Synthesis of a shortened pro-alpha 2(I) chain and decreased synthesis of pro-alpha 2(I) chains in a proband with osteogenesis imperfecta

Comprehensive genetic analyses using targeted next-generation sequencing and genotype-phenotype correlations in 53 Japanese patients with osteogenesis imperfecta

To elucidate mutation spectrum and genotype-phenotype correlations in Japanese patients with OI, we conducted comprehensive genetic analyses using NGS, as this had not been analyzed comprehensively in this patient population. Most mutations were located on COL1A1 and COL1A2. Glycine substitutions in COL1A1 resulted in the severe phenotype.

INTRODUCTION

Most cases of osteogenesis imperfecta (OI) are caused by mutations in COL1A1 or COL1A2, which encode α chains of type I collagen. However, mutations in at least 16 other genes also cause OI. The mutation spectrum in Japanese patients with OI has not been comprehensively analyzed, as it is difficult to identify using classical Sanger sequencing. In this study, we aimed to reveal the mutation spectrum and genotype-phenotype correlations in Japanese patients with OI using next-generation sequencing (NGS).

METHODS

We designed a capture panel for sequencing 15 candidate OI genes and 19 candidate genes that are associated with bone fragility or Wnt signaling. Using NGS, we examined 53 Japanese patients with OI from unrelated families.

RESULTS

Pathogenic mutations were detected in 43 out of 53 individuals. All mutations were heterozygous. Among the 43 individuals, 40 variants were identified including 15 novel mutations. We found these mutations in COL1A1 (n = 30, 69.8%), COL1A2 (n = 12, 27.9%), and IFITM5 (n = 1, 2.3%). Patients with glycine substitution on COL1A1 had a higher frequency of fractures and were more severely short-statured. Although no significant genotype-phenotype correlation was observed for bone mineral density, the trabecular bone score was significantly lower in patients with glycine substitutions.

CONCLUSION

We identified pathogenic mutations in 81% of our Japanese patients with OI. Most mutations were located on COL1A1 and COL1A2. This study revealed that glycine substitutions on COL1A1 resulted in the severe phenotype among Japanese patients with OI.

Osteogenesis imperfecta

Osteogenesis imperfecta is a phenotypically and molecularly heterogeneous group of inherited connective tissue disorders that share similar skeletal abnormalities causing bone fragility and deformity. Previously, the disorder was thought to be an autosomal dominant bone dysplasia caused by defects in type I collagen, but in the past 10 years discoveries of novel (mainly recessive) causative genes have lent support to a predominantly collagen-related pathophysiology and have contributed to an improved understanding of normal bone development. Defects in proteins with very different functions, ranging from structural to enzymatic and from intracellular transport to chaperones, have been described in patients with osteogenesis imperfecta. Knowledge of the specific molecular basis of each form of the disorder will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches. In this Seminar, together with diagnosis, management, and treatment, we describe the defects causing osteogenesis imperfecta and their mechanism and interrelations, and classify them into five groups on the basis of the metabolic pathway compromised, specifically those related to collagen synthesis, structure, and processing; post-translational modification; folding and cross-linking; mineralisation; and osteoblast differentiation.

Cell interactions in post-traumatic fibrosis

Models of reparative fibrosis, or wound healing, disclose a basic spatial relationship between inflammatory cells, fibroblasts, dividing fibroblasts and angiogenic endothelial cells. Many components of the extracellular fluid, especially steep gradients of PO2, PCO2, pH, glucose and lactate, have been measured. After components of coagulation play out their role in the first few days after injury, macrophages sense excitatory substances and signs of respiratory distress, and secrete chemoattractants for endothelial cells and fibroblasts. If excitatory substances (such as silica) are eliminated, active fibrosis ends with the restoration of the energy supply. Some of the mitogens and chemoattractants acting as intercellular messengers between these cells have been isolated and identified.

