Structure, stability and interactions of type I collagen with GLY349-CYS substitution in alpha 1(I) chain in a murine Osteogenesis Imperfecta model.

Here we report the structural and functional studies of collagen from the Brtl mouse, a heterozygous knock-in model for Osteogenesis Imperfecta, which has a G349C substitution introduced in one col1a1 allele. We observed that 25+/-5% of alpha 1(I) chains in different tissues and in different extracts from matrix deposited by cultured cells were S-S-linked mutant dimers. Apparently mutant and normal molecules are equally well incorporated into the matrix and they form mature covalent crosslinks with the same efficiency. We found different extents of post-translational overmodification of mutant molecules in different tissues, but we found no consistent differences between lethal and non-lethal animals. We did not detect any changes in the thermal stability or rate of thermal denaturation of mutant collagen. We also did not detect any changes in collagen-collagen recognition and interactions except for disruption of quasi-crystalline lateral packing of molecules in tendons from some, mostly prepubertal, mutant animals. In contrast, alpha 1(I)(3) collagen from the oim mouse--the only other non-lethal murine OI model studied by similar techniques--has altered stability, fibrillogenesis, collagen-collagen interactions and produces a more consistent and more pronounced disruption of tendon crystallinity. Nevertheless, while the G349C substitution causes moderate or lethal OI, heterozygous oim mice are much less affected. Overall, our results suggest that OI symptoms and phenotype variation in G349C animals are related to abnormal interactions of mutant collagen helices with other matrix molecules or abnormal function of osteoblasts rather than to abnormal structure, physical properties or interactions between mutant collagen helices.     

Studies on type I collagen in skin fibroblasts cultured from twins with lethal osteogenesis imperfecta

Studies on type I procollagen produced by skin fibroblasts cultured from twins with lethal type II of osteogenesis imperfecta (OI) showed that biosynthesis of collagen (measured by L-[5-(3)H]proline incorporation into proteins susceptible to the action of bacterial collagenase) was slightly increased as compared to the control healthy infant. SDS/PAGE showed that the fibroblasts synthesized and secreted only normal type I procollagen. Electrophoretic analysis of collagen chains and CNBr peptides showed the same pattern of electrophoretic migration as in the controls. The lack of posttranslational overmodification of the collagen molecule suggested a molecular defect near the amino terminus of the collagen helix. Digestion of OI type I collagen with trypsin at 30 degrees C for 5 min generated a shorter than normal alpha2 chain which melted at 36 degrees C. Direct sequencing of an asymmetric PCR product revealed a heterozygous single nucleotide change C-->G causing a substitution of histidine by aspartic acid in the alpha2 chain at position 92. Pericellular processing of type I procollagen by the twin's fibroblasts yielded a later appearance of the intermediate pC-alpha1(I) form as compared with control cells.     

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.     

Nanoscale morphology of Type I collagen is altered in the Brtl mouse model of Osteogenesis Imperfecta

Bone has a complex hierarchical structure that has evolved to serve structural and metabolic roles in the body. Due to the complexity of bone structure and the number of diseases which affect the ultrastructural constituents of bone, it is important to develop quantitative methods to assess bone nanoscale properties. Autosomal dominant Osteogenesis Imperfecta results predominantly from glycine substitutions (80%) and splice site mutations (20%) in the genes encoding the α1 or α2 chains of Type I collagen. Genotype–phenotype correlations using over 830 collagen mutations have revealed that lethal mutations are located in regions crucial for collagen–ligand binding in the matrix. However, few of these correlations have been extended to collagen structure in bone. Here, an atomic force microscopy-based approach was used to image and quantitatively analyze the D-periodic spacing of Type I collagen fibrils in femora from heterozygous (Brtl/+) mice (α1(I)G349C), compared to wild type (WT) littermates. This disease system has a well-defined change in the col1α1 allele, leading to a well characterized alteration in collagen protein structure, which are directly related to altered Type I collagen nanoscale morphology, as measured by the D-periodic spacing. In Brtl/+ bone, the D-periodic spacing shows significantly greater variability on average and along the length of the bone compared to WT, although the average spacing was unchanged. Brtl/+ bone also had a significant difference in the population distribution of collagen D-period spacings. These changes may be due to the mutant collagen structure, or to the heterogeneity of collagen monomers in the Brtl/+ matrix. These observations at the nanoscale level provide insight into the structural basis for changes present in bone composition, geometry and mechanical integrity in Brtl/+ bones. Further studies are necessary to link these morphological observations to nanoscale mechanical integrity.     

