An unfolded protein response is the initial cellular response to the expression of mutant matrilin-3 in a mouse model of multiple epiphyseal dysplasia

Multiple epiphyseal dysplasia (MED) can result from mutations in matrilin-3, a structural protein of the cartilage extracellular matrix. We have previously shown that in a mouse model of MED the tibia growth plates were normal at birth but developed a progressive dysplasia characterised by the intracellular retention of mutant matrilin-3 and abnormal chondrocyte morphology. By 3 weeks of age, mutant mice displayed a significant decrease in chondrocyte proliferation and dysregulated apoptosis. The aim of this current study was to identify the initial post-natal stages of the disease. We confirmed that the disease phenotype is seen in rib and xiphoid cartilage and, like tibia growth plate cartilage is characterised by the intracellular retention of mutant matrilin-3. Gene expression profiling showed a significant activation of classical unfolded protein response (UPR) genes in mutant chondrocytes at 5 days of age, which was still maintained by 21 days of age. Interestingly, we also noted the upregulation of arginine-rich, mutated in early stage of tumours (ARMET) and cysteine-rich with EGF-like domain protein 2 (CRELD2) are two genes that have only recently been implicated in the UPR. This endoplasmic reticulum (ER) stress and UPR did not lead to increased chondrocyte apoptosis in mutant cartilage by 5 days of age. In an attempt to alleviate ER stress, mutant mice were fed with a chemical chaperone, 4-sodium phenylbutyrate (SPB). SPB at the dosage used had no effect on chaperone expression at 5 days of age but modestly decreased levels of chaperone proteins at 3 weeks. However, this did not lead to increased secretion of mutant matrilin-3 and in the long term did not improve the disease phenotype. We performed similar studies with a mouse model of Schmid metaphyseal chondrodysplasia, but again this treatment did not improve the phenotype.

Mutations causing Greenberg dysplasia but not Pelger anomaly uncouple enzymatic from structural functions of a nuclear membrane protein

The lamin B receptor (LBR) is an inner nuclear membrane protein with a structural function interacting with chromatin and lamins, and an enzymatic function as a sterol reductase. Heterozygous LBR mutations cause nuclear hyposegmentation in neutrophils (Pelger anomaly), while homozygous mutations cause prenatal death with skeletal defects and abnormal sterol metabolism (Greenberg dysplasia). It has remained unclear whether the lethality in Greenberg dysplasia is due to cholesterol defects or altered nuclear morphology.To answer this question we characterized two LBR missense mutations and showed that they cause Greenberg dysplasia. Both mutations affect residues that are evolutionary conserved among sterol reductases. In contrast to wildtype LBR, both mutations failed to rescue C14 sterol reductase deficient yeast, indicating an enzymatic defect. We found no Pelger anomaly in the carrier parent excluding marked effects on nuclear structure. We studied Lbr in mouse embryos and demonstrate expression in skin and the developing skeletal system consistent with sites of histological changes in Greenberg dysplasia. Unexpectedly we found in disease-relevant cell types not only nuclear but also cytoplasmatic LBR localization. The cytoplasmatic LBR staining co-localized with ER-markers and is thus consistent with the sites of endogeneous sterol synthesis. We conclude that LBR missense mutations can abolish sterol reductase activity, causing lethal Greenberg dysplasia but not Pelger anomaly. The findings separate the metabolic from the structural function and indicate that the sterol reductase activity is essential for human intrauterine development.

An inducible cartilage oligomeric matrix protein mouse model recapitulates human pseudoachondroplasia phenotype

Cartilage oligomeric matrix protein (COMP) is a pentameric extracellular protein expressed in cartilage and other musculoskeletal tissues. Mutations in the COMP gene cause pseudoachondroplasia (PSACH), a severe dwarfing condition that has a growth plate chondrocyte pathology. PSACH is characterized by intracellular retention of COMP and other extracellular matrix (ECM) proteins, which form an ordered matrix within large rough endoplasmic reticulum cisternae. This accumulation is cytotoxic and causes premature chondrocyte cell death, thereby depleting chondrocytes needed for normal long bone growth. Research to define the underlying molecular mechanisms of PSACH has been hampered by the lack of a suitable model system. In this study, we achieved robust expression of human mutant (MT) or wild-type (WT) COMP in mice by using a tetracycline-inducible promoter. Normal growth plate distribution of ECM proteins was observed in 1-month-old WT-COMP and C57BL\6 control mice. In contrast, the structure of the MT-COMP growth plate recapitulated the findings of human PSACH growth plate morphology, including (1) retention of ECM proteins, (2) intracellular matrix formation in the rER cisternae, and (3) increased chondrocyte apoptosis. Therefore, we have generated the first mouse model to show extensive intracellular retention of ECM proteins recapitulating the human PSACH disease process at the cellular level.

