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Insight into Schmid metaphyseal chondrodysplasia from the crystal structure of the collagen X NC1 domain trimer.

Collagen X is expressed specifically in the growth plate of long bones. Its C1q-like C-terminal NC1 domain forms a stable homotrimer and is crucial for collagen X assembly. Mutations in the NC1 domain cause Schmid metaphyseal chondrodysplasia (SMCD). The crystal structure at 2.0 A resolution of the human collagen X NC1 domain reveals an intimate trimeric assembly strengthened by a buried cluster of calcium ions. Three strips of exposed aromatic residues on the surface of NC1 trimer are likely to be involved in the supramolecular assembly of collagen X. Most internal SMCD mutations probably prevent protein folding, whereas mutations of surface residues may affect the collagen X suprastructure in a dominant-negative manner.

Amino Acid Sequence↗

In vivo regulation of apoptosis in metaphyseal trabecular bone of young rats by synthetic human parathyroid hormone (1-34) fragment.

Osteoblast differentiation and function can be studied in situ in the metaphysis of growing long bones. Proliferation and apoptosis dominate in the primary spongiosa subjacent to the growth plate, and differentiation and function dominate in the proximal metaphysis. Apoptosis of osteocytes dominates at the termination of the trabeculae in diaphyseal marrow. As parathyroid hormone regulates all phases of osteoblast development, we studied the in vivo regulation by human parathyroid hormone (1-34) (PTH) of apoptosis in bone cells of the distal metaphysis of young male rats. Rats were given PTH at 80 microg/kg per day, once daily, for 1-28 days. Bone cells were defined for flow cytometry as PTH1-receptor-positive (PTH1R(+)) and growth factor-receptor-positive (GFR(+)) cells. Apoptotic cells stained positive for either TdT-mediated dUTP-X nick end labeling (TUNEL) or annexin V (annV(+)) were detected by either flow cytometry or immunohistochemistry. Apoptosis was also assessed at the tissue level by RNAse protection and caspase enzyme activity assays. PTH increased apoptotic osteoblasts in the proliferating zone and apoptotic osteocytes in the terminal trabecular zone, by 40%-60% within 2-6 days of PTH treatment, but values became equivalent to controls after 21-28 days of treatment. This transient increase was confirmed in PTH1R(+), GFR(+) bone cells isolated by flow cytometry. There was no detectable change in the steady-state mRNA levels of selected apoptotic genes. Starting at 3 days, at the tissue level, PTH inhibited activity of caspases, which recognize the DEVD peptide substrate (caspases 2, 3, and/or 7), but not those caspases recognizing LEHD or YVAD peptide sequences. We speculate that the localized and tissue level effects of PTH on apoptosis can be explained on the basis of its anabolic effect on bone. The transient increase in apoptosis in the proliferating zone and terminal trabecular zone may be the result of the increased activation frequency and bone turnover seen with daily PTH treatment. As once-daily PTH increases the number of differentiated osteoblasts, and as these and hematopoietic marrow cells dominate metaphyseal tissue, inhibition of caspase activity may contribute to their prolonged survival, enabling extension of trabecular bone into the diaphyseal marrow to increase bone mass.

Age Factors↗

In vivo comparison of activated protein-1 gene activation in response to human parathyroid hormone (hPTH)(1-34) and hPTH(1-84) in the distal femur metaphyses of young mice.

