A type X collagen mutation causes Schmid metaphyseal chondrodysplasia

The unfolded protein response and its relevance to connective tissue diseases

The unfolded protein response (UPR) has evolved to counter the stresses that occur in the endoplasmic reticulum (ER) as a result of misfolded proteins. This sophisticated quality control system attempts to restore homeostasis through the action of a number of different pathways that are coordinated in the first instance by the ER stress-senor proteins IRE1, ATF6 and PERK. However, prolonged ER-stress-related UPR can have detrimental effects on cell function and, in the longer term, may induce apoptosis. Connective tissue cells such as fibroblasts, osteoblasts and chondrocytes synthesise and secrete large quantities of proteins and mutations in many of these gene products give rise to heritable disorders of connective tissues. Until recently, these mutant gene products were thought to exert their effect through the assembly of a defective extracellular matrix that ultimately disrupted tissue structure and function. However, it is now becoming clear that ER stress and UPR, because of the expression of a mutant gene product, is not only a feature of, but may be a key mediator in the initiation and progression of a whole range of different connective tissue diseases. This review focuses on ER stress and the UPR that characterises an increasing number of connective tissue diseases and highlights novel therapeutic opportunities that may arise.

Targeted induction of endoplasmic reticulum stress induces cartilage pathology

Pathologies caused by mutations in extracellular matrix proteins are generally considered to result from the synthesis of extracellular matrices that are defective. Mutations in type X collagen cause metaphyseal chondrodysplasia type Schmid (MCDS), a disorder characterised by dwarfism and an expanded growth plate hypertrophic zone. We generated a knock-in mouse model of an MCDS-causing mutation (COL10A1 p.Asn617Lys) to investigate pathogenic mechanisms linking genotype and phenotype. Mice expressing the collagen X mutation had shortened limbs and an expanded hypertrophic zone. Chondrocytes in the hypertrophic zone exhibited endoplasmic reticulum (ER) stress and a robust unfolded protein response (UPR) due to intracellular retention of mutant protein. Hypertrophic chondrocyte differentiation and osteoclast recruitment were significantly reduced indicating that the hypertrophic zone was expanded due to a decreased rate of VEGF-mediated vascular invasion of the growth plate. To test directly the role of ER stress and UPR in generating the MCDS phenotype, we produced transgenic mouse lines that used the collagen X promoter to drive expression of an ER stress-inducing protein (the cog mutant of thyroglobulin) in hypertrophic chondrocytes. The hypertrophic chondrocytes in this mouse exhibited ER stress with a characteristic UPR response. In addition, the hypertrophic zone was expanded, gene expression patterns were disrupted, osteoclast recruitment to the vascular invasion front was reduced, and long bone growth decreased. Our data demonstrate that triggering ER stress per se in hypertrophic chondrocytes is sufficient to induce the essential features of the cartilage pathology associated with MCDS and confirm that ER stress is a central pathogenic factor in the disease mechanism. These findings support the contention that ER stress may play a direct role in the pathogenesis of many connective tissue disorders associated with the expression of mutant extracellular matrix proteins.

Localization of the cis-enhancer element for mouse type X collagen expression in hypertrophic chondrocytes in vivo

The type X collagen gene (Col10a1) is a specific molecular marker of hypertrophic chondrocytes during endochondral bone formation. Mutations in human COL10A1 and altered chondrocyte hypertrophy have been associated with multiple skeletal disorders. However, until recently, the cis-enhancer element that specifies Col10a1 expression in hypertrophic chondrocytes in vivo has remained unidentified. Previously, we and others have shown that the Col10a1 distal promoter (-4.4 to -3.8 kb) may harbor a critical enhancer that mediates its tissue specificity in transgenic mice studies. Here, we report further localization of the cis-enhancer element within this Col10a1 distal promoter by using a similar transgenic mouse approach. We identify a 150-bp Col10a1 promoter element (-4296 to -4147 bp) that is sufficient to direct its tissue-specific expression in vivo. In silico analysis identified several putative transcription factor binding sites including two potential activator protein-1 (AP-1) sites within its 5'- and 3'-ends (-4276 to -4243 and -4166 to -4152 bp), respectively. Interestingly, transgenic mice using a reporter construct deleted for these two AP-1 elements still showed tissue-specific reporter activity. EMSAs using oligonucleotide probes derived from this region and MCT cell nuclear extracts identified DNA/protein complexes that were enriched from cells stimulated to hypertrophy. Moreover, these elements mediated increased reporter activity on transfection into MCT cells. These data define a 90-bp cis-enhancer required for tissue-specific Col10a1 expression in vivo and putative DNA/protein complexes that contribute to the regulation of chondrocyte hypertrophy. This work will enable us to identify candidate transcription factors essential both for skeletal development and for the pathogenesis of skeletal disorders.

Surviving endoplasmic reticulum stress is coupled to altered chondrocyte differentiation and function

In protein folding and secretion disorders, activation of endoplasmic reticulum (ER) stress signaling (ERSS) protects cells, alleviating stress that would otherwise trigger apoptosis. Whether the stress-surviving cells resume normal function is not known. We studied the in vivo impact of ER stress in terminally differentiating hypertrophic chondrocytes (HCs) during endochondral bone formation. In transgenic mice expressing mutant collagen X as a consequence of a 13-base pair deletion in Col10a1 (13del), misfolded α1(X) chains accumulate in HCs and elicit ERSS. Histological and gene expression analyses showed that these chondrocytes survived ER stress, but terminal differentiation is interrupted, and endochondral bone formation is delayed, producing a chondrodysplasia phenotype. This altered differentiation involves cell-cycle re-entry, the re-expression of genes characteristic of a prehypertrophic-like state, and is cell-autonomous. Concomitantly, expression of Col10a1 and 13del mRNAs are reduced, and ER stress is alleviated. ERSS, abnormal chondrocyte differentiation, and altered growth plate architecture also occur in mice expressing mutant collagen II and aggrecan. Alteration of the differentiation program in chondrocytes expressing unfolded or misfolded proteins may be part of an adaptive response that facilitates survival and recovery from the ensuing ER stress. However, the altered differentiation disrupts the highly coordinated events of endochondral ossification culminating in chondrodysplasia.

