosumab, synthetic parathyroid hormone and growth hormone for children therapy. The main disadvantages of these therapies are their relatively weak effectiveness, lack of effects in some patients or cytotoxic side effects. Experimental approaches, particularly those based on stem cell transplantation and genetic engineering, seem to be promising to improve the therapeutic effects of OI.

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

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

Efficacy of Romosozumab for Osteoporosis in a Patient With Osteogenesis Imperfecta: A Case Report

The efficacy of romosozumab for severe osteoporosis is uncertain in patients with osteogenesis imperfecta (OI). This report introduced a severe osteoporotic case of OI to examine the effect of romosozumab on bone fragility. A 64-year-old man with OI was referred to our department for finding out the cause of his repeated fractures. He was medicated with alendronate for only one year, eight years ago, but it did not prevent repeated fractures, and thus he had not received any treatments for osteoporosis since then. However, recently, the frequency of fractures had become increased. At presentation, his lumbar and bilateral total hip bone mineral density (BMD) values were severely decreased at 0.546 and 0.209 g/cm2, respectively. Because of his severe osteoporosis, we started romosozumab treatment with eldecalcitol. Romosozumab (210 mg) was injected subcutaneously every month. At 12 months after drug initiation, his lumbar and total hip BMD increased by 22.0% and 136.4% versus pre-treatment levels, respectively. Bone formation markers increased, and bone resorption markers decreased at 12 months of the therapy. Neither hypocalcemia nor any other severe adverse effects were observed in this severe osteoporotic case. This study revealed good responses of BMD and bone turnover markers to romosozumab treatment, which can be considered as an effective treatment option for osteoporotic OI patients.

Osteogenesis imperfecta in children

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

Tendon and motor phenotypes in the Crtap-/- mouse model of recessive osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by short stature, skeletal deformities, low bone mass, and motor deficits. A subset of OI patients also present with joint hypermobility; however, the role of tendon dysfunction in OI pathogenesis is largely unknown. Using the Crtap-/- mouse model of severe, recessive OI, we found that mutant Achilles and patellar tendons were thinner and weaker with increased collagen cross-links and reduced collagen fibril size at 1- and 4-months compared to wildtype. Patellar tendons from Crtap-/- mice also had altered numbers of CD146+CD200+ and CD146-CD200+ progenitor-like cells at skeletal maturity. RNA-seq analysis of Achilles and patellar tendons from 1-month Crtap-/- mice revealed dysregulation in matrix and tendon marker gene expression concomitant with predicted alterations in TGF-β, inflammatory, and metabolic signaling. At 4-months, Crtap-/- mice showed increased αSMA, MMP2, and phospho-NFκB staining in the patellar tendon consistent with excess matrix remodeling and tissue inflammation. Finally, a series of behavioral tests showed severe motor impairments and reduced grip strength in 4-month Crtap-/- mice - a phenotype that correlates with the tendon pathology.

Metabolic Bone Diseases in the Pediatric Population

Bone plays an important role in regulating mineral balance in response to physiologic needs. In addition, bone is subject to a continuous remodeling process to maintain healthy bone mass and growth. Metabolic bone diseases are a heterogeneous group of diseases caused by abnormalities of bone mass, mineral structure homeostasis, bone turnover, or bone growth. In pediatrics, several significant advances have been made in recent years in the diagnosis of metabolic bone diseases (e.g., osteogenesis imperfecta, hyperparathyroidism, rickets, renal osteodystrophy, pediatric osteoporosis, and osteopetrosis). Imaging is fundamental in the diagnosis of these pathologies.

Genetic Impact on Bone Modulation-A Review Bridging Bioscience to Genetic Engineering

Genes control approximately 60% to 75% of the variance of peak bone mass/density and a much smaller amount of variance in rate of loss.

Bone mass increases during growth to a peak value and soon after begins to decline. Most of the genetic effect is exerted during growth and so influences peak bone mass; whether there is an additional genetic effect on the rate of bone loss is less clear. So, this article aims to place emphasis on various oral and systemic conditions that are manifested due to altered gene function. Genetic polymorphisms and mutations are simple, although the consequences of the mechanism are complex. The syndromic manifestation due to changes at genetic level will greatly affect the bone quality, which will ultimately affect any treatment prognosis. Hence, a better understanding of molecular mechanisms of bone remodeling helps to identify pathogenic causes of bone, skeletal diseases, and leads to the development of targeted therapies for these diseases. This review highlights notions on the connecting link between science and genetics as well as various oral scenarios where gene could bring about changes, resulting in deformities. There is an intense research awaited in the future which could intervene with the causes that bring about genetic modulations, so as to decrease the mortality rate of humans.

