Smad3-deficient chondrocytes have enhanced BMP signaling and accelerated differentiation

Smad3 deficiency accelerates chondrocyte maturation and leads to osteoarthritis. Primary chondrocytes without Smad3 lack compensatory increases of TGF-beta signaling factors, but BMP-related gene expression is increased. Smad2 or Smad3 overexpression and BMP blockade abrogate accelerated maturation in Smad3-/- chondrocytes. BMP signaling is increased in TGF-beta deficiency and is required for accelerated chondrocyte maturation.

INTRODUCTION

Disruption of TGF-beta signaling results in accelerated chondrocyte maturation and leads to postnatal dwarfism and premature osteoarthritis. The mechanisms involved in this process were studied using in vitro murine chondrocyte cultures.

MATERIALS AND METHODS

Primary chondrocytes were isolated from the sterna of neonatal wildtype and Smad3-/- mice. Expressions of maturational markers, as well as genes involved in TGF-beta and BMP signaling were examined. Chondrocytes were treated with TGF-beta and BMP-2, and effects on maturation-related genes and BMP/TGF-beta responsive reporters were examined. Recombinant noggin or retroviral vectors expressing Smad2 or Smad3 were added to the cultures.

RESULTS

Expression of colX and other maturational markers was markedly increased in Smad3-/- chondrocytes. Smad3-/- chondrocytes lacked compensatory increases in Smad2, Smad4, TGFRII, Sno, or Smurf2 and had reduced expression of TGF-beta1 and TGFRI. In contrast, Smad1, Smad5, BMP2, and BMP6 expression was increased, suggesting a shift from TGF-beta toward BMP signaling. In Smad3-/- chondrocytes, alternative TGF-beta signaling pathways remained responsive, as shown by luciferase assays. These non-Smad3-dependent TGF-beta pathways reduced colX expression and alkaline phosphatase activity in TGF-beta-treated Smad3-/- cultures, but only partially. In contrast, Smad3-/- chondrocytes were more responsive to BMP-2 treatment and had increased colX expression, phosphoSmads 1, 5, and 8 levels, and luciferase reporter activity. Overexpression of both Smad2 and Smad3 blocked spontaneous maturation in Smad3-deficient chondrocytes. Maturation was also abrogated by the addition of noggin, an extracellular BMP inhibitor.

CONCLUSIONS

These findings show a key role for BMP signaling during the chondrocyte maturation, occurring with loss of TGF-beta signaling with important implications for osteoarthritis and cartilage diseases.

Brachyury and chordoma: the chondroid–chordoid dilemma resolved?

Chordoma, and its relationship to the notochord, has intrigued many researchers over the last two centuries. In particular, the morphological overlap with cartilaginous tumours is striking, and developmental biology has shown a tight relationship between cartilage and the notochord. This is reflected in the expression of common genes in chordoid and chondroid tumours. Wide gene expression analyses have led to the identification of key molecules that might play a crucial role in the pathogenesis of chordoma. Brachyury, a key factor in notochord fate, is significantly differentially expressed in chordoma. This not only gives insight into the histogenesis of this tumour but may also point towards new diagnostic tools in the differential diagnosis between chordoid and chondroid tumours.

Evidence of a major gene from Bayesian segregation analyses of liability to osteochondral diseases in pigs

Bayesian segregation analyses were used to investigate the mode of inheritance of osteochondral lesions (osteochondrosis, OC) in pigs. Data consisted of 1163 animals with OC and their pedigrees included 2891 animals. Mixed-inheritance threshold models (MITM) and several variants of MITM, in conjunction with Markov chain Monte Carlo methods, were developed for the analysis of these (categorical) data. Results showed major genes with significant and substantially higher variances (range 1.384-37.81), compared to the polygenic variance (sigmau2). Consequently, heritabilities for a mixed inheritance (range 0.65-0.90) were much higher than the heritabilities from the polygenes. Disease allele frequencies range was 0.38-0.88. Additional analyses estimating the transmission probabilities of the major gene showed clear evidence for Mendelian segregation of a major gene affecting osteochondrosis. The variants, MITM with informative prior on sigmau2, showed significant improvement in marginal distributions and accuracy of parameters. MITM with a "reduced polygenic model" for parameterization of polygenic effects avoided convergence problems and poor mixing encountered in an "individual polygenic model." In all cases, "shrinkage estimators" for fixed effects avoided unidentifiability for these parameters. The mixed-inheritance linear model (MILM) was also applied to all OC lesions and compared with the MITM. This is the first study to report evidence of major genes for osteochondral lesions in pigs; these results may also form a basis for underpinning the genetic inheritance of this disease in other animals as well as in humans.