Expression of type I procollagen genes

All of the type I collagen in connective tissue is the product of one structural gene for the pro alpha 1(I) chain and another for the pro alpha 2(I) chain of type I procollagen. An intriguing question therefore is how the expression of the two genes differs in mineralizing and non-mineralizing tissues. One approach that our laboratory has pursued to answer this and related questions is to develop a new system whereby one can examine the self-assembly of collagen fibrils de novo by controlled enzymic cleavage of procollagen to collagen under physiological conditions. The system has made it possible for the first time to define thermodynamic parameters for the self-assembly process. We are now using the system to define the normal kinetics for fibril formation. The results should make it possible to study the effects of other components of extracellular matrix on fibril assembly, including the effects of bone-specific components that initiate mineralization. A second approach has been to define mutations in type I procollagen genes that cause increased brittleness of bone. Over a dozen mutations in type I procollagen genes have been found in probands with osteogenesis imperfecta. One of the surprises has been that at least 25% of the probands with lethal variants of osteogenesis imperfecta have mutations in type I procollagen genes. Another surprise has been the observation that a number of the mutations are tissue specific in terms of their phenotypic manifestations even though the same abnormal pro alpha chains are being synthesized in a variety of tissues.

Stromal stem cells: marrow-derived osteogenic precursors

Evidence is discussed for the hypothesis that there are stromal stem cells present in the soft connective tissues associated with marrow and bone surfaces that are able to give rise to a number of different cell lines including the osteogenic line. Fibroblastic colonies, each derived from a single colony-forming unit fibroblastic (CFU-F), are formed when marrow cells are cultured in vitro. In vivo assays of CFU-F have demonstrated that some CFU-F have a high ability for self renewal and multipotentiality whereas some have more limited potential. In vitro studies also support the hypothesis and have shown that CFU-F are a heterogeneous population of stem and progenitor cells and that their differentiation in vitro can be modified at the colony level. Factors added to the medium can activate osteogenesis in a range of multipotential and more committed precursors. Different stromal cell lines can be promoted under different culture conditions. The number and hierarchy of cell lines belonging to the stromal fibroblastic system are not yet fully elucidated and more specific markers for the different lines are required before a better understanding can be achieved.

Sequence of canine COL1A2 cDNA: nucleotide substitutions affecting the cyanogen bromide peptide map of the alpha 2(I) chain

The alpha2 chain of canine type I collagen was characterized with both sequence analysis of COL1A2 cDNA and chemical analysis of alpha2(I) chains. The complete sequence of canine COL1A2 cDNA was determined from reverse-transcribed and polymerase chain reaction-amplified total RNA from cultured skin fibroblasts. Pepsin-digested and cyanogen bromide-digested type I collagen peptides were analyzed with chromatography, gel electrophoresis, and mass spectrometry. Identity between the sequences of canine and human COL1A2 cDNA was 90.9%, predicting conservation of the 3 cysteine residues required for C-propeptide registration and of cleavage sites for signal peptidase, procollagen N-proteinase, vertebrate collagenase, and procollagen C-proteinase. Conservation of all 50 lysine residues was also predicted, including preservation of the 1:2 asymmetry in the X:Y distribution of the 31 lysine residues in the alpha2(I) triple helix. The human and canine sequences differed in the location of Y-position proline residues and the presence of two unique canine cyanogen bromide peptides, alpha2 CB3b and alpha2 CB3c,5. Knowledge of the conserved and unique features of canine COL1A2 will be valuable for analysis of the expression, synthesis, and structure of type I collagen as well as studies of canine osteogenesis imperfecta.

