Col2-GFP reporter mouse--a new tool to study skeletal development

Transgenic mice were generated that harbor a Col2-GFP reporter that marks chondrocytes and their immediate precursors during skeletal development. Cells engaged in chondrogenesis were identified by conventional fluorescence microscopy and confocal optical sectioning within their native environments in live embryos and in thick tissue slices. The use of these mice offers a novel approach for studying the role of chondrocytes in skeletal development.

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.

Antiproliferative effects of insulin-like growth factor-binding protein-3 in mesenchymal chondrogenic cell line RCJ3.1C5.18. relationship to differentiation stage

Chondrogenesis results from a complex equilibrium between chondrocyte proliferation and differentiation. Insulin-like growth factors (IGFs) have a crucial role in chondrogenesis, but their mechanisms of action are not well defined. IGF-binding protein-3 (IGFBP-3) is the major carrier for circulating IGFs in postnatal life, and has been shown to have IGF-independent effects on proliferation of several cancer cell lines. In this study, we have evaluated the IGF-independent and -dependent effects of IGFBP-3 on chondrocyte proliferation and the relationship of these effects with chondrocyte differentiation stage. We used the RCJ3.1C5.18 nontransformed mesenchymal chondrogenic cell line, which, over 2 weeks of culture, progresses through the differentiation pathway exhibited by chondrocytes in the growth plate. We demonstrated that IGFBP-3 inhibited, in a dose-dependent manner (1-30 nm), the proliferation of chondroprogenitors and early differentiated chondrocytes, stimulated by des-(1-3)-IGF-I and longR(3)-IGF-I (IGF-I analogs with reduced affinity for IGFBP-3), and by insulin and IGF-I. In terminally differentiated chondrocytes, IGFBP-3 retained the ability to inhibit cell proliferation stimulated by IGF-I, but had no effect on cell growth stimulated by insulin, or des-(1-3)-IGF-I or longR(3)IGF-I. By monolayer affinity cross-linking, we demonstrated a specific IGFBP-3-associated cell-membrane protein of approximately 20 kDa. We determined that IGFBP-3 has an antiproliferative effect on chondrocytes and, that this effect is related to the differentiation process. In chondroprogenitors and early differentiated chondrocytes, antiproliferative effect of IGFBP-3 is mainly IGF-independent, whereas, following terminal differentiation this effect is IGF-dependent.

Col2-GFP reporter marks chondrocyte lineage and chondrogenesis during mouse skeletal development

Mice were generated in which a Col2-GFP transgene serves as a reporter for the chondrocyte lineage and for chondrogenesis in live embryos and newborn pups. Cells actively engaged in chondrogenesis were identified by confocal optical sectioning within their native environments in embryos and in thick tissue slices. Chondrocytes exhibiting GFP fluorescence were purified from rib cages by high-speed cell sorting of crude cell suspensions. Intensity of fluorescence correlated with biosynthesis of procollagen II in these cells. The use of these mice and their cells provides a novel approach for studying chondrocyte differentiation and chondrogenesis during skeletal development.

LMX1B transactivation and expression in nail-patella syndrome

Lmx1b, a member of the LIM homeodomain protein family, is essential for the specification of dorsal limb fates at the zeugopodal and autopodal level in vertebrates. We and others have shown that a skeletal dysplasia, nail-patella syndrome (NPS), results from mutations in LMX1B. While it is a unique mesenchymal determinant of dorsal limb patterning during vertebrate development, the mechanism by which LMX1B mutations generate the NPS phenotype has not been addressed at a transcriptional level or correlated with its spatial pattern of gene expression. In this study, in situ hybridizations of Lmx1b on murine limb sections reveal strong expression in dorsal mesenchymal tissues (precursors of muscle, tendons, joints and patella) and, interestingly, also in anterior structures of the limb, explaining the anterior to posterior gradient of joint and nail dysplasia observed in NPS patients. Transfection studies showed that both the LIM domain-interacting protein, LDB1, and the helix-loop-helix protein, E47/shPan1, can regulate LMX1B action. While co--transfections of E47/shPan1 with LMX1B result in a synergistic effect on reporter activity, LDB1 down-regulated LMX1B-mediated transactivation irrespective of E47/shPan1. Mutant LMX1B proteins containing human mutations affecting each of the helices or the N-terminal arm of the homeodomain abolished transactivation, while LIM B and truncation mutations retained residual activity. These mutations fail to act in a dominant-negative manner on wild-type LMX1B in mixing studies, thereby supporting haploinsufficiency as the mechanism underlying NPS pathogenesis.

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.

