Bilateral juvenile osteochondrosis dissecans in monozygotic twins: a case report.

INTRODUCTION: The etiology of osteochondrosis dissecans (OCD), a chondropathy associated with detachment of the subchondral bone and the overlaying cartilage, is not yet fully understood. While repetitive physical exercise-related stress is usually assumed to be the main risk factor for the occurrence of OCD, genetic predisposition could have an underestimated influence on the development of the disease. CASE REPORT: We report a case of monozygotic twins with almost identical stages of bilateral osteochondrosis dissecans of the knee joint. In both patients, initially, a unilateral lesion occurred; despite restricted physical exercise, in the further course of the disease a lesion also developed on the contralateral side. While the lesion found most recently demonstrated an ongoing healing process at a 6-month follow-up, the other three lesions showed a natural course of healing under conservative treatment with significant clinical as well as radiological improvements after one year and complete consolidation in magnetic resonance imaging (MRI) after 2 years. CONCLUSION: There could be a genetic component to the development of OCD, although this has not yet been proven. Based on a two-year MRI follow-up, we were able to show the self-limiting characteristics of juvenile osteochondrosis dissecans.

microRNAs are differentially expressed in equine plasma of horses with osteoarthritis and osteochondritis dissecans versus control horses.

Osteoarthritis (OA) is a leading cause of lameness in horses with no effective disease-modifying treatment and challenging early diagnosis. OA is considered a disease of the joint involving the articular cartilage, subchondral bone, synovial membrane, and ligaments. Osteochondritis dissecans (OCD) is a joint disease consisting of focal defects in the osteochondral unit which may progress to OA later in life. MicroRNAs (miRNAs) have been recognized as small non-coding RNAs that regulate a variety of biological processes and have been detected in biological fluids. MiRNAs are currently investigated for their utility as biomarkers and druggable targets for a variety of diseases. The current study hypothesizes that miRNA profiles can be used to actively monitor joint health and differences in miRNA profiles will be found in healthy vs diseased joints and that differences will be detectable in blood plasma of tested horses. Five horses with OA, OCD, and 4 controls (C) had blood plasma and synovial fluid collected. Total RNA, including miRNA was isolated before generating miRNA libraries from the plasma of the horses. Libraries were sequenced at the Schroeder Arthritis Institute (Toronto). Differential expression analysis was done using DESeq2 and validated using ddPCR. KEGG pathway analysis was done using mirPath v.3 (Diana Tools). 57 differentially expressed miRNAs were identified in OA vs C plasma, 45 differentially expressed miRNAs in OC vs C plasma, and 21 differentially expressed miRNAs in OA vs OCD plasma. Notably, miR-140-5p expression was observed to be elevated in OA synovial fluid suggesting that miR-140-5p may serve as a protective marker early on to attenuate OA progression. KEGG pathway analysis of differentially expressed plasma miRNAs showed relationships with glycan degradation, glycosaminoglycan degradation, and hippo signaling pathway. Interestingly, ddPCR was unable to validate the NGS data suggesting that isomiRs may play an integral role in miRNA expression when assessed using NGS technologies.

Novel missense ACAN gene variants linked to familial osteochondritis dissecans cluster in the C-terminal globular domain of aggrecan.

The cartilage aggrecan proteoglycan is crucial for both skeletal growth and articular cartilage function. A number of aggrecan (ACAN) gene variants have been linked to skeletal disorders, ranging from short stature to severe chondrodyplasias. Osteochondritis dissecans is a disorder where articular cartilage and subchondral bone fragments come loose from the articular surface. We previously reported a missense ACAN variant linked to familial osteochondritis dissecans, with short stature and early onset osteoarthritis, and now describe three novel ACAN gene variants from additional families with this disorder. Like the previously described variant, these are autosomal dominant missense variants, resulting in single amino acid residue substitutions in the C-type lectin repeat of the aggrecan G3 domain. Functional studies showed that neither recombinant variant proteins, nor full-length variant aggrecan proteoglycan from heterozygous patient cartilage, were secreted to the same level as wild-type aggrecan. The variant proteins also showed decreased binding to known cartilage extracellular matrix ligands. Mapping these and other ACAN variants linked to hereditary skeletal disorders showed a clustering of osteochondritis dissecans-linked variants to the G3 domain. Taken together, this supports a link between missense ACAN variants affecting the aggrecan G3 domain and hereditary osteochondritis dissecans.

