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

Thermal stability and folding of the collagen triple helix and the effects of mutations in osteogenesis imperfecta on the triple helix of type I collagen

Osteogenesis imperfecta (OI) is an inherited disease in which 90% of the cases result from mutations in the 2 genes, pro alpha 1 and pro alpha 2, coding for type I collagen. Type I collagen is a trimeric molecule, (alpha 1)2 alpha 2, which is dominated both structurally and functionally by the 300 nm triple-helical domain. Most OI mutations occur in this domain and almost all point mutations result in the substitution of other amino acids for the obligate glycine which occurs at every third residue. The phenotypic effects of these mutations are frequently attributed in part to alterations in the stability and rate of folding of the triple helix. In order to better understand the relationship between glycine substitutions and stability we review current concepts of the forces governing triple helical stability, denaturational and predenaturational unfolding, and the techniques of measuring stability. From observations on the stability of several collagen types as well as synthetic tripeptides, we present a model for stability based on the contribution of individual and neighboring tripeptide units to the local stability. Although in preliminary form, this empirical model can account for the observed shifts in the Tm of many of the point mutations described. The folding of the triple helix is reviewed. The involvement of peptidyl prolyl cis-trans isomerase in this process in vivo is demonstrated by the inhibition of collagen folding in fibroblasts by cyclosporin A. An hypothesis based on the relationship between the thermal stability at the site of mutation and the propensity for renucleation of folding is proposed.

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.

A highly polymorphic (ACT)n VNTR (variable nucleotide of tandem repeats) locus inside intron 12 of COL1A2, one of the two genes involved in dominant osteogenesis imperfecta

A new, highly polymorphic, region consisting of variable number of tandem repeats (VNTR) is described that occurs within intron 12 of the COL1A2 gene. This VNTR consists of the trinucleotide ACT repeated from 6 to 12 times. Of the six alleles so far detected four are common in the three major races. The two rare alleles, (ACT)11 and (ACT)12, have been found only in Africans. In addition, a rapid technique has been developed that can be used successfully with very small amounts of even partially degraded DNA, thus allowing the use of this VNTR for forensic applications. Since dominant OI can be due to mutations at either of two loci (COL1A1 and COL1A2) prenatal diagnosis becomes feasible in the majority of the affected families only if a very informative marker is available for both of these genes. This VNTR provides a very powerful marker for COL1A2. In fact the heterozygosity for it ranges from 0.634 to 0.741 with PIC values from 0.562 to 0.696, respectively. Since trinucleotide repeats can be "unstable," and sometimes pathogenic, the unexplained collagenopathies (or suspected collagenopathies) should be analyzed from this point of view.

Homology-mediated recombination between type I collagen gene exons results in an internal tandem duplication and lethal osteogenesis imperfecta

It has been proposed that the structure of the exons that encode the triple helical domain of the fibrillar collagen genes arose by repeated tandem duplication of an ancestral unit exon. Because these exons encode a repeat motif [(Gly-X-Y)n], sequence homology between exons may have driven the recombinational process. We have characterized a tandem duplication mutation within a COL1A1 allele of type I collagen from an infant with the lethal form of osteogenesis imperfecta. The structure of the mutation is consistent with the occurrence of an unequal crossover within a 15 base pair region of sequence identity between exons 14 and 17 of the COL1A1 gene. The recombination produced a new 81 base pair 17/14 hybrid exon and complete duplication of exons 15 and 16. The sequence implies duplication of 60 amino acid residues within the triple helical domain with preservation of the Gly-X-Y repeat. These data suggest that a recombinational mechanism that explains the hypothetical evolutionary process is active in cells, but the lethal effect of this mutation raises questions about the role of these events in creating new structures for polymeric proteins.

Two cysteine substitutions in procollagen I: a glycine replacement near the N-terminus of alpha 1(I) chain causes lethal osteogenesis imperfecta and a glycine replacement in the alpha 2(I) chain markedly destabilizes the triple helix

Cultured skin fibroblasts were examined from two probands with type II (lethal) osteogenesis imperfecta. One proband had a single base mutation which converted the glycine codon at position alpha 1-244 in the alpha 1(I) chain of procollagen I into a cysteine codon whereas the other had a similar mutation that converted the glycine codon at position alpha 2-787 of the alpha 2(I) chain into a cysteine codon. Both mutations produced post-translational overmodification of procollagen I. The Cys alpha 1-244 mutation, however, had a minimal effect on the thermal stability or secretion of the protein whereas the Cys alpha 2-787 mutation markedly decreased the thermal stability and, apparently as a result, essentially none of the mutated protein was secreted. The results provide clear exceptions to two previous generalizations about the position-specificity of glycine substitutions in procollagen I.