Variable expression of osteogenesis imperfecta in a nuclear family is explained by somatic mosaicism for a lethal point mutation in the alpha 1(I) gene (COL1A1) of type I collagen in a parent.

Fibroblasts from a man with a mild form of osteogenesis imperfecta (OI) and from his son with perinatal lethal OI (OI type II) produced normal and abnormal type I procollagen molecules. The abnormal molecules synthesized by both cell strains contained one or two pro alpha 1(I) chains in which the glycine at position 550 of the triple-helical domain was substituted by arginine as the result of a G-to-A transition in the first base of the glycine codon. Cells from the mother produced only normal type I procollagen molecules. By allele-specific oligonucleotide hybridization to amplified genomic sequences from paternal tissues we determined that the mutant allele accounted for approximately 50% of the COL1A1 alleles in fibroblasts, 27% of those in blood, and 37% of those in sperm. These findings demonstrate that the father is mosaic for the potentially lethal mutation and suggest that the OI phenotype is determined by the nature of the mutation and the relative abundance of the normal and mutant alleles in different tissues. Furthermore, the findings make it clear that some individuals with mild to moderate forms of OI are mosaic for mutations that will be lethal in their offspring.     

Mild dominant osteogenesis imperfecta with intrafamilial variability: the cause is a serine for glycine α1(I) 901 substitution in a type-I collagen gene

Summary The molecular defect responsible for a case of mild osteogenesis imperfecta (OI) with repeated femural fractures was investigated. The proband and his mother, who presented minor OI signs but no bone fractures, were shown to produce normal and abnormal type-I procollagen molecules in their dermal fibroblasts. The molecular defect was localized in about half of the proband's pro 1(I) mRNA molecules by chemical cleavage with piperidine of hydroxylamine-reacted mRNA cDNA heteroduplexes. The corresponding region was reversetranscribed and amplified by polymerase chain reaction (PCR). Cloning and sequencing of the amplified products revealed in both subjects a G-to-A transition in the first base of codon 901 of the 1(I) triple helical domain, which led to a serine for glycine substitution. Allele-specific oligonucleotide hybridization to amplified genomic DNA from fibroblasts and leukocytes confirmed the heterozygous nature of both patients and proved the absence of mosaicism. The presence of the mutation was excluded in other healthy family members, who were reported to have bluish sclerae. The mild phenotypic outcome of this newly characterized mutation contradicts previous findings on glycine substitutions in the C-terminal region of collagen triple helix, most of which caused lethal OI.     

Severity of osteogenesis imperfecta and structure of a collagen-like peptide modeling a lethal mutation site

We show that there are correlations between the severities of osteogenesis imperfecta (OI) phenotypes and changes in the residues near the mutation site. Our results show the correlations between the severity of various forms of the inherited disease OI and alteration of residues near the site of OI causing mutations. Among our many observed correlations are particularly striking ones between the presence of nearby proline residues and lethal mutations, and the presence of nearby alanines residues and nonlethal mutations. We investigated the possibility that these correlations have a structural basis using molecular dynamics simulations of collagen-like molecules designed to mimic the site of a lethal OI mutation in collagen type I. Our significant finding is that interchain hydrogen bonding is greatly affected by variations in residue type. We found that the strength of hydrogen bond networks between backbone atoms on different chains depends on the local residue sequence and is weaker in proline-rich regions of the molecule. We also found that an alanine at a site near an OI mutation causes less structural disruption than a proline, and that residue side chains also form interchain hydrogen bonds with frequencies that are dependent on residue type. For example, arginine side chains form strong hydrogen bonds with the backbone of the subsequent peptide chain, while lysine and glutamine less frequently form similar hydrogen bonds. This decrease in the observed hydrogen bond frequency correlates with a decrease in the experimentally determined thermal stability. We contrasted general structural properties of model collagen peptides with and without the mutation to examine the effect of the single-point mutation on the surrounding residues.     

Selective retention and degradation of molecules with a single mutant α1(I) chain in the Brtl IV mouse model of OI