An effective case of growth hormone treatment on cartilage-hair hypoplasia

Cartilage-hair hypoplasia (CHH) is an autosomal recessive metaphyseal chondrodysplasia characterized by severe short-limb short stature and hypoplastic hair. The responsible gene for CHH has been identified to be ribonuclease of mitochondrial RNA-processing (RMRP) gene. We examined RMRP genes of a 3-year-old Japanese CHH boy and his family and revealed a novel mutation: 20 bp duplication (TACTCTGTGAAGCTGAGGAC), in promoter region of maternal allele, at nucleotide -3 and a reported 218A>G point mutation in transcribed region of paternal allele. No treatment for CHH has been established so far. Growth hormone (GH) action has its effect on linear growth and on bone remodeling and homeostasis. Recently, GH has been used to improve severe short stature caused by not only GH deficiency (GHD) but also some skeletal dysplasias including achondroplasia. To improve severe short stature, we treated the patient with 0.175 mg kg-1 week-1 of GH for 7 years. His height was improved from -4.2 SD to -3.0 SD by 1 year of GH treatment. Following treatment had given positive effects continuously on his height to -2.6 SD by 3.1 years GH medication. Then, when he was 6 years old, surgical lengthening was performed and his height reached to -2.0 SD. After the surgery, we continued GH treatment. Additional GH treatment of 3.6 more years had kept his height to -2.0 SD. However, when he was 8 years old, because there was an interruption of GH treatment, the velocity of his height was obviously decreased comparing before and during the interruption, which was calculated 3.4 and 2.2 cm/year, respectively, and the SD score was decreased to -2.1 SD. This result of total 7 years of GH treatment suggested that GH treatment significantly improved his disturbed bone growth and had also positive efficacy to keep growth rate. This result implies the connection between GH signal and RMRP gene. Additionally, GH may be considered to be an efficient treatment for CHH.

A newly described hereditary cartilage debonding syndrome

OBJECTIVE

We describe a hereditary chondropathy characterized by extreme cartilage friability and cartilage-bone debonding, which has not previously been described in the literature. We also describe initial studies into the molecular basis of this disorder.

METHODS

Affected family members had multiple shoulder, hip, and knee arthropathies, beginning in the pre-teen years and continuing into adulthood. Various diagnoses had been suggested, including spondyloepiphyseal dysplasia, multiple epiphyseal dysplasia, Legg-Calvé-Perthes disease, and Osgood-Schlatter disease. The affected proband father, his 3 affected children, and unaffected family members provided blood samples, which were examined for single-nucleotide polymorphisms (SNPs) in the chromosome 2 region that included the Frizzled-related protein gene, a soluble Wnt protein signaling antagonist that influences bone and cartilage development.

RESULTS

All affected individuals showed clear similarities, including effusions, large loose bodies, and bubbling and delamination of the cartilage with exposure of subchondral bone. All affected individuals exhibited radiographic changes in the hip, showing femoral head flattening and secondary degenerative arthritis, accompanied by abnormalities in the physical properties of the cartilage that were evident upon arthroscopic examination. Two SNPs were identified in subjects with the hereditary cartilage debonding syndrome. Examination of the siblings and parents of the proband demonstrated, however, that both SNPs were present in the unaffected mother and in 2 of 4 unaffected siblings of the proband.

CONCLUSION

The clinical findings reported here represent a newly defined clinical syndrome characterized by marked cartilage friability and osteochondral debonding. Because the SNPs are present in the general population, and because unaffected members of this family carry the SNPs, these polymorphisms alone are insufficient to result in the observed phenotype.