Characterization of three osteogenesis imperfecta collagen alpha 1(I) glycine to serine mutations demonstrating a position-dependent gradient of phenotypic severity

Type I collagen alpha 1(I) glycine to serine substitutions, resulting from G-to-A mutations, were defined in three cases of osteogenesis imperfecta (OI). The Gly substitutions displayed a gradient of phenotypic severity according to the location of the mutation in the collagen triple helix. The most C-terminal of these, Gly565 to Ser, led to the lethal perinatal (type II) form of OI, whereas the more N-terminal mutations, Gly415 and Gly352 to Ser, led to severe OI (type III/IV) and moderate OI (type IVB) respectively. These data support the notion that glycine substitutions towards the C-terminus of the alpha 1(I) or alpha 2(I) chains will be more clinically severe than those towards the N-terminus. This results from the more disruptive effect of the mutations at the C-terminus on helix initiation and C- and N-terminal helix directional propagation. This generalization must be modified by considering the nature of the glycine substitution and the surrounding amino acid sequence, since the helix is composed of subdomains of differing stability which will affect the ability of helix re-nucleation and propagation.

Evidence for the localization of a malignant hyperthermia susceptibility locus (MHS2) to human chromosome 17q

Malignant hyperthermia susceptibility is a lethal autosomal dominant disorder of skeletal muscle metabolism that is triggered by all potent inhalation anesthetic gases. Recent linkage studies suggest a genetic locus for this disorder on 19q13.1. We have previously reported three unrelated families diagnosed with MHS that are unlinked to markers surrounding this locus on 19q13.1. In this report we extend these observations and present linkage studies on 16 MHS families. Four families (25%) were found linked to the region 19q12-q13.2 (Zmax = 2.96 with the ryanodine receptor at theta = 0.0). Five families (31%) were found closely linked to the anonymous marker NME1 (previously designated NM23) on chromosome 17q11.2-q24 (Zmax = 3.26 at theta = 0.0). Two families (13%) were clearly unlinked to either of these chromosomal regions. In five additional families, data were insufficient to determine their linkage status (they were potentially linked to two or more sites). The results of our heterogeneity analyses are consistent with the hypothesis that MHS can be caused in humans by any one of at least three distinct genetic loci. Furthermore, we provide preliminary linkage data suggesting the localization of a gene in human MHS to 17q11.2-q24 (MHS2), with a gene frequency of this putative locus approximately equal to that of the MHS1 locus on 19q.

Extracellular matrix formation by osteoblasts from patients with osteogenesis imperfecta

Extracellular matrix proteins synthesized by bone cells isolated from 16 patients with different forms of osteogenesis imperfecta (OI) were analyzed in vitro. Specific components of the extracellular matrix by OI and age-matched cultures were investigated by steady-state radiolabeling followed by quantitation of label into specific proteins and comparison of OI cultures to those of age-matched controls. The in vitro proliferation of OI bone cells was found to be lower than that of control cells. In seven patients, abnormalities of the alpha 1(I) and/or alpha 2(I) chains of type I collagen were detected by gel electrophoresis. In two of these patients, the mutations in the COLIA1 and COLIA2 genes have been previously identified. Although the amount of total protein synthesized by the cells in culture was the same for OI bone cells and age-matched control cells, OI bone cells showed a significantly reduced synthesis of not only collagen but also other bone matrix glycoproteins. The synthesis of osteonectin (SPARC/BM40) and three proteoglycans [a large chondroitin sulfate proteoglycan, biglycan (PGI), and decorin (PGII)] was found to be decreased in OI cells. The reduction was most pronounced at the developmental age at which these macromolecules reach maximal levels during normal development.