Current Overview of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI), or brittle bone disease, is a heterogeneous disorder characterised by bone fragility, multiple fractures, bone deformity, and short stature. OI is a heterogeneous disorder primarily caused by mutations in the genes involved in the production of type 1 collagen. Severe OI is perinatally lethal, while mild OI can sometimes not be recognised until adulthood. Severe or lethal OI can usually be diagnosed using antenatal ultrasound and confirmed by various imaging modalities and genetic testing. The combination of imaging parameters obtained by ultrasound, computed tomography (CT), and magnetic resource imaging (MRI) can not only detect OI accurately but also predict lethality before birth. Moreover, genetic testing, either noninvasive or invasive, can further confirm the diagnosis prenatally. Early and precise diagnoses provide parents with more time to decide on reproductive options. The currently available postnatal treatments for OI are not curative, and individuals with severe OI suffer multiple fractures and bone deformities throughout their lives. In utero mesenchymal stem cell transplantation has been drawing attention as a promising therapy for severe OI, and a clinical trial to assess the safety and efficacy of cell therapy is currently ongoing. In the future, early diagnosis followed by in utero stem cell transplantation should be adopted as a new therapeutic option for severe OI.

Quality Control of Procollagen in Cells

Collagen is the most abundant protein in mammals. A unique feature of collagen is its triple-helical structure formed by the Gly-Xaa-Yaa repeats. Three single chains of procollagen make a trimer, and the triple-helical structure is then folded in the endoplasmic reticulum (ER). This unique structure is essential for collagen's functions in vivo, including imparting bone strength, allowing signal transduction, and forming basement membranes. The triple-helical structure of procollagen is stabilized by posttranslational modifications and intermolecular interactions, but collagen is labile even at normal body temperature. Heat shock protein 47 (Hsp47) is a collagen-specific molecular chaperone residing in the ER that plays a pivotal role in collagen biosynthesis and quality control of procollagen in the ER. Mutations that affect the triple-helical structure or result in loss of Hsp47 activity cause the destabilization of procollagen, which is then degraded by autophagy. In this review, we present the current state of the field regarding quality control of procollagen.

Skeletal Biology and Disease Modeling in Zebrafish

Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Aberrant binding of mutant HSP47 affects posttranslational modification of type I collagen and leads to osteogenesis imperfecta

Heat shock protein 47 (HSP47), encoded by the SERPINH1 gene, is a molecular chaperone essential for correct folding of collagens. We report a homozygous p.(R222S) substitution in HSP47 in a child with severe osteogenesis imperfecta leading to early demise. p.R222 is a highly conserved residue located within the collagen interacting surface of HSP47. Binding assays show a significantly reduced affinity of HSP47-R222S for type I collagen. This altered interaction leads to posttranslational overmodification of type I procollagen produced by dermal fibroblasts, with increased glycosylation and/or hydroxylation of lysine and proline residues as shown by mass spectrometry. Since we also observed a normal intracellular folding and secretion rate of type I procollagen, this overmodification cannot be explained by prolonged exposure of the procollagen molecules to the modifying hydroxyl- and glycosyltransferases, as is commonly observed in other types of OI. We found significant upregulation of several molecular chaperones and enzymes involved in procollagen modification and folding on Western blot and RT-qPCR. In addition, we showed that an imbalance in binding of HSP47-R222S to unfolded type I collagen chains in a gelatin sepharose pulldown assay results in increased binding of other chaperones and modifying enzymes. The elevated expression and binding of this molecular ensemble to type I procollagen suggests a compensatory mechanism for the aberrant binding of HSP47-R222S, eventually leading to overmodification of type I procollagen chains. Together, these results illustrate the importance of HSP47 for proper posttranslational modification and provide insights into the molecular pathomechanisms of the p.(R222S) alteration in HSP47, which leads to a severe OI phenotype.

