Differential response to intracellular stress in the skin from osteogenesis imperfecta Brtl mice with lethal and non lethal phenotype: a proteomic approach

Phenotypic variability in the presence of an identical molecular defect is a recurrent feature in heritable disorders and it was also reported in osteogenesis imperfecta (OI). OI is a prototype for skeletal dysplasias mainly caused by mutations in the two genes coding for type I collagen. No definitive cure is available for this disorder, but the understanding of molecular basis in OI phenotypic modulation will have a pivotal role in identifying possible targets to develop novel drug therapy. We used a functional proteomic approach to address the study of phenotypic variability using the skin of the OI murine model Brtl. Brtl mice reproduce the molecular defect, dominant transmission and phenotypic variability of human OI patients. In the presence of a Gly349Cys substitution in α1(I)-collagen Brtl mice can have a lethal or a moderately severe outcome. Differential expression of chaperones, proteasomal subunits, metabolic enzymes, and proteins related to cellular fate demonstrated that a different ability to adapt to cellular stress distinguished mutant from wild-type mice and mutant lethal from surviving mutant animals. Interestingly, class discovery analysis identified clusters of differentially expressed proteins associated with a specific outcome, and functional analysis contributed to a deeper investigation into biochemical and cellular pathways affected by the disease. This article is part of a Special Issue entitled: Translational Proteomics.     

Direct sequencing of PCR products for mutation detection in osteogenesis imperfecta

This work present a short and simple method for mutation detection in type I collagen genes, based on the direct sequencing of single-stranded DNA. The sequencing of type I collagen genes is complicated and difficult because of their large size and highly repetitive and GC-rich coding regions. Although many techniques have been developed for mutation screening in osteogenesis imperfecta (OI), they represent different degrees of sensitivity and are difficult to reproduce and too expensive for application in each laboratory. The method described here is short, easy and especially useful for sequencing of collagen genes in OI cases, in which the region with a suspected structural defect is localized by collagen analysis.     

Molecular heterogeneity in the mild autosomal dominant forms of osteogenesis imperfecta.

Mild osteogenesis imperfecta (OI type I and OI type IV) is characterized by postnatal onset of fractures, absence of skeletal deformity, presenile hearing loss with or without blue sclerae, and dentinogenesis imperfecta. Using one common DNA polymorphism associated with the pro alpha 2(I) human collagen gene, we found genetic heterogeneity in this disorder. In three families, the OI phenotype segregated independently of the DNA polymorphism, whereas in one family, the OI phenotype cosegregated with a DNA polymorphism in a manner suggesting linkage. Use of DNA polymorphisms associated with both type I procollagen genes should provide a tool to unravel the molecular heterogeneity of various heritable disorders of the connective tissue.     

A forward genetic screen for genes regulating mineralized tooth and bone formation in zebrafish (Danio rerio)

Our laboratory studies craniofacial skeletal and tooth regeneration. One approach we are using is to exploit the zebrafish model via a large‐scale, forward genetic, chemical N‐ethyl‐nitroso‐urea (ENU) mutagenesis screen to identify genes regulating mineralized craniofacial, axial and dental development. The fact that zebrafish continuously regenerate their teeth makes them an extremely useful model to study tooth regeneration. Our goal is to identify and characterize molecular genetic signaling pathways regulating these processes, which can be manipulated via targeted gene delivery strategies. Through these efforts, we hope to eventually define methods for effective, clinically relevant bone and tooth replacement therapies in humans. Here, we describe our studies using the zebrafish model, which are proving to be useful for the identification and characterization of genes regulating mineralized tissue formation, regeneration, and homeostasis. Although preliminary at the present time, we anticipate the elucidation of novel signaling pathways regulating bone and tooth regeneration, which will eventually facilitate the repair of human skeletal and dental dysplasias.     

Altered ratio of collagen chains in bone of a patient with non-lethal osteogenesis imperfecta.

Bone from a patient with osteogenesis imperfecta contained type III collagen which was absent in control bone. The ratio of alpha 1(I)/alpha 2(I) in type I collagen of patient's bone was increased (2.9 vs. 2.3 +/- 0.2 in controls) and the ratio of dimers beta 11/beta 12/beta 22 was altered due to the increased beta 22 content. No abnormality was observed in collagen from the patient's skin. The altered composition of collagen in bone, but the normal composition in skin suggests that the disease in the patient is due to impaired regulation of the synthesis of collagens in bone, rather than by a mutation in one of the two type I collagen genes. Unlike in skin, all the type III collagen in patient's bone was pepsin-soluble indicating an inability of the bone to incorporate type III collagen into mature highly cross-linked extracellular matrix.     