Expression of mutant alpha (I)-procollagen in osteoblast and fibroblast cultures from a proband with osteogenesis imperfecta type IV

This study compares the synthesis of mutant type I collagen in cultured dermal fibroblasts and trabecular osteoblasts that were isolated from a patient with moderately severe osteogenesis imperfecta (type IV). Previous study of this patient's dermal fibroblasts revealed a 2000 dalton deletion located in cyanogen bromide peptide 4 of alpha 2(I)-collagen. The phenotype of the bone cell cultures was defined by a 3-4 day logarithmic phase doubling time, predominantly type I collagen production over type III and alkaline phosphatase activity 13.5 times dermal fibroblast levels. The current study revealed that both fibroblasts and osteoblasts synthesized a normal and a shortened alpha 2(I) chain, each as the product of separate alleles. Following pepsin treatment of the procollagens, a shortened alpha 1(I) chain was also seen in both cell types. Cyanogen bromide peptide mapping of osteoblast alpha-chains demonstrated the same deletions in the cyanogen bromide peptide 4 as observed in the fibroblast cyanogen bromide maps. PAGE analysis of oligonucleotide-specific cDNA that was reverse transcribed from RNA isolated from fibroblasts and osteoblasts also demonstrated the presence of two bands, one the normal size of alpha 2(I) cDNA and a second species that was smaller by 54 base pairs. Sequencing of polymerase chain reaction-amplified cDNA fragments revealed an in-frame deletion of exon 12. This finding was confirmed by the RNase protection method. Genomic DNA sequencing detected a T----G point mutation in the second position of the 5' splice donor site of intron 12. Therefore, in this patient with osteogenesis imperfecta there was no qualitative alteration in the osteoblast-specific expression of this mutant alpha 2(I)-collagen allele compared to dermal fibroblasts.

Detection and characterisation of an overmodified type III collagen by analysis of non-cutaneous connective tissues in a patient with Ehlers-Danlos syndrome IV

The clinical and biochemical observations in a patient with a mild form of Ehlers-Danlos syndrome (EDS) type IV are described. The patient's skin fibroblasts produced markedly diminished amounts of type III collagen. SDS-polyacrylamide gel electrophoresis of collagens produced by cells obtained from other, non-cutaneous tissues showed two forms of collagen alpha 1(III) chains, a normal and a slow migrating, mutant form. Further analysis confirmed that the type III collagen molecules containing mutant alpha chains which were overmodified had a lower thermal stability and were poorly secreted into the extracellular medium. The protein defect was mapped by in situ cyanogen bromide digestion and was located in alpha 1(III) CB9, the C-terminal peptide of the collagen triple helix. This study shows that non-cutaneous connective tissues can be a useful source for the study of type III collagen defects in patients with EDS type IV.

Characterization of a type II collagen gene (COL2A1) mutation identified in cultured chondrocytes from human hypochondrogenesis

A subtle mutation in the type II collagen gene COL2A1 was detected in a case of human hypochondrogenesis by using a chondrocyte culture system and PCR-cDNA scanning analysis. Chondrocytes obtained from cartilage biopsies were dedifferentiated and expanded in monolayer culture and then redifferentiated by culture over agarose. Single-strand conformation polymorphism and direct sequencing analysis identified a G----A transition, resulting in a glycine substitution at amino acid 574 of the pro alpha 1(II) collagen triple-helical domain. Morphologic assessment of cartilage-like structures produced in culture and electrophoretic analysis of collagens synthesized by the cultured chondrocytes suggested that the glycine substitution interferes with conversion of type II procollagen to collagen, impairs intracellular transport and secretion of the molecule, and disrupts collagen fibril assembly. This experimental approach has broad implications for the investigation of human chondrodysplasias as well as human chondrocyte biology.

Genetic linkage of the Marfan syndrome, ectopia lentis, and congenital contractural arachnodactyly to the fibrillin genes on chromosomes 15 and 5. The International Marfan Syndrome Collaborative Study

BACKGROUND

The large glycoprotein fibrillin is a structural component of elastin-containing microfibrils found in many tissues. The Marfan syndrome has been linked to the fibrillin gene on chromosome 15, but congenital contractural arachnodactyly, which shares some of the physical features of the syndrome, has been linked to the fibrillin gene on chromosome 5.

METHODS

Using specific markers for the fibrillin genes, we performed genetic linkage analysis in 28 families with the Marfan syndrome and 8 families with four phenotypically related disorders--congenital contractural arachnodactyly (3 families), ectopia lentis (2), mitral-valve prolapse syndrome (2), and annuloaortic ectasia (1).

