Ultrastructural localization of proteoglycans in bone in osteogenesis imperfecta as demonstrated by Cuprolinic Blue staining

 The role of proteoglycans in bone in osteogenesis imperfecta (OI) has been examined. Using Cuprolinic Blue staining of whole fetal bone tissue and examining the tissue in the transmission electron microscope, the presence of proteoglycans was observed. Quantitative comparative image-analysis of the proteoglycans from electron micrographs was performed, with measurement of sizes and number of proteoglycan particles. A significant increase in the total number of proteoglycan particles in OI bone osteoid was observed when compared with normal, matched controls. The area of the proteoglycan particles, as measured by pixel-area, using image analysis, was also increased in OI bone osteoid. These findings further suggest a role for proteoglycans in mineral formation by the possible inhibition of mineral growth and alteration of collagen nucleation sites. The increased number and size of proteoglycan particles may be a contributing factor to the previously reported poor mineral formation with subsequent loss of bone strength, making it more prone to fracture, in OI. Received: August 28, 2001 / Accepted: February 8, 2002     

An ultrastructural and immunogold localization study of proteoglycans associated with the osteocytes of fetal bone in osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a rare, heterogeneous, inherited connective tissue disorder frequently caused by abnormalities of type I collagen. It is characterized by bone fragility, osteopenia, and progressive skeletal deformities. Electron microscopy of three OI type II fetal bone samples revealed numerous large osteocyte lacunae. In addition, there was a perilacunar osteoid-like band of collagen surrounding the osteocytes, which was unmineralized and morphologically unusual. Furthermore, large osteocyte lacunae contained fine particles and filamentous material similar to the expected ultrastructural appearance of proteoglycans. More detailed examination was carried out using histochemical and immunogold localization of proteoglycans at light and ultrastructural levels. These tests and the use of electron probe X-ray microanalysis confirmed that the material in the osteocyte lacunae was proteoglycan. In contrast, in the age- and site-matched normal fetal bone, all the osteocyte lacunae appeared negative for proteoglycan. Proteoglycans are regarded as inhibitors of calcification. Our observation of substantial amounts of proteoglycan in abnormally enlarged osteocytic lacunae of some OI fetal bone suggests association with the abnormal bone of this particular subtype of OI type II.     

Morphometric Analysis of Type I Collagen Fibrils in the Osteoid of Osteogenesis Imperfecta

Electron microscopy and morphometric measurements of bone osteoid collagen diameter from 42 osteogenesis imperfecta (OI) patients and 25 age- and site-matched controls were carried out. Although the mean diameter did not correlate well with the severity of the disease, it related well with the clinical types and revealed collagen fibrils of reduced diameter in the osteoid of all OI types. Thus, OI type II (the severest type) demonstrated the smallest diameter (45 nm), followed by OI type I (the mildest form) with a mean diameter of 57 nm. The diameter obtained for type III (67 nm) and type IV (64 nm) was lower than the normal control mean diameter (73 nm) but did not show a statistical difference. The thinner fibrils observed in OI bone may be unable to provide nucleating and scaffolding sites for mineral propagation and may play a role in the fragility of bone in this disease. Received: 1 June 1998 / Accepted: 23 April 1999     

Extracellular matrix formation by osteoblasts from patients with osteogenesis imperfecta.

Extracellular matrix proteins synthesized by bone cells isolated from 16 patients with different forms of osteogenesis imperfecta (OI) were analyzed in vitro. Specific components of the extracellular matrix by OI and age-matched cultures were investigated by steady-state radiolabeling followed by quantitation of label into specific proteins and comparison of OI cultures to those of age-matched controls. The in vitro proliferation of OI bone cells was found to be lower than that of control cells. In seven patients, abnormalities of the alpha 1(I) and/or alpha 2(I) chains of type I collagen were detected by gel electrophoresis. In two of these patients, the mutations in the COLIA1 and COLIA2 genes have been previously identified. Although the amount of total protein synthesized by the cells in culture was the same for OI bone cells and age-matched control cells, OI bone cells showed a significantly reduced synthesis of not only collagen but also other bone matrix glycoproteins. The synthesis of osteonectin (SPARC/BM40) and three proteoglycans [a large chondroitin sulfate proteoglycan, biglycan (PGI), and decorin (PGII)] was found to be decreased in OI cells. The reduction was most pronounced at the developmental age at which these macromolecules reach maximal levels during normal development.     