The assembly and folding of secreted proteins in the endoplasmic reticulum (ER) is exquisitely regulated by a complex mechanism that maintains an equilibrium between folded and unfolded proteins. Perturbation of this homeostasis induces ER stress, which, if not alleviated through ER stress signaling (ERSS), ultimately triggers cell death. Normal bone growth occurs through a highly coordinated differentiation program that yields specialized cartilage cells (chondrocytes); when this program is disrupted, chondrodysplasia, or malformed skeletons, can result. Chondrodysplasias caused by mutations that affect protein assembly and secretion are characterized by a disorganization of bony growth plates and distension of the ER. We tested whether these chondrodysplasia characteristics were linked to ERSS. By investigating the impact of ER stress on the cell fate of hypertrophic chondrocytes (HCs) in transgenic mice expressing mutations in collagen that prevent proper folding, we revealed a novel adaptive mechanism that helps alleviate the unfolded protein load. Instead of undergoing apoptosis, the HCs undergoing ER stress adapt, re-enter the cell cycle, and revert to a less-mature state in which expression of the mutant collagen is reduced. Our findings have broad implications for adaptive mechanisms to ER stress in vivo and for the pathophysiology underlying chondrodysplasias caused by mutations that impact on protein assembly and secretion.

When subjected to ER stress (by expression of misfolded or unfolded proteins), hypertrophic chondrocytes undergo alterations to their developmental program that may be part of an adaptive response.

Mutations of COL10A1 in Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia (SMCD) is a dominantly inherited cartilage disorder caused by mutations in the gene for the hypertrophic cartilage extracellular matrix structural protein, collagen X (COL10A1). Thirty heterozygous mutations have been described, about equally divided into two mutation types, missense mutations, and mutations that introduce premature termination signals. The COL10A1 mutations are clustered (33/36) in the 3' region of exon 3, which codes for the C-terminal NC1 trimerization domain. The effect of COL10A1 missense mutations have been examined by in vitro expression and assembly assays and cell transfection studies, which suggest that a common consequence is the disruption of collagen X trimerization and secretion, with consequent intracellular degradation. The effect of COL10A1 nonsense mutations in cartilage tissue has been examined in two patients, demonstrating that the mutant mRNA is completely removed by nonsense mediated mRNA decay. Thus for both classes of mutations, functional haploinsufficiency is the most probable cause of the clinical phenotype in SMCD.

The discoidin domain receptor DDR2 is a receptor for type X collagen

During endochondral ossification, collagen X is deposited in the hypertrophic zone of the growth plate. Our previous results have shown that collagen X is capable of interacting directly with chondrocytes, primarily via integrin alpha2beta1. In this study, we determined whether collagen X could also interact with the non-integrin collagen receptors, discoidin domain receptors (DDRs), DDR1 or DDR2. The widely expressed DDRs are receptor tyrosine kinases that are activated by a number of different collagen types. Collagen X was found to be a much better ligand for DDR2 than for DDR1. Collagen X bound to the DDR2 extracellular domain with high affinity and stimulated DDR2 autophosphorylation, the first step in transmembrane signalling. Expression of DDR2 in the epiphyseal plate was confirmed by RT-PCR and immunohistochemistry. The spatial expression of DDR2 in the hypertrophic zone of the growth plate is consistent with a physiological interaction of DDR2 with collagen X. Surprisingly, the discoidin domain of DDR2, which fully contains the binding sites for the fibrillar collagens I and II, was not sufficient for collagen X binding. The nature of the DDR2 binding site(s) within collagen X was further analysed. In addition to a collagenous domain, collagen X contains a C-terminal NC1 domain. DDR2 was found to recognise the triple-helical region of collagen X as well as the NC1 domain. Binding to the collagenous region was dependent on the triple-helical conformation. DDR2 autophosphorylation was induced by the collagen X triple-helical region but not the NC1 domain, indicating that the triple-helical region of collagen X contains a specific DDR2 binding site that is capable of receptor activation. Our study is the first to describe a non-fibrillar collagen ligand for DDR2 and will form the basis for further studies into the biological function of collagen X during endochondral ossification.

Genetic analysis of skeletal dysplasia: recent advances and perspectives in the post-genome-sequence era

Skeletal dysplasia is a group of disorders of the skeleton that result from derangement of growth, development and/or differentiation of the skeleton. Nearly 300 disorders are included; most of them are monogenic diseases. Responsible genes for skeletal dysplasia have been identified in more than 150 diseases mainly through positional cloning. Identification of disease genes would improve patient care through genetic diagnosis as well as improving our understanding of the diseases and molecular mechanism of skeletal tissue formation. Studies of skeletal dysplasia would also help identify disease genes for common diseases affecting bones and joints. In this study, the author reviews recent advances and the current status of the genetic analysis of skeletal dysplasia and its impacts on research into skeletal biology.

Selective identification of secreted and transmembrane breast cancer markers using Escherichia coli ampicillin secretion trap

Secreted and cell surface proteins play important roles in cancer and are potential drug targets and tumor markers. Here, we describe a large-scale analysis of the genes encoding secreted and cell surface proteins in breast cancer. To identify these genes, we developed a novel signal sequence trap method called Escherichia coli ampicillin secretion trap (CAST). For CAST, we constructed a plasmid in which the signal sequence of beta-lactamase was deleted such that it does not confer ampicillin resistance. Eukaryotic cDNA libraries cloned into pCAST produced tens of thousands of ampicillin-resistant clones, 80% of which contained cDNA fragments encoding secreted and membrane spanning proteins. We identified 2,708 unique sequences from cDNA libraries made from surgical breast cancer specimens. We analyzed the expression of 1,287 of the 2,708 genes and found that 166 were overexpressed in breast cancers relative to normal breast tissues. Eighty-five percent of these genes had not been previously identified as markers of breast cancer. Twenty-three of the 166 genes (14%) were relatively tissue restricted, suggesting use as cancer-specific targets. We also identified several new markers of ovarian cancer. Our results indicate that CAST is a robust, rapid, and low cost method to identify cell surface and secreted proteins and is applicable to a variety of relevant biological questions.

Schmid type of metaphyseal chondrodysplasia and COL10A1 mutations--findings in 10 patients

The Schmid type of metaphyseal chondrodyplasia (MCDS) is characterized by short stature, widened growth plates, and bowing of the long bones. It results from autosomal dominant mutations of COL10A1, the gene which encodes alpha1(X) chains of type X collagen. We report the clinical and radiographic findings in 10 patients with MCDS and COL10A1 mutations. Six patients had lower limb deformities, which necessitated orthopedic surgeries in all of them. One patient demonstrated no deformities and normal stature at age 11 years (height -1.2 SDS) while the others manifested severe short stature (<-3.5 SDS). Radiographs showed metaphyseal changes which were most pronounced at the hips and knees. Five of the identified 10 mutations in COL10A1 were novel. Six mutations resulted in truncation of the NC1 domain while four mutations were single amino-acid substitutions. Our findings suggest that COL10A1 mutations result in a uniform pattern of growth plate abnormalities. However, the clinical variability in severity among affected individuals is greater than previously thought.