RESULTS

Genetic linkage was established between the Marfan syndrome and only the fibrillin gene on chromosome 15, with a maximum lod score of 25.6 (odds for linkage, 10(25.6):1). Ectopia lentis was also linked to the fibrillin gene on chromosome 15, whereas congenital contractural arachnodactyly was linked to the fibrillin gene on chromosome 5. There was no linkage of mitral-valve prolapse to the fibrillin gene on chromosome 5; studies of chromosome 15 were not informative. Annuloaortic ectasia was not linked to either fibrillin gene.

CONCLUSIONS

The Marfan syndrome appears to be caused by mutations in a single fibrillin gene on chromosome 15. Diagnosis of the Marfan syndrome by genetic linkage and analysis is now feasible in many families.

The molecular defect in a family with mild atypical osteogenesis imperfecta and extreme joint hypermobility: exon skipping caused by an 11-bp deletion from an intron in one COL1A2 allele

We have investigated a family with an autosomal dominantly inherited connective-tissue defect causing extreme joint hypermobility, premature osteoporosis and late-onset fractures. Analysis of collagenous proteins from affected individuals showed a deletion in some alpha 2(I) chains. Peptide mapping localized this to the CB peptide alpha 2CB4, which covers the N-terminal one-third of the protein chain. Polymerase chain reaction amplification and sequencing of cDNA derived from this region of the mRNA identified a heterozygous deletion of the 54 bp comprising exon 9. Similar analysis of the genomic DNA revealed an 11-bp deletion from bp3 to bp13 of IVS-9. This disrupts the consensus 5' splice signal (GTAAGT) and leads to exon skipping. In a family study of 13 affected and unaffected family members using both heteroduplex formation and direct analysis for the deletion, all of the affected, but no unaffected individuals, were found to carry the deletion. This generated a positive Lod score of 2.6 with the Liped programme.

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.

An osteopenic nonfracture syndrome with features of mild osteogenesis imperfecta associated with the substitution of a cysteine for glycine at triple helix position 43 in the pro alpha 1(I) chain of type I collagen

Mutations affecting the pro alpha 1(I) or pro alpha 2(I) collagen genes have been identified in each of the major clinical types of osteogenesis imperfecta. This study reports the presence of a heritable connective tissue disorder in a family with an osteopenic syndrome which has features of mild osteogenesis imperfecta but was considered idiopathic osteoporosis in the proband. At age 38, while still premenopausal, she was found to have osteopenia, short stature, hypermobile joints, mild hyperelastic skin, mild scoliosis, and blue sclerae. There was no history of vertebral or appendicular fracture. Hip and vertebral bone mineral density measurements were consistent with marked fracture risk. Delayed reduction SDS-PAGE of pepsin-digested collagens from dermal fibroblast cultures demonstrated an anomalous band migrating between alpha 1(I) and alpha 1(III). This band merged with the normal alpha-chains upon prereduction, indicating an unexpected cysteine residue. Cyanogen bromide peptide mapping suggested that the mutation was in the smaller NH2-terminal peptides. cDNA was reverse transcribed from mRNA and amplified by the polymerase chain reaction. A basepair mismatch between proband and control alpha 1(I) cDNA hybrids was detected by chemical cleavage with hydroxylamine:piperidine. The cysteine substitution was thus localized to alpha 1(I) exon 9 within the cyanogen bromide 4 peptide. Nucleotide sequence analysis localized a G----T point mutation in the first position of helical codon 43, replacing the expected glycine (GGT) residue with a cysteine (TGT). The prevalence of similar NH2-terminal mutations in subjects with this phenotype which clinically overlaps idiopathic osteoporosis remains to be determined.