Coordinate patterns of expression of type I and III collagens during mouse development

The extracellular proteins types I and III collagen are abundantly expressed during development. Here, the patterns of the pro alpha 1(I), pro alpha 2(I), and pro alpha 1(III) collagen mRNAs are systematically examined from 7.5 to 17.5 days of development (E7.5 to E17.5) in the mouse using in situ hybridization with specific riboprobes. Coordinated expression of pro alpha 1(I) and pro alpha 2(I) collagen mRNA was found throughout development in all regions examined. Widespread type I collagen expression starting at E8.5 occurred in embryonic mesoderm, sclerotomes, dermatomes, and in the forming connective tissues. After E14.5, regions of ossification showed highest levels of type I collagen expression. Pro alpha 1(III) collagen expression was specific to and coordinated with patterns of type I collagen expression in many fibroblast-containing tissues. No expression of type III collagen occurred in osteoblasts. This comprehensive study of the transcripts of abundantly expressed structural proteins should provide a basis for comparison of other key extracellular matrix molecules and serve as a reference for studies on the patterns of activities of various promoter/enhancer-reporter gene constructions of type I and III collagen genes in transgenic mice.

Skin collagen defects in a patient with juvenile hyaline fibromatosis

Juvenile hyaline fibromatosis is a rare disorder characterised by multiple subcutaneous tumours, gum hypertrophy, muscle weakness, and flexion contractures of the large joints. Histology shows an abundance of a homogenous, amorphous, acidophilic extracellular matrix in which spindle shaped cells are embedded forming minute streaks. It has been previously suggested that collagen abnormalities may be involved. A 14 month old girl with this syndrome is described in whom postmortem western blot studies were performed. These studies revealed an absent pro-alpha 2(I) chain and an absent collagen type III chain in skin but not in the other organs examined.

Hereditary abnormalities of renal basement membranes

The glomerular and tubular basement membranes are the principal barriers to filtration and re-absorption of water and molecules in the nephron. They are composed primarily of type IV collagen, laminin, fibronectin, sulphated proteoglycans and collagen type I. Three common inherited diseases are associated with abnormalities of basement membrane proteins: Alport's syndrome, thin basement membrane disease (TBMD) and adult polycystic kidney disease. In this review we describe the application of molecular biological techniques to the study of these conditions. Classic Alport's syndrome is an X-linked disorder with a lamellated glomerular basement membrane (GBM) which typically results in renal failure in males. Studies with sera from patients with Goodpasture's syndrome, or monoclonal antibodies specific for the Goodpasture antigen, show that the Goodpasture antigen is absent or masked in the kidneys of individuals with Alport's syndrome. There is some evidence to suggest that the Goodpasture antigen is best represented by the non-collagenous domain of the alpha 3 chain of type IV collagen, but that other non-collagenous regions may also contribute to the antigen. It is through these non-collagenous regions that the type IV collagen chains form the typical network, and the abnormality in Alport's syndrome interferes with this network formation. However, we have recently demonstrated that the gene for the non-collagenous domain of the alpha 3 collagen chain is present in individuals with Alport's syndrome. Furthermore, other groups have shown a defect in a novel type IV collagen chain, the alpha 5 chain, in 3 unrelated cases of Alport's syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)

Osteogenesis imperfecta: translation of mutation to phenotype

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Detection and localization of base changes in RNA using a chemical cleavage method

The detection of base changes in DNA and RNA is of central importance in genetic research. Mismatched cytosines and thymines in heteroduplex DNA molecules show increased chemical reactivity with hydroxylamine and osmium tetroxide, respectively, and the DNA can then be specifically cleaved at the modified nucleotides. We show here that mismatched cytosines and thymines can be detected and located directly in RNA: DNA heteroduplex molecules. In order to detect guanosine and adenosine base changes the complementary cDNA strand must be analyzed. In addition, the sensitivity of the technique can be increased by employing the polymerase chain reaction. To test the fidelity of this method a number of known or predicted mutations were analyzed. These include single point mutations in the human collagen alpha 1(I) and rat phenylalanine hydroxylase mRNA, two engineered point mutations in a mouse collagen alpha 1(I) mRNA, and a deletion in a human collagen alpha 2(I) mRNA. All known base changes were detected and correctly localized. In addition, the predicted base changes were confirmed.