Role of gene therapy in tissue engineering procedures in rheumatology: the use of animal models

Tissue engineering is not only the application of cells and scaffolds to generate a new tissue but should also bring into play biological principles to guide cellular behavior. A way to modify cellular behavior is genetic modification of the cells used for tissue engineering (gene therapy). In the field of rheumatic diseases, cellular modification by overexpressing anabolic factors, such as insulin-like growth factor-I or transforming growth factor beta, or inhibitors of catabolic cytokines or proteolytic enzymes can protect tissues form further destruction and stimulate tissue repair. To test the effect of transgenes on tissue engineering adequate test systems have to be available. Initial testing can be done in simple in vitro systems. However, animal models are unavoidable to study the interaction between the environment and tissue engineering. Optimal models to study gene therapy in combination with tissue engineering in the field of rheumatology are not available at this moment. Arthritis models are mainly developed in small animals while high-quality tissue engineering experiments ask for a large animal model. Development of animal models that can be used for tissue engineering experiments and mimic end stage arthritic diseases is needed.

NOV (CCN3) regulation in the growth plate and CCN family member expression in cartilage neoplasia

Growth plate chondrocytes undergo a coordinated differentiation process resulting in terminal differentiation and new bone formation. Enchondromas are pre-malignant, benign cartilaginous lesions that arise from growth plate chondrocytes that fail to undergo terminal differentiation. NOV (nephroblastoma overexpressed) is a member of the CCN family of proteins, which share a common multi-modular organization. While the role of NOV in chondrocyte development and cartilage neoplasia is not known, other CCN family members play a role in chondrocyte differentiation, or are differentially regulated in cartilage neoplasia. In embryonic murine growth plates, NOV was expressed in pre-hypertrophic and early hypertrophic chondrocytes. PTHrP treatment (which inhibits terminal differentiation) decreased NOV expression in murine femurs maintained in organ culture, and decreased the activity of a NOV reporter construct in vitro. Expression of the CCN family members NOV, CTGF, CYR61, and WISP-1 was examined in 15 chondrosarcomas of various grades and in three enchondromas. Expression of all of the family members was lower in the higher-grade tumours. As identification of the grade of cartilage neoplasia can sometimes be difficult using histology alone, the level of expression of CCN family members could be a useful adjunct in the determination of tumour grade.

Growth hormone treatment in cartilage-hair hypoplasia: effects on growth and the immune system

Cartilage-hair hypoplasia (CHH) is a rare autosomal recessive disorder characterized by metaphyseal chondrodysplasia with severe growth retardation and impaired immunity. We studied the effects of growth hormone treatment on growth parameters and the immune system in four children with CHH. The effects of growth hormone on growth parameters are the most prominent in patients with the mildest growth retardation. However, the effects are temporary and last only for 1 year. There is no gain in final height. Serum immunoglobulins did not change during growth hormone treatment. We conclude that growth hormone treatment is not beneficial in children with CHH.

Clinical, radiographic, and genetic diagnosis of progressive pseudorheumatoid dysplasia in a patient with severe polyarthropathy

A 14-year-old boy presented with a 10-year history of the "sicca" form of seronegative juvenile idiopathic polyarthritis. Severely limited range of motion, pain, and capsular swelling in both small and large weight-bearing joints left him wheelchair-bound. Erythrocyte sedimentation rate and C-reactive protein were normal. Two-phase bone scan revealed tracer uptake of almost every joint at both early and late time points, indicating pathologic exudation and enhanced bone metabolism consistent with severe arthritis. However, radiographic studies revealed no erosive arthropathy but severe osteopenia, dysplastic bone changes, mega os trigonum, and platyspondylia. A magnetic resonance imaging (MRI) scan of the hips showed no signs of synovitis, pannus, or effusion but cartilage irregularities and subchondral cysts. These findings strongly suggested the diagnosis of progressive pseudorheumatoid dysplasia of childhood, an autosomal-recessive disorder of cartilage homeostasis. The patient carries a novel homozygous two-nucleotide deletion in exon 4 of the WISP3 gene. This genetic disorder is an important differential diagnosis of sicca polyarthritis.