Recurrence of lethal osteogenesis imperfecta due to parental mosaicism for a mutation in the COL1A2 gene of type I collagen. The mosaic parent exhibits phenotypic features of a mild form of the disease

We have determined that a man, ascertained because he fathered a child with lethal osteogenesis imperfecta (OI) with each of two partners, is mosaic in both his germline and somatic tissues for a mutation in the COL1A2 gene which encodes the pro alpha 2(I) chain of type I procollagen. His dermal fibroblasts were previously shown to synthesize a population of cysteine-containing alpha 2(I) chains that were posttranslationally overmodified. DNA sequence analysis of COL1A2 cDNAs demonstrated that the cysteine-containing chain resulted from a point mutation (G to T) in the first position of the codon for the glycine at residue 472 of the triple helical domain. Genomic DNA from the one available affected infant contained the mutant and normal COL1A2 alleles in equal proportion. Examination of DNA from several tissues of the father showed that the mutant allele was present in approximately 40% of his sperm, 80% of his lymphocytes, and nearly 100% of his dermal fibroblasts. Despite the high level of mosaicism detected in somatic tissues, the only phenotypic manifestation of OI in the proband was that he was shorter than his unaffected male relatives and had mild dentinogenesis imperfecta. Thermal stability of type I collagen molecules containing the substitution was decreased, but to a lesser extent than for a nonlethal cysteine for glycine substitution at residue 259 of alpha 2(I), indicating that this measure of molecular stability may be of limited use in explaining the pathogenesis of osteogenesis imperfecta.

Lethal perinatal osteogenesis imperfecta due to a type I collagen alpha 2(I) Gly to Arg substitution detected by chemical cleavage of an mRNA:cDNA sequence mismatch

A single base mismatch was detected by a chemical cleavage method in heteroduplexes formed between patient mRNA and a control collagen alpha 2(I) cDNA probe in a case of osteogenesis imperfecta type II. The region of the mRNA mismatch was amplified using the polymerase chain reaction, cloned and sequenced. A heterozygous point mutation of G to C at base pair 1,774 of the collagen alpha 2(I) mRNA resulted in the substitution of glycine with arginine at amino acid position 457 of the helix. Type I collagen of alpha 1(I)- and alpha 2(I)-chains from the patient migrated slowly on electrophoresis due to increased levels of posttranslational modification of lysine. The parents' fibroblast collagen did not contain the mRNA mismatch and the collagens showed normal electrophoretic behaviour. Two-dimensional electrophoresis of the CNBr peptides from the patient's collagen confirmed the excessive posttranslational modification of the alpha 1(I)- and alpha 2(I)-chains in the CNBr peptides N-terminal to the mutation due to disruption of the obligatory Gly-X-Y triplet repeat of the helix. The mutation led to reduced procollagen secretion and helix destabilization as evidenced by a decreased thermal stability. These data lend further support to the accumulating evidence that type I collagen alpha 2(I) glycine substitution mutations result in the same spectrum of clinical severity as those in the alpha 1(I)-chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Completion of the last half of the structure of the human gene for the Pro alpha 1 (I) chain of type I procollagen (COL1A1)

The nucleotide sequences of the 3'-half of the human gene for the pro alpha(I) chain of type I procollagen (COL1A1) is presented. The results provide the nucleotide sequences for 26 introns not previously analyzed. The sequences that are presented, together with those previously published, make it possible to design primers for the polymerase chain reaction for amplifying and sequencing the gene. The availability of such primers will greatly facilitate the current search for mutations that can cause common and rare diseases of connective tissue.

Intractable vasculitis, resorptive osteolysis, and immunity to type I collagen in type VIII Ehlers-Danlos syndrome

A unique patient with type VIII Ehlers-Danlos syndrome and cutaneous vasculitis, resorptive osteolysis, and cardiac valvular disease is described. Collagen analyses identified morphologic and physical abnormalities of type I collagen. The patient's T lymphocytes could be propagated in vitro with type I collagen and produced a 60-kd lymphokine that bound this protein. Cellular autoimmunity to type I collagen may be responsible for this patient's intractable clinical condition.

Reduced amounts of cartilage collagen fibrils and growth plate anomalies in transgenic mice harboring a glycine-to-cysteine mutation in the mouse type II procollagen alpha 1-chain gene