Differential effects of fibroblast growth factor (FGF) 9 and FGF2 on proliferation, differentiation and terminal differentiation of chondrocytic cells in vitro

Fibroblast growth factor (FGF) 9 was compared with FGF2 in its ability to influence proliferation, differentiation, terminal differentiation and apoptosis in a rat calvaria-derived cell line (RCJ 3.1C5.18) that spontaneously undergoes chondrocyte differentiation in vitro. Like FGF2, FGF9 promoted proliferation, but to a lesser extent. In contrast to FGF2, which blocked chondrocytic differentiation, FGF9 had no effect on differentiation but inhibited terminal differentiation. FGF9 also stimulated expression of the mitotic inhibitor p21 to a greater extent than FGF2. Neither ligand influenced apoptosis. The results indicate that FGF9 could account for many of the physiological responses attributed to FGF-receptor activation in the growth plate.

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.

Craniofacial and otic capsule abnormalities in a transgenic mouse strain with a Col2a1 mutation

Abnormal craniofacial features of a transgenic mouse model of chondrodysplasia with a type II collagen mutation (Gly574Ser) are described in this report. In addition to a shortened mandible and cleft palate, a misshapen otic capsule was observed. Interestingly, hearing impairment is often a component of the chondrodysplasia phenotype that results from mutations in COL2A1. To identify a potential mechanism in the hearing loss associated with type II collagen mutations, we examined the development of the otic capsule in the transgenic mice. It appeared to be smaller overall, relative to the skull proportions, and rather than the normal rounded dimensions, the transgenic capsule was flattened and elongated. We speculate that the cartilage of the developing otic capsule was less able to resist the mechanical forces from the developing brain and other tissues within the cranium and thus became deformed under pressure. We further speculate that the hearing loss associated with the chondrodysplasia phenotype is at least partially due to these defects in the developing cartilage matrix of the otic capsule.

Cloning, characterization, and chromosomal assignment of the human ortholog of murine Zfp-37, a candidate gene for Nager syndrome

In an effort to identify putative transcription factors involved in chondrocyte differentiation during human endochondral bone formation, a human fetal cartilage-specific cDNA library was screened with a degenerate oligonucleotide probe corresponding to a conserved stretch of eight amino acids from the zinc finger region of the Drosophila Krüppel gene family of DNA-binding proteins. Using this strategy, we have identified a novel zinc finger gene ZFP-37. ZFP-37 corresponds to a putative transcription factor containing 12 tandemly repeated zinc finger motifs and a Krüppel-associated box (KRAB) domain. The KRAB domain has been reported to function as a transcriptional repressor and is located in the amino terminus, while the zinc finger repeats are positioned at the carboxy-terminal end of ZFP-37. Gene mapping with a somatic cell hybrid panel and fluorescence in situ hybridization (FISH) localized ZFP-37 to human Chr 9q32. The gene is expressed at low level as a 3.2-kb mRNA in several tissues including fetal human cartilage. Sequence comparison revealed that ZFP-37 may represent the human homolog of the mouse gene Zfp-37. The map location and expression pattern suggest ZFP-37 as a candidate gene for a craniofacial-limb malformation, Nager syndrome (acrofacial dysostosis).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Exclusion of human proteoglycan link protein (CRTL1) and type II collagen (COL2A1) genes in pseudoachondroplasia

Patients with pseudoachondroplasia have a skeletal dysplasia with marked short stature. The most common cause of this condition is an autosomal dominant mutation, although autosomal recessive inheritance has been reported. Linkage to 2 cartilage-specific candidate genes, type II collagen (COL2A1) and proteoglycan link protein genes (CRTL1), was tested in 9 autosomal dominant families with pseudoachondroplasia. Tight linkage to these candidate genes was excluded with LOD scores for COL2A1 of -2.45 at theta = 0.05 and for CRTL1 of -7.28 at theta = 0.001. Discordant inheritance of the disease phenotype with each of these genes was also observed. Thus, these 2 candidate genes can be excluded as the cause of disease in these families.

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.

Xeroderma pigmentosum and Cockayne syndrome: overlapping clinical and biochemical phenotypes

Two siblings are described whose clinical presentation of cutaneous photosensitivity and central nervous system dysfunction is strongly reminiscent of the DeSanctis-Cacchione syndrome (DCS) variant of xeroderma pigmentosum. An extensive clinical evaluation supported a diagnosis of DCS and documented previously unreported findings. In vitro fibroblast studies showed UV sensitivity that was two to three times that of normal controls. However, neither a post-UV-irradiation DNA excision-repair defect indicative of XP nor a semiconservative DNA replication defect indicative of XP variant was found. Rather, a failure of RNA synthesis to recover to normal levels after UV exposure was observed, a biochemical abnormality seen in Cockayne syndrome (CS), one of the premature-aging syndromes with clinical UV sensitivity. These patients, therefore, clinically have XP, but their biochemical characteristics suggest CS. The reason(s) for the severe neurologic disease, in light of the relatively mild cutaneous abnormalities, is unclear. Other cases with unusual fibroblast responses to irradiation have been noted in the literature and, along with the data from our patients, reinforce the notion of the complexity of DNA maintenance and repair.

