The molecular defect in an autosomal dominant form of osteogenesis imperfecta. Synthesis of type I procollagen containing cysteine in the triple-helical domain of pro-alpha 1(I) chains

Bilateral congenital cataracts result from a gain-of-function mutation in the gene for aquaporin-0 in mice

Cataract Tohoku (Cat(Tohm)) is a dominant cataract mutation that leads to severe degeneration of lens fiber cells. Linkage analysis showed that the Cat(Tohm) mutation is located on mouse chromosome 10, close to the gene for aquaporin-0 (Aqp0), which encodes a membrane protein that is expressed specifically in lens fiber cells. Sequence analysis of Aqp0 revealed a 12-bp deletion without any change in the reading frame, which resulted in a deletion of four amino acids within the second transmembrane region of the AQP0 protein. Targeted expression of the mutated Aqp0 caused lens opacity in transgenic mice, the pathological severity of which depended on the expression level of the transgene. The mutated AQP0 protein was localized to the intracellular and perinuclear spaces rather than to the plasma membranes of the lens fiber cells. The cataract phenotype of Cat(Tohm) is caused by a gain-of-function mutation in the mutated AQP0 protein and not by a loss-of-function mutation.

Somatic cell mosaicism: another source of phenotypic heterogeneity in nuclear families with osteogenesis imperfecta

Mutations in the genes coding for the pro alpha 1 and pro alpha 2 chains of type I procollagen have been found in many patients with osteogenesis imperfecta (OI), a heritable disorder of connective tissue. The severity of the disease varies between families and even among members of the same family. This phenotypic variability covers a spectrum extending from very mild forms that cannot be easily detected to perinatally lethal forms. One explanation for this phenotypic variability is the nature of the mutation in the type I procollagen genes. Another explanation is mosaicism. Here we report on 2 families with propositi who have OI, whereas their mothers had a milder form of the disease. In one family, the molecular defect was previously shown to be a substitution of alpha 1(904) by cysteine [Constantinou et al., 1990]. The biochemical phenotype was characterized by significant post-translational overmodification of the mutated type 1 collagen molecules which also had a 3-4 degrees C decrease in their thermal unfolding. Also, secretion of the procollagen into the culture media was delayed. In the second family, the proposita's muscle fibroblasts synthesized and secreted type I procollagen molecules that were highly over-modified along the entire length of their triple-helical domain. Cells from the mother also synthesized normal and over-modified protein, although the amount of over-modified protein was less than that synthesized by her daughter's cells. The exact molecular defect has not yet been defined.(ABSTRACT TRUNCATED AT 250 WORDS)

A cysteine for glycine substitution at position 175 in an alpha 1 (I) chain of type I collagen produces a clinically heterogeneous form of osteogenesis imperfecta

The molecular basis for Osteogenesis Imperfecta in a large kindred with a highly variable phenotype was identified by sequencing the mutant pro alpha 1 (I) protein, cDNA and genomic DNA from the proband. Fibroblasts from different affected individuals all synthesize both normal Type I procollagen molecules and abnormal Type I procollagen molecules in which one or both pro alpha 1 (I) chain(s) contain a cysteine residue within the triple helical domain. Protein studies of the proband localized the mutant cysteine residue to the alpha 1 (I) CB 8 peptide. We now report that cysteine has replaced glycine at triple helical residue 175 disrupting the invariant Gly-X-Y structural motif required for perfect triple helix formation. The consequences include post-translational overmodification, decreased thermal stability, and delayed secretion of mutant molecules. The highly variable phenotype in the present kindred cannot be explained solely on the basis of the cysteine for glycine substitution but will require further exploration.