Inherited disorders of collagen gene structure and expression

As a result of investigations completed during the last 15 years, the molecular bases of most form of osteogenesis imperfecta (OI) and of some forms of the Ehlers-Danlos syndrome (EDS) are now known. Most forms of OI result from point mutations in the genes (COL1A1 and COL1A2) that encode the chains of type I procollagen or mutations that affect the expression of these genes. Less frequently, mutations that affect the size of the chain can also result in these phenotypes. The phenotypic presentation appears to be determined by the nature of the mutation, the chain in which it occurs, and, for point mutations, the position of the substitution and the nature of the substituting amino acid in the protein product. Similar mutations in the gene (COL3A1) that encodes the chains of type III procollagen result in the EDS type IV phenotype. Mutations which result in deletion of the cleavage site for the aminoterminal procollagen protease result in the EDS type VII phenotype and other mutations which affect the structure of the triple-helical domain by deletions and alter the conformation of the substrate at the site of proteolytic conversion can produce mixed phenotypes. Alterations in post-translational processing of collagenous proteins can result in the EDS type VI and EDS type IX phenotypes. Linkage analysis and study of type II collagen proteins from individuals with a variety of skeletal dysplasias suggest that similar mutations in these genes also result in clinically apparent phenotypes. Mutations in the majority of the 20 known collagen genes have not yet been identified.

Type I procollagen: the gene-protein system that harbors most of the mutations causing osteogenesis imperfecta and probably more common heritable disorders of connective tissue

Recent data from several laboratories have established that most variants of osteogenesis imperfecta (OI) are caused by mutations in the 2 structural genes for type I procollagen. There are 2 general reasons for the large number of mutations in type I procollagen in OI. One reason is that most of the structure of the procollagen monomer is essential for normal biological function of the protein. The second reason is that most of the mutations cause synthesis of structurally altered pro alpha chains of type I procollagen. The deleterious effects of the structurally altered pro alpha chains are then amplified by at least 3 mechanisms. One mechanism is a phenomenon referred to as "procollagen suicide" whereby altered pro alpha chains cause degradation of normal pro alpha chains synthesized by the same cell. Another mechanism involves the fact that many of the structurally altered pro alpha chains prevent normal processing of the N-propeptides of procollagen and persistence of the N-propeptide interferes with normal fibril assembly. A third mechanism is a recently discovered phenomenon in which a substitution of a bulkier amino acid for glycine can cause a kink in the triple helix of the molecule. The kinked collagen, in turn, causes formation of abnormally branched fibrils. Because the deleterious effects of abnormal pro alpha chains are amplified by these 3 mechanisms, most of the mutations are dominant and many are dominant lethal. The conclusion that most variants of OI are caused by mutations in the structural genes for type I procollagen has broad implications for other diseases that affect connective tissue, diseases such as chondrodystrophies, osteoarthritis, and osteoporosis.

Differential expression of type IV procollagen and laminin genes by fetal vs adult skin fibroblasts in culture: determination of subunit mRNA steady-state levels

Basement membrane zone gene expression by fibroblast cultures, established from individuals varying in age from 14 fetal weeks to 61 years, was examined by molecular hybridizations with human sequence specific cDNAs corresponding to type IV procollagen and laminin subunit polypeptides. Northern transfer analysis with poly(A)+RNA revealed the presence of specific mRNA transcripts for alpha 1(IV) and alpha 2(IV) chains of type IV procollagen as well as B1 and B2 chains of laminin. Laminin A chain mRNAs were not detected using the same RNA preparations. Quantitative estimates of the steady-state levels of type IV procollagen and laminin mRNAs indicated that they were of relatively low abundance, as compared with mRNA to type I procollagen. The expression of alpha 1(IV) and alpha 2(IV) collagen genes was high in fetal fibroblasts but was reduced to low, yet detectable, levels in cultures established from 3-d to 61-year-old individuals. In contrast, the laminin B1 and B2 chain mRNA levels showed little age-associated variation within the cultures examined. These results provide evidence for differential regulation of the expression of different basement membrane zone macromolecules during chronologic aging.