Osteogenesis imperfecta type IV. Detection of a point mutation in one alpha 1(I) collagen allele (COL1A1) by RNA/RNA hybrid analysis

We have identified a point mutation in one alpha 1(I) collagen allele (COL1A1) of a child with the type IV osteogenesis imperfecta phenotype. When compared to parental and control samples, skin fibroblasts of the proband synthesized two populations of type I collagen molecules. One population was normal; the other was delayed in secretion and electrophoretic migration due to post-translational overmodification. Two-dimensional gel electrophoresis of the CNBr peptides demonstrated a gradient of overmodification beginning near the carboxyl-terminal CB peptides. This predicts that the mutation delaying helix formation is near the carboxyl-terminal end of one of the component chains of type I collagen. The mRNA of the patient was probed with overlapping antisense riboprobes to type I collagen cDNA. Cleavage of a mismatch in RNA/RNA hybrids of RNase A allowed the location of the mutation to a 225-base pair region of alpha 1(I) cDNA. The mismatch was not present in RNA/RNA hybrids from either parent. This region of both alpha 1(I) alleles of the patient was isolated by screening a lambda ZAP cDNA library. Sequence determination of both alleles demonstrated a single nucleotide change, G----A, resulting in the substitution of a serine for a glycine at amino acid residue 832. This point mutation occurs in the coding region for alpha 1(I) CB6 and is concordant with the protein data. The finding of a glycine substitution in an alpha 1(I) chain of a patient with the milder type IV osteogenesis imperfecta phenotype requires modification of current molecular models for types II and IV osteogenesis imperfecta.     

Changes in collagen stability and folding in lethal perinatal osteogenesis imperfecta. The effect of alpha 1 (I)-chain glycine-to-arginine substitutions.

The effect of glycine-to-arginine mutations in the alpha 1 (I)-chain on collagen triple-helix structure in lethal perinatal osteogenesis imperfecta was studied by determination of the helix denaturation temperature and by computerized molecular modelling. Arginine substitutions at glycine residues 391 and 667 resulted in similar small decreases in helix stability. Molecular modelling suggested that the glycine-to-arginine-391 mutant resulted in only a relatively small localized disruption to the helix structure. Thus the glycine-to-arginine substitutions may lead to only a small structural abnormality of the collagen helix, and it is most likely that the over-modification of lysine, poor secretion, increased degradation and other functional sequelae result from a kinetic defect in collagen helix formation resulting from the mutation.     

Altered triple helical structure of type I procollagen in lethal perinatal osteogenesis imperfecta

Cultured dermal fibroblasts from an infant with the lethal perinatal form of osteogenesis imperfecta (type II) synthesize normal and abnormal forms of type I procollagen. The abnormal type I procollagen molecules are excessively modified during their intracellular stay, have a lower than normal melting transition temperature, are secreted at a reduced rate, and form abnormally thin collagen fibrils in the extracellular matrix in vitro. Overmodification of the abnormal type I procollagen molecules was limited to the NH2-terminal three-fourths of the triple helical domain. Two-dimensional mapping of modified and unmodified alpha chains of type I collagen demonstrated neither charge alterations nor large insertions or deletions in the region of alpha 1(I) and alpha 2(I) in which overmodification begins. Both the structure and function of type I procollagen synthesized by cells from the parents of this infant were normal. The simplest interpretation of the results of this study is that the osteogenesis imperfecta phenotype arose from a new dominant mutation in one of the genes encoding the chains of type I procollagen. Given the requirement for glycine in every third position of the triple helical domain, the mutation may represent a single amino acid substitution for a glycine residue. These findings demonstrate further heterogeneity in the biochemical basis of osteogenesis imperfecta type II and suggest that the nature and location of mutations in type I procollagen may determine phenotypic variation.     

Mutation characteristics in type I collagen genes in Chinese patients with osteogenesis imperfecta

Osteogenesis imperfecta is normally caused by an autosomal dominant mutation in the type I collagen genes COL1A1 and COL1A2. The severity of osteogenesis imperfecta varies, ranging from perinatal lethality to a very mild phenotype. Although there have been many reports of COL1A1 and COL1A2 mutations, few cases have been reported in Chinese people. We report on five unrelated families and three sporadic cases. The mutations were detected by PCR and direct sequencing. Four mutations in COL1A1 and one in COL1A2 were found, among which three mutations were previously unreported. The mutation rates of G>C at base 128 in intron 31 of the COL1A1 gene and G>A at base 162 in intron 30 of the COL1A2 gene were higher than normal. The patients' clinical characteristics with the same mutation were variable even in the same family. We conclude that mutations in COL1A1 and COL1A2 also have an important role in osteogenesis imperfecta in the Chinese population. As the Han Chinese people account for a quarter of the world's population, these new data contribute to the type I collagen mutation map.     

