Normal long bone growth and development in type X collagen-null mice.

To investigate the role of type X collagen in skeletal development, we have generated type X collagen-null mice. Surprisingly, mice without type X collagen were viable and fertile and had no gross abnormalities in long bone growth or development. No differences were detected between the type X collagen-null mice and controls when growth plates of both newborn and 3-week old mice were examined by histology and by immunostaining for extracellular matrix components of bone including osteopontin, osteocalcin and type II collagen. Our results suggest that type X collagen is not required for long bone development. However, mice and humans with dominant acting type X collagen mutations have bone abnormalities, suggesting that only the presence of abnormal type X collagen can modify bone growth and development.

Cloning of the putative tumour suppressor gene for hereditary multiple exostoses (EXT1).

Hereditary multiple exostoses is an autosomal dominant disorder that is characterized by short stature and multiple, benign bone tumours. In a majority of families, the genetic defect (EXT1) is linked to the Langer-Giedion syndrome chromosomal region in 8q24.1. From this region we have cloned and characterized a cDNA which spans chromosomal breakpoints previously identified in two multiple exostoses patients. Furthermore, the gene harbours frameshift mutations in affected members of two EXT1 families. The cDNA has a coding region of 2,238 bp with no apparent homology to other known gene sequences and thus its function remains elusive. However, recent studies in sporadic and exostosis-derived chondrosarcomas suggest that the 8q24.1-encoded EXT1 gene may have tumour suppressor function.

A recurrent mutation in the tyrosine kinase domain of fibroblast growth factor receptor 3 causes hypochondroplasia.

Hypochondroplasia (MIM 146000) is an autosomal dominant skeletal dysplasia with skeletal features similar to but milder than those seen in achondroplasia. Within the past year, the achondroplasia locus has been mapped to 4p 16.3 (refs 5-7) and mutations in the fibroblast growth factor receptor 3 (FGFR3) gene have been identified in patients with the disorder. More than 95% of 242 cases reported so far are accounted for by a single Gly380Arg mutation. McKusick et al. proposed that achondroplasia and hypochondroplasia are allelic based on the similarities in phenotype between the two disorders and the identification of a severely dwarfed individual whose father had achondroplasia and whose mother had hypochondroplasia. There is also genetic linkage evidence that hypochondroplasia and achondroplasia map to the same locus. We therefore began a systematic screening of FGFR3 to detect mutations in patients with hypochondroplasia. We now report a single FGFR3 mutation found in 8 out of 14 unrelated patients with hypochondroplasia. This mutation causes a C to A transversion at nucleotide 1620, resulting in an Asn540Lys substitution in the proximal tyrosine kinase domain.

Von Hippel-Lindau disease: clinical and pathological manifestations in nine families with 50 affected members.

Fifty individuals in nine families had von Hippel-Lindau disease. Nearly all of the morbidity and mortality of the entity is associated with six of its manifestations, each of which can be successfully treated. Retinal angiomatosis, which occurs in more than half of those affected, can produce blindness if not treated. Cerebellar hemangioblastoma, which is observed in one third of patients, is the most common source of initial symptoms and caused more than half of the deaths in the series. Medullary and spinal hemangioblastomas occur infrequently. Pheochromocytoma is common in certain families and is usually bilateral. Renal cell carcinoma, which generally arises at a later age, may befall the patient who is successfully treated for the tumors that occurred earlier. However, this tumor can be treated also, if there is early detection.

Skeletal dysplasia and defective chondrocyte differentiation by targeted overexpression of fibroblast growth factor 9 in transgenic mice.

Mutations in fibroblast growth factor receptor 3 (FGFR3) cause several human chondrodysplasias, including achondroplasia, the most common form of dwarfism in humans. From in vitro studies, the skeletal defects observed in these disorders have been attributed to constitutive activation of FGFR3. Here we show that FGF9 and FGFR3, a high-affinity receptor for this ligand, have similar developmental expression patterns, particularly in areas of active chondrogenesis. Targeted overexpression of FGF9 to cartilage of transgenic mice disturbs postnatal skeletal development and linear bone growth. The growth plate of these mice exhibits reduced proliferation and terminal differentiation of chondrocytes similar to that observed in the human disorders. The observations provide evidence that targeted, in vivo activation of endogenous FGFR3 inhibits bone growth and demonstrate that signals derived from FGF9-FGFR3 interactions can physiologically block endochondral ossification to produce a phenotype characteristic of the achondroplasia group of human chondrodysplasias.