Intermittent parathyroid hormone (PTH) treatment increases bone mass in humans and animals. Although intact human PTH has 84 amino acids, the N-terminal 31 to 38 amino acids are sufficient for bone anabolic activity in vivo. Prior studies have evaluated hPTH(1-34) and hPTH(1-84) with respect to bone mass increase and quality, but there have been no in vivo comparisons of dose-dependent molecular responses. After confirming that young male BALB/c mice respond to daily PTH with increased bone mass, we profiled the steady-state mRNA levels of activating protein-1 (AP-1) genes regulated by hPTH(1-34) and hPTH(1-84) at doses ranging from 0 to 19.4 nmol/kg in the distal femur metaphyses. We selected AP-1 genes, which include jun and fos, as they play a fundamental role mediating signals for proliferation, differentiation, and apoptosis in cells of different origins, including bone, and are known to be regulated by PTH. Human PTH(1-34) and hPTH(1-84) increased steady-state mRNA expression of c-jun, junB, c-fos, and fra-2 in an equivalent dose- and time-dependent manner. Expression of fosB or fra-1 was not detected with either peptide. When averaged across dose and time, responses to hPTH(1-34) and hPTH(1-84) were not significantly different from each other. Expression of c-jun, junB, and c-fos peaked 30 minutes after the injection while fra-2 expression peaked 30 minutes later. All AP-1 genes stimulated by PTH returned to the levels of vehicle treated controls by 3 h after injection. The expression level of junD, which was abundant in the distal metaphysis, was not altered by either peptide. No change in magnitude was observed after 1, 3, or 7 days of once-daily subcutaneous treatment of either peptide. When individual comparisons for each dose between peptides were made, the minimum effective dose necessary to stimulate a significant increase in c-fos and junB expression was equivalent for both peptides. The minimum effective dose for hPTH(1-34) was at least tenfold lower than hPTH(1-84) in stimulating c-jun and fra-2 expression. Area under the curve for the highest dose (19.4 nmol/kg) of either peptide showed no significant differences in the expression of any of the genes. In conclusion, in young mice given once-daily subcutaneous injections up to 7 days, hPTH(1-34) and hPTH(1-84) induced equivalent responses by time and dose in the selected AP-1 genes. These data on molecular regulation in mouse bone confirm and extend prior data from rat studies showing equivalence on bone mass at equimolar doses.

Age Factors↗

A type X collagen mutation causes Schmid metaphyseal chondrodysplasia.

The expression of type X collagen is restricted to hypertrophic chondrocytes in regions undergoing endochondral ossification, such as growth plates. The precise function of type X collagen is unknown but the tissue-specific expression prompted us to examine the gene in hereditary disorders of cartilage and bone growth (osteochondrodysplasias). We have identified a 13 base pair deletion in one type X collagen allele segregating with autosomal dominant Schmid metaphyseal chondrodysplasia in a large Mormon kindred (lod score = 18.2 at theta = 0). The mutation produces a frameshift which alters the highly conserved C-terminal domain of the alpha 1(X) chain and reduces the length of the polypeptide by nine residues. This mutation may prevent association of the mutant polypeptide during trimer formation, resulting in a decreased amount of normal protein.

Alleles↗

Metaphyseal chondrodysplasia, type Jansen.

The Jansen type of metaphyseal dysplasia is a rare disorder with significant clinical and radiographic variability. Two cases of classical Jansen disease and one with some distinctive features suggestive of the Jansen variant are reported.

Child↗

Growth disturbance following metaphyseal bending fractures of the proximal tibia--an experimental study in the mini pig.

Based on our clinical experience, we postulate that the unilateral "posttraumatic genu valgum" develops on the grounds of an unreduced primary valgus deformity. This induces a disturbance of the consolidation on the medial aspect of the fracture. Subsequently, a partial medial stimulation of the epiphyseal plate develops resulting in a secondary valgus deformity. We succeeded in proving experimentally this postulate using the Mini Pig as an experimental model. We investigated 33 tibiae of 17 Mini Pigs. With radiological follow-up studies we were able to show that the operatively created primary valgus deformity induces an increased valgus deformity of functional significance. This is a model of the unreduced fracture in men. However, the transection of the pes anserinus and the periost per se did not provoke a significant valgus formation. Based on these experimental results and our clinical findings we postulate the following treatment for the metaphyseal bending fractures of the proximal tibia in men: 1. Accurate, usually conservative, reduction of any primary valgus malposition of the fracture. 2. Retention of the corrected position of the tibia in a plaster cast. 3. Compression of the medial aspect of the fracture to prevent disturbance of consolidation and subsequent development of valgus deformity.

Animals↗

[Fibrous metaphyseal defect--a stage- and age-dependent differential diagnosis].