Growth hormone therapy in achondroplasia

A pilot study was carried out to examine the safety and efficacy of recombinant human growth hormone for growth-promoting therapy of achondroplasia. The data suggest that the agent in doses used to treat non-GH-deficient forms of short stature (0.3 mg/kg/wk) modestly increases overall height velocity in some children with achondroplasia. The effect was seen mainly in children with the lowest growth velocities prior to treatment. No untoward effects were noted. Several questions were raised that require further study.

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.

Factor analysis in patients with a history of failed tracheal intubation during pregnancy

Eight patients with a history of failed tracheal intubation during pregnancy were investigated by x-ray laryngoscopy after delivery. Partial elevation of the epiglottis with no view of glottic structures was found in five patients who were therefore considered to still present difficulty. In each of these five patients the blade tip failed to make contact with the hyoid and in four this was explained by the tongue being compressed into a pear shape such that it prevented sight of the larynx. Relatively few abnormal anatomical indices were seen in these patients and this was in keeping with the level of difficulty encountered. An angular measure of jaw protrusion from a line joining the upper incisors and a point just above and anterior to the vocal cords, to the mid-point on the inner surface of the mandible was useful: the lower angle of this triangle was as important as the angle at the incisors.

Factor analysis in difficult tracheal intubation: laryngoscopy-induced airway obstruction

We have studied eight patients with a history of difficult tracheal intubation, using x-ray laryngoscopy and local anaesthesia, a curved Macintosh blade and a standard intubating position. The view obtained was better than recorded previously during general anaesthesia in two patients, and in a third the x-ray showed that positioning the blade tip beneath the epiglottis would have improved vision, suggesting that reproducibility of the assessment may not be consistent. The "ease of intubation" and "complementary" angles may be helpful in the assessment of such patients. A "peardrop" effect is described whereby during laryngoscopy, the epiglottis became pressed against the posterior pharyngeal wall as a result of tongue compression. In the absence of muscle paralysis, removal of the blade caused immediate correction. However, during anaesthesia with neuromuscular block it is suggested that this not only occurs more readily but, may not correct when the blade is removed. Iatrogenic airway obstruction during moderately difficult tracheal intubation may be common and should be anticipated.

Human ceruloplasmin. Tissue-specific expression of transcripts produced by alternative splicing

Ceruloplasmin (CP) is a plasma glycoprotein that transports copper throughout the body. In previous studies (Yang, F., Naylor, S., Lum, J., Cutshaw, S., McCombs, J., Naberhaus, K., McGill, J., Adrian, G., Moore, C., Barnett, D., and Bowman, B. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 3277-3261), two CP cDNA clones, CP-1 and CP-2, from a human cDNA library, differed from each other by the presence or absence, respectively, of 12 nucleotide bases encoding a deduced sequence of Gly-Glu-Tyr-Pro near the carboxyl-terminal region of the ceruloplasmin molecule. Examination of genomic DNA demonstrates that the two CP mRNAs are produced from a single gene by alternative spliced patterns. The additional amino acids deduced in CP-1 are products of alternative splicing within an intron of the CP gene at a site 12 nucleotide bases 3' to the commonly used site of CP-2. The CP-1 mRNA transcript encoding four extra amino acids appeared as a minor species accompanying CP-2 mRNA in placenta and chondrocytes. CP-1 mRNA was the predominant CP transcript in a lymphoblastic leukemia cell line, CEM. The mRNA examined from other tissues contained only CP-2 mRNA transcripts. These findings predict that alternative RNA splicing may lead to the differential expression of CP genomic sequences and produce alternate isoforms from a single CP gene in specific tissues.

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.

Disposition of cervical vertebrae, atlanto-axial joint, hyoid and mandible during x-ray laryngoscopy

Ten healthy volunteers underwent direct laryngoscopy using topical anaesthesia and a curved Macintosh laryngoscope blade. A lateral x-ray was performed during laryngoscopy in a standard intubating position. In this position the lower neck was relatively straight and increasing curvature occurred from the mid cervical spine upwards. Extension at the atlanto-axial joint was probably near maximum; this has implications for conditions associated with laxity of this joint. The position of the mandible was influenced by the direction of pull on the laryngoscope handle. The hyoid was drawn forward and its body tilted downwards so as to lie halfway between the lower border of the mandible and the glottis. The relative length of epiglottis projecting above the hyoid was variable.