Type II collagen pro-alpha-chains containing a Gly574Ser mutation are not incorporated into the cartilage matrix of transgenic mice.

The biochemical consequences of a type II procollagen mutation that contained a Gly574Ser amino acid substitution were analyzed in a transgenic mouse strain. The mutation correlated with one previously characterized in a patient with the lethal human chondrodysplasia, hypochondrogenesis (Horton et al., 1992), and resulted in a similar shortlimbed phenotype. There were fewer collagen fibrils present in the transgenic cartilage and reduced immunofluorescence of cartilage matrix using a type II collagen antibody. Pepsin-extracted collagen from transgenic mouse embryo cartilage was analyzed electrophoretically and indicated less type II as well as type XI collagen compared to their wild-type littermates. A pulse-chase experiment was performed to evaluate the biosynthesis and fate of type II collagen. Chondrocytes isolated from transgenic tissue synthesized fewer stable molecules, resulting in decreased secretion of the procollagen chains. By amino acid sequence analysis of the type II collagen peptides from cartilage of transgenic mouse embryos, serine was not detected at residue 574, the site mutated in the transgene. Based on sequence data, we believe that the molecules incorporated into collagen fibrils of the extracellular matrix, while fewer in number, were composed of normal alpha 1(II) chains.

Selective intracellular retention of extracellular matrix proteins and chaperones associated with pseudoachondroplasia.

Mutations in the cartilage oligomeric matrix protein (COMP) gene result in pseudoachondroplasia (PSACH), which is a chondrodysplasia characterized by early-onset osteoarthritis and short stature. COMP is a secreted pentameric glycoprotein that belongs to the thrombospondin family of proteins. We have identified a novel missense mutation which substitutes a glycine for an aspartic acid residue in the thrombospondin (TSP) type 3 calcium-binding domain of COMP in a patient diagnosed with PSACH. Immunohistochemistry and immunoelectron microscopy both show abnormal retention of COMP within characteristically enlarged rER inclusions of PSACH chondrocytes, as well as retention of fibromodulin, decorin and types IX, XI and XII collagen. Aggrecan and types II and VI collagen were not retained intracellularly within the same cells. In addition to selective extracellular matrix components, the chaperones HSP47, protein disulfide isomerase (PDI) and calnexin were localized at elevated levels within the rER vesicles of PSACH chondrocytes, suggesting that they may play a role in the cellular retention of mutant COMP molecules. Whether the aberrant rER inclusions in PSACH chondrocytes are a direct consequence of chaperone-mediated retention of mutant COMP or are otherwise due to selective intracellular protein interactions, which may in turn lead to aggregation within the rER, is unclear. However, our data demonstrate that retention of mutant COMP molecules results in the selective retention of ECM molecules and molecular chaperones, indicating the existence of distinct secretory pathways or ER-sorting mechanisms for matrix molecules, a process mediated by their association with various molecular chaperones.

Molecular genetics of the human chondrodysplasias-1995.

A number of gene loci have recently been shown to harbor mutations that cause human chondrodysplasias. They encode proteins that occupy cartilage matrix, such as types II, IX, X and XI collagens and COMP, that transduce signals in the growth plate, i.e., FGFR3 and PTHrP receptor, that influence the transport and metabolism of sulfate ions in relevant cells, e.g., DTDST and arylsulfatase E, and that regulate transcription of other genes, such as SOX9. Mutations at two loci, COL2A1 and FGFR3, account for most patients with chondrodysplasias--those with spondyloepiphyseal dysplasia and the achondroplasia classes of disorders, respectively. Mutations in the former tend to be dispersed throughout the gene and in other functionally related genes, whereas mutations in the latter are restricted to a few codons that seem to be very mutable.

Familial motor neuron disease. Evidence for at least three different types.

Based on a clinical, pathologic, and genetic study of 14 families, at least three types of familial motor neuron disease can be distinguished, all apparently of autosomal dominant transmission. The first is characterized by rapid, progressive loss of motor function with predominantly lower motor neuron manifestations and a course lasting less than 5 years. Pathologic changes are limited to the anterior horn cells and pyramidal tracts. The second type is clinically identical to the first, but at autopsy additional changes are found in the posterior columns, Clarke's column, and spinocerebellar tracts. The third type is characterized by a much longer survival usually beyond 10 and after more than 20 years in affected family members but is otherwise similar to the second type.