Elevated accumulation of hyaluronate in the tubular bones of osteogenesis imperfecta

The content and composition of glycosaminoglycans in the tubular bones of osteogenesis imperfecta were compared to those in the tubular bones of age-matched controls. Chondroitin sulfate was the major glycosaminoglycan (70-80% of total) both in the normal and pathological bones, and its level, based on the tissue wet weight, was slightly less in the pathological bones. The composition of chondroitin sulfate disaccharide units in the pathological samples was different from those of the control; a lower proportion of chondroitin 4-sulfate unit. Hyaluronate accounted for at most 7% of total glycosaminoglycans from the normal bones. The hyaluronate content of the pathological bones was 1.5- to 3-fold higher than that of the controls. Glycosaminoglycans have been shown to participate in the formation of a functional supramolecular complex in extracellular matrices. Therefore, it may be postulated that the abnormalities in glycosaminoglycan composition in the tubular bones of osteogenesis imperfecta is implicated in some clinical aspects of this connective tissue disorder such as the bony fragility.

Lethal osteogenesis imperfecta: abnormal collagen metabolism and biochemical characteristics of hypophosphatasia

We have examined collagen from a patient with the rare type IIC form of perinatally lethal osteogenesis imperfecta, in whom biochemical characteristics of hypophosphatasia were also apparent. In addition to normal alpha 1(I) and alpha 2(I) chains, there were chains overmodified along their lengths. Unexpectedly, the thermal stability of molecules containing these chains was normal. This suggests the existence of a structural mutation causing delayed triple helix formation, situated in either the alpha 1(I) or alpha 2(I) C-terminal propeptide. Since collagen synthesised by fibroblasts from each of the patient's parents was normal, the mutation was probably newly arising and dominant. In contrast to other reported cases of lethal osteogenesis imperfecta, not only was the secretion of collagen by cultured fibroblasts considerably retarded, but that of non-collagenous proteins was also severely impaired.

Existence of malfunctioning pro alpha2(I) collagen genes in a patient with a pro alpha 2(I)-chain-defective variant of Ehlers-Danlos syndrome

Collagen synthesis was examined in skin fibroblasts from a patient with a variant of Ehlers-Danlos syndrome. The relative rate of collagen synthesis to total protein synthesis in the patient's fibroblasts was always one-half of that in fibroblasts from normal controls. Total collagen synthesis, as assessed by quantification of total hydroxyproline, was also significantly lower than that of controls, indicating that the rate of collagen synthesis by the patient's fibroblasts was decreased compared with that by normal fibroblasts. Analysis of procollagen and collagen components showed the absence of the pro alpha 2(I) chain and its derivatives. Dot-blot and Northern-blot analyses showed the patient's fibroblasts to contain less than 10% of the mRNAs for pro alpha 2(I) found in control fibroblasts. In spite of these results, Southern blot analysis of genomic DNA indicated the presence of the same number of genes for the pro alpha 2(I) collagen chain in the patient's fibroblasts as in control fibroblasts, suggesting malfunctioning pro alpha 2(I) collagen genes as the cause for failure of the patient's fibroblasts to synthesize pro alpha 2(I) collagen chains.