A highly conserved enhancer in mammalian type X collagen genes drives high levels of tissue-specific expression in hypertrophic cartilage in vitro and in vivo

Previously we have identified a cis-acting regulatory domain in the human type X collagen gene upstream of the transcription start site which acts as a strong enhancer in hypertrophic, but not in resting chondrocytes. Here we show that this enhancer is highly conserved also in the murine and bovine Col10a1 genes, but not found in the known promoter sequences of chicken Col10a1. It contains a functionally active AP-1 site (TPA Responsive Element, TRE) which is essential for the high transcriptional activity of the COL10A1 enhancer in transiently transfected hypertrophic chondrocytes. Gel-shift experiments with nuclear extracts of hypertrophic chondrocytes revealed FosB and Fra-1 as candidates regulating AP-1 factors binding to the TRE site. In fact, coexpression of FosB and Fra-1 in reporter gene assays greatly stimulated transcriptional activity of enhancer bearing reporter genes. Quantitative analysis of AP-1 factor mRNA levels in distinct fractions of fetal bovine epiphyseal chondrocytes by real-time PCR confirmed significant levels of FosB and Fra-1 mRNA besides other AP-1 factors in hypertrophic chondrocytes. A key role of the enhancer element in regulating tissue-specific expression of the Col10a1 gene was shown by establishing transgenic mouse lines with a reporter gene containing a 4.6 kb murine Col10a1 promoter fragment which included the enhancer, exon 1, part of exon 2 and the first intron. Reporter gene expression was seen exclusively in hypertrophic cartilages in the growth plates of long bones, ribs, vertebrae, sternum and mandibles of 17.5-18.5 dpc embryos, confirming that the 4.6 kb promoter is able to drive specific expression of Col10a1 in hypertrophic cartilage. These established transgenic lines should facilitate the genetic analysis of regulatory pathways of chondrocyte maturation and Col10a1 gene expression in the future.

SP3/SP1 transcription activity regulates specific expression of collagen type X in hypertrophic chondrocytes

Previously, we have shown that two non-canonical specificity protein (SP)-binding sites within the proximal promoter (nucleotide (nt) -139 to +5) of the chicken Col10a1 gene are involved in conferring tissue-specific expression of type X collagen to hypertrophic chondrocytes. In the present study, we examined the role of SP3/SP1 transcription factors in the regulation of the Col10a1 promoter. The SP3/SP1 ratio is higher in hypertrophic versus non-hypertrophic chondrocytes, due to the significant decrease in SP1 in hypertrophic cells detected by real-time PCR and Western blot analyses. Functional analyses by transfection-mediated overexpression of SP1 and SP3 suggest that SP1 inhibits the Col10a1 promoter. This effect is negated by an interaction with SP3 in hypertrophic chondrocytes. Additionally, mutation analysis showed that the 40-bp intervening sequence (nt -115 to -75) is required for expression of the Col10a1 gene. In this sequence, a binding site for Dlx5/6 transcription factors (nt -99 to -87) retards a protein specific for hypertrophic chondrocytes in electrophoretic mobility shift assay. Endogenous levels of Dlx5 are 3-fold higher in hypertrophic versus non-hypertrophic cells by real-time PCR analysis, and overexpression of Dlx5 in non-hypertrophic chondrocytes activates the proximal Col10a1 promoter 3-fold. These results indicate that the SP3/SP1 ratio and Dlx5 are important regulators of the proximal Col10a1 promoter in hypertrophic cartilage and suggest that interactions between SP3 and SP1 regulate expression of different types of collagen during chondrocyte differentiation.

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.

Hand involvement in Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia (Schmid MCD, MIM 156500) is caused by mutations in the COL10A1 gene and is clinically characterized by short stature, bowed legs, and a waddling gait. Radiographic findings include anterior cupping, sclerosis and splaying of the ribs, diffuse metaphyseal flaring, and irregularity that is most pronounced at the knees, coxa vara, and femoral bowing. We reviewed the radiographs of Schmid MCD patients at the International Skeletal Dysplasia Registry in Los Angeles for evidence of hand involvement. We found hand involvement in 47% (7/15) of cases included in our analysis. These changes were subtle and consisted of shortening of the tubular bones and metaphyseal cupping of the proximal phalanges and metacarpals. Mild hand involvement is a common feature of Schmid MCD.

Critical roles for collagenase-3 (Mmp13) in development of growth plate cartilage and in endochondral ossification

Collagenase-3 (MMP13), a member of the matrix metalloproteinase (MMP) family of neutral endopeptidases, is expressed in the skeleton during embryonic development and is highly overexpressed in human carcinomas and in chondrocytes and synovial cells in rheumatoid arthritis and osteoarthritis. To determine the functional roles of Mmp13, we generated Mmp13-null mice that showed profound defects in growth plate cartilage with markedly increased hypertrophic domains as well as delay in endochondral ossification and formation and vascularization of primary ossification centers. Absence of Mmp13 resulted in significant interstitial collagen accumulation due, in part, to the lack of appropriate collagenase-mediated cleavage that normally occurs in growth plates and primary ossification centers. Cartilaginous growth plate abnormalities persisted in adult mice and phenocopied defects observed in human hereditary chondrodysplasias. Our findings demonstrate a unique role of Mmp13 in skeletal development.

Chicken collagen X regulatory sequences restrict transgene expression to hypertrophic cartilage in mice