Abnormal ossification in thanatophoric dysplasia.

Thanatophoric dysplasia (TD) is a lethal human bone dysplasia characterized by severe dwarfism. It pathogenesis is thought to involve an abnormal ossifying fibrous tissue that disrupts the skeletal growth plate. We employed a combined morphologic, immunohistochemical and biochemical approach to better define the nature of this tissue in growth plate cartilage from 15 TD fetuses and infants in whom the abnormality was observed. The tissue was organized into tufts comprised of a cap of interstitial connective tissue, a transition region containing preosteoblastic cells near the cap and osteoblastic cells near the base, and a mineralized base which formed the subchondral bone trabeculae. The morphology of the cells and the presence of type I collagen in all regions of the tufts suggested that they were foci of membraneous ossification. However, elements of cartilage matrix (type II collagen, proteoglycan, link protein) were identified in the pericellular matrix of the osteoblastic cells and adjacent osteocytic cells. These observations suggest that a peculiar form of ossification occurs in the TD growth plate and may be involved in the pathogenesis of the disorder.

Bovine achondrogenesis: evidence for defective chondrocyte differentiation.

A survey study of growth cartilage abnormalities in bovine bone dysplasias revealed that a disorder in Holstein cattle called bulldog calf closely resembles human achondrogenesis Type II. Substantial amounts of Type I collagen and other non Type II collagens were detected in the bulldog cartilage which was comprised primarily of extensive vascular canals and cells having the characteristics of hypertrophic and degenerative chondrocytes normally found in the growth plate. It is proposed that chondrocytes throughout the bulldog growth cartilage prematurely differentiate into hypertrophic cells that degenerate and predispose the cartilage to vascular invasion and the formation of cartilage canals. The presence of these canals probably accounts for most of the observed collagen abnormalities.

Molecular genetic basis of the human chondrodysplasias.

Considerable progress has been made in delineating the molecular genetic basis of the human chondrodysplasias. Two genes emerge as harboring mutations found in patients with the most common disorders. Mutations in the type II collagen gene account for most spondyloepiphyseal dysplasia and spondyloepiphyseal dysplasia-like clinical disorders, whereas mutations in the fibroblast growth factor receptor 3 gene are responsible for achondroplasia, thanatophoric dysplasia, and hypochondroplasia. A substantial portion of remaining patients have mutations of the genes encoding cartilage oligomeric matrix protein or diastrophic dysplasia sulfate transporter.

Immunohistochemistry of types I and II collagen in undecalcified skeletal tissues.

Types I and II collagen were demonstrated in semithin sections of undecalcified human endochondral growth plate, articular cartilage, and subchondral bone. The effects of several different methods for fixation, embedding, exposing of antigenic determinants, and immunoperoxidase staining were examined. Fixation in buffered formalin and paraformaldehyde-lysine-periodate solution gave more intense staining for collagens than fixation in paraformaldehyde-gluaraldehyde or Bouin's solution. Specimens embedded in Spurr epoxy resin yielded intense and uniform staining of areas known to contain the particular collagens after the resin had been removed by sodium ethoxide. The staining was enchanced following enzymatic digestion, especially with protease V (Sigma). Staining sensitivity and specificity were comparable with the indirect conjugate and double peroxidase-antiperoxidase (PAP) techniques; the PAP method was less sensitive. Embedment in methacrylate resins proved unsatisfactory because of exaggerated immunostaining of mineralized sites in comparison to unmineralized areas of the same tissues. In the growth plate specimens, type I collagen was identified in the matrices of bone, periosteum, perichondrium, and in the cytoplasm of hypertrophic and degenerative chondrocytes. Type II collagen was found uniformly throughout the cartilage matrix and in spicules of unresorbed cartilage matrix located in subchondral bone. A similar staining pattern was observed for the articular cartilage, except that type I collagen was not detected in chondrocytes.

Chondrocyte differentiation in a rat mesenchymal cell line.