We investigated the secretion, matrix incorporation and interactions of molecules with one and two mutant alpha1(I) collagen chains in the Brtl IV murine model for Osteogenesis Imperfecta, carrying a Gly-349 to Cys substitution in one col1a1 allele. We detected a significant deviation from the expected 25 and 50% content of the molecules with no (37-46%) and one (26-40%) mutant chains in skin and bone as well as in fibroblast and osteoblast cell culture media. Steady-state labeling with (35)S-Cys demonstrated incomplete secretion of the mutant collagen in cell culture, particularly molecules containing one mutant chain. Pulse and pulse-chase experiments revealed slower secretion of the latter. An enlargement of endoplasmic reticulum in skin fibroblasts from Brtl IV mice, clearly visible by electron microscopy, supported the abnormal secretion identified by biochemical studies. We observed increased susceptibility of molecules with one mutant chain to proteolytic degradation in vitro, but we did not detect significant selective degradation in cell culture media. Mutant collagen molecules incorporated from the media into newly deposited fibers and into fully crosslinked and mature matrix in the same ratio as they were secreted. Specific labeling of reactive -SH demonstrated that about half of the Cys349-SH groups in the mutant molecules were exposed and potentially available for aberrant interactions with other molecules inside or outside the cells. Based on these and our previous findings, we argue that the outcome in Brtl IV may be significantly affected by cellular stress and malfunction caused by the retention and degradation of newly synthesized mutant collagen.     

Collagen defects in lethal perinatal osteogenesis imperfecta.

Quantitative and qualitative abnormalities of collagen were observed in tissues and fibroblast cultures from 17 consecutive cases of lethal perinatal osteogenesis imperfecta (OI). The content of type I collagen was reduced in OI dermis and bone and the content of type III collagen was also reduced in the dermis. Normal bone contained 99.3% type I and 0.7% type V collagen whereas OI bone contained a lower proportion of type I, a greater proportion of type V and a significant amount of type III collagen. The type III and V collagens appeared to be structurally normal. In contrast, abnormal type I collagen chains, which migrated slowly on electrophoresis, were observed in all babies with OI. Cultured fibroblasts from five babies produced a mixture of normal and abnormal type I collagens; the abnormal collagen was not secreted in two cases and was slowly secreted in the others. Fibroblasts from 12 babies produced only abnormal type I collagens and they were also secreted slowly. The slower electrophoretic migration of the abnormal chains was due to enzymic overmodification of the lysine residues. The distribution of the cyanogen bromide peptides containing the overmodified residues was used to localize the underlying structural abnormalities to three regions of the type I procollagen chains. These regions included the carboxy-propeptide of the pro alpha 1(I)-chain, the helical alpha 1(I) CB7 peptide and the helical alpha 1(I) CB8 and CB3 peptides. In one baby a basic charge mutation was observed in the alpha 1(I) CB7 peptide and in another baby a basic charge mutation was observed in the alpha 1(I) CB8 peptide. The primary defects in lethal perinatal OI appear to reside in the type I collagen chains. Type III and V collagens did not appear to compensate for the deficiency of type I collagen in the tissues.     

Defects of type I procollagen metabolism correlated with decrease of prolidase activity in a case of lethal osteogenesis imperfecta.

We have studied the structure and metabolism of type I procollagen in a case of perinatal lethal osteogenesis imperfecta (OI) type II. Cultured skin fibroblasts from the proband synthesized both normal and abnormal forms of type I procollagen. Some abnormal, overmodified molecules were secreted by OI cells, although less efficiently than normal molecules from control cells. The OI fibroblasts accumulated large amounts of abnormal proalpha1(I) and proalpha2(I) chains intracellularly. The extracellular collagenolytic activity was decreased compared to control cells. Furthermore, OI cells produced less type I procollagen and demonstrated lower capacity to synthesize DNA than control cells. We have found that in contrast to prolinase activity, the activity of prolidase (an enzyme essential for collagen synthesis and cell growth) is also significantly reduced in OI cells. No differences were found in the amount of the enzyme protein recovered from both the OI and control cells. However, we found that expressions of beta1 integrin and insulin-like growth factor-I receptor (receptors known to play an important role in up regulation of prolidase activity) were decreased in OI cells compared to control cells. The decrease in prolidase activity may provide an important mechanism of altered cell growth and collagen metabolism involved in producing the perinatal lethal form of the OI phenotype.     

Abnormal type I collagen metabolism by cultured fibroblasts in lethal perinatal osteogenesis imperfecta.

