成骨不全及其分子机制

成骨不全作为罕见性遗传性结缔组织疾病,具有临床异质性与遗传异质性,迄今已经分为15个亚型.有常染色体显性遗传与常染色体隐性遗传两种遗传方式.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,涉及到胶原合成后异常修饰,胶原蛋白分子伴侣及羧基端前肽剪切酶缺陷、成骨细胞与破骨细胞分化及转录因子异常、钙离子通道与Wnt信号通路分子等诸多方面.致病基因及其机制的研究,对于成骨不全的基因确诊及个体化药物治疗意义重大.     

成骨不全的分子机制

成骨不全是一类临床表现为骨质脆弱、易骨折等特征的罕见遗传性疾病.绝大多数（90%以上）显性患者发病系由Ⅰ型前胶原α链COL1A1和COL1A2基因突变引起胶原合成量不足 ,或结构改变.少数隐性患者发病为其他相关基因突变导致胶原翻译后过度修饰、折叠、装配和分泌过程异常.本文就成骨不全发病的遗传学及分子生物学机制作一综述.     

Instability of Polymeric Skin Collagen in Osteogenesis Imperfecta

The structural polymeric collagen of the skin of 19 patients with osteogenesis imperfecta has been examined. In those with severe bone disease, who often have white sclerae, this collagen fraction is less resistant to depolymerization than that of age-matched controls, though the total amount is normal. In patients with less severe bone disease, whose sclerae are usually blue, the polymeric collagen may have normal stability but the total amount is reduced. These results suggest defective cross-linking of collagen in severe osteogenesis imperfecta.     

PPIB Mutations Cause Severe Osteogenesis Imperfecta

Deficiency of cartilage-associated protein (CRTAP) or prolyl 3-hydroxylase 1(P3H1) has been reported in autosomal-recessive lethal or severe osteogenesis imperfecta (OI). CRTAP, P3H1, and cyclophilin B (CyPB) form an intracellular collagen-modifying complex that 3-hydroxylates proline at position 986 (P986) in the α1 chains of collagen type I. This 3-prolyl hydroxylation is decreased in patients with CRTAP and P3H1 deficiency. It was suspected that mutations in the PPIB gene encoding CyPB would also cause OI with decreased collagen 3-prolyl hydroxylation. To our knowledge we present the first two families with recessive OI caused by PPIB gene mutations. The clinical phenotype is compatible with OI Sillence type II-B/III as seen with COL1A1/2, CRTAP, and LEPRE1 mutations. The percentage of 3-hydroxylated P986 residues in patients with PPIB mutations is decreased in comparison to normal, but it is higher than in patients with CRTAP and LEPRE1 mutations. This result and the fact that CyPB is demonstrable independent of CRTAP and P3H1, along with reported decreased 3-prolyl hydroxylation due to deficiency of CRTAP lacking the catalytic hydroxylation domain and the known function of CyPB as a cis-trans isomerase, suggest that recessive OI is caused by a dysfunctional P3H1/CRTAP/CyPB complex rather than by the lack of 3-prolyl hydroxylation of a single proline residue in the α1 chains of collagen type I.     

Recessive Osteogenesis Imperfecta Caused by Missense Mutations in SPARC

Secreted protein, acidic, cysteine-rich (SPARC) is a glycoprotein that binds to collagen type I and other proteins in the extracellular matrix. Using whole-exome sequencing to identify the molecular defect in two unrelated girls with severe bone fragility and a clinical diagnosis of osteogenesis imperfecta type IV, we identified two homozygous variants in SPARC (GenBank: {"type":"entrez-nucleotide","attrs":{"text":"NM_003118.3","term_id":"365777426","term_text":"NM_003118.3"}}NM_003118.3; c.497G>A [p.Arg166His] in individual 1; c.787G>A [p.Glu263Lys] in individual 2). Published modeling and site-directed mutagenesis studies had previously shown that the residues substituted by these mutations form an intramolecular salt bridge in SPARC and are essential for the binding of SPARC to collagen type I. The amount of SPARC secreted by skin fibroblasts was reduced in individual 1 but appeared normal in individual 2. The migration of collagen type I alpha chains produced by these fibroblasts was mildly delayed on SDS-PAGE gel, suggesting some overmodification of collagen during triple helical formation. Pulse-chase experiments showed that collagen type I secretion was mildly delayed in skin fibroblasts from both individuals. Analysis of an iliac bone sample from individual 2 showed that trabecular bone was hypermineralized on the material level. In conclusion, these observations show that homozygous mutations in SPARC can give rise to severe bone fragility in humans.     