Tissue and cell studies of the growth plate in the chondrodysplasias

As the morphologic expression of the chondrocytic differentiation pathway responsible for bone development and growth, the growth plate has been investigated extensively in the chondrodysplasias. Unique morphologic abnormalities identified in many disorders have provided insight into pathogenetic mechanisms and have been useful diagnostically and nosologically. Biochemical studies have detected evidence of type II collagen defects in patients having disorders in the achondrogenesis type II-spondyloepiphyseal dysplasias (SED) congenita family of chondrodysplasias. Most promising may be the cell culture systems now being developed for human chondrocytes. Preliminary results suggest that they will allow the cellular and molecular biology of the dysplastic growth plate to be directly analyzed.

Expression of the human chondrocyte phenotype in vitro

We report a culture scheme in which human epiphyseal chondrocytes lose their differentiated phenotype in monolayer and subsequently reexpress the phenotype in an agarose gel. The scheme is based on a method using rabbit chondrocytes. Culture in monolayer allowed small quantities of cells to be amplified and provided a starting point to study expression of the differentiated human chondrocyte phenotype. The cells cultured in monolayer produced type I procollagen, fibronectin, and small noncartilaginous proteoglycans. Subsequent culture in agarose was associated with the acquisition of typical chondrocyte ultrastructural features and the synthesis of type II collagen and cartilage-specific proteoglycans. The switch from the nonchondrocyte to the differentiated chondrocyte phenotype occurred under these conditions between 1 and 2 wk of agarose culture and was not necessarily homogeneous throughout a culture. This culture technique will facilitate direct investigation of human disorders of cartilage that have been addressed in the past by alternative approaches.

Growth of the foramen magnum in achondroplasia

Foramen magnum growth curves in achondroplasia and in the general population are presented. The achondroplastic foramen magnum is small at birth, and during the first year it has a severely impaired rate of growth especially in the transverse dimension. This markedly diminished growth results not only from abnormal endochondral bone growth but also because of abnormal placement and premature fusion of the synchondroses. Evaluation of the foramen magnum in achondroplasia should address absolute size of the transverse and sagittal dimensions, shape, and growth centers to determine growth potential of this area.

Defining a standard intubating position using "angle finder"

Study of patients who exhibit only limited morphological abnormality yet present difficulty with direct laryngoscopy is facilitated by a standard intubating position. The "Angle Finder" instrument allows implementation of a simple reproducible geometric standard which is applied easily in formal research work and in clinical practice and teaching. The proposed standard relates to the curved (Macintosh) laryngoscope blade and a supine patient. The lower neck flexion is 35 degrees and extension of the plane of the face 15 degrees, each angle measured relative to horizontal. Initially, the standard was derived from a review of the literature, then validated in a study of the intubating practices of 10 senior anaesthetists. A more detailed study of 10 normal volunteers confirmed reproducibility and, for nine patients with a history of difficult direct laryngoscopy, the standard was shown to be appropriate.

Obesity in achondroplasia

Obesity is a significant and potentially serious health problem in achondroplasia. Body mass indices, weight-to-square of the height ratio (W/H2), and triceps skinfold measurements show that obesity is common. It begins in early childhood and is prevalent at all ages. We recommend that weight be monitored closely in all persons with achondroplasia and that dietary intervention be instituted whenever the body mass indices, W/H2, and triceps skinfold measurements exceed the 95th centile for the general population.

Neurological basis of respiratory complications in achondroplasia

Evaluation of 32 individuals with achondroplasia revealed that 28% had a history of apnea and 22% had respiratory abnormalities on polysomnography. In those patients requiring posterior fossa decompressive surgery, improvement was noted in follow-up polysomnograms. Multimodality studies suggested that brainstem compression was common in achondroplasia and could account in part for the abnormal respiratory function in this disorder, including obstructive apnea, central apnea, and hypoxemia.

An additional tactile test. Further developments in tactile tests to confirm laryngeal placement of tracheal tubes

An additional clinical test to confirm laryngeal placement of tracheal tubes is described. Using the new test, placement was confirmed in all of 50 patients studied in whom difficulty would have been anticipated using previously described tactile tests (male patients with lower molar teeth). Two anaesthetists with small hands averaged 98% confirmations in two series each of 100 consecutive unselected intubations. A simple modification of the tests enables their application after nasotracheal intubation; even with small hands, a confirmation rate of 96% in 50 consecutive cases was found. The three tactile tests are reviewed and analysed. In the authors' combined experience of 14 cases of difficult laryngoscopy the tests gave reliable confirmation in 12 patients. Familiarity with these tests is stressed to be important for their reliable implementation.