Heat shock protein 47 (HSP47) is essential for correct collagen folding. We report a homozygous p.(R222S) substitution in HSP47 in a child with severe osteogenesis imperfecta. The highly conserved p.R222 residue is located within the collagen interacting surface and HSP47-R222S shows a significantly reduced affinity for type I collagen. This altered interaction leads to posttranslational overmodification of type I procollagen. In contrast to other types of OI, this overmodification is not caused by prolonged exposure of procollagen to modifying enzymes, since the intracellular folding rate of type I procollagen appears to be normal. We show significant upregulation of several molecular chaperones and collagen-modifying enzymes and increased binding of several of these molecules to unfolded type I collagen chains upon abnormal HSP47-R222S binding. This suggests a compensatory mechanism for aberrant HSP47-R222S binding, eventually leading to overmodification of type I procollagen chains, and underscores the importance of HSP47 for proper posttranslational modification.

A novel P3H1 mutation is associated with osteogenesis imperfecta type VIII and dental anomalies

OBJECTIVE

Our objective was to investigate the molecular etiology of osteogenesis imperfecta type VIII and dental anomalies in 4 siblings of a Karen tribe family.

MATERIALS AND METHODS

Four patients and their unaffected parents were studied by clinical and radiographic examination. In situ hybridization of P3h1 during early murine tooth development, whole-exome sequencing, and Sanger direct sequencing were performed.

RESULTS

A novel homozygous missense P3H1 mutation (NM_001243246.1; c.2141A>G; NP_001230175.1; p.Lys714Arg) was identified in all patients. Their unaffected parents were heterozygous for the mutation. The mutation is hypothesized to belong to isoform c of P3H1. Mutations in P3H1 are associated with autosomal recessive osteogenesis imperfecta type VIII. Hypodontia, a mesiodens, and single-rooted permanent second molars found in our patients have never been reported in patients with P3H1 mutations. Single-rooted second permanent molars or failure to form multiple roots implies effects of the P3H1 mutation on root development.

CONCLUSIONS

We report a novel P3H1 mutation as the underlying cause of osteogenesis imperfecta type VIII with dental anomalies. Our study suggests that isoform c of P3H1 is also a functional isoform of P3H1. We report, for the first time, to our knowledge, the association of P3H1 mutation and osteogenesis imperfecta type VIII with dental anomalies.

Intravenous pamidronate for treatment of osteogenesis imperfecta in Indian children

Osteogenesis imperfecta is characterised by low bone mineral density, bone fragility, fractures and deformity. We present five such children treated with intravenous pamidronate, which resulted in a decrease of fracture rate and increase in spinal bone mineral density.

Human dentin characteristics of patients with osteogenesis imperfecta: insights into collagen-based biomaterials

Osteogenesis imperfecta (OI), also known as "brittle bone disease", is a rare genetic disorder of the skeleton, whose most benign form I corresponds to autosomal dominant mutations in the genes encoding type I collagen (COLA1, COLA2). Several associated skeletal manifestations are often observed but, surprisingly, while dentin defects often reflect genetic bone disorders, about half of OI patients have no obvious oral manifestations. Here, we investigated the collagen, mineral and mechanical properties of dentin from deciduous teeth collected from patients with mild form of OI and displaying no obvious clinical signs of dentinogenesis imperfecta. For the first time, an increase in the hardness of OI dentin associated with an increase in mineral content compared to healthy patients was reported. In addition, OI altered the tissue characteristics of the dentin-enamel junction but the interfacial gradient was preserved. The impact of changes in molecular structure due to mutations in OI was assessed by Raman microspectroscopy. Our results highlighted a change in the hydroxyproline-proline ratio in direct association with collagen mineralization. Our findings suggest that the evaluation of teeth could be an important aid for mild types of OI that are often difficult to diagnose clinically and provide experimental evidence that hydroxyproline content should be considered in future studies on collagen-based biomaterials.