Expression of basement membrane zone genes coding for type IV procollagen and laminin by human skin fibroblasts in vitro: elevated alpha 1 (IV) collagen mRNA levels in lipoid proteinosis

Basement membrane zone gene expression by fibroblast cultures established from adult human skin was examined by molecular hybridizations with human sequence-specific cDNAs corresponding to pro-alpha 1 (IV) chain of type IV collagen and B2 chain of laminin. Northern transfer analysis of poly(A)+RNA isolated from fibroblast cultures clearly revealed the presence of specific mRNA transcripts, indicating the expression of the genes coding for pro-alpha 1 (IV) and laminin B2 polypeptides. Quantitative estimates of the relative levels of these mRNAs coding for basement membrane zone components indicated that they are of relatively low abundance in control fibroblasts, as compared with mRNAs coding for fibronectin or pro-alpha 1 (I) chain of type I procollagen. In fibroblast cultures established from the lesional skin of a 32-year-old patient with lipoid proteinosis, the levels of mRNA coding for pro-alpha 1 (IV) polypeptides were increased approximately 4.5-fold, as compared with age- and passage-matched control cultures. This increase was selective in that the levels of fibronectin, pro-alpha 1 (I), and beta-actin mRNAs were unaltered in the same cultures. The increase in pro-alpha 1 (IV) mRNA level was also uncoordinate with the expression of the laminin B2 chain gene, which was unaltered in lipoid proteinosis. The selective increase in pro-alpha 1 (IV) mRNA may have relevance to the accumulation of this basement membrane component in the skin in lipoid proteinosis.

Osteogenesis imperfecta

Major advances have occurred in the classification of OI and in the definition of underlying molecular defects. A clearer understanding of the pathogenesis of OI and of the relationships between the phenotypes and genotypes should emerge. The study of induced mutations in selected regions of the collagen genes with expression in cultured cells or transgenic mice should hasten this process. These advances will also provide a basis for studies into the large number of other genetically determined connective tissue disorders that are grouped together as the skeletal dysplasias. The results of recent studies in OI are providing a unique insight into many aspects of collagen and connective tissue biochemistry, physiology and pathology.

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

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.

Osteonectin content in human osteogenesis imperfecta bone shows a range similar to that of two bovine models of OI

Samples of human osteogenesis imperfecta (OI) bone were analyzed for osteonectin content by SDS gel electrophoresis and immunodetection on Western blots. The OI bone osteonectin content varied from normal to severely depressed. Previously, we showed that two clinically identical but genetically unrelated bovine models of OI were differentiated biochemically by their bone osteonectin content: one OI model had normal bone osteonectin while the other was severely depressed in this parameter. The data in this pilot study suggest that further investigation of bone osteonectin content may prove useful in the clinical assessment of human OI cases.

Two bovine models of osteogenesis imperfecta exhibit decreased apatite crystal size

In recent years advances have been made in detailing the changes in both collagen and noncollagenous proteins caused by a variety of mutations leading to osteogenesis imperfecta. Much less, however, is known about the mineral phase in the affected bone. In this report, we measured the crystallinity of the apatite in bovine OI bone. Line broadening of the 002 reflection (which estimates changes in the long or c axis of the crystals) and of the 310 reflection (which estimates changes in the thickness of the crystals) both show large decreases (30 and 35% respectively). Transmission electron micrograph measurements indicate that these changes were most probably a result of smaller crystals. No decrease in the ash weight of the bone was observed.

Changes in the types of collagen synthesized during chondrogenesis of the mouse otic capsule

We have investigated the temporal relationship between the morphological differentiation of the mouse otic capsule and the pattern of collagen synthesis by mouse otocyst-mesenchyme complexes labeled in vitro. In 10.5- to 12-day embryos the mesenchyme surrounding the otocyst was loosely organized except for a few lateroventral condensations; explants from these embryos synthesized only small amounts of collagen. Collagen synthesis by whole explants increased by more than 50% between 12 and 13 days concomitant with metachromatic staining of the lateral periotic mesenchyme. Cartilage specific type II collagen was the predominant collagen synthesized by these explants as confirmed by SDS-PAGE, densitometry, CNBr cleavage, and V8 protease digestion. This biochemical expression of the cartilage phenotype preceded morphologic recognition of otic capsular cartilage by almost 2 days. Type II collagen synthesis continued to increase and predominate through Day 16 of gestation by which time the otic labyrinth was surrounded by mature cartilage. The minor cartilage collagen chains, 1 alpha, 2 alpha, and 3 alpha, first appeared on different days of gestation. The 1 alpha, and 3 alpha chains were synthesized by explants from 11-day embryos while the 2 alpha chain appeared during Day 13, just before overt differentiation of mature cartilage. These results suggested that the 1 alpha, 2 alpha, and 3 alpha chains may not form heterotrimers containing all three chains and that synthesis of the 2 alpha chain may be associated with stabilization of the cartilaginous matrix. Comparison of these data with the patterns of collagen production by mutant, diseased, or experimentally manipulated inner ear tissues may provide insights into the molecular basis of chondrogenic tissue interactions.