Severe Osteogenesis Imperfecta in Cyclophilin B–Deficient Mice

Osteogenesis Imperfecta (OI) is a human syndrome characterized by exquisitely fragile bones due to osteoporosis. The majority of autosomal dominant OI cases result from point or splice site mutations in the type I collagen genes, which are thought to lead to aberrant osteoid within developing bones. OI also occurs in humans with homozygous mutations in Prolyl-3-Hydroxylase-1 (LEPRE1). Although P3H1 is known to hydroxylate a single residue (pro-986) in type I collagen chains, it is unclear how this modification acts to facilitate collagen fibril formation. P3H1 exists in a complex with CRTAP and the peptidyl-prolyl isomerase cyclophilin B (CypB), encoded by the Ppib gene. Mutations in CRTAP cause OI in mice and humans, through an unknown mechanism, while the role of CypB in this complex has been a complete mystery. To study the role of mammalian CypB, we generated mice lacking this protein. Early in life, Ppib-/- mice developed kyphosis and severe osteoporosis. Collagen fibrils in Ppib-/- mice had abnormal morphology, further consistent with an OI phenotype. In vitro studies revealed that in CypB–deficient fibroblasts, procollagen did not localize properly to the golgi. We found that levels of P3H1 were substantially reduced in Ppib-/- cells, while CRTAP was unaffected by loss of CypB. Conversely, knockdown of either P3H1 or CRTAP did not affect cellular levels of CypB, but prevented its interaction with collagen in vitro. Furthermore, knockdown of CRTAP also caused depletion of cellular P3H1. Consistent with these changes, post translational prolyl-3-hydroxylation of type I collagen by P3H1 was essentially absent in CypB–deficient cells and tissues from CypB–knockout mice. These data provide significant new mechanistic insight into the pathophysiology of OI and reveal how the members of the P3H1/CRTAP/CypB complex interact to direct proper formation of collagen and bone.     

A Missense Mutation in the SERPINH1 Gene in Dachshunds with Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a hereditary disease occurring in humans and dogs. It is characterized by extremely fragile bones and teeth. Most human and some canine OI cases are caused by mutations in the COL1A1 and COL1A2 genes encoding the subunits of collagen I. Recently, mutations in the CRTAP and LEPRE1 genes were found to cause some rare forms of human OI. Many OI cases exist where the causative mutation has not yet been found. We investigated Dachshunds with an autosomal recessive form of OI. Genotyping only five affected dogs on the 50 k canine SNP chip allowed us to localize the causative mutation to a 5.82 Mb interval on chromosome 21 by homozygosity mapping. Haplotype analysis of five additional carriers narrowed the interval further down to 4.74 Mb. The SERPINH1 gene is located within this interval and encodes an essential chaperone involved in the correct folding of the collagen triple helix. Therefore, we considered SERPINH1 a positional and functional candidate gene and performed mutation analysis in affected and control Dachshunds. A missense mutation (c.977C>T, p.L326P) located in an evolutionary conserved domain was perfectly associated with the OI phenotype. We thus have identified a candidate causative mutation for OI in Dachshunds and identified a fifth OI gene.     

一成骨不全家系的COL1A1基因突变检测

成骨不全(Osteogenesisimperfecta,OI)是一种由于Ⅰ型胶原形成障碍,导致骨脆性增强为主要症状的 常染色体显性遗传性疾病。临床上主要表现为骨质脆弱、蓝巩膜、耳聋和中等程度的关节畸形等症状。成骨不全 基因分别定位于17q21.31 q22和7q22.1,其致病基因分别为COL1A1和COL1A2。对一常染色体显性遗传的 成骨不全家系进行连锁分析,在COL1A1遗传位点发现紧密连锁(LOD=9.31;θ=.00)。突变检测发现在 COL1A1基因第26内含子5′端剪接位点处存在一由GT转换为AT的致病突变,该突变引起的异常剪接是导致成 骨不全的致病原因之一。     

Molecular and Mesoscale Mechanisms of Osteogenesis Imperfecta Disease in Collagen Fibrils