成骨不全的研究进展

成骨不全(OI)以骨骼脆性增加、反复骨折为主要临床表现的遗传性结缔组织疾病,亦可累及眼、耳、皮肤等,有常染色体显性遗传与常染色体隐性遗传两种遗传方式。目前根据临床表型、遗传方式和致病基因可分为15型,新发现的致病基因有待进一步明确分型。多数的OI患者是常染色体显性遗传,以Ⅰ型胶原蛋白结构基因COL1A1、COL1A2突变为主,非Ⅰ型胶原蛋白突变的常染色体隐性遗传的成骨不全患者数量少,但致病基因种类多,机制较为繁复,主要是前胶原蛋白的合成代谢异常所致。通过典型的临床特征及脆性骨折不难建立OI的诊断,为预防新生患儿,产前基因诊断极为重要。OI的治疗需多项学科的优化协同,畸形严重者可采取措施矫正畸形,改善负重力线,联合药物治疗可减轻疼痛、降低骨折风险。目前双膦酸盐在OI药物治疗中起主导地位,甲状旁腺素氨基端片段、抗硬化蛋白抗体等药物,有望增加骨密度、改善骨骼微结构且降低骨折风险,基因治疗、干细胞移植是新型治疗手段。本文综述成骨不全症的研究进展。     

Osteogenesis imperfecta: changes in noncollagenous proteins in bone

The noncollagenous proteins osteonectin, bone sialoprotein, osteocalcin, the small proteoglycan decorin (PG II), and alpha 2-HS glycoprotein (which is synthesized in the liver but highly concentrated in bone) were measured in extracts of cortical bone from 3 type I, 2 type II, 8 type III and 13 type IV patients with osteogenesis imperfecta (OI) and from 7 control subjects. Osteonectin was found to be reduced in the bone of all OI patients. The bone from severely affected type III OI patients contained the lowest levels of osteonectin. In contrast, bone sialoprotein was found to be elevated in the bones of OI patients. The highest levels were found in individuals classified as type IV patients. Osteocalcin and alpha 2-HS glycoprotein concentrations were increased in all OI patients. Decorin levels were not significantly altered in OI bones compared to controls. These changes in the concentrations of the noncollagenous proteins may contribute to the fragility of the OI bone by interfering with complete mineralization and/or normal tissue architecture.     

Defective Proteolytic Processing of Fibrillar Procollagens and Prodecorin Due to Biallelic BMP1 Mutations Results in a Severe,Progressive Form of Osteogenesis Imperfecta

Whereas the vast majority of osteogenesis imperfecta (OI) is caused by autosomal dominant defects in the genes encoding type I procollagen, mutations in a myriad of genes affecting type I procollagen biosynthesis or bone formation and homeostasis have now been associated with rare autosomal recessive OI forms. Recently, homozygous or compound heterozygous mutations in BMP1, encoding the metalloproteases bone morphogenetic protein‐1 (BMP1) and its longer isoform mammalian Tolloid (mTLD), were identified in 5 children with a severe autosomal recessive form of OI and in 4 individuals with mild to moderate bone fragility. BMP1/mTLD functions as the procollagen carboxy‐(C)‐proteinase for types I to III procollagen but was also suggested to participate in amino‐(N)‐propeptide cleavage of types V and XI procollagens and in proteolytic trimming of other extracellular matrix (ECM) substrates. We report the phenotypic characteristics and natural history of 4 adults with severe, progressive OI characterized by numerous fractures, short stature with rhizomelic shortening, and deformity of the limbs and variable kyphoscoliosis, in whom we identified novel biallelic missense and frameshift mutations in BMP1. We show that BMP1/mTLD‐deficiency in humans not only results in delayed cleavage of the type I procollagen C‐propeptide but also hampers the processing of the small leucine‐rich proteoglycan prodecorin, a regulator of collagen fibrillogenesis. Immunofluorescent staining of types I and V collagen and transmission electron microscopy of the dermis show impaired assembly of heterotypic type I/V collagen fibrils in the ECM. Our study thus highlights the severe and progressive nature of BMP1‐associated OI in adults and broadens insights into the functional consequences of BMP1/mTLD‐deficiency on ECM organization. © 2015 American Society for Bone and Mineral Research.     