Collagen X is produced by hypertrophic cartilage undergoing endochondral ossification. Transgenic mice expressing defective collagen X under the control of 4.7- or 1.6-kb chicken collagen X regulatory sequences yielded skeleto-hematopoietic defects (Jacenko O, LuValle P, Olsen BR: Spondylometaphyseal dysplasia in mice carrying a dominant-negative mutation in a matrix protein specific for cartilage-to-bone transition. Nature 1993, 365:56-61; Jacenko O, Chan D, Franklin A, Ito S, Underhill CB, Bateman JF, Campbell MR: A dominant interference collagen X mutation disrupts hypertrophic chondrocyte pericellular matrix and glycosaminoglycan and proteoglycan distribution in transgenic mice. Am J Pathol 2001, 159:2257-2269; Jacenko O, Roberts DW, Campbell MR, McManus PM, Gress CJ, Tao Z: Linking hematopoiesis to endochondral ossification through analysis of mice transgenic for collagen X. Am J Pathol 2002, 160:2019-2034). Current data indicate that the hematopoietic abnormalities do not result from extraskeletal expression of endogenous collagen X or the transgene. Organs from mice carrying either promoter were screened by immunohistochemistry, in situ hybridization, and Northern blot; transgene and mouse collagen X proteins and messages were detected only in hypertrophic cartilage. Likewise, reverse transcriptase-polymerase chain reaction revealed both transgene and mouse collagen X amplicons only in the endochondral skeleton of mice with the 4.7-kb promoter; however, in mice with the 1.6-kb promoter, multiple organs were transgene-positive. Collagen X and transgene amplicons were also detected in marrow, but likely resulted from contaminating trabecular bone; this was supported by reverse transcriptase-polymerase chain reaction analysis of rat tibial zones free of trabeculae. Our data demonstrate that in mice, the 4.7-kb chicken collagen X promoter restricts transcription temporo-spatially to that of endogenous collagen X, and imply that murine skeleto-hematopoietic defects result from transgene co-expression with collagen X. Moreover, the 4.7-kb hypertrophic cartilage-specific promoter could be used for targeting transgenes to this tissue site in mice.

Skeletal phenotype in patients with Shwachman-Diamond syndrome and mutations in SBDS

Pancreatic exocrine and bone marrow dysfunctions are considered to be universal features of Shwachman-Diamond syndrome (SDS) whereas the associated skeletal dysplasia is variable and not consistently observed. The genetic defect in SDS has recently been identified; causative mutations have been shown in the SBDS gene. The aims of this study were to characterize the nature, frequency, and age-related changes of radiographic skeletal abnormalities in patients with SBDS mutations and to assess genotype-phenotype correlation. Fifteen patients (mean age 9.7 years) with a clinical diagnosis of SDS and documented SBDS gene mutations were included. Review of their skeletal radiographs showed abnormalities in all patients. The skeletal changes were variable, even in patients with identical genotypes. The typical features were (1) delayed appearance of secondary ossification centers, (2) variable widening and irregularity of the metaphyses in early childhood, followed by progressive thickening and irregularity of the growth plates, and (3) generalized osteopenia. There was a tendency towards normalization of the epiphyseal maturation defect and progression of the metaphyseal changes with age. The results suggest that the characteristic skeletal changes are present in all patients with SDS and SBDS mutations, but their severity and localization varies with age. No phenotype-genotype correlation was observed.

Expression and activity of the CDK inhibitor p57Kip2 in chondrocytes undergoing hypertrophic differentiation

Growth plates of p57-null mice exhibit several abnormalities, including loss of collagen type X (CollX) expression. The phenotypic consequences of p57 expression were assessed in an in vitro model of hypertrophic differentiation. Adenoviral p57 expression was not sufficient for CollX expression but did augment induction of CollX by BMP-2.

INTRODUCTION

During hypertrophic differentiation, chondrocytes pass from an actively proliferative state to a postmitotic, hypertrophic phenotype. The induction of growth arrest is a central feature of this phenotypic transition. Mice lacking the cyclin dependent-kinase inhibitor p57Kip2 exhibit several developmental abnormalities including chondrodysplasia. Although growth plate chondrocytes in p57-null mice undergo growth arrest, they do not express collagen type X, a specific marker of the hypertrophic phenotype. This study was carried out to investigate the link between p57 expression and the induction of collagen type X in chondrocytes and to determine whether p57 overexpression is sufficient for the induction of hypertrophic differentiation.

MATERIALS AND METHODS

Neonatal rat epiphyseal or growth plate chondrocytes were maintained in an aggregate culture model, in defined, serum-free medium. Protein and mRNA levels were monitored by Western and Northern blot analyses, respectively. Proliferative activity was assessed by fluorescent measurement of total DNA and by 3H-thymidine incorporation rates. An adenoviral vector was used to assess the phenotypic consequences of p57 expression.

RESULTS AND CONCLUSIONS

During in vitro hypertrophic differentiation, levels of p57 mRNA and protein were constant despite changes in chondrocyte proliferative activity and the induction of hypertrophic-specific genes in response to bone morphogenetic protein (BMP)-2. Adenoviral p57 overexpression induced growth arrest in prehypertrophic epiphyseal chondrocytes in a dose-dependent manner but was not sufficient for the induction of collagen type X, either alone or when coexpressed with the related CDKI p21Cip1. Similar results were obtained with more mature tibial growth plate chondrocytes. p57 overexpression did augment collagen type X induction by BMP-2. These data indicate that p57-mediated growth arrest is not sufficient for expression of the hypertrophic phenotype, but rather it occurs in parallel with other aspects of the differentiation pathway. Our findings also suggest a contributing role for p57 in the regulation of collagen type X expression in differentiating chondrocytes.

Cartilage morphogenetic proteins: role in joint development, homoeostasis, and regeneration

BACKGROUND

Articular cartilage homoeostasis is critical for joint function. The steady state homoeostasis of articular cartilage is a balance between anabolic morphogens such as cartilage derived morphogenetic proteins (CDMPs) and bone morphogenetic proteins (BMPs) of the BMP family and catabolic cytokines such as interleukin (IL)1, IL17, and tumour necrosis factor alpha. Although bone and articular cartilage are adjacent tissues, there is a profound difference in their regeneration potential. Bone has the highest potential for regeneration. On the other hand, articular cartilage is recalcitrant to repair.

OBJECTIVE

To examine the hypothesis that the feeble innate regeneration ability of cartilage is due to the preponderance of catabolic cytokines such as IL1 and IL17.

RESULTS

During a systematic investigation of CDMPs and cytokines IL17B (chondroleukin) was found in bovine articular cartilage.

DISCUSSION AND CONCLUSIONS

BMP-7 and IL17B are present in articular cartilage and synthesised in chondrocytes as shown by northern blots and real-time reverse transcription-polymerase chain reaction. The coexistence of anabolic morphogens and catabolic cytokines in articular cartilage has important implications for cartilage homoeostasis and regeneration. The networks of signalling systems of morphogens and cytokines determine the net capacity for regenerative morphogenesis of articular cartilage. Finally, the feeble innate capacity for articular cartilage may be improved by targeted therapy by soluble receptors to block catabolic cytokines.

Genetic changes in the RNA components of RNase MRP and RNase P in Schmid metaphyseal chondrodysplasia

BACKGROUND

The Schmid type of metaphyseal chondrodysplasia (MCDS) is generally due to mutations in COL10A1 encoding for type X collagen of cartilage.

METHODS

We performed a study on the genes coding for the RNA components of RNase MRP (MRPR) and RNase P (H1RNA) among 20 patients with diagnosis of MCDS and no mutations in COL10A1.