Identification of novel mutations in WISP3 gene in two unrelated Chinese families with progressive pseudorheumatoid dysplasia

INTRODUCTION

Progressive pseudorheumatoid dysplasia (PPD) is an autosomal recessive genetic disease and it has been reported that PPD is caused by mutations of the Wnt1-inducible signaling pathway protein 3 (WISP3) gene which is located on chromosome 6q22. Up to date, 16 different mutations in the WISP3 have been identified in patients with PPD in different countries previously, but only two mutations in exon 5 were previously identified from Asian origin. Our study aimed to characterize the clinical manifestations and features of PPD and screen the mutations of the disease causing WISP3, and try to elucidate the molecular pathogenesis of PPD.

MATERIALS AND METHODS

Altogether, 153 persons, including 4 affected individuals, 49 unaffected individuals from two unrelated Chinese families, and 100 healthy donors were recruited and genomic DNA was extracted. PPD was diagnosed based on the clinical manifestations, physical examination, characteristics of their bones on X-ray and laboratory results. All 5 exons and their exon-intron boundaries of the WISP3 gene were amplified by polymerase chain reaction (PCR) and sequenced directly.

RESULTS

In family 1, we identified that the proband (IV4) carried a novel non-sense mutation (G46X) which consisted of a homozygous C to T transition at c.8004 in exon 3. This mutation changed codon CAG to TAG and resulted in a subsequent change of the glutamine codon to stop codon and truncation at p. 46. In family 2, a novel missense mutation (C114Y) was found in the three patients (IV6, IV7, IV8), namely, a homozygous G to A transition at c.8209 in exon 3, which resulted in a cysteine (TGT) to tyrosine (TAT) substitution at p.114. Neither G46X nor C114Y was found in 100 normal controls. Meanwhile, we found that these patients had some different phenotypes, compared with the affected individuals with PPD from cases reported previously.

CONCLUSIONS

Our study suggests that the novel G46X and C114Y mutations in exon 3 in WISP3 gene are responsible for PPD in Chinese patients. Furthermore, many heterozygous carriers (c.8004C>T and c.8209G>A) are found in the two families, suggesting the existence of a founder effect in the locality where they live, respectively.

[Lethal osteo-chondro-dysplasia: feto-pathological study of 32 cases]

BACKGROUND

The lethal osteochondrodysplasias are rare, their prevalence is estimated at 1 per 10 000 births. Mostly have genetic determinism.

AIMS

To describe the malformations and dysmorphic features in lethal osteochondrodysplasias

METHODS

Our study involved 32 cases of lethal fetal Osteochondrodysplasias, collected over a period of 14 years in the pathological department of Sousse.

RESULTS

Our series consisted of 23 foetuses from a medical termination of pregnancy, 6 newborns and 3 stillbirths. The mean age of mothers was 28 years old, consanguinity was observed in 61%. 3 cases of recurrence of the disease in three families were noted. The bone abnormalities were detected in antenatal ultrasonography in 25 cases (87%) and at birth in 7 cases. Ultrasound showed micromelia in all cases, a narrow chest in 5 cases and spina bifida in 3 cases. The feto-pathological exam, including a macroscopic examination, radiological and histological samples of bone, has allowed us, based on the International Classification of 2001 to classify the 32 cases of Osteochondrodysplasias in: 8 cases of Achondrogenesis type I (type Parenti-Fraccaro), 3 cases of Achondrogenesis type II (Langer Saldino), 9 cases of lethal osteogenesis imperfecta, 8 cases of thanatophoric dysplasia, 4 cases of Schneckenbecken dysplasia, 2 cases of Short rib polydactyly syndrome, Majewski type and 1 case of asphyxiating thoracic dysplasia.

