Mutations in type I procollagen genes that cause osteogenesis imperfecta

Pyridinium cross-links in bone of patients with osteogenesis imperfecta: evidence of a normal intrafibrillar collagen packing

The brittleness of bone in patients with osteogenesis imperfecta (OI) has been attributed to an aberrant collagen network. However, the role of collagen in the loss of tissue integrity has not been well established. To gain an insight into the biochemistry and structure of the collagen network, the cross-links hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) and the level of triple helical hydroxylysine (Hyl) were determined in bone of OI patients (types I, III, and IV) as well as controls. The amount of triple helical Hyl was increased in all patients. LP levels in OI were not significantly different; in contrast, the amount of HP (and as a consequence the HP/LP ratio and the total pyridinoline level) was significantly increased. There was no relationship between the sum of pyridinolines and the amount of triple helical Hyl, indicating that lysyl hydroxylation of the triple helix and the telopeptides are under separate control. Cross-linking is the result of a specific three-dimensional arrangement of collagens within the fibril; only molecules that are correctly aligned are able to form cross-links. Inasmuch as the total amount of pyridinoline cross-links in OI bone is similar to control bone, the packing geometry of intrafibrillar collagen molecules is not disturbed in OI. Consequently, the brittleness of bone is not caused by a disorganized intrafibrillar collagen packing and/or loss of cross-links. This is an unexpected finding, because mutant collagen molecules with a random distribution within the fibril are expected to result in disruptions of the alignment of neighboring collagen molecules. Pepsin digestion of OI bone revealed that collagen located at the surface of the fibril had lower cross-link levels compared with collagen located at the inside of the fibril, indicating that mutant molecules are not distributed randomly within the fibril but are located preferentially at the surface of the fibril.

The Human Genome Project: implications for the treatment of musculoskeletal disease

The ultimate goal of the Human Genome Project is the determination of the molecular sequence of the entire human chromosomal complement. Realization of this goal will include characterization of all the genes that cause or predispose to disease, which will most certainly lead to the development of powerful new tools for diagnosis, prevention, and treatment in all medical fields, including orthopaedics. The authors review the fundamentals of human genetics and gene mapping, summarize the progress of the Human Genome Project thus far, and discuss the implications of this research as it relates to the treatment of musculoskeletal diseases.

Hydroxylation of collagen type I: evidence that both lysyl and prolyl residues are overhydroxylated in osteogenesis imperfecta

The composition of the collagens secreted into the media of fibroblast cultures of 39 patients with osteogenesis imperfecta (OI) was the same in controls and OI cultures. An abnormal migration pattern of collagens upon SDS-PAGE was evident in one third of the cultures investigated. Lysyl and prolyl hydroxylation of HPLC-purified alpha 1(I) chains was elevated in about 60% of cultures. The degree of hydroxylation was highest in the lethal forms. The extent of lysyl and prolyl hydroxylation showed a strong correlation (r = 0.74, P < 0.001). While high levels of hydroxylation are frequently observed in OI patients, a direct correlation between lysyl or prolyl hydroxylation and fracture rate or growth retardation could not be established.

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)

Genetic linkage of recessive dystrophic epidermolysis bullosa to the type VII collagen gene

Generalized mutilating recessive dystrophic epidermolysis bullosa (RDEB) is characterized by extreme skin fragility owing to loss of dermal-epidermal adherence. Immunohistochemical studies have implicated type VII collagen, the major component of anchoring fibrils, in the etiology of RDEB. In this study, we demonstrate genetic linkage of the type VII collagen gene and the generalized mutilating RDEB phenotype. We first identified a Pvull polymorphic site by digestion of an amplified product of the type VII collagen gene, which was shown to reside within the coding region. Genetic linkage analysis between this marker and the RDEB phenotype in 19 affected families which were informative for this polymorphism showed no recombination events, and gave a maximum lod score of 3.97 at a recombination fraction (theta) of 0, demonstrating that this DNA region is involved in this form of RDEB. These data provide strong evidence that the type VII collagen gene, which has also been linked with the dominant form of the disease, harbors the mutation(s) causing the generalized mutilating form of RDEB in these families, thus underscoring the major functional importance of type VII collagen in basement membrane zone stability.

Chondrodysplasia in transgenic mice harboring a 15-amino acid deletion in the triple helical domain of pro alpha 1(II) collagen chain

We have generated transgenic mice by microinjection of a 39-kb mouse pro alpha 1(II) collagen gene construct containing a deletion of exon 7 and intron 7. This mutation was expected to disturb the assembly and processing of the homotrimeric type II collagen molecule in cartilage. Expression of transgene mRNA at levels equivalent or higher than the endogenous mRNA in the offspring of two founder animals resulted in a severe chondrodysplastic phenotype with short limbs, hypoplastic thorax, abnormal craniofacial development, and other skeletal deformities. The affected pups died at birth due to respiratory distress. Light microscopy of epiphyseal growth plates of transgenic pups demonstrated a marked reduction in cartilaginous extracellular matrix and disruption of the normal organization of the growth plate. The zone of proliferating chondrocytes was greatly reduced whereas the zone of hypertrophic chondrocytes was markedly increased extending deep into the diaphysis suggestive of a defect in endochondral ossification. Electron microscopic examination revealed chondrocytes with extended RER, a very severe reduction in the amount of cartilage collagen fibrils, and abnormalities in their structure. We postulate that the deletion in the alpha 1(II) collagen acts as a dominant negative mutation disrupting the assembly and secretion of type II collagen molecules. The consequences of the mutation include interference with normal endochondral ossification. These mice constitute a valuable model to study the mechanisms underlying human chondrodysplasias and normal bone formation.

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.

Transgenic mice as models for heritable diseases

The development of methods for introduction of foreign DNA stably into genome of experimental animals has opened new possibilities to study the effects of mutations in complex gene-protein systems at the level of the entire organism. Information from such experiments is directly applicable to understanding of pathogenetic mechanisms in hereditary diseases and to designing of new therapeutic approaches. Techniques are currently available for studying both dominant mutations, introduced usually by microinjection, and recessive mutations introduced by homologous recombination employing the pluripotent embryonic stem cells. It remains to be emphasized, however, that the information which can be obtained with these techniques is optimal when sufficient background information is available on the protein system in question. The purpose of this review is to describe how the transgenic mouse methodology has increased our understanding of molecular basis of diseases both at the level of protein function and gene regulation.

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 exonic mutation in the HuP2 paired domain gene causes Waardenburg's syndrome

Here we report the identification and characterization of a gene defect causing Waardenburg's syndrome with hearing loss in a large Brazilian family. This demonstrates a mutation causing Waardenburg's syndrome as well as a mutation causing a form of congenital deafness. The mutation was found in the HuP2 gene, a member of the paired domain family of proteins that bind DNA and regulate gene expression. The mutation occurred in 100% of the cases with the disease in this family and was absent in a random sample of 50 unrelated control subjects. Identification of the Waardenburg's syndrome gene and future characterization of its gene product is likely to increase our understanding of the pathogenesis of this disorder and may allow prevention of deafness of this type.

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.