RESULTS

Two patients were found to be homozygous for a base substitution G for A at nucleotide 70 of RMRP, which is the major mutation causing cartilage-hair hypoplasia. No pathogenic mutations were detected in H1RNA.

CONCLUSION

Cartilage-hair hypoplasia diagnosis should be considered in patients with metaphyseal chondrodysplasia even in the absence of any extra-skeletal manifestations if no mutation in COL10A1 can be found and the family history is compatible with autosomal recessive inheritance. Correct diagnosis is important for genetic counselling and for proper follow up of the patients.

Distinctive metaphyseal chondrodysplasia with severe distal radius and ulna involvement (upper extremity mesomelia) and normal height

Metaphyseal chondrodysplasias (MCD) are skeletal disorders characterized by metaphyseal irregularities and, usually, by short stature. In MCD, wide heterogeneity exists with regard to clinical and radiological changes. We report on a patient with clinical and radiological findings of MCD who had coxa valga and normal height with metaphyseal involvement of the long bones. The short radii and ulnae showed a very severe change in their distal metaphyses, leading to mesomelic shortening confined to the upper limbs. Hematological, ophthalmological, and hearing examinations were normal. This type of MCD appears to represent a yet undescribed syndrome.

Adiponectin as a novel determinant of bone mineral density and visceral fat

Growing evidence suggests that positive associations between fat mass (FM) and bone mineral density (BMD) are mediated by not only biomechanical but also biochemical factors. Adiponectin is a novel adipocyte-derived hormone that regulates energy homeostasis and has anti-inflammatory and anti-atherogenic effects. Unlike other adipokines such as leptin, adiponectin levels decrease in obesity and type 2 diabetes. The purpose of our study was to investigate associations of serum adiponectin with BMD (DXA and QCT), FM (DXA and QCT), and serum leptin and soluble leptin receptor levels in 38 women and 42 men (age 39-81, BMI 17-55, 86% with type 2 diabetes). After adjusting for age, gender, race, smoking, and diabetes status, serum adiponectin was inversely associated with areal BMD (r = -0.20 to -0.3, all P < 0.01), volumetric BMD (r = -0.35 to -0.44, all P < 0.01), and visceral fat volume (r = -0.30, P < 0.01). These associations remained significant after adjusting for whole body fat mass. The associations of adiponectin with subcutaneous fat volume, whole body FM, and serum leptin level were not significant (all P > 0.1). These data suggest that adiponectin may play a role in the protective effects of visceral fat on BMD.

Type X collagen gene regulation by Runx2 contributes directly to its hypertrophic chondrocyte-specific expression in vivo

The alpha1(X) collagen gene (Col10a1) is the only known hypertrophic chondrocyte-specific molecular marker. Until recently, few transcriptional factors specifying its tissue-specific expression have been identified. We show here that a 4-kb murine Col10a1 promoter can drive beta-galactosidase expression in lower hypertrophic chondrocytes in transgenic mice. Comparative genomic analysis revealed multiple Runx2 (Runt domain transcription factor) binding sites within the proximal human, mouse, and chick Col10a1 promoters. In vitro transfection studies and chromatin immunoprecipitation analysis using hypertrophic MCT cells showed that Runx2 contributes to the transactivation of this promoter via its conserved Runx2 binding sites. When the 4-kb Col10a1 promoter transgene was bred onto a Runx2(+/-) background, the reporter was expressed at lower levels. Moreover, decreased Col10a1 expression and altered chondrocyte hypertrophy was also observed in Runx2 heterozygote mice, whereas Col10a1 was barely detectable in Runx2-null mice. Together, these data suggest that Col10a1 is a direct transcriptional target of Runx2 during chondrogenesis.

Genetically altered mouse models: the good, the bad, and the ugly

Targeted gene disruption in mice is a powerful tool for generating murine models for human development and disease. While the human genome program has helped to generate numerous candidate genes, few genes have been characterized for their precise in vivo functions. Gene targeting has had an enormous impact on our ability to delineate the functional roles of these genes. Many gene knockout mouse models faithfully mimic the phenotypes of the human diseases. Because some models display an unexpected or no phenotype, controversy has arisen about the value of gene-targeting strategies. We argue in favor of gene-targeting strategies, provided they are used with caution, particularly in interpreting phenotypes in craniofacial and oral biology, where many genes have pleiotropic roles. The potential pitfalls are outweighed by the unique opportunities for developing and testing different therapeutic strategies before they are introduced into the clinic. In the future, we believe that genetically engineered animal models will be indispensable for gaining important insights into the molecular mechanisms underlying development, as well as disease pathogenesis, diagnosis, prevention, and treatment.

Partial characterization of cell-type X collagen interactions

Type X collagen is a short-chain non-fibrillar collagen that is deposited exclusively at sites of new bone formation. Although this collagen has been implicated in chondrocyte hypertrophy and endochondral ossification, its precise function remains unclear. One possible function could be to regulate the processes of chondrocyte hypertrophy through direct cell-type X collagen interactions. Adhesions of embryonic chick chondrocytes, and cell lines with known expression of collagen-binding integrins (MG63 and HOS), were assayed on chick type X collagen substrates, including the native, heat-denatured and pepsin-digested collagen, and the isolated C-terminal non-collagenous (NC1) domain. Type X collagen supported the greatest level of adhesion for all cell types tested. The involvement of the alpha2beta1 integrin in type X collagen-cell interaction was demonstrated by adhesion studies in the presence of Mg(2+) and Ca(2+) ions and integrin-function-blocking antibodies. Cells expressing alpha2beta1 integrin (chick chondrocytes and MG63 cells) also adhered to heat-denatured type X collagen and the isolated NC1 domain; however, removal of the non-collagenous domains by limited pepsinization of type X collagen resulted in very low levels of adhesion. Both focal contacts and actin stress-fibre formation were apparent in cells plated on type X collagen. The presence of alpha2 and beta1 integrin subunits in isolated chondrocytes and epiphyseal cartilage was also confirmed by immunolocalization. Our results demonstrate, for the first time, that type X collagen is capable of interacting directly with chondrocytes and other cells, primarily via alpha2beta1 integrin. These findings are atypical from the fibrillar collagen-cell interactions via collagen binding integrins in that: (1) the triple-helical conformation is not strictly required for cell adhesion; (2) the NC1 domain is also involved in the adhesion of alpha2beta1-expressing cells. These data form the basis for further studies into the mechanism and biological significance of type X collagen deposition in the growth plate.