Expression of cartilage growth plate signalling molecules in chondroblastoma

Chondroblastoma (CB) is a rare benign tumour (<1% of all bone tumours) involving epiphyseal long bones (male:female 1.5:1). During development, and in the postnatal period, IHh/PTHrP and FGF signalling molecules control the space and timing of chondrocyte differentiation. Considering the close relationship of CB with the growth plate (age and location), the expression of proteins involved in epiphyseal growth regulation was studied. Twelve cases of CB were retrieved. Immunohistochemistry was performed using antibodies against fibroblast growth factor-2 (FGF-2), fibroblast growth factor receptor-1 (FGFR-1), FGFR-3, bcl-2, p21, parathyroid hormone-related peptide (PTHrP), and parathyroid hormone-related peptide receptor (PTHR1). Three observers evaluated haematoxylin and eosin (H&E)-stained and immunostained slides independently. Semi-quantitative estimation of the matrix, the type of matrix, and immunostaining was performed. Cellular and matrix-rich areas were evaluated separately. Diverse amounts and types of matrix were present in different tumours, as well as within individual tumours. Signalling molecules were expressed in 50-100% of the cases. Higher levels of expression were found in cellular areas than in matrix-rich areas, especially for PTHR1, bcl-2, and FGFR-3. CB is an unusual entity affecting specific sites, showing that both IHh/PTHrP and FGF signalling are active. Higher expression was found in cellular than in matrix-rich areas, as in the proliferating/pre-hypertrophic growth plate zone in comparison with the hypertrophic/calcifying zone. Previous studies have shown the same molecules to be expressed with a similar pattern in chondrosarcomas. The sum of the evaluated features indicates that CB is a neoplasm originating from a mesenchymal cell committed towards chondrogenesis via active growth plate signalling pathways.

Juvenile chondrodermatitis nodularis helicis: a case report and literature review

Chondrodermatitis nodularis helicis (CNH) is a disorder that affects adults. Only one case of juvenile CNH has been reported, in an 8-year-old child who suffered from dermatomyositis. We report another child with juvenile CNH who was not afflicted with dermatomyositis or other systemic disorders. The clinical and histologic evaluations demonstrated CNH on the helix of the right ear in a 16-year-old Caucasian girl who was otherwise healthy. Serologic analysis ruled out an underlying autoimmune disorder. We conclude that juvenile CNH is extremely rare and may occur in patients without dermatomyositis or other systemic disorders.

Neoepitopes as biomarkers of cartilage catabolism

Progressive degradation of articular cartilage is a central feature of arthritis and a major determinant of long term joint dysfunction. There are no treatments able to halt the progression of cartilage destruction presently available, and monitoring the benefit of potential therapies is hampered by our inability to measure the "health" of articular cartilage. Serial radiographic assessment of joint space narrowing, the current gold standard, requires measurements over a prolonged time (1-5 years) and is prone to technical difficulties. Other strategies for evaluating cartilage degradation are needed to enable both short and long term monitoring of disease progression and response to therapy. One avenue that holds promise is the use of biomarkers that accurately reflect the degradative state of the articular cartilage. Antibodies that recognise terminal amino acid sequences generated by proteolysis at specific sites in the core protein of both aggrecan and type II collagen (neoepitope antibodies) have become available in recent years. These antibodies have been invaluable for identifying the proteinases responsible for cartilage breakdown both in vitro and in vivo. The presence of neoepitope sequences generated by specific metalloenzyme cleavage of aggrecan and type II collagen correlates well with the progression of cartilage degeneration, both in vitro and in mouse models of arthritis. Preliminary results with quantitative assays of type II collagen neoepitopes suggest that they may be useful markers of joint disease in humans. Long term studies correlating neoepitope concentration with clinical and radiographic disease are now required to validate the utility of neoepitopes as surrogate markers of cartilage degeneration and joint disease.

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.