[Jeune syndrome (asphyxiating thoracic dystrophy): a case report]

Jeune syndrome or asphyxiating thoracic dystrophy is a rare autosomal recessive skeletal dysplasia characterized by a small thorax, short-limbed dwarfism, renal and hepatic anomalies. It is estimated to occur in 1 per 100.000-130.000 live births. Radiological confirmation of diagnosis is essential. Prognosis of the disease is due to the severity of chest deformities and most of the patients are lost in their first year because of respiratory problems. We presented a 19-month-old female patient who admitted to our clinic because of recurrent lower respiratory tract infections and diagnosed as Jeune syndrome and reported due to its rarity.

Demonstration of the interaction of transforming growth factor beta 2 and type X collagen using a modified tandem affinity purification tag

Like other members of the transforming growth factor beta (TGF-beta) family of growth factors, the biological activity of TGF-beta2 is believed to be regulated by the formation and dissociation of multiprotein complexes. To isolate the molecular complex formed by TGF-beta2 secreted by hypertrophic chondrocytes we have used expression of TGF-beta2 fused with the humanized, tandem affinity purification (hTAP) tag and mass spectrometry for the identification of interacting proteins. The hTAP synthetic gene was assembled by systematically replacing the rare codons of the original TAP tag with codons most preferred in highly expressed human genes to circumvent the poor translation efficiency of the original TAP tag in animal cells. TGF-beta2 was shown to interact with Type X collagen and this interaction confirmed using V5 tagged TGF-beta2. Functional interaction was suggested by the inhibition of TGF-beta2 activity by type X collagen in culture and the influence of a mutation in type X collagen on the distribution of TGF-beta2 in growth cartilage.

A novel mutation in the sulfate transporter gene SLC26A2 (DTDST) specific to the Finnish population causes de la Chapelle dysplasia

BACKGROUND

Mutations in the sulfate transporter gene SLC26A2 (DTDST) cause a continuum of skeletal dysplasia phenotypes that includes achondrogenesis type 1B (ACG1B), atelosteogenesis type 2 (AO2), diastrophic dysplasia (DTD), and recessive multiple epiphyseal dysplasia (rMED). In 1972, de la Chapelle et al reported two siblings with a lethal skeletal dysplasia, which was denoted "neonatal osseous dysplasia" and "de la Chapelle dysplasia" (DLCD). It was suggested that DLCD might be part of the SLC26A2 spectrum of phenotypes, both because of the Finnish origin of the original family and of radiographic similarities to ACG1B and AO2.

OBJECTIVE

To test the hypothesis whether SLC26A2 mutations are responsible for DLCD.

METHODS

We studied the DNA from the original DLCD family and from seven Finnish DTD patients in whom we had identified only one copy of IVS1+2T>C, the common Finnish mutation. A novel SLC26A2 mutation was found in all subjects, inserted by site-directed mutagenesis in a vector harbouring the SLC26A2 cDNA, and expressed in sulfate transport deficient Chinese hamster ovary (CHO) cells to measure sulfate uptake activity.

RESULTS

We identified a hitherto undescribed SLC26A2 mutation, T512K, homozygous in the affected subjects and heterozygous in both parents and in the unaffected sister. T512K was then identified as second pathogenic allele in the seven Finnish DTD subjects. Expression studies confirmed pathogenicity.

CONCLUSIONS

DLCD is indeed allelic to the other SLC26A2 disorders. T512K is a second rare "Finnish" mutation that results in DLCD at homozygosity and in DTD when compounded with the milder, common Finnish mutation.

[A novel mutation in the SEDL gene leading to X-linked spondyloepiphyseal dysplasia tarda in a large Chinese pedigree]

OBJECTIVE

To identify the genetic defect in a four-generation pedigree with X-linked recessive spondyloepiphyseal dysplasia tarda (SEDT) from Southwest China.

METHODS

Linkage analysis with one panel of fluorescently labeled microsatellite markers on chromosome X and mutation screening of SEDL gene by direct sequencing were performed.

RESULTS

Linkage between SEDT and Xp22.2-Xp23.1 was established with maximum LOD score of 3.82 (theta = 0) between DXS987 and DXS8051. Upon sequence analysis, a point mutation within exon 4 of the SEDL gene (c.239A to G) was found which resulted in substitution of histidine with arginine at codon 80 (His80Arg).

CONCLUSION

A novel missense mutation (H80R) was identified for SEDL gene in the large Chinese SEDT pedigree.