Cultured skin fibroblasts from seven consecutive cases of lethal perinatal osteogenesis imperfecta (OI) expressed defects of type I collagen metabolism. The secretion of [14C]proline-labelled collagen by the OI cells was specifically reduced (51-79% of control), and collagen degradation was increased to twice that of control cells in five cases and increased by approx. 30% in the other two cases. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that four of the OI cell lines produced two forms of type I collagen consisting of both normally and slowly migrating forms of the alpha 1(I)- and alpha 2(I)-chains. In the other three OI cell lines only the 'slow' alpha (I)'- and alpha 2(I)'-chains were detected. In both groups inhibition of the post-translational modifications of proline and lysine resulted in the production of a single species of type I collagen with normal electrophoretic migration. Proline hydroxylation was normal, but the hydroxylysine contents of alpha 1(I)'- and alpha 2(I)'-chains purified by h.p.l.c. were greater than in control alpha-chains. The glucosylgalactosylhydroxylysine content was increased approx. 3-fold while the galactosylhydroxylysine content was only slightly increased in the alpha 1(I)'-chains relative to control alpha 1(I)-chains. Peptide mapping of the CNBr-cleavage peptides provided evidence that the increased post-translational modifications were distributed throughout the alpha 1(I)'- and alpha 2(I)'-chains. It is postulated that the greater modification of these chains was due to structural defects of the alpha-chains leading to delayed helix formation. The abnormal charge heterogeneity observed in the alpha 1 CB8 peptide of one patient may reflect such a structural defect in the type I collagen molecule.     

Molecularly engineered vaccine which expresses an immunodominant T-cell epitope induces cytotoxic T lymphocytes that confer protection from lethal virus infection

Identification of a single viral T-cell epitope, associated with greater than 95% of the virus-specific cytotoxic T-lymphocyte (CTL) activity in BALB/c (H-2d) mice (J. L. Whitton, A. Tishon, H. Lewicki, J. Gebhard, T. Cook, M. Salvato, E. Joly, and M. B. A. Oldstone, J. Virol. 63:4303-4310, 1989), permitted us to design a CTL vaccine and test its ability to protect against a lethal virus challenge. Here we show that a single immunization with a recombinant vaccinia virus-lymphocytic choriomeningitis virus (LCMV) vaccine (VVNPaa1-201) expressing the immunodominant epitope completely protected H-2d mice from lethal infection with LCMV but did not protect H-2b mice. Furthermore, we show that the success or failure of immunization was determined entirely by the host class I major histocompatibility glycoproteins. The difference in outcome between mice of these two haplotypes was consistent with the presence or absence in the immunizing sequences of an epitope for CTL recognition and is correlated with the induction of LCMV-specific H-2-restricted CTL in H-2d mice. Protection is not conferred by a humoral immune response, since LCMV-specific antibodies were not detectable in sera from VVNPaa1-201-immunized mice. In addition, passive transfer of sera from vaccinated mice did not confer protection upon naive recipients challenged with LCMV. Hence, the molecular dissection of viral proteins can uncover immunodominant CTL epitope(s) that can be engineered into vaccines that elicit CTL. A single CTL epitope can protect against a lethal virus infection, but the efficacy of the vaccine varies in a major histocompatibility complex-dependent manner.     

Recessive osteogenesis imperfecta: clinical, radiological, and molecular findings

Osteogenesis imperfecta (OI) or "brittle bone disease" is currently best described as a group of hereditary connective tissue disorders related to primary defects in type I procollagen, and to alterations in type I procollagen biosynthesis, both associated with osteoporosis and increased susceptibility to bone fractures. Initially, the autosomal dominant forms of OI, caused by mutations in either COL1A1 or COL1A2, were described. However, for decades, the molecular defect of a small percentage of patients clinically diagnosed with OI has remained elusive. It has been in the last 6 years that the genetic causes of several forms of OI with autosomal recessive inheritance have been characterized. These comprise defects of collagen chaperones, and proteins involved in type I procollagen assembly, processing and maturation, as well as proteins involved in the formation and homeostasis of bone tissue. This article reviews the recently characterized forms of recessive OI, focusing in particular on their clinical and molecular findings, and on their radiological characterisation. Clinical management and treatment of OI in general will be discussed, too.     

Mutations in type I collagen genes resulting in osteogenesis imperfecta in humans

Osteogenesis imperfecta (OI), commonly known as "brittle bone disease", is a dominant autosomal disorder characterized by bone fragility and abnormalities of connective tissue. Biochemical and molecular genetic studies have shown that the vast majority of affected individuals have mutations in either the COL1A1 or COL1A2 genes that encode the chains of type I procollagen. OI is associated with a wide spectrum of phenotypes varying from mild to severe and lethal conditions. The mild forms are usually caused by mutations which inactivate one allele of COL1A1 gene and result in a reduced amount of normal type I collagen, while the severe and lethal forms result from dominant negative mutations in COL1A1 or COL1A2 which produce structural defects in the collagen molecule. The most common mutations are substitutions of glycine residues, which are crucial to formation and function of the collagen triple helix, by larger amino acids. Although type I collagen is the major structural protein of both bone and skin, the mutations in type I collagen genes cause a bone disease. Some reports showed that the mutant collagen can be expressed differently in bone and in skin. Since most mutations identified in OI are dominant negative, the gene therapy requires a fundamentally different approach from that used for genetic-recessive disorders. The antisense therapy, by reducing the expression of mutant genes, is able to change a structural mutation into a null mutation, and thus convert severe forms of the disease into mild OI type I.     