Abnormal procollagen synthesis in fibroblasts from three patients of the same family with a severe form of osteogenesis imperfecta (type III)

Dermal fibroblast cultures from three siblings with a severe form of osteogenesis imperfecta were established in order to analyze their procollagen and collagen synthesis. Cell strains from clinically normal consanguineous parents (first cousins), were also obtained for comparison. Total collagen production in culture media was diminished by 55% in the patients fibroblasts and to a lesser extent in the parents. This decrease was specific for collagenous proteins. From polyacrylamide gel electrophoresis, it appeared that the three children had not only the same defective secretion of pro alpha 1(I) molecules but that their pro alpha 1(I) migrated slightly faster than the parental and control counterparts. Analysis of secretion confirmed a reduced rate in procollagen synthesis and the absence of intracellular storage. Upon pepsin treatment, extracellular alpha 1(I) and alpha 2(I) chains were found in the expected ratio of 2:1 and migrated normally, suggesting that the altered mobility of pro alpha 1(I) chains was related to COOH or NH2 terminal propeptides. In agreement with the reduced type I collagen production, an increase in the alpha 1(III)/alpha 1(I) ratio was also detected. Furthermore, after a 2.5-h labelling followed by alkylation with iodoacetamide, free intracellular pro alpha 2(I) and alpha 1(I) chains were detected in the absence of reduction, consistent with an abnormal intracellular ratio of pro alpha 1(I)/pro alpha 2(I) that was measured after dithiothreitol reduction. Analysis of intracellular collagen chains from parental strains following a 4-h incubation demonstrated that pro alpha 1(I) appeared as a doublet, one band with normal mobility and a less intense band migrating faster and corresponding to the defective chain found in the patients. Absence of the abnormal molecules in culture media was related to the demonstration of a defective collagen secretion by parental fibroblasts. Correlation between these biochemical findings and clinical data strongly support a recessive inheritance of the disease that could be classified as a type III form of osteogenesis imperfecta. Patients would be homozygous for the same defective allele and the asymptomatic parents would most likely be heterozygous carriers of the mutation. Although the exact location of the alteration is not yet elucidated, a splicing mutation is suggested.

Mutations linked to the pro alpha 2(I) collagen gene are responsible for several cases of osteogenesis imperfecta type I

We have analysed six South African families with osteogenesis imperfecta type I using three DNA polymorphisms associated with the pro alpha 2(I) collagen gene. In four of these families linkage of the pro alpha 2(I) gene and the osteogenesis imperfecta phenotype was suggested, whereas in the remaining two families there was a lack of linkage. No distinct correlation could be made between the phenotypic features of the families studied and linkage or lack of linkage to the pro alpha 2(I) gene. Two different haplotypes were found to be associated with the mutant pro alpha 2(I) alleles. These findings suggest that molecular heterogeneity exists within osteogenesis imperfecta type I and that in a significant proportion of cases the defect is linked to the pro alpha 2(I) gene.