Gene expression analysis of subchondral bone, cartilage, and synovium in naturally occurring equine palmar/plantar osteochondral disease.

Osteoarthritis (OA) is a disease of the entire joint but the relationship between pathological events in various joint tissues is poorly understood. We examined concurrent changes in bone, cartilage, and synovium in a naturally occurring equine model of joint degeneration. Joints (n = 64) were grossly assessed for palmar/plantar osteochondral disease (POD) in racehorses that required euthanasia for unrelated reasons and assigned a grade of 0 (n = 34), 1 (n = 17), 2 or 3 (n = 13) using a recognized grading scheme. Synovium, cartilage, and subchondral bone were collected for histological and gene expression analysis. Relations between POD grade, cartilage histological score, and gene expression levels were examined using one-way analysis of variance or Kruskal-Wallis test and Spearman's correlation coefficient with corrections for multiple comparisons. Cartilage histological score increased in joints with POD grade 1 (p = 0.002) and 2 or 3 (p < 0.001) compared to 0. At grade 1, expression of COL1A1, COL2A1, and MMP1 increased and BGN decreased in subchondral bone while expression of BGN and ACAN decreased in cartilage. These changes further progressed at grades 2 and 3. POD grades 2 and 3 were associated with decreased expression of osteoclast inhibitor OPG and increased markers of cartilage degeneration (MMP13, COL1A1). Expression of the vascular endothelial growth factor decreased with POD grade and negatively correlated with cartilage histological score. Synovium showed no histological or transcriptomic changes related to pathology grade. Cartilage degeneration in POD is likely to be secondary to remodeling of the subchondral bone. Limited activation of proinflammatory and catabolic genes and moderate synovial pathology suggests distinct molecular phenotype of POD compared with OA.

Variable expressivity in a family with an aggrecanopathy.

BACKGROUND: Osteochondritis dissecans is a condition wherein there is a subchondral bone lesion that causes pain, inflammation, and cartilage damage. Dominant Familial Osteochondritis Dissecans is a rare and severe form of osteochondritis dissecans (OCD). It is caused by heterozygous pathogenic variants in the gene encoding Aggrecan; ACAN. Aggrecan, a proteoglycan, is an essential component of the articular and growth plate cartilage. METHODS: Herein, we report three individuals from one family; the proband who presented with short stature, a lower limb bone exostosis, and bilateral knee and elbow OCD at the age of 13 years old. His twin brother presented with isolated short stature and his father with short stature and lumbar disc herniation. RESULTS: Next-generation sequencing of the ACAN gene in the proband identified a frameshift variant which is also present in the brother and father with short stature. The proband was treated surgically with bilateral elbow microfracture, after the failure of conservative therapy. CONCLUSION: To the best of our knowledge, this is the first patient with an aggrecanopathy who presents with osteochondritis dissecans due to a frameshift variant. This family presents with variable expressivity which might be attributed to modifier genes.

Osteochondritis Dissecans of the Knee: Etiology and Pathogenetic Mechanisms. A Systematic Review.

OBJECTIVE: The purpose of this manuscript is to analyze the evidence regarding etiopathogenesis of knee osteochondritis dissecans (OCD) lesions through a systematic review, so to summate the current understanding of the origin and progression of this pathologic articular processes. DESIGN: A systematic review of the literature was performed on the PubMed and Cochrane databases on October 2017 by 2 independent authors and included all levels of evidence. This included all English language literature, pertaining specifically to etiopathology of knee OCD with exclusions for review articles and expert opinion. Of 965 identified records, 154 full-text articles were assessed for eligibility and 86 studies met the inclusion criteria. RESULTS: According to these studies, the etiology of OCD can be of a biological or mechanical origin: 40 articles proposed a biological hypothesis, including genetic causes (27), ossification center deficit (12), and endocrine disorders (9); conversely, 52 articles supported a mechanical hypothesis, including injury/overuse (18), tibial spine impingement (5), discoid meniscus (16), and biomechanical alterations (20) as the cause of the onset of OCD. The pathogenic processes were investigated by 36 of these articles, with a focus on subchondral bone fracture and ischemia as the ultimate events leading to OCD. CONCLUSIONS: Biological and mechanical factors are found to result in subchondral bone remodeling alterations, acting independently or more likely synergically in the progression of knee OCD. The former includes genetic causes, deficit of ossification centers and endocrine disorders; the latter, tibial spine impingement, discoid meniscus, and biomechanical alterations, together with injuries and overuse. The resultant subchondral bone ischemia and/or fracturing appears to determine the onset and progression of OCD. LEVEL OF EVIDENCE: Systematic review of level II-IV studies, level IV.