A distinctive type of metaphyseal chondrodysplasia with characteristic thickening of the distal ulna and radius: possible metaphyseal chondrodysplasia-Rosenberg

We report an 8-year-old boy with a distinctive form of metaphyseal chondrodysplasia (MCD). He presented with moderate disproportionate short stature and bony swelling of his wrists, knees, and ankles. There were severe metaphyseal abnormalities with a honeycomb appearance affecting the distal tibiae and fibulae, proximal tibiae, distal femurs, distal ulnae and radii, and both hands. His thoracolumbar spine was normal. Radiological examination of the mother's forearms revealed widening of the distal radii and short ulnae with hypoplastic distal ends. Rosenberg and Löhr [1986: Eur J Pediatr 145:40-45] reported a four-generational kindred in which affected members had thickening of the wrist proximal to the styloid process of the ulna and thickening of the dorsum sellae. Although many of the radiographic features of this patient are those of MCD-Rosenberg, the skeletal features of our patient do not appear to represent any known classified forms of MCD.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Physiology and pathophysiology of the growth plate

Longitudinal growth of the skeleton is a result of endochondral ossification that occurs at the growth plate. Through a sequential process of cell proliferation, extracellular matrix synthesis, cellular hypertrophy, matrix mineralization, vascular invasion, and eventually apoptosis, the cartilage model is continually replaced by bone as length increases. The regulation of longitudinal growth at the growth plate occurs generally through the intimate interaction of circulating systemic hormones and locally produced peptide growth factors, the net result of which is to trigger changes in gene expression by growth plate chondrocytes. This review highlights recent advances in genetics and cell biology that are illuminating the important regulatory mechanisms governing the structure and biology of the growth plate, and provides selected examples of how studies of human mutations have yielded a wealth of new knowledge regarding the normal biology and pathophysiology of growth plate cartilage.

Skeletal dysplasias caused by a disruption of skeletal patterning and endochondral ossification

Identification of a number of the genes that cause skeletal dysplasias has helped clinicians to provide accurate diagnoses, genetic counseling, and pre-natal diagnosis for this complex group of disorders. This review considers how some of the recent advances in human and murine genetics have led to an increased understanding of normal bone development and, in particular, the processes of skeletal patterning and endochondral ossification.

Crystal structure of the collagen alpha1(VIII) NC1 trimer

Collagen VIII is a major component of Descemet's membrane and is also found in vascular subendothelial matrices. The C-terminal non-collagenous domain (NC1) domain of collagen VIII, which is a member of the C1q-like protein family, forms a stable trimer and is thought to direct the assembly of the collagen triple helix, as well as polygonal supramolecular structures. We have solved the crystal structure of the mouse alpha1(VIII)(3) NC1 domain trimer at 1.9 A resolution. Each subunit of the intimate NC1 trimer consists of a ten-stranded beta-sandwich. The surface of the collagen VIII NC1 trimer presents three strips of partially exposed aromatic residues shown to interact with the non-ionic detergent CHAPS, which are likely to be involved in supramolecular assemblies. Equivalent strips exist in the NC1 domain of the closely related collagen X, suggesting a conserved assembly mechanism. Surprisingly, the collagen VIII NC1 trimer lacks the buried calcium cluster of the collagen X NC1 trimer. The mouse alpha1(VIII) and alpha2(VIII) NC1 domains are 71.5% identical in sequence, with the differences being concentrated on the NC1 trimer surface. A few non-conservative substitutions map to the subunit interfaces near the surface, but it is not obvious from the structure to what extent they determine the preferred assembly of collagen VIII alpha1 and alpha2 chains into homotrimers.

Site-specific immunostaining for type X collagen in noncalcified articular cartilage of canine stifle knee joint

Type X collagen is a short-chain collagen that is strongly expressed in hypertrophic chondrocytes. In this study, we used an immunohistochemical technique exploiting a prolonged hyaluronidase unmasking of type X collagen epitopes to show that type X collagen is not restricted to calcified cartilage, but is also present in normal canine noncalcified articular cartilage. A 30 degrees valgus angulation procedure of the right tibia was performed in 15 dogs at the age of 3 months, whereas their nonoperated sister dogs served as controls. Samples were collected 7 and 18 months after the surgery and immunostained for type X collagen. The deposition of type X collagen increased during maturation from age 43 weeks to 91 weeks. In the patella, most of the noncalcified cartilage stained for type X collagen, whereas, in the patellar surface of the femur, it was present mainly in the femoral groove close to cartilage surface. In femoral condyles, the staining localized mostly in the superficial cartilage on the lateral and medial sides, but not in the central weight-bearing area. In tibial condyles, type X collagen was often observed close to the cartilage surface in medial parts of the condyles, although staining could also be seen in the deep zone of the cartilage. Staining for type X collagen appeared strongest at sites where the birefringence of polarized light was lowest, suggesting a colocalization of type X collagen with the collagen fibril arcades in the intermediate zone. No significant difference in type X collagen immunostaining was observed in lesion-free articular cartilage between controls and dogs that underwent a 30 degrees valgus osteotomy. In osteoarthritic lesions, however, there was strong immunostaining for both type X collagen and collagenase-induced collagen cleavage products. The presence of type X collagen in the transitional zone of cartilage in the patella, femoropatellar groove, and in tibial cartilage uncovered by menisci suggests that it may involve a modification of collagen fibril arrangement at the site of collagen fibril arcades, perhaps providing additional support to the collagen network.

An enhancer complex confers both high-level and cell-specific expression of the human type X collagen gene

Type X collagen expression is restricted to hypertrophic chondrocytes in the endochondral growth plate. Transient transfection of reporter constructs containing the human collagen X promoter into primary growth plate chondrocytes identified a cis-acting positive regulatory DNA element(s) that has cell-specific enhancer properties and binds a nuclear protein expressed specifically in growth plate chondrocytes. Functional disruption of this region results in a significant reduction in the activation of reporter gene transcription. The identified enhancer is a major element controlling both high-level and cell-specific expression of type X collagen gene.

Positional cloning of the gene LIMBIN responsible for bovine chondrodysplastic dwarfism

Chondrodysplastic dwarfism in Japanese brown cattle is an autosomal recessive disorder characterized by short limbs. Previously, we mapped the locus responsible for the disease on the distal end of bovine chromosome 6. Here, we narrowed the critical region to approximately 2 cM by using linkage analysis, constructed a BAC and YAC contig covering this region, and identified a gene, LIMBIN (LBN), that possessed disease-specific mutations in the affected calves. One mutation was a single nucleotide substitution leading to an activation of a cryptic splicing donor site and the other was a one-base deletion resulting in a frameshift mutation. Strong expression of the Lbn gene was observed in limb buds of developing mouse embryos and in proliferating chondrocytes and bone-forming osteoblasts in long bones. These findings indicate that LBN is responsible for bovine chondrodysplastic dwarfism and has a critical role in a skeletal development.