Substitution of murine type I collagen A1 3-hydroxylation site alters matrix structure but does not recapitulate osteogenesis imperfecta bone dysplasia

Null mutations in CRTAP or P3H1, encoding cartilage-associated protein and prolyl 3-hydroxylase 1, cause the severe bone dysplasias, types VII and VIII osteogenesis imperfecta. Lack of either protein prevents formation of the ER prolyl 3-hydroxylation complex, which catalyzes 3Hyp modification of types I and II collagen and also acts as a collagen chaperone. To clarify the role of the A1 3Hyp substrate site in recessive bone dysplasia, we generated knock-in mice with an α1(I)P986A substitution that cannot be 3-hydroxylated. Mutant mice have normal survival, growth, femoral breaking strength and mean bone mineralization. However, the bone collagen HP/LP crosslink ratio is nearly doubled in mutant mice, while collagen fibril diameter and bone yield energy are decreased. Thus, 3-hydroxylation of the A1 site α1(I)P986 affects collagen crosslinking and structural organization, but its absence does not directly cause recessive bone dysplasia. Our study suggests that the functions of the modification complex as a collagen chaperone are thus distinct from its role as prolyl 3-hydroxylase.     

Identification of a novel COL1A1 frameshift mutation, c.700delG,in a Chinese osteogenesis imperfecta family

Osteogenesis imperfecta (OI) is a family of genetic disorders associated with bone loss and fragility. Mutations associated with OI have been found in genes encoding the type I collagen chains. People with OI type I often produce insufficient α1-chain type I collagen because of frameshift, nonsense, or splice site mutations in COL1A1 or COL1A2. This report is of a Chinese daughter and mother who had both experienced two bone fractures. Because skeletal fragility is predominantly inherited, we focused on identifying mutations in COL1A1 and COL1A2 genes. A novel mutation in COL1A1, c.700delG, was detected by genomic DNA sequencing in the mother and daughter, but not in their relatives. The identification of this mutation led to the conclusion that they were affected by mild OI type I. Open reading frame analysis indicated that this frameshift mutation would truncate α1-chain type I collagen at residue p263 (p.E234KfsX264), while the wild-type protein would contain 1,464 residues. The clinical data were consistent with the patients’ diagnosis of mild OI type I caused by haploinsufficiency of α1-chain type I collagen. Combined with previous reports, identification of the novel mutation COL1A1-c.700delG in these patients suggests that additional genetic and environmental factors may influence the severity of OI.     

Crtap and p3h1 knock out zebrafish support defective collagen chaperoning as the cause of their osteogenesis imperfecta phenotype

Prolyl 3-hydroxylation is a rare collagen type I post translational modification in fibrillar collagens. The primary 3Hyp substrate sites in type I collagen are targeted by an endoplasmic reticulum (ER) complex composed by cartilage associated protein (CRTAP), prolyl 3-hydroxylase 1 (P3H1) and prolyl cis/trans isomerase B, whose mutations cause recessive forms of osteogenesis imperfecta with impaired levels of α1(I)3Hyp986. The absence of collagen type I 3Hyp in wild type zebrafish provides the unique opportunity to clarify the role of the complex in vertebrate. Zebrafish knock outs for crtap and p3h1 were generated by CRISPR/Cas9. Mutant fish have the typical OI patients’ reduced size, body disproportion and altered mineralization. Vertebral body fusions, deformities and fractures are accompanied to reduced size, thickness and bone volume. Intracellularly, collagen type I is overmodified, and partially retained causing enlarged ER cisternae. In the extracellular matrix the abnormal collagen type I assembles in disorganized fibers characterized by altered diameter. The data support the defective chaperone role of the 3-hydroxylation complex as the primary cause of the skeletal phenotype.     

