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Biomedical subjects

W A Horton

Publications and source records attributed to W A Horton.

At least 19 recordsLinked to original sources
PubMed1997

The fate of cartilage oligomeric matrix protein is determined by the cell type in the case of a novel mutation in pseudoachondroplasia.

We have identified a novel missense mutation in a pseudoachondroplasia (PSACH) patient in one of the type III repeats of cartilage oligomeric matrix protein (COMP). Enlarged lamellar rough endoplasmic reticulum vesicles were shown to contain accumulated COMP along with type IX collagen, a cartilage-specific component. COMP was secreted and assembled normally into the extracellular matrix of tendon, demonstrating that the accumulation of COMP in chondrocytes was a cell-specific phenomenon. We believe that the intracellular storage of COMP causes a nonspecific aggregation of cartilage-specific molecules and results in a cartilage matrix deficient in required structural components leading to impaired cartilage growth and maintenance. These data support a common pathogenetic mechanism behind two clinically related chondrodysplasias, PSACH and multiple epiphyseal dysplasia.

Achondroplasia↗
PubMed1997

Activation of Stat1 by mutant fibroblast growth-factor receptor in thanatophoric dysplasia type II dwarfism.

The achondroplasia class of chondrodysplasias comprises the most common genetic forms of dwarfism in humans and includes achondroplasia, hypochondroplasia and thanatophoric dysplasia types I and II (TDI and TDII), which are caused by different mutations in a fibroblast growth-factor receptor FGFR3 (ref. 1). The molecular mechanism and the mediators of these FGFR3-related growth abnormalities are not known. Here we show that mutant TDII FGFR3 has a constitutive tyrosine kinase activity which can specifically activate the transcription factor Stat1 (for signal transducer and activator of transcription). Furthermore, expression of TDII FGFR3 induced nuclear translocation of Stat1, expression of the cell-cycle inhibitor p21(WAF1/CIP1), and growth arrest of the cell. Thus, TDII FGFR3 may use Stat1 as a mediator of growth retardation in bone development. Consistent with this, Stat1 activation and increased p21(WAF1/CIP1) expression was found in the cartilage cells from the TDII fetus, but not in those from the normal fetus. Thus, abnormal STAT activation and p21(WAF1/CIP1) expression by the TDII mutant receptor may be responsible for this FGFR3-related bone disease.

Animals↗
PubMed1997

Skeletal development in transgenic mice expressing a mutation at Gly574Ser of type II collagen.

Skeletal development of transgenic mice with a type II collagen mutation was analyzed and compared with wild-type littermates. The single base substitution in Col2a1 resulted in a glycine to serine mutation within the helical domain and corresponded to one previously identified in a patient with the lethal human chondrodysplasia, hypochondrogenesis (Horton et al. [1992] Proc. Natl. Acad. Sci. U.S.A. 89:4583-4587). Skeletal staining of embryos from 14.5 through 18.5 days of gestation demonstrated a dwarf phenotype in the transgenic embryos, most notably short limb bones and vertebral column that was first detected at 15.5 days post-coitus. In addition to the reduced length, the extent of ossification was less in the transgenic mice. The architecture of the long bone growth plate was abnormal in the transgenic tissue, in particular there was no discernible proliferative zone. There were few stacks of characteristically flattened cells and the overall length of the growth plate in the mutant embryos was reduced. At the ultrastructural level, there were fewer collagen fibrils present in the transgenic mouse cartilage compared to that of wild-type littermates. Ultrastructural localization of collagen types II, IX and XI revealed a similar pattern between the transgenic and wild-type pups, suggesting that the collagen fibrils present in the matrix of littermates with both phenotypes had a similar composition. Skeletal analysis and cartilage histochemistry indicated that effect of the type II collagen mutation was to reduce the density of the collagen fibrils within the cartilage matrix which was associated with delayed bone formation and resulted in a short-limbed phenotype.

Animals↗
PubMed1997

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.

Amino Acid Sequence↗
PubMed1997

Fibroblast growth factor receptor 3 and the human chondrodysplasias.