A child with a novel ACAN missense variant mimicking a septic arthritis.

Heterozygous mutations of the ACAN gene have been associated with a broad spectrum of non-lethal skeletal dysplasias, called Aggrecanopathies. We report a case of a child with severe inflammatory elbow involvement mimicking septic arthritis who carried the new ACAN missense variant c.6970 T > C, p.Trp2324Arg. The comprehensive clinical evaluation of the patient and his family, focused on the associated clinical features (facial dysmorphisms, short stature, brachydactily), led us to suspect a hereditary condition. Our findings suggest that Aggrecanopathies should be considered in children with familial short stature, poor growth spurt and joint involvement.

Osteochondritis dissecans (OCD) in Horses - Molecular Background of its Pathogenesis and Perspectives for Progenitor Stem Cell Therapy.

Osteochondrosis (osteochondrosis dissecans; OCD) is a disease syndrome of growing cartilage related to different clinical entities such as epiphysitis, subchondral cysts and angular carpal deformities, which occurs in growing animals of all species, including horses. Nowadays, these disorders are affecting increasing numbers of young horses worldwide. As a complex multifactorial disease, OCD is initiated when failure in cartilage canals because of existing ischemia, chondrocyte biogenesis impairment as well as biochemical and genetic disruptions occur. Recently, particular attention have been accorded to the definition of possible relations between OCD and some metabolic disorders; in this way, implication of mitochondrial dysfunctions, endoplasmic reticulum disruptions, oxidative stress or endocrinological affections are among the most considered axes for future researches. As one of the most frequent cause of impaired orthopaedic potential, which may result in a sharp decrease in athletic performances of the affected animals, and lead to the occurrence of complications such as joint fragility and laminitis, OCD remains as one of the primary causes of considerable economic losses in all sections of the equine industry. It would therefore be important to provide more information on the exact pathophysiological mechanism(s) underlying early OC(D) lesions, in order to implement innovative strategies involving the use of progenitor stem cells, which are considered nowadays as a promising approach to regenerative medicine, with the potential to treat numerous orthopaedic disorders, including osteo-degenerative diseases, for prevention and reduction of incidence of the disease, not only in horses, but also in human medicine, as the equine model is already widely accepted by the scientific community and approved by the FDA, for the research and application of cellular therapies in the treatment of human conditions.

Candidate Loci are Revealed by an Initial Genome-wide Association Study of Juvenile Osteochondritis Dissecans.

BACKGROUND: Osteochondritis dissecans (OCD) is a condition that oftentimes causes significant knee pain in pediatric patient populations. If left untreated, OCD significantly increases the risk of developing degenerative osteoarthritis along with its associated consequences and costs. Although a genetic component has been suggested to play a role in this disorder, few studies have been carried out in order to determine the underlying genetic etiology of this relatively common complex trait. The goal of our study was to perform an initial genome-wide association study (GWAS) to uncover candidate loci associated with the pathogenesis of OCD. METHODS: Blood samples were acquired from 2 cohorts, aged 0 to 18 years old, consisting of 209 OCD cases and 1855 population-matched controls. Agencourt Genfind DNA isolation technology was used to isolate high-quality DNA from each sample. Genotype data was then generated utilizing the Illumina Infinium BeadChip array to examine single-nucleotide polymorphisms (SNPs). RESULTS: In an initial GWAS analysis of our cohort, where a SNP was excluded if the Hardy-Weinberg Equilibrium test P<0.0001, the minor allele frequency<5%, and the genotyping call rate<90%, we obtained our first results for OCD. Although there was no SNP strictly reaching the threshold for genome-wide significance at this early stage, multiple SNPs (35) at several loci revealed evidence of suggestive association with OCD (P<5.0×10). CONCLUSIONS: The results from our preliminary study are encouraging. Herein we not only discuss the relevance and applicability of GWAS in studying a genetic basis for OCD, but have also identified top signals that may suggest loci involved in coordinated expression as well as a transcription factor involved in development that may be highly relevant to this trait. CLINICAL RELEVANCE: If genetic predispositions for OCD are detected early enough in life, attempts at activity modification, counseling, and orthopaedic monitoring may successfully reduce progression of this condition, which may lead to progressive osteoarthritis in the third to fourth decade in at-risk patients.