Metatropic dysplasia in children

Metatropic dysplasia is a rare genetic condition characterized by progressive dwarfism. Metatropic dysplasia is defined as a short limb skeletal dysplasia characterized by dumbell like configuration of long bones, a narrow but normal length of thorax and occasionally a coccygeal appendage similar to a tail. Children born with this condition show different signs and symptoms throughout the childhood. An eight years old boy was admitted in the Paediatric ward of Bangbandhu Sheikh Mujib Medical University (BSMMU), Dhaka with the complain of swelling in the back for 7 1/2 years. On examination the boy had features suggestive of Metatropic dysplasia. Radiological findings included exaggerated curvature of the spine which was different at the different levels giving rise to "S" shaped deformity, pectus carinatum and flattening of the vertebral bodies which were variable at different levels. From the history, clinical examination and radiological findings the boy was diagnosed as a case of metatropic dysplasia.

Tibial dyschondroplasia 40 years later

Tibial dyschondroplasia is a disease of rapid growth rate that occurs in many avian species. It is characterized by an avascular lesion in which the life span of the growth plate chondrocyte is essentially doubled. A characteristic pattern of gene expression and gene product localization has emerged that mimics the pattern observed with endoplasmic reticulum (ER) stress in growth plate chondrocytes. This activates a cell-survival mechanism called autophagy. The initial phases of this mechanism appear to originate in the avascular transition zone of the growth plate. Because specific genes and gene products are associated with autophagy and ER stress, it should now be possible to identify the mechanisms involved in the development of this cartilage abnormality. The potential biochemical pathways responsible for initiating ER stress are discussed.

Effect of temperature during the incubation period on tibial growth plate chondrocyte differentiation and the incidence of tibial dyschondroplasia

Tibial dyschondroplasia (TD) is one of the most prevalent skeletal abnormalities in avian species, causing enormous economic losses and major animal welfare problems. Irregular cell differentiation of the chondrocytes that populate the growth plate has been hypothesized to be involved in the etiology of the disease. We evaluated the effect of incubation temperature at various stages of embryo development and bone formation on growth plate chondrocyte differentiation and the incidence of TD. Eggs were incubated either at a control temperature of 37.8 degrees C, or at 36.9 or 39 degrees C, each for 6 h/ d, during early (0 to 8 d) or late (10 to 18 d) embryo development. At 14 d of incubation and at hatch, tibias were collected and weighed, and their ash and calcium contents were determined. Growth plate chondrocyte differentiation was evaluated by alkaline phosphatase activity and collagen type II and osteopontin gene expression. In addition, the level of the heat-shock protein 90 (Hsp90) was evaluated by immunohistochemistry. The rest of the chicks were raised to 49 d and the incidence of TD was recorded. The incidence of TD increased only when the temperature was altered at the early stages of embryo development, and it was correlated with an increase in tibia ash but not with tibia weight or calcium content. Moreover, increased TD incidence was correlated with delayed chondrocyte differentiation. Early changes in incubation temperature caused an increase in the level of Hsp90 in articular and differentiated chondrocytes of the hypertrophic zone and in the numbers of distinct undifferentiated chondrocytes arranged in columns in the proliferative zone of the growth plate. In summary, the early stages of embryo development and bone formation are of utmost importantance for appropriate growth plate chondrocyte differentiation, and any temperature deviation will increase the subsequent incidence of TD. The increase in TD incidence is probably the result of delayed Hsp90-driven chondrocyte differentiation, supporting the hypothesis that TD is the result of abnormal chondrocyte differentiation.

Czech dysplasia metatarsal type: another type II collagen disorder

Czech dysplasia metatarsal type is an autosomal-dominant disorder characterized by an early-onset, progressive spondyloarthropathy with normal stature. Shortness of third and/or fourth toes is a frequently observed clinical feature. Similarities between individuals with this dysplasia and patients with an R275C mutation in the COL2A1 gene, prompted us to analyze the COL2A1 gene in the original families reported with Czech dysplasia. Targeted sequencing of exon 13 of the COL2A1 gene was performed, followed by sequencing of the remaining exons in case the R275C mutation was not identified. We identified the R275C substitution in two of the original patients reported with Czech dysplasia and three additional patients. All affected individuals had a similar phenotype characterized by normal height, spondyloarthropathy, short postaxial toes and absence of ocular and orofacial anomalies. The R275C mutation was excluded in a third patient reported with Czech dysplasia. However, the identification of the Y1391C mutation in this patient with disproportionate short stature made the diagnosis of spondyloperipheral dysplasia (SPD) more probable. The Y1391C mutation is located in the C-propeptide of the procollagen chain and has been reported before in a patient with the Torrance type of lethal platyspondylic skeletal dysplasia (PLSD-T). Our observation of the same Y1391C mutation in an additional unrelated patient with SPD further supports the evidence that PLSD-T and SPD represent a phenotypic continuum. The R275C mutation in the COL2A1 gene causes a specific type II collagen disorder that was recently delineated as Czech dysplasia.