Worldwide mutation spectrum in cartilage-hair hypoplasia: ancient founder origin of the major70A-->G mutation of the untranslated RMRP

Pleiotropic, recessively inherited cartilage-hair hypoplasia (CHH) is due to mutations in the untranslated RMRP gene on chromosome 9p13-p12 encoding the RNA component of RNase MRP endoribonuclease. We describe 36 different mutations in this gene in 91 Finnish and 44 non-Finnish CHH families. Based on their nature and localisation, these mutations can be classified into three categories: mutations affecting the promoter region, small changes of conserved nucleotides in the transcript, and insertions and duplications in the 5' end of the transcript. The only known functional region that seemed to avoid mutations was a nucleolar localisation signal region between nucleotides 23-62. The most common mutation in CHH patients was a base substitution G for A at nucleotide 70. This mutation contributed 92% of the mutations in the Finnish CHH patients. Our results using linkage disequilibrium based maximum likelihood estimates with close markers, genealogical studies, and haplotype data suggested that the mutation was introduced to Finland some 3900-4800 years ago, and before the expansion of the population. The same major mutation accounted for 48% of the mutations among CHH patients from other parts of Europe, North and South America, the Near East, and Australia. In the non-Finnish CHH families, the A70G mutation segregated with the same major haplotype, although shorter, as in most of the Finnish families. In 23 out of these 27 chromosomes, the common region extended over 60 kb, and, therefore, all the chromosomes most likely arose from a solitary event many thousands of years ago.

Cytokine imbalance in non-immunological chronic disease

Over the last decade, cytokine imbalances have been associated with a plethora of diseases. While the Th(1)/Th(2) paradigm is widely used to explain the pathogenesis of immunological diseases, the role of cytokine imbalances for non-immunological diseases is still incompletely defined. The major obstacle here is to assess the extent to which non-immunological diseases are influenced by inflammation. Non-immunological diseases cover the whole spectrum from those triggered by infection-as may be the case for Alzheimer's disease-to those where the immune system has no apparent impact at all. Examples of the latter are bone diseases, including post-menopausal osteoporosis and skeletal malformations. In between there are diseases such as intrinsic asthma and osteoarthritis where the impact of the immune system is unclear. Thus far, imbalances affecting tumour necrosis factor (TNF)-alpha and members of the interleukin (IL)-1 and the TGF superfamily have been found in association with all of these diseases. We speculate here that cytokine imbalance will be found in additional diseases and touch on the role in phylogeny of cytokines outside the immune system.

Role of parathyroid hormone-related peptide and Indian hedgehog in skeletal development

Parathyroid hormone-related peptide (PTHrP), which frequently causes the humoral hypercalcemia of malignancy syndrome, is an autocrine/paracrine regulator of chondrocyte proliferation and differentiation that acts through the PTH/PTHrP receptor (PTH1R). PTHrP is generated in response to Indian hedgehog (Ihh), which mediates its actions through the membrane receptor patched, but interacts also with hedgehog-interacting protein (Hip). Mice lacking PTHrP show accelerated chondrocyte differentiation, and thus premature ossification of those bones that are formed through an endochondral process, and similar but more-severe abnormalities are observed in PTH1R-ablated animals. The mirror image of these skeletal findings, i.e., a severe delay in chondrocyte differentiation and endochondral ossification, is observed in transgenic mice that overexpress PTHrP under the control of the alpha1(II) procollagen promoter. Severe abnormalities in chondrocyte proliferation and differentiation are also observed in two genetic disorders in humans that are most likely caused by mutations in the PTH1R. Heterozygous PTH1R mutations that lead to constitutively activity were identified in Jansen metaphyseal chondrodysplasia, and homozygous or compound heterozygous mutations that lead to less-active or completely inactive receptors were identified in patients with Blomstrand lethal chondrodysplasia. Based on the growth plate abnormalities observed in these human disorders and in mice with abnormal expression of either PTHrP or the PTH1R, it appears plausible that impaired expression of PTHrP and/or its receptor contributes to the growth abnormalities in children with end-stage renal disease. In fact, mild-to-moderate renal failure leads in animals to a reduction in PTH1R expression in growth plates and impaired growth, but it remains uncertain whether this contributes to altered chondrocyte growth and differentiation.