Substitution of cysteine for glycine at residue 415 of one allele of the alpha 1(I) chain of type I procollagen in type III/IV osteogenesis imperfecta

We have examined the type I collagen in a patient with type III/IV osteogenesis imperfecta. Two forms of alpha 1(I) chain were produced, one normal and the other containing a cysteine residue within the triple helical domain of the molecule. Cysteine is not normally present in this domain of type I collagen. Peptide mapping experiments localised the mutation to peptide alpha 1(I)CB3 which spans residues 403 to 551 of the triple helix. Subsequent PCR amplification of cDNA covering this region followed by sequencing showed a G to T single base change in the GGC codon for glycine 415 generating TGC, the codon for cysteine. The effect of the mutation on the protein is to delay secretion from the cell, reduce the thermal stability of the molecule by 2 degrees C, and cause excessive post-translational modification of all chains in molecules containing one or more mutant alpha 1(I) chains. The clinical phenotype observed in this patient and the position of the mutation conform to the recent prediction of Starman et al that Gly----Cys mutations in the alpha 1(I) chain have a gradient of severity decreasing from the C-terminus to the N-terminus.

Cysteine in the triple helical domain of the pro alpha 2(I) chain of type-I collagen in nonlethal forms of osteogenesis imperfecta

To determine if some individuals with deforming varieties of osteogenesis imperfecta (OI) carry point mutations in the COL1A2 gene of type-I collagen, we examined collagens synthesized by cell strains from affected individuals for the presence of cysteine in the triple helical domain of the alpha 2 (I) chain, a domain from which it is normally excluded. We identified 4 individuals out of 60 whose cells synthesized a population of alpha 2(I) chains with a cysteine residue in the triple helix. The clinical differences among the affected individuals and the heterogeneity in the locations of the cysteine residues suggest that the position of the substitution within the chain is important in determining the clinical phenotype. These data confirm that individuals with nonlethal OI may commonly harbor defects in the COL1A2 gene, and suggest that many of the defects are substitutions for glycine residues in the alpha 2(I) triple helical domain.

Brittle bones--fragile molecules: disorders of collagen gene structure and expression

Mutations in the genes that encode the chains of type I collagen, the major structural protein in most tissues, usually produce brittle bones. The consequences of even apparently minor mutations--single base substitutions--can range from lethal to mild, and the phenotypic consequences reflect the nature and position of the mutation. The manner in which phenotypes are produced depends on the effect of the mutation on the structural integrity of the molecule and on whether or how the abnormal molecules can be incorporated into an extracellular matrix.

Moderately severe osteogenesis imperfecta: biochemical studies showing variable defect localization in the triple-helical domain of type I collagen

This report describes the biochemical investigations on six patients affected by a moderate form of Osteogenesis Imperfecta (type IV according to the Sillence classification). Biochemical characterization of type I collagen produced by skin fibroblasts showed considerable heterogeneity: in three patients out of six, collagen appeared normal; while in the three others a structural defect in the protein was present. In these probands the mutations were localized in different regions of the triple helix domain (corresponding to peptides alpha 1(I)CB6 and alpha 1(I)CB7). In two probands showing the defect in alpha 1(I)CB7, a decrease of the thermal stability of the protein was present.

Anomalous cysteine in type I collagen. Localisation by chemical cleavage of the protein using 2-nitro-5-thiocyanobenzoic acid and by mismatch analysis of cDNA heteroduplexes

A method is presented for the localisation of an anomalous cysteine inside the triple helical domain of type I collagen from a patient affected with Osteogenesis Imperfecta. The chemical cleavage used relies on the specificity and reactivity of the thiol side chain versus 2-nitro-5-thiocyanobenzoic acid, to yield cyanocysteine; in mild alkaline conditions this derivative will undergo the breakdown of its N-side peptide bond. This method could allow a more precise localisation of anomalous cysteine in both type I collagen alpha chains, alpha 1(I) and alpha 2(I), compared to previous analytical methods on CNBr peptides. For the mutant alpha 1(I) chains from a patient affected by Osteogenesis Imperfecta, we found a location of cysteine in the peptide alpha 1(I)CB8, between amino acids 170-200. Biochemical localisation was confirmed by a chemical cleavage method for mismatched cytosines on heteroduplexes obtained after denaturation and annealing of a 233 bp cDNA fragment amplified by PCR from the heterozygote patient.

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.