Osteochondritis Dissecans Lesions in Family Members: Does a Positive Family History Impact Phenotypic Potency?

BACKGROUND: Although repetitive microtrauma and athletic overuse patterns are most commonly associated with osteochondritis dissecans (OCD), recent studies have identified a potential genetic predisposition for OCD. Several case series have documented family pedigrees that support autosomal-dominant inheritance, but the families in these studies were all selected as a result of unique histories that may not accurately represent OCD inheritance patterns at large. Because there has been little investigation beyond these case reports, we aimed to describe a broader, more representative pattern of OCD inheritance applicable to all affected patients. QUESTIONS/PURPOSES: (1) What proportion of patients treated for OCD of the knee have one or more immediate and/or extended family members with a history of OCD lesions? (2) Do patients with more phenotypically potent lesions, which we defined as patients with bilateral OCD lesions or patients who have undergone multiple procedures for OCD, have a higher frequency of affected relatives than those with less potent lesions? METHODS: This retrospective study queried patient databases, diagnosis codes (International Classification of Diseases, 9th Revision), and surgical logs at a regional, tertiary care children's hospital to identify all patients treated over a 10-year period (March 2004-March 2014) by the senior author for OCD of the knee. All patients aged 0-18 years at the time of diagnosis were included. At our institution, patients with intact lesions are treated with a trial of conservative therapy; conversely, patients with a break in the articular cartilage and/or loose fragments of bone/cartilage are treated surgically. There were no OCD-specific contraindications to surgery. This search identified 543 patients. After patient identification, a questionnaire was designed that asked for the number, age, and gender of all immediate family members and the history of OCD lesions in any family member (immediate or extended). For all positive family members, patients were further queried regarding relevant clinical details to affirm a history of OCD. Patients were contacted by mailed questionnaires and phone calls for survey completion. All 543 patients received the survey, of which 103 (19%) responded to it and were included here. Responders were approximately 1 year younger than nonresponders; there was no difference in gender distribution. A retrospective chart review was then conducted to collect demographic information, phenotypic disease severity, and treatment details. Of the 103 included patients, 20 patients (19%) with unilateral lesions were managed nonoperatively ("conservative" group), 50 patients (49%) had unilateral lesions advanced to surgery ("unilateral" group), 21 patients (20%) had bilateral lesions managed either conservatively or surgically ("bilateral" group), and 12 patients (12%) underwent multiple procedures for the same lesion ("multiple" group). Of those included, 75 patients (73%) were treated surgically. With the numbers available, there were no baseline differences among the groups in terms of gender, lesion laterality, lesion location, or number of secondary procedures at the time of the initial surgical intervention. RESULTS: In total, 14 of 103 (14%) patients treated for OCD in this study had an immediate and/or extended family member with a history of OCD lesions. This included four of 20 (20%) patients in the conservative group, five of 50 (10%) in the unilateral group, four of 21 (19%) in the bilateral group, and one of 12 (8%) in the multiple group. With the numbers available, we did not identify a higher proportion of immediate and/or extended family members with a positive history of OCD in those patients with phenotypically potent lesions (bilateral and multiple) as compared with those with patients phenotypically less potent lesions (conservative and unilateral; five of 33 [15%] versus nine of 70 [13%]; odds ratio, 1.2; 95% confidence interval, 0.4-3.9; p = 0.751). CONCLUSIONS: In this broad, heterogeneous cohort of pediatric patients with OCD, the proportion of patients with a positive family history of OCD was 14%, which appeared to be much higher than the reported prevalence of OCD in the general population according to prior research. Therefore, our data provide preliminary support for a familial inheritance pattern for OCD, suggesting that further clinical, biologic, and genomic investigation might help to improve our understanding of who is at highest risk for OCD and what moderating factors might influence their disease severity and risk of progression. Furthermore, our data suggest that expanded patient education and screening regarding inheritance patterns might enhance identification of potential familial disease burden and improve access to timely and appropriate treatment. LEVEL OF EVIDENCE: Level IV, prognostic study.

Clinical Characterization of Patients With Autosomal Dominant Short Stature due to Aggrecan Mutations.