Based on 57 fibrous metaphyseal defects the conventional radiologic signs are described as diagnostically reliable including asymptomatic patients as well as patients with a history of pain and lesions with positive scintigraphy. The radiologic morphology of the fibrous endosteal and medullary defects differs with the age of the patient. Beyond the age of 18 years there is a sclerosis of the lesion and a less marked arrosion of the corticalis. The differential diagnosis depends on the cortical, endosteal of medullary site of the fibrous defect as well as on the age of patient and includes the cortical metastasis, the juxta-articular osteoid-osteoma, the periosteal chondroma, the cortical irregularity, the solitary bone cyst, the fibrous dysplasia, the chronic osteomyelitis and the osteosclerotic osteosarcoma.

Aging↗

[Fibrous metaphyseal defect (fibrous cortical defect, non-ossifying fibroma). Paper II: differential diagnosis (author's transl)].

FMD, whether in the stage of a fibrous cortical defect or a non-ossifying fibroma, possesses very characteristic radiological appearances which rarely make it necessary to resort to biopsy. In order to avoid mistakes, it is necessary to observe strictly the known radiological features: metaphyseal position, clearcut relationships to the cortex, well defined margins, maximal size 6 to 7 cm., presence during growth, rarely observed in the upper extremity. The differential diagnosis, which needs to be considered only rarely, is discussed.

Adolescent↗

Type X collagen multimer assembly in vitro is prevented by a Gly618 to Val mutation in the alpha 1(X) NC1 domain resulting in Schmid metaphyseal chondrodysplasia.

Type X collagen is a homotrimer of alpha 1(X) chains encoded by the COL10A1 gene. It is a highly specialized extracellular matrix component, and its synthesis is restricted to hypertrophic chondrocytes in the calcifying cartilage of the growth plate and in zones of secondary ossification. Our studies on a family with Schmid metaphyseal chondrodysplasia demonstrated that the affected individuals were heterozygous for a single base substitution in the COL10A1 gene, which changed the codon GGC for glycine 618 to GTC for valine in the highly conserved region of the carboxyl-terminal NC1 domain and altered the amino acid sequence in the putative oligosaccharide attachment site. Since hypertrophic cartilage tissues or cell cultures were not available to assess the effect of the mutation, an in vitro cDNA expression system was used to study normal and mutant type X collagen biosynthesis and assembly. Full-length cDNA constructs of the normal type X collagen sequence and also cDNA containing the specific Gly to Val NC1 mutation found in the patient were produced and expressed by in vitro transcription and translation. While the control construct produced type X collagen, which formed trimeric collagen monomers and assembled into larger multimeric assemblies, the mutant collagen was unable to form these larger aggregates. These experiments demonstrated that the mutation disturbed type X collagen NC1 domain interaction and assembly, a finding consistent with the abnormal disorganized cartilage growth plate seen in the patient. These studies provide the first evidence of the effect of a type X collagen mutation on protein structure and function and directly demonstrate the critical role of interactions between NC1 domains in the formation of type X collagen multimeric structures in vitro.

Amino Acid Sequence↗

Interaction of collagen alpha1(X) containing engineered NC1 mutations with normal alpha1(X) in vitro. Implications for the molecular basis of schmid metaphyseal chondrodysplasia.

Collagen X is a short-chain homotrimeric collagen expressed in the hypertrophic zone of calcifying cartilage. The clustering of mutations in the carboxyl-terminal nonhelical NC1 domain in Schmid metaphyseal chondrodysplasia (SMCD) suggests a critical role for NC1 in collagen X structure and function. In vitro collagen X DNA expression, using T7-driven coupled transcription and translation, demonstrated that although alpha1(X) containing normal NC1 domains can form electrophoretically stable trimers, engineered SMCD NC1 missense or premature termination mutations prevented the formation of electrophoretically stable homotrimers or heterotrimers when co-expressed with normal alpha1(X). To allow the detection of more subtle interactions that may interfere with assembly but not produce SDS-stable final products, we have developed a competition-based in vitro co-expression and assembly approach. Our studies show that alpha1(X) chains containing SMCD mutations reduce the efficiency of normal alpha1(X) trimer assembly, indicating that interactions do occur between mutant and normal NC1 domains, which can impact on the formation of normal trimers. This finding has important implications for the molecular pathology of collagen X mutations in SMCD. Although we have previously demonstrated haploinsufficiency as one in vivo mechanism (Chan, D., Weng, Y. M., Hocking, A. M., Golub, S., McQuillan, D. J., and Bateman, J. F. (1998) J. Clin. Invest. 101, 1490-1499), the current study suggests dominant interference is also possible if the mutant protein is expressed in vivo. Furthermore, we establish that a conserved 13-amino acid aromatic motif (amino acids 589-601) is critical for the interaction between the NC1 domains, suggesting that this region may initiate assembly and the other NC1 mutations interfered with secondary interactions important in folding or in stabilizing the assembly process.