Impaired pyridinoline cross-link formation in patients with osteogenesis imperfecta

Patients with osteogenesis imperfecta (OI) show various degrees of bone fragility. Nevertheless, details of the mechanisms causing bone fragility remain unclear. We hypothesized that differences in pyridinoline cross-link formation at the N-and C-termini in type I collagen molecules partly contribute to bone fragility of OI. To verify this hypothesis, urinary N and C terminal telopeptides of type I collagen (uNTx and ubetaCTx, respectively) and urinary hydroxyproline (uHyp) were measured using second morning void urine samples obtained from OI patients and healthy control children. Ratios of uNTx and ubetaTx to uHyp (uNTx/uHyp and ubetaCTx/uHyp, respectively) of OI patients and healthy normal control children were analyzed. Ratios of uNTx to ubetaCTx (uNTx/ubetaCTx) were also analyzed. In OI patients, uNTx and ubetaCTx were lower than in healthy control children. Also, uNTx/uHyp and ubetaCTx/uHyp were significantly lower than in healthy children. Among OI patients, uNTx/uHyp and uNTx/ubetaCTx of type III OI were significantly lower than of either type I or type IV OI. These results show that pyridinoline cross-link formation is lower than in healthy control children and that pyridinoline cross-link formation at the N-and C-termini in type I collagen molecules might be differently disrupted in OI patients according to the severity of OI.     

Mineralized tissue protein profiles in the Australian form of bovine osteogenesis imperfecta

Summary Noncollagenous proteins and proteoglycan from the bone and dentin of affected and normal half-siblings neonatal calves with the Australian variant of bovine osteogenesis imperfecta (BOI-A) were quantitated. In contrast to a clinically similar syndrome of BOI in the progeny of a second progenitor bull identified in Texas (BOI-T), osteonectin and bone sialoprotein levels were normal in the affected BOI-A bone, and dentin phosphophoryn levels were normal in affected BOI-A teeth. However, bone proteoglycan in the affected BOI-A calves was depleted to one-third of the level found in normal siblings, a finding qualitatively similar to the depletion of bone proteoglycan in the BOI-T syndrome. These results suggest that quantitation of noncollagenous proteins and proteoglycans may be a useful technique for differentiating clinically similar syndromes of OI in man.     

Collagen defect of bone in osteogenesis imperfecta (Type I). An electron microscopic study

Osteogenesis imperfecta (OI) is an inherited disorder of connective tissue metabolism characterized by fragility of the bones, resulting in multiple fractures. In this study electron microscopic examination of the osteoid region of iliac crest bone biopsy specimens from a 55-year-old man and his three sons (18, 21, and 25 years of age) affected with the common Type I (autosomal dominant) form of OI revealed collagen fibrils consistently smaller in diameter than those from normal age-matched control subjects, with the majority of OI fibrils measuring 0.04-0.06 micron, as opposed to 0.06-0.08 micron in the control subjects. This defect may be due to the biosynthesis of Type III collagen, not normally found in adult lamellar bone or, more likely, to a failure of maturation of Type I fibrils to their normal diameters, possibly owing to the increased levels of hydroxylysine previously reported in OI collagen, a factor known to be inversely related to fibril diameter.     

Increased intra-cortical porosity reduces bone stiffness and strength in pediatric patients with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable disease occurring in one out of every 20,000 births. Although it is known that Type I collagen mutation in OI leads to increased bone fragility, the mechanism of this increased susceptibility to fracture is not clear. The aim of this study was to assess the microstructure of cortical bone fragments from patients with osteogenesis imperfecta (OI) using polarized light microscopy, and to correlate microstructural observations with the results of previously performed mechanical compression tests on bone from the same source. Specimens of cortical bone were harvested from the lower limbs of three (3) OI patients at the time of surgery, and were divided into two groups. Group 1 had been subjected to previous micro-mechanical compression testing, while Group 2 had not been subjected to any prior testing. Polarized light microscopy revealed disorganized bone collagen architecture as has been previously observed, as well as a large increase in the areal porosity of the bone compared to typical values for healthy cortical bone, with large (several hundred micron sized), asymmetrical pores. Importantly, the areal porosity of the OI bone samples in Group 1 appears to correlate strongly with their previously measured apparent Young's modulus and compressive strength. Taken together with prior nanoindentation studies on OI bone tissue, the results of this study suggest that increased intra-cortical porosity is responsible for the reduction in macroscopic mechanical properties of OI cortical bone, and therefore that in vivo imaging modalities with resolutions of ~ 100 μm or less could potentially be used to non-invasively assess bone strength in OI patients. Although the number of subjects in this study is small, these results highlight the importance of further studies in OI bone by groups with access to human OI tissue in order to clarify the relationship between increased porosity and reduced macroscopic mechanical integrity.     