Does heat damage fetuses?

Temperature affects phenotypic variation during critical developmental stages in all forms of life that have been studied thus far. In animal studies of heat teratogenicity, adverse effects have ranged from disruption of the normal cell cycle leading to decreased numbers of cells, to the induction of developmental abnormalities by means of embryonic cell death. The heat shock response is a universal cellular stress reaction in which the transcriptional and translational mechanisms of the cell are pre-empted by preferential induction of heat shock protein synthesis. Occurrence of such a phenomenon during prenatal life could lead to the absence of essential gene products at critical stages of development. The crucial question of whether temperature induced cellular and genetic effects ever occur during human fetal development has been considered only in relation to maternal hyperthermia, which is generally viewed as not being of significance in human teratology. We propose that teratogenicity may result from fetal hyperthermia unrelated to maternal hyperthermia, caused either by impaired fetomaternal heat dissipation due to reduced placental blood flow (extrinsic fetal hyperthermia) or by increased fetal heat production during hypermetabolic states (intrinsic fetal hyperthermia). The need for further studies in this regard is emphasized.

Prenatal diagnosis and prevention of inherited abnormalities of collagen

There is now strong evidence for the implication of collagen alpha 1(I), alpha 2(I) and alpha 1(III) mutations in many forms of osteogenesis imperfecta and inherited arterial aneurysms (Ehlers Danlos syndrome type IV). A sizeable proportion of these disorders have detectable abnormalities by conventional protein chemistry, immunofluorescence, or more sophisticated DNA analysis. Everyone of them with specific defects or with linkage to appropriate gene markers is therefore amenable to prevention using conventional prenatal diagnosis by chorionic villus biopsy (with fibroblast culture), fetoscopic biopsy (with fibroblast culture), ultrasound diagnosis of the severely deformed fetus, or gene linkage studies by chorionic villus biopsy or amniocentesis. Already many collagen alpha 1(I), alpha 2(I) and alpha 1(III) mutations have been characterized including point mutations, small and large deletions and regulatory mutations. Many others are likely to be rapidly studied by exploiting recent advances in DNA technology, and other strong candidate genes include collagen II (some chondrodystrophies), collagen VI (certain arterial and cardiovascular diseases) and collagen VII (dystrophic epidermolysis bullosa). Other important common diseases are likely to include osteoporosis, osteoarthritis and cerebral aneurysms. A detailed review is provided of collagen interstitial genes and proteins, together with a description of the various forms of osteogenesis imperfecta and Ehlers Danlos syndrome in which either collagen alpha 1(I), alpha 2(I) or alpha 1(III) mutations have been identified. Appropriate restriction length polymorphisms (RFLPs) useful in identifying carriers of these mutant genes are also described.

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.

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.

The A and B fragments of normal type I procollagen have a similar thermal stability to proteinase digestion but are selectively destabilized by structural mutations

Previous studies demonstrated that the thermal stability of the procollagen triple helix can be assayed by digesting the protein for short periods with high concentrations of trypsin and chymotrypsin. Here we cleaved human type I procollagen or collagen with vertebrate collagenase to generate A fragments from the three-quarter amino termini and B fragments from the one-quarter carboxy termini of the molecules. The thermal stabilities of the fragments were then assayed by rapid trypsin/chymotrypsin digestion. Both fragments were resistant up to 36 degrees C and completely degraded between 37 degrees C and 39 degrees C. In subsequent experiments the same assay was carried out with type I procollagens synthesized by fibroblasts from two patients with lethal variants of osteogenesis imperfecta. With one, the A fragments were selectively destabilized, an observation consistent with previous data indicating that the mutation in the patient produced a deletion of 84 amino acids from the middle of the alpha 1(I) chain. With procollagen synthesized by fibroblasts from the second patient the B fragments were selectively destabilized, an observation consistent with preliminary data indicating a mutation that alters the primary structure of the carboxy-terminal region of the alpha 1(I) chain. Therefore, the procedures described here present a simple and direct method for locating mutations that destabilize the collagen triple helix.