Base Sequence↗

Schmid's metaphyseal chondrodysplasia mutations interfere with folding of the C-terminal domain of human collagen X expressed in Escherichia coli.

Human collagen X contains a highly conserved 161-amino acid C-terminal non-triple helical domain that is homologous to the C-terminal domain of collagen VIII and to the C1q module of the human C1 enzyme. We have expressed this domain (residues 545-680) in Escherichia coli as a glutathione S-transferase fusion protein. The purified fusion protein trimerizes spontaneously in vitro, and after thrombin cleavage, the purified C-terminal domain trimer (46.2 kDa) is extremely stable and trypsin-resistant. Mutations within the C-terminal domain have been observed in patients with Schmid's metaphyseal chondrodysplasia (SMCD). Some of these mutations (Y598D, G618V, W651X, or H669X; X is the stop codon) were constructed by site-directed mutagenesis. Each mutation had identical consequences regarding the fusion protein: 1) absence of trimeric formation, 2) copurification of the approximately 60-kDa GroEL chaperone protein, and 3) sensitivity of the monomeric fusion protein to trypsin digestion. These results show that the C-terminal domain of collagen X is sufficient to produce a very stable and compact trimer in the absence of collagen Gly-X-Y repeats. Moreover, mutations causing SMCD interfere in this system with the correct folding of the C-terminal domain. The existence of a similar mechanism in chondrocytes might explain the relative homogeneity of phenotypes in SMCD despite the diversity of mutations.

Chaperonin 60↗

Metaphyseal chondrodysplasia type Schmid mutations are predicted to occur in two distinct three-dimensional clusters within type X collagen NC1 domains that retain the ability to trimerize.

Metaphyseal chondrodysplasia type Schmid (MCDS) is caused by mutations in COL10A1 that are clustered in the carboxyl-terminal non-collagenous (NC1) encoding domain. This domain is responsible for initiating trimerization of type X collagen during biosynthesis. We have built a molecular model of the NC1 domain trimer based on the crystal structure coordinates of the highly homologous trimeric domain of ACRP30 (adipocyte complement-related protein of 30 kDa or AdipoQ). Mapping of the MCDS mutations onto the structure reveals two specific clusters of residues as follows: one on the surface of the monomer which forms a tunnel through the center of the assembled trimer and the other on a patch exposed to solvent on the exterior surface of each monomeric unit within the assembled trimer. Biochemical studies on recombinant trimeric NC1 domain show that the trimer has an unusually high stability not exhibited by the closely related ACRP30. The high thermal stability of the trimeric NC1 domain, in comparison with ACRP30, appears to be the result of a number of factors including the 17% greater total buried solvent-accessible surface and the increased numbers of hydrophobic contacts formed upon trimerization. The 27 amino acid sequence present at the amino terminus of the NC1 domain, which has no counterpart in ACRP30, also contributes to the stability of the trimer. We have also shown that NC1 domains containing the MCDS mutations Y598D and S600P retain the ability to homotrimerize and heterotrimerize with wild type NC1 domain, although the trimeric complexes formed are less stable than those of the wild type molecule. These studies suggest strongly that the predominant mechanism causing MCDS involves a dominant interference of mutant chains on wild type chain assembly.

Amino Acid Sequence↗

Collagen X chains harboring Schmid metaphyseal chondrodysplasia NC1 domain mutations are selectively retained and degraded in stably transfected cells.