Tracheopathia osteoplastica misdiagnosed as asthma

Tracheopathia osteoplastica (TPO) is a rare disorder characterized by cartilaginous or bony mucosal nodules in the tracheobronchial tree. Due to most patients being asymptomatic, the majority are diagnosed post-mortem. If symptoms are present, they typically include chronic cough, dyspnea, hemoptysis, hoarseness, and wheezing, with some of these symptoms overlapping with asthma. This case report describes an adult female with chronic cough who was diagnosed with asthma that was refractory to therapy for 8 years. Computed tomography (CT) of the chest revealed several soft tissue masses in the trachea with bronchoscopy revealing TPO in the upper portion of the trachea. She was diagnosed with TPO and started on airway clearance and a nasal corticosteroid spray with resolution of the cough at follow-up 3 months later.

Collagen XI chain misassembly in cartilage of the chondrodysplasia (cho) mouse

Molecular mechanisms controlling the assembly of cartilage-specific types II, IX and XI collagens into a heteropolymeric network of uniformly thin, unbanded fibrils are not well understood, but collagen XI has been implicated. The present study on cartilage from the homozygous chondrodysplasia (cho/cho) mouse adds support to this concept. In the absence of alpha1(XI) collagen chains, thick, banded collagen fibrils are formed in the extracellular matrix of cho/cho cartilage. A functional knock-out of the type XI collagen molecule has been assumed. We have re-examined this at the protein level to see if, rather than a complete knock-out, alternative type XI chain assemblies were formed. Mass spectrometry of purified triple-helical collagen from the rib cartilage of cho/cho mice identified alpha1(V) and alpha2(XI) chains. These chains were recovered in roughly equal amounts based on Coomassie Blue staining of SDS-PAGE gels, in addition to alpha1(II)/alpha3(XI) collagen chains. Using telopeptide-specific antibodies and Western blot analysis, it was further shown that type V/XI trimers were present in the matrix cross-linked to each other and to type II collagen molecules to form heteropolymers. Cartilage from heterozygous (cho/+) mice contained a mix of alpha1(V) and alpha1(XI) chains and a mix of thin and thick fibrils on transmission electron microscopy. In summary, the results imply that native type XI collagen molecules containing an alpha1(XI) chain are required to form uniformly thin fibrils and support a role for type XI collagen as the template for the characteristic type II collagen fibril network of developing cartilage.

Achondrogenesis

In this issue of Pediatric and Developmental Pathology, Aigner and colleagues report a detailed investigation of cartilage matrix changes in a 14-week fetus with achondrogenesis type IA. The changes reported differ from matrix alterations observed in achondrogenesis types IB or II and provide insight into the phenotypic and genotypic differences within this group of skeletal dysplasias.

Achondrogenesis Type IA (Houston-Harris): a still-unresolved molecular phenotype

Achondrogenesis type IA (Houston-Harris) is an extremely rare lethal chondrodysplasia with a characteristic severe disarrangement of endochondral ossification. The growth plate cartilage completely lacks columnar-zone formation and shows chondrocyte expansion due to intracellular vacuoles. This article on a new case of achondrogenesis type IA confirms these findings and demonstrates, on the ultrastructural level, the retention of fine fibrillar material within the rough endoplasmic reticulum (rER). Molecular analysis in the presented case of achondrogenesis type IA did not reveal mutations in the COL2A1 and SLC26A2 genes, which are known to cause achondrogenesis types IB and type II. Although the extracellular cartilage matrix was severely altered, all of the investigated matrix molecules (collagens, aggrecan, matrilins, cartilage oligomeric protein [COMP]) showed a normal distribution pattern. The only exception was type-X collagen, which was significantly reduced. Overall, our study suggests a disturbance in cartilage matrix assembly in the present case due to the retention of some sort of matrix component within the rER. Presumably, as a consequence of this event, processes of chondrocyte maturation and differentiation and endochondral bone formation are severely affected in this case of achondrogenesis type IA.