Lethal perinatal osteogenesis imperfecta due to the substitution of arginine for glycine at residue 391 of the alpha 1(I) chain of type I collagen

A baby with the lethal perinatal form of osteogenesis imperfecta was shown to have a structural defect in the alpha 1(I) chain of type I procollagen. Normal and mutant alpha 1(I) CB8 cyanogen bromide peptides, from the helical part of the alpha 1(I) chains, were purified from bone. Amino acid sequencing of tryptic peptides derived from the mutant alpha 1(I) CB8 peptide showed that the glycine residue at position 391 of the alpha 1(I) chain had been replaced by an arginine residue. This substitution accounted for the more basic charged form of this peptide that was observed on two-dimensional electrophoresis of the collagen peptides obtained from the tissues. The substitution was associated with increased enzymatic hydroxylation of lysine residues in the alpha 1(I) CB8 and the adjoining CB3 peptides but not in the carboxyl-terminal CB6 and CB7 peptides. This finding suggested that the sequence abnormality had interfered with the propagation of the triple helix across the mutant region. The abnormal collagen was not incorporated into the more insoluble fraction of bone collagen. The baby appeared to be heterozygous for the sequence abnormality and as the parents did not show any evidence of the defect it is likely that the baby had a new mutation of one allele of the pro-alpha 1(I) gene. The amino acid substitution could result from a single nucleotide mutation in the codon GGC (glycine) to produce the codon CGC (arginine).     

The effects of different cysteine for glycine substitutions within alpha 2(I) chains. Evidence of distinct structural domains within the type I collagen triple helix

Affected individuals from two apparently distinct, mild osteogenesis imperfecta families were heterozygous for a G to T transition in the COL1A2 gene that resulted in cysteine for glycine substitutions at position 646 in the alpha 2(I) chain of type I collagen. A child with a moderately severe form of osteogenesis imperfecta was heterozygous for a G to T transition that resulted in a substitution of cysteine for glycine at position 259 in the COL1A2 gene. Type I collagen molecules containing an alpha 2(I) chain with cysteine at position 259 denaturated at a lower temperature than molecules containing an alpha 2(I) chain with cysteine at position 646. In contrast to cysteine for glycine substitutions in the alpha 1(I) chain, the severity of the osteogenesis imperfecta phenotype is not directly proportional to the distance of the mutation from the amino-terminal end of the triple helix. These findings could be explained if the type I collagen triple helix contains discontinuous domains that differ in their contributions to maintaining helix stability.     

A Gly238Ser substitution in the α2 chain of type I collagen results in osteogenesis imperfecta type III

In general, osteogenesis imperfecta (brittle bone disease) is caused by heterozygous mutations in the genes encoding the 1 or 2 chains of type I collagen (COL1A1 and COL1A2, respectively). In this study we screened these genes in a proband presenting with the severe form (type III) of osteogenesis imperfecta for mutations which might result in the phenotype. Single-strand conformation polymorphism mapping analysis was used to identify a region suspected of harbouring the mutation and subsequent sequence analysis revealed a heterozygous G to A transition in the 2(I) gene of type I collagen in the individual. The resulting substitution of the glycine at position 238 of the chain by serine is the most N-terminal yet reported for this chain.     

Neural networks for modeling gene-gene interactions in association studies

Background

The monogenic disease osteogenesis imperfecta (OI) is due to single mutations in either of the collagen genes ColA1 or ColA2, but within the same family a given mutation is accompanied by a wide range of disease severity. Although this phenotypic variability implies the existence of modifier gene variants, genome wide scanning of DNA from OI patients has not been reported. Promising genome wide marker-independent physical methods for identifying disease-related loci have lacked robustness for widespread applicability. Therefore we sought to improve these methods and demonstrate their performance to identify known and novel loci relevant to OI.

Results

We have improved methods for enriching regions of identity-by-descent (IBD) shared between related, afflicted individuals. The extent of enrichment exceeds 10- to 50-fold for some loci. The efficiency of the new process is shown by confirmation of the identification of the Col1A2 locus in osteogenesis imperfecta patients from Amish families. Moreover the analysis revealed additional candidate linkage loci that may harbour modifier genes for OI; a locus on chromosome 1q includes COX-2, a gene implicated in osteogenesis.

Conclusion

Technology for physical enrichment of IBD loci is now robust and applicable for finding genes for monogenic diseases and genes for complex diseases. The data support the further investigation of genetic loci other than collagen gene loci to identify genes affecting the clinical expression of osteogenesis imperfecta. The discrimination of IBD mapping will be enhanced when the IBD enrichment procedure is coupled with deep resequencing.     