Cell biology and evolution: molecular modules link it all?

Classical studies comparing developing embryos have suggested the importance of modified cell biological processes in the evolution of new phenotypes. Here, I revisit this connection focusing on embryonic development, in particular nematode embryogenesis. I compare phenotypic differences in nematode embryogenesis in two basic cell biological processes, the cell cycle and the localization of the first division axis. The analysis of these and other processes shows that, at the cell biological level, exhaustive variation is found that does not necessarily translate into morphological differences. Modern molecular analyses have led to a view in which molecular complexes, made up of groups of proteins, or modules, that are working together, are responsible for the proper execution of cell biological programs. I discuss how this modular architecture could facilitate the phenotypic changes observed in cell biological processes. Ultimately, understanding the connection between cellular behavior and phenotypic outcome will further elucidate the mechanisms responsible for phenotypic evolution.     

An Arabidopsis embryonic lethal mutant with reduced expression of alanyl—t RNA synthetase gene

In present paper,one of the T-DNA insertional embryonic lethal mutant of Arabidopsis is identified and designated as acd mutant.The embryo developmant of this mutant is arrested in globular stage,The cell division pattern is abnormal during early embryogenesis and results in distubed cellular differentiation.Most of mutant embryos are finally degenerated and aborted in globular stage,However,a few of them still can germinate in agar palte and produce seedlings with shoter hypoctyl and distorted shoot meristem.To understand the molecular basis of the phenotype of this mutant,the joint fragment of T-DNA/plant DNA is isolated by plasmid rescue and Dig-labeled as probe for cDNA library screening.According to the sequence analysis and similarity searching,a 936 bp cDNA sequence(EMBL accession #:Y12555)from selectoed positive clone shows a 99.8%(923/925bp) sequence homolgy with Alanyl-tRNA Synthetase(AlaRS) gene of Arabidopsis thaliana.Furthermore,the data of in situ hybridization experiment indicate that the expression of Ala RS gene is weak in early embryogenesis and declines along with globular embryodevelopment in this mutant Accordingly,the reduced expression of Ala RS gene may be closely related to the morphological changes in early embryogenesis of this lethal mutant.     

Synthetic lethal hubs associated with vincristine resistant neuroblastoma

Chemotherapy of cancer experiences a number of shortcomings including development of drug resistance. This fact also holds true for neuroblastoma utilizing chemotherapeutics as vincristine. We performed a comparative analysis of molecular and cellular mechanisms associated with vincristine resistance utilizing cell line as well as human tissue data. Differential gene expression analysis revealed molecular features, processes and pathways afflicted with drug resistance mechanisms in general, and specifically with vincristine significantly involving actin associated features. However, specific mode of resistance as well as underlying genotype of parental, vincristine sensitive cells apparently exhibited significant heterogeneity. No consensus profile for vincristine resistance could be derived, but resistance-associated changes on the level of individual neuroblastoma cell lines as well as individual patient profiles became clearly evident. Based on these prerequisites we utilized the concept of synthetic lethality aimed at identifying hub proteins which when inhibited promise to induce cell death due to a synthetic lethal interaction with down-regulated, chemoresistance associated features. Our screening procedure identified synthetic lethal hub proteins afflicted with actin associated processes holding synthetic lethal interactions to down-regulated features individually found in all chemoresistant cell lines tested, therefore promising an improved therapeutic window. Verification of such synthetic lethal hub candidates in human neuroblastoma tissue expression profiles indicated the feasibility of this screening approach for addressing vincristine resistance in neuroblastoma.     

Dominant mutations in familial lethal and severe osteogenesis imperfecta

Summary Four families presenting with familial osteogenesis imperfecta (OI) have been studied: 2 with the lethal type II and 2 with the severe type III form. Fibroblasts of the patients, all issue from non-consanguineous parents, produced normal and abnormal (I) chains. These heterozygous mutations differentiate the recurrent forms from homozygous mutations characteristic of autosomal recessive forms. Although the identity of the mutations could not be determined, such recurrence of autosomal dominant OI is probably the result of germinal mosaicism in one of the parents. Biochemical results were consistent with a somatic mosaicism in the father's fibroblasts in one family. Moreover, our studies show that not only OI type II but also severe OI type III can arise from gonadal mosaicism. We discuss the importance of such a phenomenon for genetic counseling.