CONTEXT: Heterozygous mutations in the aggrecan gene (ACAN) cause autosomal dominant short stature with accelerated skeletal maturation. OBJECTIVE: We sought to characterize the phenotypic spectrum and response to growth-promoting therapies. PATIENTS AND METHODS: One hundred three individuals (57 females, 46 males) from 20 families with autosomal dominant short stature and heterozygous ACAN mutations were identified and confirmed using whole-exome sequencing, targeted next-generation sequencing, and/or Sanger sequencing. Clinical information was collected from the medical records. RESULTS: Identified ACAN variants showed perfect cosegregation with phenotype. Adult individuals had mildly disproportionate short stature [median height, -2.8 standard deviation score (SDS); range, -5.9 to -0.9] and a history of early growth cessation. The condition was frequently associated with early-onset osteoarthritis (12 families) and intervertebral disc disease (9 families). No apparent genotype-phenotype correlation was found between the type of ACAN mutation and the presence of joint complaints. Childhood height was less affected (median height, -2.0 SDS; range, -4.2 to -0.6). Most children with ACAN mutations had advanced bone age (bone age - chronologic age; median, +1.3 years; range, +0.0 to +3.7 years). Nineteen individuals had received growth hormone therapy with some evidence of increased growth velocity. CONCLUSIONS: Heterozygous ACAN mutations result in a phenotypic spectrum ranging from mild and proportionate short stature to a mild skeletal dysplasia with disproportionate short stature and brachydactyly. Many affected individuals developed early-onset osteoarthritis and degenerative disc disease, suggesting dysfunction of the articular cartilage and intervertebral disc cartilage. Additional studies are needed to determine the optimal treatment strategy for these patients.

Chondrocytes Derived From Mesenchymal Stromal Cells and Induced Pluripotent Cells of Patients With Familial Osteochondritis Dissecans Exhibit an Endoplasmic Reticulum Stress Response and Defective Matrix Assembly.

UNLABELLED: : Familial osteochondritis dissecans (FOCD) is an inherited skeletal defect characterized by the development of large cartilage lesions in multiple joints, short stature, and early onset of severe osteoarthritis. It is associated with a heterozygous mutation in the ACAN gene, resulting in a Val-Met replacement in the C-type lectin domain of aggrecan. To understand the cellular pathogenesis of this condition, we studied the chondrogenic differentiation of patient bone marrow mesenchymal stromal cells (BM-MSCs). We also looked at cartilage derived from induced pluripotent stem cells (iPSCs) generated from patient fibroblasts. Our results revealed several characteristics of the differentiated chondrocytes that help to explain the disease phenotype and susceptibility to cartilage injury. First, patient chondrogenic pellets had poor structural integrity but were rich in glycosaminoglycan. Second, it was evident that large amounts of aggrecan accumulated within the endoplasmic reticulum of chondrocytes differentiated from both BM-MSCs and iPSCs. In turn, there was a marked absence of aggrecan in the extracellular matrix. Third, it was evident that matrix synthesis and assembly were globally dysregulated. These results highlight some of the abnormal aspects of chondrogenesis in these patient cells and help to explain the underlying cellular pathology. The results suggest that FOCD is a chondrocyte aggrecanosis with associated matrix dysregulation. The work provides a new in vitro model of osteoarthritis and cartilage degeneration based on the use of iPSCs and highlights how insights into disease phenotype and pathogenesis can be uncovered by studying differentiation of patient stem cells. SIGNIFICANCE: The isolation and study of patient stem cells and the development of methods for the generation of iPSCs have opened up exciting opportunities in understanding causes and exploring new treatments for major diseases. This technology was used to unravel the cellular phenotype in a severe form of inherited osteoarthritis, termed familial osteochondritis dissecans. The phenotypic abnormalities that give rise to cartilage lesions in these patients were able to be described via the generation of chondrocytes from bone marrow-derived mesenchymal stromal cells and iPSCs, illustrating the extraordinary value of these approaches in disease modeling.

A suspected genetic form of bilateral osteochondritis dissecans of the knee in a Dutch family.