The molecular genetics of bone formation: implications for therapeutic interventions in bone disorders

Skeletal biology is a complex process involving the developmental commitment and differentiation of chondrocytes and osteoblasts which produce and mineralize cartilage and bone matrix during growth and postnatal life. Several genes are involved in controlling osteogenesis by acting on target cells in a very complex manner. Manipulation of genes in mice and studies of genetic mutations affecting the skeleton in humans have enabled the assessment of the role of transcription factors, bone matrix proteins and regulatory factors involved in the control of chondrocyte and osteoblast differentiation, and have considerably improved our understanding of the bone formation process. Clinical studies and gene polymorphism analyses suggest that the variable expression of particular genes may be linked to clinical osteoporosis. A major challenge in the future will be to develop molecularly targeted approaches to stimulating bone formation and increasing bone mass. The use of mouse strain models and transgenic animals with variable bone density may be useful to identify genetic determinants of bone mass which may serve as a basis for drug discovery and development. On the other hand, the availability of gene microarrays and other emerging genomic techniques are promising tools to identify genes that are distinctly expressed in health and disease. These technologies may also serve to test the mechanisms of action of drugs aimed at increasing bone formation. Genetic studies of the molecular signaling pathways involved in normal and pathological osteogenesis may also help to identify genes that could be targeted for therapeutic intervention. Candidate approaches include selective gene transfection in target cells and the use of drugs acting on gene promoters to selectively enhance gene expression in osteoblasts. The development of these strategies is expected not only to bring new insight into the molecular mechanisms that govern bone formation in normal and pathological situations but, in the long term, may also result in the identification of novel molecular targets for therapeutic interventions for bone formation disorders.

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.

Chondrodysplasias due to proteoglycan defects

The proteoglycans, especially the large chondroitin sulfate proteoglycan aggrecan, have long been viewed as important components of the extracellular matrix of cartilage. The drastic change in expression during differentiation from mesenchyme to cartilage, the loss of tissue integrity associated with proteoglycan degradation in several disease processes and, most important, the demonstration of abnormalities in proteoglycan production concomitant with the aberrant growth patterns exhibited by the brachymorphic mouse, the cartilage matrix deficient mouse, and the nanomelic chick provide the strongest evidence that the proteoglycan aggrecan is essential during differentiation and for maintenance of the skeletal elements. More recently, mutations associated with proteoglycans other than aggrecan, especially the heparan sulfate proteoglycans, glypican and perlecan, suggest an important role for these molecules in skeletal development as well. This review focuses on the molecular bases of the hereditary proteoglycan defects in animal models, as well as of some human chondrodysplasias, that collectively are providing a better understanding of the role of proteoglycans in the development and maintenance of the skeletal elements.

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.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.

Alterations of cartilage and collagen expression during fracture healing in experimental diabetes

We investigated alterations in the expression of mRNA for type II and type X collagen in fracture callus of experimentally induced diabetic animals compared with controls and performed radiographic, histological, immunocytochemical and biomechanical studies. Experimentally induced diabetic rats exhibited an alteration in the temporal expression of type II and type X collagen mRNA and a decrease in type X mRNA expression as compared to controls. Radiographs showed a more intense periosteal reaction and a more rapid reconstitution of cortices in control versus diabetic animals. Histologically there was a delay in chondrocyte maturation and hypertrophy seen in diabetics. Immunolocalization of type X collagen demonstrated a delay in type X collagen expression around the hypertrophic chondrocytes. Biomechanical analysis showed a decrease in the strength of healing fractures in diabetic animals. Fracture healing in diabetic patients is compromised and may lead to delays in bone union. Though the exact mechanisms are unknown, we present evidence of decreased mechanical strength of the fracture and suggest that associated changes in collagen expression and chondrocyte maturation are mechanisms leading to delayed healing in untreated and poorly controlled diabetes.

PTHrP modulates chondrocyte differentiation through AP-1 and CREB signaling

During the process of differentiation, chondrocytes integrate a complex array of signals from local or systemic factors like parathyroid hormone-related peptide (PTHrP), Indian hedgehog, bone morphogenetic proteins and transforming growth factor beta. While PTHrP is known to be a critical regulator of chondrocyte proliferation and differentiation, the signaling pathways through which this factor acts remain to be elucidated. Here we show that both cAMP response element-binding protein (CREB) and AP-1 activation are critical to PTHrP signaling in chondrocytes. PTHrP treatment leads to rapid CREB phosphorylation and activation, while CREB DNA binding activity is constitutive. In contrast, PTHrP induces AP-1 DNA binding activity through induction of c-Fos protein expression. PTHrP activates CRE and TRE reporter constructs primarily through PKA-mediated signaling events. Both signaling pathways were found to be important mediators of PTHrP effects on chondrocyte phenotype. Alone, PTHrP suppresses maturation and stimulates proliferation of the chondrocyte cultures. However, in the presence of dominant negative inhibitors of CREB and c-Fos, these PTHrP effects were suppressed, and chondrocyte maturation was accelerated. Moreover, in combination, the effects of dominant negative c-Fos and CREB are synergistic, suggesting interaction between these signaling pathways during chondrocyte differentiation.

Aberrant signal peptide cleavage of collagen X in Schmid metaphyseal chondrodysplasia. Implications for the molecular basis of the disease

Schmid metaphyseal chondrodysplasia results from mutations in the collagen X (COL10A1) gene. With the exception of two cases, the known mutations are clustered in the C-terminal nonhelical (NC1) domain of the collagen X. In vitro and cell culture studies have shown that the NC1 mutations result in impaired collagen X trimer assembly and secretion. In the two other cases, missense mutations that alter Gly(18) at the -1 position of the putative signal peptide cleavage site were identified (Ikegawa, S., Nakamura, K., Nagano, A., Haga, N., and Nakamura, Y. (1997) Hum. Mutat. 9, 131-135). To study their impact on collagen X biosynthesis using in vitro cell-free translation in the presence of microsomes, and cell transfection assays, these two mutations were created in COL10A1 by site-directed mutagenesis. The data suggest that translocation of the mutant pre-alpha1(X) chains into the microsomes is not affected, but cleavage of the signal peptide is inhibited, and the mutant chains remain anchored to the membrane of microsomes. Cell-free translation and transfection studies in cells showed that the mutant chains associate into trimers but cannot form a triple helix. The combined effect of both the lack of signal peptide cleavage and helical configuration is impaired secretion. Thus, despite the different nature of the NC1 and signal peptide mutations in collagen X, both result in impaired collagen X secretion, probably followed by intracellular retention and degradation of mutant chains, and causing the Schmid metaphyseal chondrodysplasia phenotype.