Signaling pathways affected by mutations causing osteogenesis imperfecta

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

Over-Representation of Recessive Osteogenesis Imperfecta in Asian Indian Children

Several genes are implicated in the etiology of early onset osteogenesis imperfecta (OI). The various genes causing severe OI include WNT1 , SERPINF1 , P3H1 , CREB3L1 , and CRTAP , although glycine substitutions in COL1A1chains have also been predicted to cause perinatal lethal OI . Patients with early onset OI present decreased mobility, recurrent rib fractures, bony deformities, and chest infections that lead to an early death. We reported our experience in children with OI in Asian Indian families, which includes two patients with SERPINF1 pathogenic variants; and another two patients with severe OI and antenatal fractures caused by pathogenic variants in the CRTAP gene, identified by next generation sequencing (NGS). For one affected fetus, medical termination of pregnancy was done. The other baby was started on zoledronate therapy just after birth and is now 3 years old. Prenatal diagnosis was subsequently done on chorionic villus sample in the latter family.

Novel Compound Heterozygous Mutations in CRTAP Cause Rare Autosomal Recessive Osteogenesis Imperfecta

Whole-exome sequencing (WES) has advantages over the traditional molecular test by screening 20,000 genes simultaneously and has become an invaluable tool for genetic diagnosis in clinical practice. Here, we reported a family with a child and a fetus presenting undiagnosed skeletal dysplasia phenotypes, while the parents were asymptomatic. WES was applied to the parents and affected fetus to identify the genetic cause of the phenotypes. We identified novel compound heterozygous mutations consisting of a single-nucleotide variant (SNV) and a large deletion in the CRTAP gene (NM_006371.4:c.1153-3C > G/hg19 chr3:g.32398837_34210906del). Genetic alterations of CRTAP are known to cause osteogenesis imperfecta (OI) in an autosomal recessive manner. Further examination of the proband’s elder sibling who was diagnosed as OI after birth found that she shares the inherited compound heterozygous mutations of CRTAP; thus, the findings support the disease-causing role of CRTAP mutations. Through the in vitro molecular test and in silico analysis, the deleterious effects of the splicing-altering SNV in CRTAP (c.1153-3C > G) on gene product were confirmed. Collectively, our WES-based pathogenic variant discovery pipeline identifies the SNVs and copy number variation to delineate the genetic cause on the proband affected with OI. The data not only extend the knowledge of mutation spectrum in patients with skeletal dysplasia but also demonstrate that WES holds great promise for genetic screening of rare diseases in clinical settings.

Fracture Healing in Collagen-Related Preclinical Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a genetic bone dysplasia characterized by bone deformities and fractures caused by low bone mass and impaired bone quality. OI is a genetically heterogeneous disorder that most commonly arises from dominant mutations in genes encoding type I collagen (COL1A1 and COL1A2). In addition, OI is recessively inherited with the majority of cases resulting from mutations in prolyl-3-hydroxylation complex members, which includes cartilage-associated protein (CRTAP). OI patients are at an increased risk of fracture throughout their lifetimes. However, non-union or delayed healing has been reported in 24% of fractures and 52% of osteotomies. Additionally, refractures typically go unreported, making the frequency of refractures in OI patients unknown. Thus, there is an unmet need to better understand the mechanisms by which OI affects fracture healing. Using an open tibial fracture model, our study demonstrates delayed healing in both Col1a2 ^G610c/+ and Crtap ^-/- OI mouse models (dominant and recessive OI, respectively) that is associated with reduced callus size and predicted strength. Callus cartilage distribution and chondrocyte maturation were altered in OI, suggesting accelerated cartilage differentiation. Importantly, we determined that healed fractured tibia in female OI mice are biomechanically weaker when compared with the contralateral unfractured bone, suggesting that abnormal OI fracture healing OI may prime future refracture at the same location. We have previously shown upregulated TGF-β signaling in OI and we confirm this in the context of fracture healing. Interestingly, treatment of Crtap ^-/- mice with the anti-TGF-β antibody 1D11 resulted in further reduced callus size and predicted strength, highlighting the importance of investigating dose response in treatment strategies. These data provide valuable insight into the effect of the extracellular matrix (ECM) on fracture healing, a poorly understood mechanism, and support the need for prevention of primary fractures to decrease incidence of refracture and deformity in OI patients. © 2020 American Society for Bone and Mineral Research.