Contraction of collagen lattices by fibroblasts from patients and animals with heritable disorders of connective tissue

Fibroblasts derived from patients and animals presenting various heritable connective tissue disorders were investigated for the ability to retract a reconstituted collagen matrix. When seeded into gels, dermatosparactic calf and sheep fibroblasts did not exhibit the elongated shape of normal fibroblasts and did not contract the collagen lattice to the same extent as control fibroblasts. In contrast, several cell strains obtained from patients with Ehlers-Danlos syndrome type VII displayed contractile properties for collagen gels similar to controls. Delayed contraction was noted by two strains of fibroblasts from patients with Ehlers-Danlos syndrome type IV, whereas fibroblasts from patients with osteogenesis imperfecta, Marfan syndrome and cutis laxa had normal retraction properties.

Lethal osteogenesis imperfecta with amniotic band lesions: collagen studies

An infant was born with osteogenesis imperfecta (OI) and died after 7 days. In addition, there were amniotic constriction bands and amputations of several digits of the upper and lower limbs. The radiologic picture was suggestive of type III OI. Histomorphometric analysis of the bone showed a trabecular bone volume of 15.1% compared to 26.9% for age-matched controls. This was due to a decreased apposition of matrix by the osteoblasts. Because abnormal collagen synthesis has been suggested as the underlying defect in most forms of OI, collagen studies were undertaken using intact tissues. Bone and skin collagen solubilities were strikingly reduced. Shortened type I collagen molecules, representing 25% of the total type I collagen, were produced by pepsin digestion of the demineralized bone matrix. The molecular weight of the shortened collagen, was 10 kd lower than normal for both the alpha 1 and alpha 2 chains as determined by gel electrophoresis. The bone acetic acid-soluble collagen showed few shortened alpha-chains. Twenty-five percent of the acid-soluble bone collagen was cleaved into shortened molecules by a pepsin digestion. The shortened alpha 1 chain was purified by high-performance liquid chromatography (HPLC) and digested with CNBr. The analysis of the resulting fragments by HPLC and by gel electrophoresis unequivocally demonstrated that the shortened alpha 1 chain was derived from the alpha 1(I) chains and that the pepsin sensitivity extends from the amino terminal end of the chain to the alpha 1(I) CB5 peptide, approximately 120 residues inside the triple helix. These studies show a distinct structural abnormality of type I collagen in the bone matrix of this patient resulting in an increased sensitivity of the collagen to general enzymatic proteolysis. The importance of correlating clinical and biochemical information in OI is emphasized; classification and genetic counseling based only on clinical observations are inaccurate.

The differential symptomatology of errors of collagen metabolism: a tentative classification

We address the confusion in the current classification of inherited disorders of collagen and the excessive extension of the concept of the Ehlers-Danlos "syndrome" that tends to cover many facts and conditions frequently without strong clinical connection. We propose to subdivide the collagen disorders into four main classes depending on whether skin, joints, bone, or blood vessels are mainly involved. The class with mainly skin involvement includes the different forms of cutis laxa, Ehlers-Danlos syndrome types I and II (autosomal dominant), types V and IX (X-linked recessive), type VI (autosomal recessive), and type VIII (autosomal dominant). The group with mainly articular involvement includes Larsen and related syndromes and other types with a more benign course. The conditions with mainly skeletal involvement include the different forms of osteogenesis imperfecta. The class with mainly blood vessel involvement includes disorders of type III collagen and the Marfan syndrome. This tentative classification proposes a logical clinical framework that will allow easier integration of molecular biology data.

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

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.

Isolation and characterization of the human fibrillar collagen genes

In order to elucidate some of the mechanisms leading to the pathological expression of the human fibrillar collagens, as well as to understand the evolution of these loci, specific cDNA and genomic clones have been isolated. The primary structure of the COOH-terminal propeptide of the four collagen chains and either part or the entire exon/intron arrangement of the genes have been determined. Interspecies and pairwise comparison revealed that the four loci have evolved at slightly different rates, maintaining, however, remarkably similar exon/intron arrangement. The fibrillar genes, albeit sharing the same elaborate structure, exhibit different sizes that correlate with the average length of their intron sequences, possibly because of their different chromosomal origin.