Heterozygous mutations of the gene encoding the fibroblast growth factor receptor 3 (FGFR3) have been found in persons with achondroplasia, thanatophoric dysplasia, and hypochondroplasia. They exhibit considerable genetic homogeneity, and specific mutations strongly correlate with the clinical severity of disease. The mutations activate the FGFR3 by promoting dimerization, by stimulating intrinsic tyrosine kinase activity, and perhaps by altering ligand and dimerization specificity. The downstream signals regulate events in the growth plate, ultimately inhibiting linear bone growth.

Achondroplasia↗
PubMed1996

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.

Achondroplasia↗
PubMed1995

Molecular cloning of the alpha 3 chain of human type IX collagen: linkage of the gene COL9A3 to chromosome 20q13.3.

Type IX collagen is composed of three polypeptides derived from the human genes COL9A1, COL9A2, and COL9A3 that assemble to form a mature collagen molecule with the structure alpha 1(IX)alpha 2(IX)alpha 3(IX). We have identified overlapping cDNA and genomic clones that encode for the entire alpha 3 chain of human type IX collagen. Tryptic peptides from the human alpha 3(IX) collagen chain were subjected to N-terminal amino acid sequencing, and a stretch of 124 contiguous amino acids that included the NC1, COL1, and NC2 domains was obtained. Degenerate oligonucleotide primers were designed based on the amino acid sequences of the human tryptic peptides as well as bovine peptides and sequences from chicken cDNA clones. These primers were used to amplify three overlapping PCR products that covered the majority of the human alpha 3(IX) collagen. PCR products were then used to identify overlapping cDNA clones from a human chondrocyte library. A lambda genomic clone was identified that contained the 5'-most exon that encodes the signal peptide to complete the entire structure of the human alpha 3(IX) collagen chain. Genomic amplification identified a single-strand conformational polymorphism in COL1 that was used to map COL9A3 to chromosome 20q13.3 by linkage analysis. The present study completes the structure of human type IX collagen, and linkage for COL9A3 completes the genomic mapping of cartilage collagen genes. These data will greatly assist the genetic screening of families with degenerative cartilage and eye diseases by allowing investigators to screen for a complete set of candidate collagen gene markers.

Amino Acid Sequence↗
PubMed1995

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.

Achondroplasia↗
PubMed1995

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.

Amino Acid Sequence↗
PubMed1995

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.

Animals↗
PubMed1995

Achondroplasia is defined by recurrent G380R mutations of FGFR3.

Genomic DNA from 154 unrelated individuals with achondroplasia was evaluated for mutations in the fibroblast growth factor receptor 3 (FGFR3) transmembrane domain. All but one, an atypical case, were found to have a glycine-to-arginine substitution at codon 380. Of these, 150 had a G-to-A transition at nt 1138, and 3 had a G-to-C transversion at this same position. On the basis of estimates of the prevalence of achondroplasia, the mutation rate at the FGFR3 1138 guanosine nucleotide is two to three orders of magnitude higher than that previously reported for tranversions and transitions in CpG dinucleotides. To date, this represents the most mutable single nucleotide reported in the human genome. The homogeneity of mutations in achondroplasia is unprecedented for an autosomal dominant disorder and may explain the relative lack of heterogeneity in the achondroplasia phenotype.

Achondroplasia↗
PubMed1994

Extending the nosology of the chondrodysplasias to the cellular and molecular levels.

Rapid advances in molecular genetics have created and will continue to create problems in the classification of human bone dysplasias. It is proposed that the functional significance of mutations be taken into account in future nosologies for these disorders. Chondrocyte culture and transgenic mouse strategies are briefly discussed as approaches to evaluating the functional consequences of chondrodysplasia mutations on skeletel development and growth.

Animals↗
PubMed1994

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.

Animals↗
PubMed1993

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.

Cartilage↗
PubMed1993

Linkage of typical pseudoachondroplasia to chromosome 19.

Pseudoachondroplasia (PSACH) is an autosomal dominant dwarfing condition associated with disproportionate short stature, marked joint deformities, and early onset osteoarthritis. Previous linkage studies have excluded linkage to cartilage and noncartilagenous extracellular matrix candidate genes. Here, we report mapping the pseudoachondroplasia gene to chromosome 19. Maximum lod scores of 4.70, 4.15, and 4.86 at theta = 0.00 were found for D19S212, D19S215, and D19S49, respectively. Multipoint analysis suggests the following order: D19S253-D19S199-(D19S212/PSACH/D19S215)-++ +D19S222-D19S49.

Achondroplasia↗