Compositional analysis of collagen from patients with diverse forms of osteogenesis imperfecta

Summary Collagen was extracted by pepsin treatment from various tissues and skin fibroblasts of 23 patients belonging to different types of osteogenesis imperfecta (OI), and characterized by molecular sieve and ion exchange chromatography, gel electrophoresis, and amino acid analysis. We found an elevated collagen III/I ratio in the skin of one patient with OI type I but almost normal values in skin fibroblasts of two other patients of this OI type. Five patients with OI type II had a normal collagen III/I ratio in their skin and skin fibroblasts, but the degree of hydroxylation of lysine residues in collagen I and III from their skin, bone, calvarium, and noncalcified calvarial tissue was increased. Patients belonging to OI types II, III, and IV had also considerable amounts of collagen III in their long bones, while bone tissue from controls contained only type I collagen. The content of type V in calcified tissues was virtually the same in controls and patients.     

A Fourier transform infrared spectroscopic and solid-state NMR study of bone mineral in osteogenesis imperfecta

Fourier transform infrared spectroscopy and ³¹P solid-state nuclear magnetic resonance spectroscopy were used to determine if any structural or compositional differences in osteogenesis imperfecta (OI) bone mineral could be detected that might help to explain the bone fragility observed in this disease. A previous study by Cassella et al. used an electron probe X-ray microanalytical technique to compare the calcium to phosphorus (Ca/P) molar ratios in normal bone and bone from patients with OI. It was demonstrated that bone from OI patients had a lower Ca/P molar ratio. This study demonstrated that OI bone mineral had a general hydroxyapatite structure and that isomorphous substitutions in the carbanoapatite lattice could account for the low Ca/P molar ratio. Received: December 28, 1999 / Accepted: March 22, 2000     

A novel splicing mutation in FKBP10 causing osteogenesis imperfecta with a possible mineralization defect

Osteogenesis imperfecta (OI) is a group of hereditary disorders characterized by bone fragility and osteopenia, with a broad spectrum of clinical severity. The majority of cases are dominantly inherited and due to mutations in type I collagen genes, whereas recessive forms are less frequent and attributable to mutations in different genes involved in collagen I post translational modifications and folding (prolyl-3-hydroxylase complex, SERPINH1, FKBP10).We report the case of a patient with an initially mild and then progressively severe form of osteogenesis imperfecta due to a novel homozygous splicing mutation in FKBP10 (intron 8 c.1399+1G>A), which results in aberrant mRNA processing and consequent lack of FKBP65 chaperone.Although this mutation does not affect collagen type I post translational modifications in dermal fibroblasts, the histomorphometric pattern of our patient's bone sample showed a mineralization defect possibly due to the mutation in FKBP10.     

Fracture failure mechanisms in patients with osteogenesis imperfecta

This investigation examines the failure mechanism of bone from patients with osteogenesis imperfecta (OI). Mechanical testing and ultrastructural analysis of surgically obtained bone specimens, from patients with OI and from approximately age matched normal controls, were performed. Small rectangular bone samples (1.0 mm X 1.2 mm X 15 mm) with a prismatic cross section were put under three-point bending to determine the work to fracture. Furthermore, the fracture surface was observed with scanning electron microscopy (SEM). The SEM observation of partially dissected lamellae was also performed to determine the collagen organization of the bone. The OI bone has a consistently lower work to fracture and reduced collagen organization than the normal controls. The fracture surface of the OI bone is rough and exhibits a "pull-out" type of morphology, as opposed to the smoother fracture surface of the controls. This study shows that a relatively simple mechanical test utilizing a small amount of bone can differentiate between a diseased state (OI) and a normal state of bone.     