Respiratory defects in the CrtapKO mouse model of osteogenesis imperfecta

Respiratory disease is a leading cause of mortality in patients with osteogenesis imperfecta (OI), a connective tissue disease that causes severely reduced bone mass and is most commonly caused by dominant mutations in type I collagen genes. Previous studies proposed that impaired respiratory function in OI patients was secondary to skeletal deformities; however, recent evidence suggests the existence of a primary lung defect. Here, we analyzed the lung phenotype of Crtap knockout (KO) mice, a mouse model of recessive OI. While we confirm changes in the lung parenchyma that are reminiscent of emphysema, we show that CrtapKO lung fibroblasts synthesize type I collagen with altered posttranslation modifications consistent with those observed in bone and skin. Unrestrained whole body plethysmography showed a significant decrease in expiratory time, resulting in an increased ratio of inspiratory time over expiratory time and a concomitant increase of the inspiratory duty cycle in CrtapKO compared with WT mice. Closed-chest measurements using the forced oscillation technique showed increased respiratory system elastance, decreased respiratory system compliance, and increased tissue damping and elasticity in CrtapKO mice compared with WT. Pressure-volume curves showed significant differences in lung volumes and in the shape of the curves between CrtapKO mice and WT mice, with and without adjustment for body weight. This is the first evidence that collagen defects in OI cause primary changes in lung parenchyma and several respiratory parameters and thus negatively impact lung function.

Dental and craniofacial defects in the Crtap-/- mouse model of osteogenesis imperfecta type VII

BACKGROUND

Inactivating mutations in the gene for cartilage-associated protein (CRTAP) cause osteogenesis imperfecta type VII in humans, with a phenotype that can include craniofacial defects. Dental and craniofacial manifestations have not been a focus of case reports to date. We analyzed the craniofacial and dental phenotype of Crtap^-/- mice by skull measurements, micro-computed tomography (micro-CT), histology, and immunohistochemistry.

RESULTS

Crtap^-/- mice exhibited a brachycephalic skull shape with fusion of the nasofrontal suture and facial bones, resulting in mid-face retrusion and a class III dental malocclusion. Loss of CRTAP also resulted in decreased dentin volume and decreased cellular cementum volume, though acellular cementum thickness was increased. Periodontal dysfunction was revealed by decreased alveolar bone volume and mineral density, increased periodontal ligament (PDL) space, ectopic calcification within the PDL, bone-tooth ankylosis, altered immunostaining of extracellular matrix proteins in bone and PDL, increased pSMAD5, and more numerous osteoclasts on alveolar bone surfaces.

CONCLUSIONS

Crtap^-/- mice serve as a useful model of the dental and craniofacial abnormalities seen in individuals with osteogenesis imperfecta type VII.

Characterization of PPIB interaction in the P3H1 ternary complex and implications for its pathological mutations

The P3H1/CRTAP/PPIB complex is essential for prolyl 3-hydroxylation and folding of procollagens in the endoplasmic reticulum (ER). Deficiency in any component of this ternary complex is associated with the misfolding of collagen and the onset of osteogenesis imperfecta. However, little structure information is available about how this ternary complex is assembled and retained in the ER. Here, we assessed the role of the KDEL sequence of P3H1 and probed the spatial interactions of PPIB in the complex. We show that the KDEL sequence is essential for retaining the P3H1 complex in the ER. Its removal resulted in co-secretion of P3H1 and CRTAP out of the cell, which was mediated by the binding of P3H1 N-terminal domain with CRTAP. The secreted P3H1/CRTAP can readily bind PPIB with their C-termini close to PPIB in the ternary complex. Cysteine modification, crosslinking, and mass spectrometry experiments identified PPIB surface residues involved in the complex formation, and showed that the surface of PPIB is extensively covered by the binding of P3H1 and CRTAP. Most importantly, we demonstrated that one disease-associated pathological PPIB mutation on the binding interface did not affect the PPIB prolyl-isomerase activity, but disrupted the formation of P3H1/CRTAP/PPIB ternary complex. This suggests that defects in the integrity of the P3H1 ternary complex are associated with pathological collagen misfolding. Taken together, these results provide novel structural information on how PPIB interacts with other components of the P3H1 complex and indicate that the integrity of P3H1 complex is required for proper collagen formation.

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.