Premature termination codon in the aggrecan gene of nanomelia and its influence on mRNA transport and stability

AIM

To analyze the influence of the premature termination codon on mRNA transport and stability

METHODS

Chondrocyte mRNA was isolated from homozygous and heterozygous nanomelic 17-days old embryos and examined by RT-PCR analysis. To analyze aggrecan mRNA stability, mRNA synthesis was inhibited with DRB [5,6 dichloro-1-(-D-ribofuranosyl benzimidazole)], a specific inhibitor of RNA polymerase II. Visualization of the aggrecan alleles was performed by in situ hybridization.

RESULTS

The level of mutant aggrecan mRNA within the nucleus was equal to that of the control, but no mutant mRNA was observed in the cytoplasm. RT-PCR revealed that the mutant transcript was only detectable in the nucleus, compared with house-keeping glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene or collagen type II. A restriction site induced by premature termination codon TAA allowed the distinction of normal and mutant transcripts in chondrocytes derived from embryos heterozygous for the nanomelic mutation. After the treatment with DRB, identical decay rates were demonstrated for both transcripts within the heterozygous nucleus. In situ hybridization showed no abnormal mRNA accumulation.

CONCLUSION

This is the first evidence suggesting that the transcript of the mRNA with the premature termination codon within an exon does exit the nucleus.

Integrated high-resolution BAC, P1, and transcript map of the CHH region in chromosome 9p13

A bacterial artificial chromosome (BAC) and P1 contig of the proximal part of chromosome 9p centromeric of markers D9S165 and D9S304 is described. This 1.1- to 1.7-Mb portion of chromosome 9p13 was previously not physically mapped. It contains 24 genes or expressed sequence tags, five polymorphic AC repeats, and three new polymorphic single-strand conformation polymorphism variants. Several of the genes thus mapped are excellent candidates for disease-causing genes whose loci have previously been assigned to proximal 9p. Our primary interest is in the cartilage-hair hypoplasia gene (CHH) that resides within the contig between markers D9S163 and D9S1791 based on linkage evidence.

Roles of aggrecan, a large chondroitin sulfate proteoglycan, in cartilage structure and function

Aggrecan, a large aggregating proteoglycan, is one of the major structural components of cartilage. Its core protein contains three glubular domains and two glycosaminoglycan-attachment domains. These domains play various roles to maintain cartilage structure and function. An N-terminal globular domain binds hyaluronan and link protein to form huge aggregates. The chondroitin sulfate (CS) chains attach to the CS domain and provide a hydrated, viscous gel that absorbs compressive load. Two autosomal recessive chondrodysplasias, cartilage matrix deficiency (cmd) in mice and nanomelia in chicken are both caused by aggrecan gene mutations. Cmd homozygotes die shortly after birth, while the heterozygotes are born normal. However, cmd heterozygotes develop late onset of spinal disorder, which suggests aggrecan as a candidate gene predisposing individuals to spinal problems. Nanomelia is a useful model to elucidate intracellular trafficking of proteoglycans. Further studies on aggrecan will lead to prophylaxis and treatment of joint destructive diseases such as osteoarthrosis and to elucidation of cartilage development, which is essential for skeletal formation.

Dwarfism and age-associated spinal degeneration of heterozygote cmd mice defective in aggrecan

Mouse cartilage matrix deficiency (cmd) is an autosomal recessive disorder caused by a genetic defect of aggrecan, a large chondroitin sulfate proteoglycan in cartilage. The homozygotes (-/-) are characterized by cleft palate and short limbs, tail, and snout. They die just after birth because of respiratory failure, and the heterozygotes (+/-) appear normal at birth. Here we report that the heterozygotes show dwarfism and develop spinal misalignment with age. Within 19 months of age, they exhibit spastic gait caused by misalignment of the cervical spine and die because of starvation. Histological examination revealed a high incidence of herniation and degeneration of vertebral discs. Electron microscopy showed a degeneration of disc chondrocytes in the heterozygotes. These findings may facilitate the identification of mutations in humans predisposed to spinal degeneration.