Osteogenesis imperfecta (OI) is a genetic disorder in collagen characterized by mechanically weakened tendon, fragile bones, skeletal deformities, and in severe cases, prenatal death. Although many studies have attempted to associate specific mutation types with phenotypic severity, the molecular and mesoscale mechanisms by which a single point mutation influences the mechanical behavior of tissues at multiple length scales remain unknown. We show by a hierarchy of full atomistic and mesoscale simulation that OI mutations severely compromise the mechanical properties of collagenous tissues at multiple scales, from single molecules to collagen fibrils. Mutations that lead to the most severe OI phenotype correlate with the strongest effects, leading to weakened intermolecular adhesion, increased intermolecular spacing, reduced stiffness, as well as a reduced failure strength of collagen fibrils. We find that these molecular-level changes lead to an alteration of the stress distribution in mutated collagen fibrils, causing the formation of stress concentrations that induce material failure via intermolecular slip. We believe that our findings provide insight into the microscopic mechanisms of this disease and lead to explanations of characteristic OI tissue features such as reduced mechanical strength and a lower cross-link density. Our study explains how single point mutations can control the breakdown of tissue at much larger length scales, a question of great relevance for a broad class of genetic diseases.     

Immunocytochemical Detection of Dentin Matrix Proteins in Primary Teeth from Patients with Dentinogenesis Imperfecta Associated with Osteogenesis Imperfecta

Dentinogenesis imperfecta determines structural alterations of the collagen structure still not completely elucidated. Immunohisto-chemical analysis was used to assay type I and VI collagen, various non-collagenous proteins distribution in human primary teeth from healthy patients or from patients affected by type I dentinogenesis imperfecta (DGI-I) associated with osteogenesis imperfecta (OI). In sound primary teeth, an organized well-known ordered pattern of the type I collagen fibrils was found, whereas atypical and disorganized fibrillar structures were observed in dentin of DGI-I affected patients. Expression of type I collagen was observed in both normal and affected primary teeth, although normal dentin stained more uniformly than DGI-I affected dentin. Reactivity of type VI collagen was significantly lower in normal teeth than in dentin from DGI-I affected patients (P<0.05). Expressions of dentin matrix protein-1 (DMP1) and osteopontin (OPN) were observed in both normal dentin and dentin from DGI-I affected patients, without significant differences, being DMP1 generally more abundantly expressed. Immuno labeling for chondroitin sulfate (CS) and biglycan (BGN) was weaker in dentin from DGI-I-affected patients compared to normal dentin, this decrease being significant only for CS. This study shows ultra-structural alterations in dentin obtained from patients affected by DGI-I, supported by immunocytochemical assays of different collagenous and non-collagenous proteins.Key words: Osteogenesis imperfecta, dentinogenesis imperfecta, immuno-electron microscopy, collagen, non-collagenous proteins     

First use of the RANKL antibody denosumab in Osteogenesis Imperfecta Type VI

Osteogenesis imperfecta (OI) is a genetically heterogeneous disease leading to bone fragility. OI-VI is an autosomal-recessive form caused by mutations in SERPINF1. There is experimental evidence suggesting that loss of functional SERPINF1 leads to an activation of osteoclasts via the RANK/RANKL pathway. Patients with OI-VI show a poor response to bisphosphonates. We report on four children with OI-VI who had shown continuously elevated urinary bone resorption markers during a previous treatment with bisphosphonates. We treated these children with the RANKL antibody denosumab to reduce bone resorption. Intervention and results: Denosumab (1 mg/kg body weight) was injected s.c. every 3 months. There were no severe side effects. Markers of bone resorption decreased to the normal range after each injection. N-terminal Propeptide of collagen 1 was measured in the serum during the first treatment cycle and decreased also. Urinary deoxypyridinoline/creatinine was monitored in a total of seven treatment cycles and indicated that bone resorption reached the pre-treatment level after 6-8 weeks. Conclusion: This was the first use of denosumab in children with OI-VI. Denosumab was well tolerated, and laboratory parameters provided evidence that the treatment reversibly reduced bone resorption. Therefore, denosumab may be a new therapeutic option for patients with OI-VI.     

Genotype-Phenotype Relationship and Follow-up Analysis of a Chinese Cohort With Osteogenesis Imperfecta