We have generated transgenic mice harboring a glycine-to-cysteine mutation in residue 85 of the triple helical domain of mouse type II collagen. The offspring of different founders displayed a phenotype of severe chondrodysplasia characterized by short limbs and trunk, cranio-facial deformities, and cleft palate. The affected pups died of acute respiratory distress caused by an inability to inflate lungs at birth. Staining of the skeleton showed a severe retardation of growth for practically all bones. Light microscopic examination indicated a decrease in cartilage matrix density, a severe disorganization of growth plate architecture, and the presence of streaks of fibrillar material in the cartilage matrix. Electron microscopic analysis showed a pronounced decrease in the number of typical thin cartilage collagen fibrils, distension of the rough endoplasmic reticulum of chondrocytes, and the presence of abnormally large banded collagen fibril bundles. The level of expression of the mutant type II procollagen alpha 1 chain transgene in cartilage tissues was approximately equal to that of the endogenous gene in two of the strains. We propose that the principal consequence of the mutation is a considerable reduction in density of the typical thin cartilage collagen fibrils and that this phenomenon causes the severe disorganization of the growth plate. We also postulate that the abnormal thick collagen fibrils are probably related to a defect in crosslinking between the collagen molecules. The cartilage anomalies displayed by these transgenic mice are remarkably similar to those of certain human chondrodysplasias.

Substitution of cysteine for glycine-alpha 1-691 in the pro alpha 1(I) chain of type I procollagen in a proband with lethal osteogenesis imperfecta destabilizes the triple helix at a site C-terminal to the substitution

Skin fibroblasts from a proband with lethal osteogenesis imperfecta synthesized a type I procollagen containing a cysteine residue in the alpha 1(I) helical domain. Assay of thermal stability of the triple helix by proteinase digestion demonstrated a decreased temperature for thermal unfolding of the protein. Of special importance was the observation that assays of thermal stability by proteinase digestion revealed two bands present in a 2:1 ratio of about 140 and 70 kDa; the 140 kDa band was reducible to a 70 kDa band. Further analysis of the fragments demonstrated that the cysteine mutation produced a local unfolding of the triple helix around residue 700 and apparently exposed the arginine residue at position 704 in both the alpha 1(I) and alpha 2(I) chains. Analysis of cDNAs and genomic DNAs demonstrated a single-base mutation that changed the GGT codon for glycine-691 of the alpha 1(I) chain to a TGT codon for cysteine. The mutation was not found in DNA from either of the proband's parents. Since the proteinase assay of helical stability generated a fragment of 700 residues that retained disulphide-bonded cysteine residues at alpha 1-691, the results provide one of the first indications that glycine substitutions in type I procollagen can alter the conformation of the triple helix at a site that is C-terminal to the site of the substitution.

Expression of a partially deleted gene of human type II procollagen (COL2A1) in transgenic mice produces a chondrodysplasia

A minigene version of the human gene for type II procollagen (COL2A1) was prepared that lacked a large central region containing 12 of the 52 exons and therefore 291 of the 1523 codons of the gene. The construct was modeled after sporadic in-frame deletions of collagen genes that cause synthesis of shortened pro alpha chains that associate with normal pro alpha chains and thereby cause degradation of the shortened and normal pro alpha chains through a process called procollagen suicide. The gene construct was used to prepare five lines of transgenic mice expressing the minigene. A large proportion of the mice expressing the minigene developed a phenotype of a chondrodysplasia with dwarfism, short and thick limbs, a short snout, a cranial bulge, a cleft palate, and delayed mineralization of bone. A number of mice died shortly after birth. Microscopic examination of cartilage revealed decreased density and organization of collagen fibrils. In cultured chondrocytes from the transgenic mice, the minigene was expressed as shortened pro alpha 1(II) chains that were disulfide-linked to normal mouse pro alpha 1(II) chains. Therefore, the phenotype is probably explained by depletion of the endogenous mouse type II procollagen through the phenomenon of procollagen suicide.

Marfan syndrome caused by a recurrent de novo missense mutation in the fibrillin gene

Marfan syndrome is an inherited disorder of connective tissue manifested in the ocular, skeletal and cardiovascular systems. It is inherited as an autosomal dominant with high penetrance, but has great clinical variability. Linkage studies have mapped the Marfan locus to chromosome 15q15-21.3. There have been no reports of genetic heterogeneity in the syndrome. Following the identification of fibrillin (a glycoprotein component of the extracellular microfibril), immunohistopathological quantification of the protein in skin and fibroblast culture, and examination of fibrillin synthesis, extracellular transport, and incorporation into the extracellular matrix (D. M. Milewicz, R.E.P., E. S. Crawford and P. H. Byers, manuscript in preparation) have demonstrated abnormalities of fibrillin metabolism in most patients. A portion of the complementary DNA encoding fibrillin has been cloned and mapped by in situ hybridization to chromosome 15. Here we report that the fibrillin gene is linked to the Marfan phenotype (theta = 0.00; logarithm of the odds (lod) = 3.9) and describe a de novo missense mutation in the fibrillin gene in two patients with sporadic disease. We thus implicate fibrillin as the protein defective in patients with the Marfan syndrome.