A structural mutation of the collagen alpha 1(I)CB7 peptide in lethal perinatal osteogenesis imperfecta

Structurally abnormal type I collagen was identified in the dermis, bone, and cultured fibroblasts obtained from a baby with lethal perinatal osteogenesis imperfecta. Two-dimensional gel electrophoresis of the CNBr peptides demonstrated that the alpha 1(I)CB7 peptide from the alpha 1(I)-chain of type I collagen existed in a normal form and a mutant form with a more basic charge distribution. This heterozygous peptide defect was not detected in the collagens from either parent. The defect was localized to a 224-residue region at the NH2 terminus of the alpha 1(I)CB7 peptide by mammalian collagenase digestion. Analysis of unhydroxylated collagens produced in cell culture indicated that the mutant alpha 1(I)CB7 migrated faster on electrophoresis suggesting that the abnormality may be a small deletion or a mutation that alters sodium dodecyl sulfate binding. The post-translational hydroxylation of lysine residues was increased in the CB7 peptide and also in peptides CB3 and CB8 which are toward the NH2 terminus of the alpha 1(I)-chain. The COOH-terminal CB6 peptide was normally hydroxylated. These findings support the proposal that the lysine overhydroxylation resulted from a perturbation of helix propagation from the COOH to NH2 terminus of the collagen trimer caused by the structural defect in alpha 1(I)CB7.     

Osteogenesis imperfecta

Osteogenesis imperfecta (OI) is the most frequently occurring congenital disorder with an increased fracture rate and systemic skeletal involvement. The vast majority of patients have an autosomal dominant form of OI resulting from a mutation in one of the two type I collagen genes COL1A1 or COL1A2. Since 2006, eight genes for autosomal recessive forms of the disorder have been identified, as well as one additional gene for autosomal dominant OI. Our knowledge concerning molecular pathophysiology has been substantially broadened, such that the paradigm of OI as a pure ??collagenopathy?? no longer applies and the clinical classification system will have to be revised. Standard therapy for the more severe forms of OI comprises intravenous administration of bisphosphonates. Additional elements of a multimodal therapeutic concept include surgical intervention for bone deformities or fractures and physiotherapy.     

Les complications orthopédiques de l’ostéogenèse imparfaite

Osteogenesis imperfecta is a genetic disease characterized by bone frailty. It is generally caused by an abnormal production of collagen, which is the main fibrous protein of the bone. Collagen is also present in the skin, tendons, the sclera of the eye and dentin. The most frequent manifestation of osteogenesis imperfecta is the occurrence of multiple fractures without major trauma. Severity and timing of the attack varies widely: some patients sustain a significant number of fractures during early childhood which may have a serious impact on growth, while others will have some fractures separated by a few years. In all cases, the bone strength improves in adulthood. The bone fractures cause pain and bone deformities sometimes result in a smaller size. Scoliosis is frequent and associated with painful vertebral collapses. We present a case of osteogenesis imperfecta in a 40-year-old adult and we describe the various orthopaedic complications of the disease, stressing the role of bone scintigraphy in the diagnosis and monitoring of these complications.     

Incorporation of type I collagen molecules that contain a mutant alpha 2(I) chain (Gly580-->Asp) into bone matrix in a lethal case of osteogenesis imperfecta.

To understand more directly the tissue defect in osteogenesis imperfecta (OI), bone matrix was analyzed from an infant with lethal OI (type II) of defined mutation (collagen alpha 2(I)Gly580-->Asp). Pepsin-solubilized alpha 1(I) and alpha 2(I) chains and derived CNBr-peptides migrated more slowly on sodium dodecyl sulfate-polyacrylamide gel electrophoresis compared with normal human controls. The peptide alpha 2(I)CB3,5, predicted to contain the mutation site, ran as a retarded doublet band and was purified by high performance liquid chromatography and digested with V8 protease. Two peptides with amino-terminal sequences beginning at residue 576 of the alpha 2(I) chain were isolated. One had the normal sequence. The other differed in that aspartic acid replaced glycine at residue 580 as predicted from cDNA analysis, and in having an unhydroxylated proline at residue 579. From yields on microsequencing and the relative intensities of the two forms of alpha 2(I)CB3,5 on SDS-polyacrylamide gel electrophoresis, the ratio of mutant to normal alpha 2(I) chains in the infant's bone matrix was 0.7/1. Although the effects of an efficient incorporation of mutant chains on the properties of the bone matrix are unknown, it may be that in this OI case the tissue abnormalities result more from the presence of mutant protein than from an underexpression of matrix.