ObjectiveTo evaluate the genotype-phenotype relationship and the effect of treatment on the clinical course of osteogenesis imperfecta (OI).MethodsWe established a Chinese hospitalized cohort with OI and followed them up for an average of 6 years. All patients were confirmed as having OI using whole-exome sequencing. We analyzed the genotype-phenotype relationship based on different types, pathogenic mechanisms, and gene inheritance patterns of OI. Additionally, we assessed whether there was a difference in treatment efficacy based on genotype.ResultsOne hundred sixteen mutations in 6 pathogenic genes (COL1A1, COL1A2, IFITM5, SERPINF1, FKBP10, and WNT1) were identified in 116 patients with type I, III, IV, V, VI, XI, or XV OI. Compared with patients with COL1A1 mutations, patients with COL1A2 mutations were younger at the time of the first fracture, whereas other phenotypes were similar. When 3 groups (helical, haploinsufficiency, and non-collagen I gene mutations) were compared, patients with helical mutations were the shortest and most prone to dentinogenesis imperfecta. Patients with haploinsufficiency mutations were the oldest at the time of the first fracture. Moreover, patients with non-collagen I gene mutations were least susceptible to blue sclerae and had the highest fracture frequency. Furthermore, there were some minor phenotypic differences among non-collagen I gene mutations. Interestingly, pamidronate achieved excellent results in the treatment of patients with OI, and the treatment effect appeared to be unrelated to their genotypes.ConclusionOur findings indicated a genotype-phenotype relationship and a similar effect of pamidronate treatment in patients with OI, which could provide a basis for guiding clinical treatment and predicting OI prognosis.     

Phenotype and Genotype Analysis of Chinese Patients with Osteogenesis Imperfecta Type V

Osteogenesis imperfecta (OI) type V is an autosomal-dominant disease characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation. The causative mutation, c.-14C>T in the 5''-untranslated region of IFITM5, was recently discovered to be involved in this disease. However, in spite of the little genotypic variability, considerable phenotypic variability has been recognized in two cohorts of patients, the majority of whom were Caucasians. Using exome sequencing, we identified the same heterozygous mutation in four Chinese families with OI type V. This study confirms the molecular cause of OI type V and describes the phenotype of Chinese patients with this disorder. In conclusion, the phenotype of Chinese patients was generally similar to that of Caucasian patients.     

Atypical Femoral Fracture in an Osteogenesis Imperfecta Patient Successfully Treated with Teriparatide

ObjectiveWe report a case of a successfully healed atypical femoral fracture (AFF) following treatment with teriparatide in a patient with osteogenesis imperfecta (OI). To our knowledge, no successful treatment of AFFs with teriparatide in this subpopulation has ever been described.MethodsThis is a case report of an AFF treated with teriparatide.ResultsThe patient was treated with hormone replacement therapy for 18 years and bisphosphonates for 9 years before suffering a spontaneous AFF in the form of a dislocated noncomminute transverse fracture of the right femoral shaft, and an open reduction and internal fixation (ORIF) with a T2 Femoral Nail was done. Due to nonunion and another fracture distal to the nail, the patient was reoperated on with exchange ORIF and off-label treatment with teriparatide 20 μg/day was started. An X-ray 1 month later showed early signs of fracture healing. A subsequent X-ray 6 months after the last operation showed a solid healing of both right femoral fractures.ConclusionThis is a rare case that highly suggests a potential fracture healing effect of teriparatide treatment and highlights a potential significant practical therapeutic consideration in relation to the management of AFF with delayed healing. (Endocr Pract. 2014;20:e187-e190)     

A Single Recurrent Mutation in the 5'-UTR of IFITM5 Causes Osteogenesis Imperfecta Type V

Osteogenesis imperfecta (OI) is a heterogenous group of genetic disorders of bone fragility. OI type V is an autosomal-dominant disease characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation; the causative mutation involved in this disease has not been discovered yet. Using linkage analysis in a four-generation family and whole-exome sequencing, we identified a heterozygous mutation of c.-14C>T in the 5'-untranslated region of a gene encoding interferon-induced transmembrane protein 5 (IFITM5). It completely cosegregated with the disease in three families and occurred de novo in five simplex individuals. Transfection of wild-type and mutant IFITM5 constructs revealed that the mutation added five amino acids (Met-Ala-Leu-Glu-Pro) to the N terminus of IFITM5. Given that IFITM5 expression and protein localization is restricted to the skeletal tissue and IFITM5 involvement in bone formation, we conclude that this recurrent mutation would have a specific effect on IFITM5 function and thus cause OI type V.     