Collagen X is a short chain, homotrimeric collagen expressed specifically by hypertrophic chondrocytes during endochondral bone formation and growth. Although the exact role of collagen X remains unresolved, mutations in the COL10A1 gene disrupt growth plate function and result in Schmid metaphyseal chondrodysplasia (SMCD). With the exception of two mutations that impair signal peptide cleavage during alpha1(X) chain biosynthesis, SMCD mutations are clustered within the carboxyl-terminal NC1 domain. The formation of stable NC1 domain trimers is a critical stage in collagen X assembly, suggesting that mutations within this domain may result in subunit mis-folding or reduce trimer stability. When expressed in transiently transfected cells, alpha1(X) chains containing SMCD mutations were unstable and presumed to be degraded intracellularly. More recently, in vitro studies have shown that certain missense mutations may exert a dominant negative effect on alpha1(X) chain assembly by formation of mutant homotrimers and normal-mutant heterotrimers. In contrast, analysis of cartilage tissue from two SMCD patients revealed that the truncated mutant message was fully degraded, resulting in 50% reduction of functional collagen X within the growth plate. Therefore, in the absence of data that conclusively demonstrates the full cellular response to mutant collagen X chains, the molecular mechanisms underlying SMCD remain controversial. To address this, we closely examined the effect of two NC1 domain mutations, one frameshift mutation (1963del10) and one missense mutation (Y598D), using both semi-permeabilized cell and stable cell transfection expression systems. Although able to assemble to a limited extent in both systems, we show that, in intact cells, collagen X chains harboring both SMCD mutations did not evade quality control mechanisms within the secretory pathway and were degraded intracellularly. Furthermore, co-expression of wild-type and mutant chains in stable transfected cells demonstrated that, although wild-type chains were secreted, mutant chains were largely excluded from hetero-trimer formation. Our data indicate, therefore, that the predominant effect of the NC1 mutations Y598D and 1963del10 is a reduction in the amount of functional collagen X within the growth cartilage extracellular matrix.

Base Sequence↗

Misfolding of collagen X chains harboring Schmid metaphyseal chondrodysplasia mutations results in aberrant disulfide bond formation, intracellular retention, and activation of the unfolded protein response.

Collagen X is a short chain collagen expressed specifically by the hypertrophic chondrocytes of the cartilage growth plate during endochondral bone formation. Accordingly, COL10A1 mutations disrupt growth plate function and cause Schmid metaphyseal chondrodysplasia (SMCD). SMCD mutations are almost exclusively located in the NC1 domain, which is crucial for both trimer formation and extracellular assembly. Several mutations are expected to reduce the level of functional collagen X due to NC1 domain misfolding or exclusion from stable trimer formation. However, other mutations may be tolerated within the structure of the assembled NC1 trimer, allowing mutant chains to exert a dominant-negative impact within the extracellular matrix. To address this, we engineered SMCD mutations that are predicted either to prohibit subunit folding and assembly (NC1del10 and Y598D, respectively) or to allow trimerization (N617K and G618V) and transfected these constructs into 293-EBNA and SaOS-2 cells. Although expected to form stable trimers, G618V and N617K chains (like Y598D and NC1del10 chains) were secreted very poorly compared with wild-type collagen X. Interestingly, all mutations resulted in formation of an unusual SDS-stable dimer, which dissociated upon reduction. As the NC1 domain sulfhydryl group is not solvent-exposed in the correctly folded NC1 monomer, disulfide bond formation would result only from a dramatic conformational change. In cells expressing mutant collagen X, we detected significantly increased amounts of the spliced form of X-box DNA-binding protein mRNA and up-regulation of BiP, two key markers for the unfolded protein response. Our data provide the first clear evidence for misfolding of SMCD collagen X mutants, and we propose that solvent exposure of the NC1 thiol may trigger the recognition and degradation of mutant collagen X chains.

Amino Acid Sequence↗

Tissue-specific RNA surveillance? Nonsense-mediated mRNA decay causes collagen X haploinsufficiency in Schmid metaphyseal chondrodysplasia cartilage.