We used a combination of morphologic and histochemical methods to demonstrate that rat calvaria-derived mesenchymal cells, RCJ 3.1C5. 18, in culture progress through the differentiation pathway exhibited by chondrocytes in the endochondral growth plate. The cells were grown either as monolayer or suspension cultures. Subconfluent monolayer cultures did not express markers typical of chondrocyte phenotypes. However, after reaching confluency the cells formed nodules of chondrocytic cells separated by cartilage-appearing matrix and encapsulated by fibroblast-like cells. Suspension culture produced cell aggregates with similar characteristics. Matrix in both the nodules and aggregates stained for collagen Types II and XI and aggrecan, and some cells displayed a distinctive pericellular matrix that stained for Type X collagen. Mineralization was evident in older cultures. By electron microscopy, most cells in the aggregates appeared as typical chondrocytes. However, some larger cells were surrounded by a "mat" of matrix comprised of hexagonal arrays of dense nodules interconnected by a filamentous network. Immunogold localization confirmed the presence of collagen Type X in this matrix. Analysis of markers of chondrocyte differentiation and terminal differentiation over time showed that these markers were acquired sequentially over 2 weeks of culture. This model system will be useful to study the regulation of various steps in the chondrocyte differentiation pathway.

In vitro chondrogenesis in human chondrodysplasias.

Bone morphogenesis depends on the sequential expression of multiple genes that first allow formation of mesenchymal anlagen, their replacement by cartilage models, and finally, the synthesis of new bone at growth plates. These processes require orchestration of synthesis of multiple collagens, proteoglycans and glycoproteins, the genes for many of which have been isolated and are being studied. Several genes carry mutations which are responsible for the chondrodysplasia phenotypes and can be studied at the gene level or by examining their expression in cultured chondrocytes.

Type II collagen screening in the human chondrodysplasias.

Abnormalities of type II collagen have been considered strong candidates for causing human condrodysplasias. We have employed peptide mapping to screen for several types of type II colagen abnormalities in cartilage samples from 66 patients with 20 separate disorders. Except for achondrogenesis type II (Langer-Saldino) and spondyloepiphyseal dysplasia (SED) congenita in which abnormalities have been described and diastrophic dysplasia in which the changes were probably secondary, no abnormalities were detected. Within the limitations of the screening technique, the results combined with other data from the literature suggest that abnormalities of this molecule are not common causes of chondrodysplasias outside of the achondrogenesis type II-SED congenita family of disorders.

Discordance for the Kleeblattschädel anomaly in monozygotic twins with thanatophoric dysplasia.

We describe monozygotic twins with thanatophoric dysplasia who were discordant for the Kleeblattschädel anomaly, but were identical in all other respects, including the histologic appearance of the endochondral growth plate. The observation suggests that the skull anomaly represents a variable manifestation of thanatophoric dysplasia, and that its occurrence in an infant with this bone dysplasia should not alter the low recurrence risk generally given.

Brachyolmia, recessive type (Hobaek): a clinical, radiographic, and histochemical study.

Brachyolmia recessive type (Hobaek) is a rare bone dysplasia characterized by short trunk dwarfism that becomes evident during childhood. The radiographic manifestations are primarily limited to the spine, and consist of universal platyspondyly with lateral extension of the vertebral bodies beyond the pedicles and irregularity of the vertebral end plates. Histologic changes seen on growth plate biopsy permit the diagnosis to be confirmed. The inheritance pattern appears to be autosomal-recessive.

A new X-linked syndrome comprising progressive basal ganglion dysfunction, mental and growth retardation, external ophthalmoplegia, postnatal microcephaly and deafness.

We report on 4 boys (3 in one family) who have a remarkably constant syndrome of childhood-onset choreoathetosis with later spasticity, postnatal microcephaly, growth and mental retardation, apparent external ophthalmoplegia and varying degrees of deafness. The pedigrees are consistent with X-linked inheritance. The syndrome is compared and contrasted with others comprising basal ganglion dysfunction in childhood. It is concluded that clinically and genetically the condition is unique.

Cognitive and motor skills in achondroplastic infants: neurologic and respiratory correlates.

Thirteen infants with achondroplasia underwent psychometric testing as part of a comprehensive neurologic assessment. As a group, mental development was average and motor development was delayed, although a wide range of scores was obtained. Foramen magnum measurements were correlated with respiratory dysfunction, abnormal somatosensory evoked potentials, and delayed motor development. Abnormal polysomnogram outcome was associated with reduced mental capacity. In light of the reported increased frequency of respiratory dysfunction in achondroplasia, these findings warrant careful attention and further study.