Osteochondritis dissecans (OCD) mostly has an idiopathic origin, but syndromic and familial forms have been reported. Mutations of the aggrecan (ACAN) and COL9A2 genes are associated with familial OCD, but these patients present with syndromic features. This article describes a mother and a daughter who both have bilateral OCD of the medial femoral condyles, and the monozygotic twin sister of the mother who has confirmed unilateral OCD (and possible bilateral OCD) of the medial femoral condyle. No short stature or any other syndromic features were present. None of the syndromic features associated with ACAN or COL9A2 mutations or any other known syndromes were present in this case. This case suggests a possible unknown genetic anomaly.

Emerging genetic basis of osteochondritis dissecans.

Genome-wide association studies (GWAS) provide an unbiased approach in the identification of genes that increase the risk for osteochondritis dissecans (OCD). Recent GWAS in humans, horses, and pigs are reviewed and genes identified. The identified genes tended to cluster with respect to function and biologic processes. GWAS in humans are a critical next step in the effort to provide a better understanding of the causes of OCD, which will, in turn, allow preventive strategies for treatment of adolescents and young adults who are at risk for the development of degenerative joint disease due to the effects of OCD.

Involvement of mitochondrial dysfunction and ER-stress in the physiopathology of equine osteochondritis dissecans (OCD).

Osteochondrosis (OC) is a developmental bone disorder affecting several mammalian species including the horse. Equine OC is described as a focal disruption of endochondral ossification, leading to osteochondral lesions (osteochondritis dissecans, OCD) that may release free bodies within the joint. OCD lesions trigger joint swelling, stiffness and lameness and affects about 30% of the equine population. OCD is considered as multifactorial but its physiopathology is still poorly understood and genes involved in genetic predisposition are still unknown. Our study compared two healthy and two OC-affected 18-month-old French Trotters diagnosed with OCD lesions at the intermediate ridge of the distal tibia. A comparative shot-gun proteomic analysis of non-wounded cartilage and sub-chondral bone from healthy (healthy samples) and OC-affected foals (predisposed samples) identified 83 and 53 modulated proteins, respectively. These proteins are involved in various biological pathways including matrix structure and maintenance, protein biosynthesis, folding and transport, mitochondrial activity, energy and calcium metabolism. Transmission electron microscopy revealed typical features of mitochondrial swelling and ER-stress, such as large, empty mitochondria, and hyper-dilated rough endoplasmic reticulum, in the deep zone of both OC lesions and predisposed cartilage. Abnormal fibril organization surrounding chondrocytes and abnormal features at the ossification front were also observed. Combining these findings with quantitative trait loci and whole genome sequencing results identified about 140 functional candidate genes carrying putative damaging mutations in 30 QTL regions. In summary, our study suggests that OCD lesions may result from defective hypertrophic terminal differentiation associated with mitochondrial dysfunction and ER-stress, leading to impaired cartilage and bone biomechanical properties, making them prone to fractures. In addition, 11 modulated proteins and several candidate mutations located in QTL regions were identified, bringing new insight into the molecular physiopathology and genetic basis of OCD.

Identical osteochondritis dissecans lesions of the knee in sets of monozygotic twins.

Osteochondritis dissecans (OCD) is a focal, idiopathic alteration of subchondral bone structure with the risk for secondary damage to adjacent articular cartilage and the development of premature osteoarthritis. The exact etiology of OCD is unknown, although repetitive microtrauma and vascular insufficiency have been previously described. A genetic predisposition has been suggested, but the existing evidence is sparse. There are multiple case reports of twins and siblings with OCD and a few large family series in the literature, promoting the theory that OCD may have a genetic component to its etiology. This article describes 2 sets of monozygotic twins presenting concurrently with OCD of their dominant knees, offering further support for a genetic component to the etiology of OCD. Interestingly, in both sets of twins, 1 was left-handed and 1 was right-handed. Both sets of twins had simultaneous presentations and clinical courses, lending support to a genetic element to OCD. The development of the OCD lesion in the dominant knee of each patient suggests an environmental influence, perhaps due to repetitive microtrauma and overuse. Recently, a genome-wide linkage study identified a prime candidate locus for OCD. However, despite the suggested association, genetic and developmental factors in the development of OCD remain relatively unstudied. The authors believe monozygotic twins provide an excellent clinical opportunity for future examination of the role of familial inheritance in the etiology of OCD.

The different roles of aggrecan interaction domains.