Bone development

Early development of the vertebrate skeleton depends on genes that pattern the distribution and proliferation of cells from cranial neural crest, sclerotomes, and lateral plate mesoderm into mesenchymal condensations at sites of future skeletal elements. Within these condensations, cells differentiate to chondrocytes or osteoblasts and form cartilages and bones under the control of various transcription factors. In most of the skeleton, organogenesis results in cartilage models of future bones; in these models cartilage is replaced by bone by the process of endochondral ossification. Lastly, through a controlled process of bone growth and remodeling the final skeleton is shaped and molded. Significant and exciting insights into all aspects of vertebrate skeletal development have been obtained through molecular and genetic studies of animal models and humans with inherited disorders of skeletal morphogenesis, organogenesis, and growth.

Mapping of the autosomal recessive (AR) craniometaphyseal dysplasia locus to chromosome region 6q21-22 and confirmation of genetic heterogeneity for mild AR spondylocostal dysplasia

We report on a four-generation inbred family including 10 individuals affected with a form of craniotubular dysplasia (CTD). All affected patients were born to consanguineous healthy parents; this finding, together with the equal sex ratio among affected individuals and the occurrence of only normal individuals among their offspring, indicates that the disease in this family is an autosomal recessive (AR) trait. Taking into account the segregation pattern of the disease in the family and the radiological characteristics of two young CTD patients, the most likely diagnosis for the defect is AR craniometaphyseal dysplasia (CMD). CMD is a CTD, with both autosomal dominant (AD) and recessive forms. The description of the present genealogy confirms the AR pattern of inheritance of some cases of CMD and contributes to a better delineation of the clinical spectrum of AR CMD, suggesting a more pronounced diaphyseal involvement in the AR compared with the AD CMD. Through genomewide scanning, we mapped the AR CMD to a 7 cM interval, between D6S302 and D6S1639, at 6q21-22 region. We have also excluded the positional candidate COL10A1 gene as being the responsible for this disorder. Curiously, a form of AR spondylocostal dysplasia (SD) also segregates in the family, including one affected individual with both conditions. The gene DLL3, mapped to 19q13 region, was recently found to be responsible for one form of AR SD; however, we did not find evidence of linkage between this 19q region and the SD segregating in our family, thus implying in genetic heterogeneity for AR SD.

The genetic basis of the osteochondrodysplasias

The osteochondrodysplasias are a heterogeneous group of disorders characterized by abnormal growth and remodeling of cartilage and bone, affecting from 2 to 4.7 per 10,000 individuals. Most osteochondrodysplasias are heritable and many have elaborate patterns of genetic transmission. Affected individuals generally require management by multidisciplinary teams of specialists. In this review, we divide the osteochondrodysplasias into groups based on their genetic relationships, including mutations in various types of collagen, fibroblast growth factor, cartilage oligomeric matrix protein, parathyroid hormone receptor, the diastrophic dysplasia sulfate transporter, enzymes such as steroid sulfatases, transcription factor SOX9, and a cysteine proteinase, cathepsin K. We describe the major osteochondrodysplasias, define their causes and clinical manifestations, and provide the orthopaedic surgeon with an understanding of the underlying molecular defects as well as the anatomical aspects of these disorders.

Mutations in the N-terminal globular domain of the type X collagen gene (COL10A1) in patients with Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia (SMCD) is a relatively common, heritable osteochondrodysplasia characterized by short-limbed short stature with normal facies, and generalized metaphyseal dysplasias of the long and short tubular bones. Several mutations of the type X collagen gene (COL10A1) have been reported in patients with SMCD, all in the C-terminal globular domain. To address whether mutations in other domains can cause SMCD, we examined the coding region of the COL10A1 gene in DNA samples from six Japanese families affected with SMCD, by direct sequencing. We detected novel mutations in three unrelated SMCD patients; one was a one-base deletion in the C-terminal globular domain and others were de novo missense mutations in the N-terminal globular domain. All three cases revealed a typical clinical phenotype for SMCD. Thus, we have demonstrated that mutations of COL10A1 in regions other than the C-terminal globular domain can cause SMCD, and the results suggest that the N-terminal globular domain also plays an important role in formation of type X collagen.

Genetics of skeletogenesis

Major advances have been made in the last 10 years in the genetics of skeletogenesis. This has followed the general progress in our understanding of the genetic control of development in chicken and mouse and more recent advances in human genetics. This large field now encompasses three smaller but distinct fields of investigation. Those are skeleton patterning, cell differentiation in the skeleton, and cell function in the skeleton. This review focuses primarily on advances in understanding cell differentiation and cell function in the skeleton at the genetic level.

Growth plate compressions and altered hematopoiesis in collagen X null mice

A variable skeleto-hematopoietic phenotype was observed in collagen X null mice which mirrored the defects in transgenic (Tg) mice with dominant interference collagen X mutations (Jacenko, O., P. LuValle, and B.R. Olsen. 1993. Nature. 365:56-61). Specifically, perinatal lethality was seen in approximately 10.8% of null mutants at week three after birth, and in another subset by 12 wk. In perinatal lethal mutants, growth plates were compressed, trabecular bone reduced, and hematopoietic aplasia and erythrocyte-filled vascular sinusoids were apparent in marrows. Lymphatic organs, reduced to approximately 80% that of controls, displayed altered architecture and lymphocyte content. In thymuses, a paucity of cortical CD3(+)/CD4(+)/CD8(+) lymphocytes was consistent with the marrow's inability to replenish maturing T cells. In spleens, an unaltered T cell distribution was coupled with diffuse staining for IgD(+)/B220(+) B cells, whose reduction was prominent in poorly organized lymphatic nodules. Disorderly arrays of splenic macrophages surrounding periarteriolar lymphatic sheaths and a red pulp depletion further complemented the Tg perinatal lethal phenotype. Moreover, subtle growth plate compressions and hematopoietic changes were seen in all null mice. Data from Tg and null mice implicate the disruption of collagen X function in the observed skeleto-hematopoietic defects, and suggest that hypertrophic cartilage and endochondral skeletogenesis may contribute to the marrow microenvironment prerequisite for blood cell differentiation.