Mutations resulting in a premature termination codon (PTC) are a major cause of inherited disorders, and the majority of these mutant RNA transcripts are subjected to nonsense-mediated mRNA decay (NMD). This RNA surveillance results in reduced mutant allele expression, the extent of which can impact on the clinical severity. The molecular mechanisms of NMD in mammalian cells, its relationship to splicing and translation, downstream sequence elements and binding factors remains only partially understood. Currently there is little information on whether the extent of NMD is gene- or tissue-specific, although nonsense mutation inhibition of RNA splicing has been shown to exhibit some tissue and gene specificity in vitro. Schmid metaphyseal chondrodysplasia results from heterozygous mutations in the gene for collagen X (COL10A1), expressed by the hypertrophic chondrocytes of growth plate cartilage. In one patient a PTC mutation has been shown to result in complete NMD and collagen X haploinsufficiency in cartilage. Here we show that, in this patient, and in another with a different collagen X PTC mutation also leading to complete NMD in cartilage, the mutant mRNAs were not subjected to NMD in non-cartilage cells (lymphoblasts and bone cells). These data suggest that novel RNA surveillance mechanisms may exist in cartilage and that tissue specificity of NMD could be of importance in understanding the molecular pathology of nonsense mutations. Furthermore, the demonstration of collagen X haploinsufficiency in the second patient to be studied at the level of tissue expression, confirms that nonsense mutations leading to complete mutant collagen X mRNA degradation in cartilage is an important molecular cause of SMCD.

Cartilage↗

Indications for epiphyseal preservation in metaphyseal malignant bone tumors of children: relationship between image methods and histological findings.

We compared several image methods in the evaluation of the possible physeal effect in 47 osteosarcomas and 18 Ewing's sarcomas in children. The minimal follow-up was 3 years (range, 3-17). In the histological study, the physis was affected in 53% of the cases. We correlated the histological findings and the findings from the different image methods. There were more false-positive than false-negative results, and in the computed tomography and magnetic resonance imaging (MRI) studies, there were no false negatives. The accuracy of MRI (predictive positive value plus predictive negative value) was the best (90.3%), and it is the technique that we prefer. According to these findings, we can safely preserve the epiphysis in cases of metaphyseal tumors showing no contact between the tumor and the growth plate in the MRI images. If the tumor shows contact with part of the physis, it is also possible to preserve the epiphysis.

Adolescent↗

Congenital coxa vara: computed tomographic analysis of femoral retroversion and the triangular metaphyseal fragment.

Three patients with congenital coxa vara studied with two- and three-dimensional computed tomographic (2DCT and 3DCT) methods are reported. In all cases, the femoral retroversion was documented and subsequently corrected by proximal femoral osteotomy. In two patients with isolated coxa vara, the physeal-femoral neck angle was decreased as seen in slipped capital femoral epiphysis in adolescents. Our studies suggest that the triangular metaphyseal fragment reflects a Salter-Harris type II separation pattern through the defective femoral neck. The epiphysis and attached triangular fragment slip from the normal superoanterior portion of the neck in an inferior-posterior direction. The treating surgeon should be aware of the often marked femoral retroversion component present in severe congenital coxa vara. This knowledge allows surgical planning for corrective osteotomies that will better normalize hip mechanics. A combination of marked valgus and flexion with internal rotation of the distal fragment are required to fully correct the deformity.

Acetabulum↗

Treatment of nonunion of fractures in the epiphyseal-metaphyseal region of long bones.

Twenty-four consecutive patients with fracture nonunion in the metaphyseal-epiphyseal areas of long bones were surgically treated. Average time from injury to treatment of the nonunion was 10 months, and average follow-up time after surgical treatment was 29 months. Eight patients with infected nonunions had initial debridement procedures; three of these patients then had placement of external fixators and bone grafting. The remaining five patients and 13 others were then treated by open reduction and internal fixation alone or with the addition of autogenous cancellous bone grafting. Single or double plates and screws were used. Arthrolysis, joint manipulation, and intensive postsurgical exercises were considered necessary to regain joint function. One patient underwent a hemiarthroplasty, and two others underwent arthrodesis as the initial nonunion treatment. Twenty of the 21 patients not treated by arthrodesis or arthroplasty healed their fractures in an average time of 7 months. Fifty-two percent of the patients achieved good or excellent range of motion (ROM) of the contiguous joint, with 70% of the patients reporting no pain in this joint. These fractures have excellent intrinsic healing capability because they occur in anatomical regions with a normally abundant circulation. We recommend stable fixation, with the need for bone grafting only in defect nonunions, together with intra- and postoperative joint mobilization to obtain a satisfactory functional end result.

Adult↗