Mutation in a gene for type I procollagen (COL1A2) in a woman with postmenopausal osteoporosis: evidence for phenotypic and genotypic overlap with mild osteogenesis imperfecta

Mutations in the two genes for type I collagen (COL1A1 or COL1A2) cause osteogenesis imperfecta (OI), a heritable disease characterized by moderate to extreme brittleness of bone early in life. Here we show that a 52-year-old postmenopausal woman with severe osteopenia and a compression fracture of a thoracic vertebra had a mutation in the gene for the alpha 2(I) chain of type I collagen (COL1A2) similar to mutations that cause OI. cDNA was prepared from the woman's skin fibroblast RNA and assayed for the presence of a mutation by treating DNA heteroduplexes with carbodiimide. The results indicated a sequence variation in the region encoding amino acid residues 660-667 of the alpha 2(I) chain. Further analysis demonstrated a single-base mutation that caused a serine-for-glycine substitution at position 661 of the alpha 2(I) triple-helical domain. The substitution produced posttranslational overmodification of the collagen triple helix, as is seen with most glycine substitutions that cause OI. The patient had a history of five previous fractures, slightly blue sclerae, and slight hearing loss. Therefore, the results suggest that there may be phenotypic and genotypic overlap between mild osteogenesis imperfecta and postmenopausal osteoporosis, and that a subset of women with postmenopausal osteoporosis may have mutations in the genes for type I procollagen.

Characterization of a type I collagen alpha 2(I) glycine-586 to valine substitution in osteogenesis imperfecta type IV. Detection of the mutation and prenatal diagnosis by a chemical cleavage method

A chemical cleavage method for detecting mismatched bases in heteroduplexes formed between patient mRNA and control cDNA probes was employed to identify a single base mutation in a heterozygous case of osteogenesis imperfecta type IV. The parents' fibroblast mRNA did not contain the mutation. The region of the mRNA mismatch was amplified by using the polymerase chain reaction, cloned and sequenced. A point mutation of G to U at base-pair 2162 of the collagen alpha 2(I) mRNA resulted in the substitution of glycine by valine at amino acid position 586 of the helix. This substitution disrupted the critical Gly-Xaa-Yaa repeating unit of the collagen triple helix and resulted in helix destabilization, as evidenced by a decreased thermal stability. This local disturbance to helix propagation from the C-terminus to the N-terminus led to the overmodification of the collagen helix downstream towards the N-terminus. However, collagen secretion in vitro was normal, and the clinical phenotype probably resulted from the secretion into the extracellular matrix of the mutant collagen combined with a decrease in collagen production to 65% of control values. The rapid detection of the osteogenesis imperfecta mutation by using the chemical cleavage method afforded the opportunity to apply the technique to prenatal diagnosis in the next pregnancy of the mother of the osteogenesis imperfecta patient. The absence of a mismatched base in chorionic villus mRNA and control cDNA heteroduplexes indicated that the foetus did not carry the mutation, which was confirmed by the subsequent delivery of a normal baby.

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.

Osteogenesis imperfecta due to recurrent point mutations at CpG dinucleotides in the COL1A1 gene of type I collagen

Most individuals with osteogenesis imperfecta (OI) are heterozygous for dominant mutations in one of the genes that encode the chains of type I collagen. Each of the more than 30 mutations characterized to date has been unique to the affected member(s) of the family. We have determined that two individuals with a progressive deforming variety of OI, OI type III, have the same new dominant mutation [alpha 1(I)gly154 to arg] and that two unrelated infants with perinatal lethal OI, OI type II, share a second new dominant mutation [alpha 1(I)gly1003 to ser]. These mutations occurred at CpG dinucleotides, in a manner consistent with deamination of a methylated cytosine residue, and raise the possibility that CpG dinucleotides are common sites of recurrent mutations in collagen genes. Further, these findings confirm that the OI type-III phenotype, previously thought to be inherited in an autosomal recessive manner, can result from new dominant mutations in the COL1A1 gene of type-I collagen.