The aggregating proteoglycans of the lectican family are important components of extracellular matrices. Aggrecan is the most well studied of these and is central to cartilage biomechanical properties and skeletal development. Key to its biological function is the fixed charge of the many glycosaminoglycan chains, that provide the basis for the viscoelastic properties necessary for load distribution over the articular surface. This review is focused on the globular domains of aggrecan and their role in anchoring the proteoglycans to other extracellular matrix components. The N-terminal G1 domain is vital in that it binds the proteoglycan to hyaluronan in ternary complex with link protein, retaining the proteoglycan in the tissue. The importance of the C-terminal G3 domain interactions has recently been emphasized by two different human hereditary disorders: autosomal recessive aggrecan-type spondyloepimetaphyseal dysplasia and autosomal dominant familial osteochondritis dissecans. In these two conditions, different missense mutations in the aggrecan C-type lectin repeat have been described. The resulting amino acid replacements affect the ligand interactions of the G3 domain, albeit with widely different phenotypic outcomes.

A genome-wide association study of osteochondritis dissecans in the Thoroughbred.

Osteochondrosis is a developmental orthopaedic disease that occurs in horses, other livestock species, companion animal species, and humans. The principal aim of this study was to identify quantitative trait loci (QTL) associated with osteochondritis dissecans (OCD) in the Thoroughbred using a genome-wide association study. A secondary objective was to test the effect of previously identified QTL in the current population. Over 300 horses, classified as cases or controls according to clinical findings, were genotyped for the Illumina Equine SNP50 BeadChip. An animal model was first implemented in order to adjust each horse's phenotypic status for average relatedness among horses and other potentially confounding factors which were present in the data. The genome-wide association test was then conducted on the residuals from the animal model. A single SNP on chromosome 3 was found to be associated with OCD at a genome-wide level of significance, as determined by permutation. According to the current sequence annotation, the SNP is located in an intergenic region of the genome. The effects of 24 SNPs, representing QTL previously identified in a sample of Hanoverian Warmblood horses, were tested directly in the animal model. When fitted alongside the significant SNP on ECA3, two of these SNPs were found to be associated with OCD. Confirmation of the putative QTL identified on ECA3 requires validation in an independent sample. The results of this study suggest that a significant challenge faced by equine researchers is the generation of sufficiently large data sets to effectively study complex diseases such as osteochondrosis.

Type IX collagen gene mutations can result in multiple epiphyseal dysplasia that is associated with osteochondritis dissecans and a mild myopathy.

Multiple epiphyseal dysplasia (MED) is a clinically variable and genetically heterogeneous disease that is characterized by mild short stature and early onset osteoarthritis. Autosomal dominant forms are caused by mutations in the genes that encode type IX collagen, cartilage oligomeric matrix protein, and matrilin-3: COL9A1, COL9A2, COL9A3, COMP, and MATN3, respectively. Splicing mutations have been identified in all three genes encoding type IX collagen and are restricted to specific exons encoding an equivalent region of the COL3 domain in all three alpha(IX) chains. MED has been associated with mild myopathy in some families, in particular one family with a COL9A3 mutation and two families with C-terminal COMP mutations. In this study we have identified COL9A2 mutations in two families with MED that also have osteochondritis dissecans and mild myopathy. This study therefore extends the range of gene-mutations that can cause MED-related myopathy. (c) 2010 Wiley-Liss, Inc.

A missense mutation in the aggrecan C-type lectin domain disrupts extracellular matrix interactions and causes dominant familial osteochondritis dissecans.

Osteochondritis dissecans is a disorder in which fragments of articular cartilage and subchondral bone dislodge from the joint surface. We analyzed a five-generation family in which affected members had autosomal-dominant familial osteochondritis dissecans. A genome-wide linkage analysis identified aggrecan (ACAN) as a prime candidate gene for the disorder. Sequence analysis of ACAN revealed heterozygosity for a missense mutation (c.6907G > A) in affected individuals, resulting in a p.V2303M amino acid substitution in the aggrecan G3 domain C-type lectin, which mediates interactions with other proteins in the cartilage extracellular matrix. Binding studies with recombinant mutated and wild-type G3 proteins showed loss of fibulin-1, fibulin-2, and tenascin-R interactions for the V2303M protein. Mass spectrometric analyses of aggrecan purified from patient cartilage verified that V2303M aggrecan is produced and present in the tissue. Our results provide a molecular mechanism for the etiology of familial osteochondritis dissecans and show the importance of the aggrecan C-type lectin interactions for cartilage function in vivo.