Cellular stress due to impairment of collagen prolyl hydroxylation complex is rescued by the chaperone 4-phenylbutyrate

Osteogenesis imperfecta (OI) types VII, VIII and IX, caused by recessive mutations in cartilage-associated protein (CRTAP), prolyl-3-hydroxylase 1 (P3H1) and cyclophilin B (PPIB), respectively, are characterized by the synthesis of overmodified collagen. The genes encode for the components of the endoplasmic reticulum (ER) complex responsible for the 3-hydroxylation of specific proline residues in type I collagen. Our study dissects the effects of mutations in the proteins of the complex on cellular homeostasis, using primary fibroblasts from seven recessive OI patients. In all cell lines, the intracellular retention of overmodified type I collagen molecules causes ER enlargement associated with the presence of protein aggregates, activation of the PERK branch of the unfolded protein response and apoptotic death. The administration of 4-phenylbutyrate (4-PBA) alleviates cellular stress by restoring ER cisternae size, and normalizing the phosphorylated PERK (p-PERK):PERK ratio and the expression of apoptotic marker. The drug also has a stimulatory effect on autophagy. We proved that the rescue of cellular homeostasis following 4-PBA treatment is associated with its chaperone activity, since it increases protein secretion, restoring ER proteostasis and reducing PERK activation and cell survival also in the presence of pharmacological inhibition of autophagy. Our results provide a novel insight into the mechanism of 4-PBA action and demonstrate that intracellular stress in recessive OI can be alleviated by 4-PBA therapy, similarly to what we recently reported for dominant OI, thus allowing a common target for OI forms characterized by overmodified collagen.

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

Editor's choice: Mutations in the collagen 3-prolyl hydroxylation complex cause a cellular stress that is rescued by the chaperone ability of 4-phenylbutyrate.

A Case of Osteogenesis Imperfecta Type II With Additional Balanced Translocation t(1;20)(p13;p11.2)

BACKGROUND

Osteogenesis imperfect (OI) type II is a genetic disorder of bone characterized by bone fragility, multiple fractures, severe bowing and shortening of long bones, and perinatal death due to respiratory insufficiency. It is mainly caused by mutations in the COL1A1 or COL1A2 genes, inherited in an autosomal dominant manner.

CASE REPORT

A fetal form of this disorder that included brachydactyly, macrocephaly, frontal bossing, soft calvarium, saddle nose, micrognathia, low set ears, and narrow thoracic cavity is described. A postmortem skeletal survey revealed multiple fractures, unossified skull, and long crumpled bones. The fetal karyotype revealed a balanced translocation t(1;20)(p13;p11.2). DNA sequencing detected a c.3065G > T transversion in exon 42 of the COL1A1 gene, a mutation associated with OI type II.

CONCLUSION

Although the balanced translocation t(1:20)(p13;p11.2) appears to be incidental in our case, identification of the specific mutation and translocation is important for estimation of genetic risk for another afflicted child.

COL1A1 C-propeptide mutations cause ER mislocalization of procollagen and impair C-terminal procollagen processing

Mutations in the type I procollagen C-propeptide occur in ~6.5% of Osteogenesis Imperfecta (OI) patients. They are of special interest because this region of procollagen is involved in α chain selection and folding, but is processed prior to fibril assembly and is absent in mature collagen fibrils in tissue. We investigated the consequences of seven COL1A1 C-propeptide mutations for collagen biochemistry in comparison to three probands with classical glycine substitutions in the collagen helix near the C-propeptide and a normal control. Procollagens with C-propeptide defects showed the expected delayed chain incorporation, slow folding and overmodification. Immunofluorescence microscopy indicated that procollagen with C-propeptide defects was mislocalized to the ER lumen, in contrast to the ER membrane localization of normal procollagen and procollagen with helical substitutions. Notably, pericellular processing of procollagen with C-propeptide mutations was defective, with accumulation of pC-collagen and/or reduced production of mature collagen. In vitro cleavage assays with BMP-1 ± PCPE-1 confirmed impaired C-propeptide processing of procollagens containing mutant proα1(I) chains. Overmodified collagens were incorporated into the matrix in culture. Dermal fibrils showed alterations in average diameter and diameter variability and bone fibrils were disorganized. Altered ER-localization and reduced pericellular processing of defective C-propeptides are expected to contribute to abnormal osteoblast differentiation and matrix function, respectively.