Abnormal Mineral Composition of Osteogenesis Imperfecta Bone as Determined by Electron Probe X-ray Microanalysis on Conventional and Cryosections

Osteogenesis imperfecta (OI) is a genetic disorder of the connective tissue characterized by frequent bone fractures. The cause of bone fragility is still unknown even though substantial work on collagen has been done. We measured the calcium to phosphorus ratio (Ca/P) of bone mineral from 35 OI bone samples and 25 age- and site-matched control specimens, using electron probe X-ray microanalysis in the transmission electron microscope. Ultra-thin cryosections and conventionally prepared resin sections were used. Cryo-ultramicrotomy avoids any possible artifactual demineralization that may occur in conventional aqueous media. The Ca/P ratio obtained by these two methods was compared and there was no statistical difference between them. The results were differentiated according to the clinical types of OI for the first time. The Ca/P ratio of OI bone mineral was lower than normal in both resin and cryosections, and mirrored the severity of the disease. OI type II had the lowest ratio (Ca/P = 1.49) compared with normal age- and site-matched controls (Ca/P = 1.69). This abnormal mineral composition in OI type II could be a contributory factor to bone fragility in OI bone. Received: 1 June 1998 / Accepted: 23 December 1998     

Pulmonary hypoplasia and osteogenesis imperfecta type II with defective synthesis of alpha I(1) procollagen

Perinatal lethal osteogenesis imperfecta (OI type II), a heritable disorder of connective tissue occurs approximately once in 60,000 live births. Phenotypic characteristics include defective cranial ossification and severe skeletal deformity due to intrauterine rib and long bone fractures. Lethal OI may be associated with intracranial hemorrhage or severe respiratory insufficiency. Pulmonary hypoplasia has been previously observed in OI type II, but has not been defined clinically. The infant described herein was born with OI type II and pulmonary hypoplasia. Pathological examination of airway branching patterns indicated that lung development had progressed to only the 10 week stage with immature acinar development. Investigation type I collagen synthesis by cultured dermal fibroblasts revealed the presence of electrophoretically abnormal alpha 1(I) polypeptides. These findings suggest that biochemically regulated processes, as well as mechanical factors, may impeded pulmonary development in similar cases of OI type II.     

A morphological and ultrastructural study of bone in osteogenesis imperfecta

A morphological and electron microscopic study of bone from patients with osteogenesis imperfecta (OI) has been performed. Bone from OI patients from various anatomical sites has been compared with that from normal, age-, site-, and sex-matched controls. The morphology of OI bone appeared variable among patients and sites of bone examined. Immature woven bone and a poor lamellar pattern were the significant morphological features and demonstrated that OI could not be characterized on the basis of a single histological pattern. At the ultrastructural level, a number of previously unreported features were evident. Abnormal collagen fibers and an altered mineral composition were found in many OI patients, however, the panoramic heterogeneity between clinical types and indeed within a single clinical type made it difficult to classify OI in this manner. The presence of intermitochondrial inclusions containing calcium and phosphorus and the presence of a stromal calcification in the bone in some OI patients suggested an abnormal mineral formation. Qualitatively, no obvious difference in the number of osteoblasts or osteoclasts was observed. The morphology and ultrastructure of OI bone were good indicators of the disease and serve a role in assessing the progress of a patient through diagnosis and treatment. This report presents new ultrastructural findings in collagen and in mineral formation in OI compared with normal human bone.     

Limb lengthening and correction of deformity in the lower limbs of children with osteogenesis imperfecta

We performed limb lengthening and correction of deformity of nine long bones of the lower limb in six children (mean age, 14.7 years) with osteogenesis imperfecta (OI). All had femoral lengthening and three also had ipsilateral tibial lengthening. Angular deformities were corrected simultaneously. Five limb segments were treated using a monolateral external fixator and four with the Ilizarov frame. In three children, lengthening was done over previously inserted femoral intramedullary rods. The mean lengthening achieved was 6.26 cm (mean healing index, 33.25 days/cm). Significant complications included one deep infection, one fracture of the femur and one anterior angulation deformity of the tibia. The abnormal bone of OI tolerated the external fixators throughout the period of lengthening without any episodes of migration of wires or pins through the soft bone. The regenerate bone formed within the time which is normally expected in limb-lengthening procedures performed for other conditions. We conclude that despite the abnormal bone characteristics, distraction osteogenesis to correct limb-length discrepancy and angular deformity can be performed safely in children with OI.