Zoledronic Acid Versus Alendronate In The Treatment of Children With Osteogenesis Imperfecta: A 2-Year Clinical Study

Objective: Bisphosphonates have been demonstrated to increase the bone mineral density (BMD) of osteogenesis imperfecta (OI) patients. We aimed to compare the efficacy and safety of intravenous zoledronic acid and oral alendronate in patients with OI.Methods: A total of 161 patients with OI ranging from 2 to 16 years old were included and randomized at a 2:1 ratio to receive either weekly oral alendronate (ALN) 70 mg or a once-yearly infusion of zoledronic acid (ZOL) for 2 years. The primary endpoints were percentage change from baseline in lumbar spine (LS) BMD and change in Z-scores of LS BMD.Results: A total of 136 patients with OI completed the 2-year clinical study, 90 of whom were assigned to receive ALN, while 46 received ZOL treatment. The percentage change in LS BMD was 60.01 ± 7.08% in the ALN group and 62.04 ± 5.9% in the ZOL group (P = .721). The corresponding BMD Z-score increased by 0.50 ± 0.05 in the ALN group and 0.71 ± 0.06 in the ZOL group (P = .013). ZOL was superior to ALN in reducing the clinical fracture rate (hazard ratio, 0.23; 95% confidence interval, 0.118 to 0.431). There was no difference in the incidence of severe side effects between the two groups.Conclusion: A once-yearly 5 mg infusion of ZOL and weekly oral ALN had similar effects in increasing BMD and reducing bone resorption in children and adolescents with OI. ZOL was superior to ALN in reducing the clinical fracture rate.Abbreviations: 25OHD = 25-hydroxyvitamin D; ALN = alendronate; ALP = alkaline phosphatase; β-CTX = cross-linked C-telopeptide of type I collagen; BMD = bone mineral density; BP = bisphosphonate; FN = femoral neck; LS = lumbar spine; OI = osteogenesis imperfecta; SAE = severe adverse event; ZOL = zoledronic acid     

Development of a High-Throughput Resequencing Array for the Detection of Pathogenic Mutations in Osteogenesis Imperfecta

Objective

Osteogenesis imperfecta (OI) is a rare inherited skeletal disease, characterized by bone fragility and low bone density. The mutations in this disorder have been widely reported to be on various exonal hotspots of the candidate genes, including COL1A1, COL1A2, CRTAP, LEPRE1, and FKBP10, thus creating a great demand for precise genetic tests. However, large genome sizes make the process daunting and the analyses, inefficient and expensive. Therefore, we aimed at developing a fast, accurate, efficient, and cheaper sequencing platform for OI diagnosis; and to this end, use of an advanced array-based technique was proposed.

Method

A CustomSeq Affymetrix Resequencing Array was established for high-throughput sequencing of five genes simultaneously. Genomic DNA extraction from 13 OI patients and 85 normal controls and amplification using long-range PCR (LR-PCR) were followed by DNA fragmentation and chip hybridization, according to standard Affymetrix protocols. Hybridization signals were determined using GeneChip Sequence Analysis Software (GSEQ). To examine the feasibility, the outcome from new resequencing approach was validated by conventional capillary sequencing method.

Result

Overall call rates using resequencing array was 96–98% and the agreement between microarray and capillary sequencing was 99.99%. 11 out of 13 OI patients with pathogenic mutations were successfully detected by the chip analysis without adjustment, and one mutation could also be identified using manual visual inspection.

Conclusion

A high-throughput resequencing array was developed that detects the disease-associated mutations in OI, providing a potential tool to facilitate large-scale genetic screening for OI patients. Through this method, a novel mutation was also found.     

成骨不全显性遗传一家系五代28例报告

对成骨不全先证者上溯5代372人进行家系调查,发现该家系共有28例成骨不全,其中已死亡10例(成人、儿童各5例),均非死于本病;现生存18例(成人12例、儿童6例)。该28例均身材矮小、身高显著低于同龄正常者;均有蓝色巩膜;符合常染色体显性遗传缺陷的结缔组织病。现存活18例中,耳聋、听力下降6例;骨折9例(22例次),对照文献中分型标准,本组28例与Ⅰ型、Ⅲ型的部分临床特征相符,是否系介于Ⅰ型、Ⅲ型之间的亚型,有待于进一步研究。 Abstract:A pedigree investigation was carried out on 372 relatives in five generations of a proband of osteogenesis imperfecta.The results showed that there were 28 patients in the family.10 of them died from other causes(5 adults and 5 children respectively).Other 18 cases are alive(12 adults and 6 children respectively).The height of the 28 cases were significantly lower than those of the same age.Furthermore,they all had blue scleras.Their clinical manifestations conformed to those of autosomal dominant hereditary connective tissue disease.Among the living 18 cases,6 suffered from deafness or hearing loss and 0 suffered from fracture(22 person-time altogether).According to the classification criteria in the literature,the clinical manifestations of the 28 cases conformed to those of type I and type III,It is necessary to study further whether they belonged to the subtype between type I and type III.