Reduced cell proliferation and increased apoptosis are significant pathological mechanisms in a murine model of mild pseudoachondroplasia resulting from a mutation in the C-terminal domain of COMP

Pseudoachondroplasia (PSACH) is one of the more common skeletal dysplasias and results from mutations in cartilage oligomeric matrix protein (COMP). Most COMP mutations identified to date cluster in the TSP3 repeat region of COMP and the mutant protein is retained in the rough endoplasmic reticulum (rER) of chondrocytes and may result in increased cell death. In contrast, the pathomolecular mechanism of PSACH resulting from C-terminal domain COMP mutations remain largely unknown. This study describes the generation and analysis of a murine model of mild PSACH resulting from a p.Thr583Met mutation in the C-terminal globular domain (CTD) of COMP. Mutant animals are normal at birth, but grow slower than their wild-type littermates and by 9 weeks of age they have mild short-limb dwarfism. Furthermore, by 16 months of age mutant animals exhibit severe degeneration of articular cartilage, which is consistent with early onset osteoarthritis seen in PSACH patients. In the growth plates of mutant mice the chondrocyte columns are sparser and poorly organized. Mutant COMP is secreted into the extracellular matrix, but its localization is disrupted along with the distribution of several COMP-binding proteins. Although mutant COMP is not retained within the rER there is an unfolded protein/cell stress response and chondrocyte proliferation is significantly reduced, while apoptosis is both increased and spatially dysregulated. Overall, these data suggests a mutation in the CTD of COMP exerts a dominant-negative effect on both intra- and extracellular processes. This ultimately affects the morphology and proliferation of growth plate chondrocytes, eventually leading to chondrodysplasia and reduced long bone growth.

Human immortalized chondrocytes carrying heterozygous FGFR3 mutations: an in vitro model to study chondrodysplasias

Achondroplasia and thanatophoric dysplasia are human chondrodysplasias caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed an immortalized human chondrocyte culture model to study the regulation of chondrocyte functions. One control and eight mutant chondrocytic lines expressing different FGFR3 heterozygous mutations were obtained. FGFR3 signaling pathways were modified in the mutant lines as revealed by the constitutive activation of the STAT pathway and an increased level of P21(WAF1/CIP1) protein. This model will be useful for the study of FGFR3 function in cartilage studies and future therapeutic approaches in chondrodysplasias.

COL10A1 nonsense and frame-shift mutations have a gain-of-function effect on the growth plate in human and mouse metaphyseal chondrodysplasia type Schmid

Missense, nonsense and frame-shift mutations in the collagen X gene (COL10A1) result in metaphyseal chondrodysplasia type Schmid (MCDS). Complete degradation of mutant COL10A1 mRNA by nonsense-mediated decay in human MCDS cartilage implicates haploinsufficiency in the pathogenesis for nonsense mutations in vivo. However, the mechanism is unclear in situations where the mutant mRNA persist. We show that nonsense/frame-shift mutations can elicit a gain-of-function effect, affecting chondrocyte differentiation in the growth plate. In an MCDS proband, heterozygous for a p.Y663X nonsense mutation, the growth plate cartilage contained 64% wild-type and 36% mutant mRNA and the hypertrophic zone was disorganized and expanded. The in vitro translated mutant collagen X chains, which are truncated, were misfolded, unable to assemble into trimers and interfered with the assembly of normal alpha1(X) chains into trimers. Unlike Col10a1 null mutants, transgenic mice (FCdel) bearing the mouse equivalent of a human MCDS p.P620fsX621 mutation, displayed typical characteristics of MCDS with disproportionate shortening of limbs and early onset coxa vara. In FCdel mice, the degree of expansion of the hypertrophic zones was transgene-dosage dependent, being most severe in mice homozygous for the transgene. Chondrocytes in the lower region of the expanded hypertrophic zone expressed markers uncharacteristic of hypertrophic chondrocytes, indicating that differentiation was disrupted. Misfolded FCdel alpha1(X) chains were retained within the endoplasmic reticulum of hypertrophic chondrocytes, activating the unfolded protein response. Our findings provide strong in vivo evidence for a gain-of-function effect that is linked to the activation of endoplasmic reticulum-stress response and altered chondrocyte differentiation, as a possible molecular pathogenesis for MCDS.