COMP Mutations Research Articles

Principles of the differential diagnosis of achondroplasia and pseudoachondroplasia

BACKGROUND: Achondroplasia and pseudoachondroplasia are hereditary systemic skeletal dysplasias characterized by a certain similarity of clinical manifestations; however, they have different etiopathogenetic mechanisms and confirmation methods for molecular genetic diagnosis. Their common phenotypic features often make differential diagnosis difficult during the clinical examination of patients, planning DNA diagnostics, and appropriate time detection of neurosurgical and orthopedic complications.
 AIM: This study aimed to identify differential diagnostic criteria for achondroplasia and pseudoachondroplasia and optimize the strategy for their molecular genetic diagnosis.
 MATERIALS AND METHODS: A comprehensive examination of 76 children from 74 unrelated families aged 1 month to 18 years with phenotypic signs of achondroplasia and pseudoachondroplasia was conducted. To clarify the diagnosis through genealogical and amnestic analysis, clinical and neurological examination data according to the standard method and radiographic data were used. Molecular genetic confirmation of diseases was conducted by searching for hotspot mutations in the FGFR3 gene, assessing the number of GAC repeats located in exon 13 of the COMP gene, and new-generation sequencing of the target panel consisting of 166 genes responsible for hereditary skeletal pathology.
 RESULTS: Based on a comparative analysis of the specific phenotypic characteristics, the criteria for the differential diagnosis of achondroplasia and pseudoachondroplasia were identified. The leading signs of achondroplasia are disproportionate nanism from birth, macrocrania, and facial dysmorphism, which are not specific to pseudoachondroplasia. Certain radiological features are essential in the differential diagnosis of pseudoachondroplasia, which should be considered when referring to patients for molecular genetic analysis. A deletion of the GAC repeat c.1417_1419del in the COMP gene was identified in 27% of patients with pseudoachondroplasia. Thus, the analyses of these two mutations in FGFR3 and COMP were conducted first. In the absence of target mutations, further diagnostic search should be continued with a target panel consisting of 166 genes responsible for hereditary skeletal pathology or whole-exome sequencing.
 CONCLUSIONS: The analysis of the clinical, radiological, and molecular genetic characteristics of patients with achondroplasia and pseudoachondroplasia, together with the literature data analysis, made it possible to clarify the differential diagnostic criteria for these diseases and optimize the algorithm for their molecular genetic diagnosis.

Differential diagnosis of myopathy and multiple epiphysal dysplasia caused by mutations in the <i>COMP</i> gene in children

Background. Multiple epiphysal dysplasia (MED) type 1 (OMIM: 132400) is one of 7 genetic variants of this group of skeletal dysplasias described to date. The disease is caused by mutations in the COMP gene located on chromosome 19p13.1. The presence of muscle hypotonia and ligamentous laxity, as well as a moderate increase in the level of creatinephosphokinase activity, can lead to misdiagnosis of myopathy.Objective: to analyze the clinical and genetic characteristics of type 1 MED caused by mutations in the COMP gene in a series of Russian patients. Differential diagnosis was focused on the distinctive features of the disorder and hereditary myopathies.Materials and methods. We observed 8 patients from 7 families aged 7 to 15 years with MED type 1 caused by heterozygous mutations in the COMP gene. To confirm the diagnosis, the following methods were used: genealogical analysis, clinical examination, neurological examination with psycho-emotional testing, radiography and targeted sequencing of a panel consisting of 166 genes responsible for the development of inherited skeletal pathology.Results. Case history, clinical, radiological and genetic characteristics of 8 patients with MED type 1 caused by mutations in the COMP gene were analyzed. The first clinical manifestations of the disease were recorded from the age of 2–3 years and were characterized by gait disturbances, muscle weakness, difficulties with climbing stairs, frequent falls when walking, the inability to get up from the floor and from a squatting position and hypermobility of the joints. Electroneuromyographic study did not reveal the signs of miopathy. In two patients, a moderate increase in the creatinekinase level of up to 250–360 u / l was found. All patients were surveyed by neurologists for several years with a clinical diagnosis of congenital myopathy. At the age of 5–6 years patients COMPlained knee and ankle pain, which was assumed as rheumatic arthropathy. X-ray examination revealed typical signs of deficient ossification of the epiphyses. The next-generation sequencing analysis revealed seven single nucleotide variants in the COMP gene that lead to MED type 1. Three of the found variants here identified for the first time. As previously described, the majority of nucleotide variants (six out of seven) were localized in the 8–14 exons of the COMP gene and led to amino acid substitutions in calmodulin-like protein domain repeats, and only one substitution was localized in the C-terminal region of the protein molecule.Conclusion. In most patients with MED caused by mutations in the COMP gene, the first symptoms of the disease are gait disturbance, muscle weakness, and Gowers» maneuvers. The presence of these symptoms, along with a moderate increase in the level of creatinephosphokinase activity, often precedes the onset of clinical manifestations of skeletal dysplasia, leading to a misdiagnosis with myopathies. Accession of expressive arthralgias to these symptoms was mistakenly identified as reactive arthritis. X-ray examination of patients’ long bones helps to suspect the presence of MED. This X-ray imaging shows specific signs of epiphyses damage. A molecular-genetic analysis needs to be done to diagnose the genetic variant, caused by mutations in gene COMP.

A Novel COMP Mutated Allele Identified in a Chinese Family with Pseudoachondroplasia

Pseudoachondroplasia (PSACH) is an autosomal dominant skeletal dysplasia with an estimated incidence of ~1/60000 that is characterized by disproportionate short stature, brachydactyly, joint laxity, and early-onset osteoarthritis. COMP encodes the cartilage oligomeric matrix protein, which is expressed predominantly in the extracellular matrix (ECM) surrounding the cells that make up cartilage, ligaments, and tendons. Mutations in COMP are known to give rise to PSACH. In this study, we identified a novel nucleotide mutation (NM_000095.2: c.1317C>G, p.D439E) in COMP responsible for PSACH in a Chinese family by employing whole-exome sequencing (WES) and built the structure model of the mutant protein to clarify its pathogenicity. The novel mutation cosegregated with the affected individuals. Our study expands the spectrum of COMP mutations and further provides additional genetic testing information for other PSACH patients.

Diversity in heritable disorders of connective tissue at a single center

ABSTRACT Background Heritable disorders of connective tissue (HDCT) is a heterogeneous group of conditions caused by defects in genes responsible for extracellular matrix elements. Although next-generation sequencing (NGS) technology can be used to analyze many genes at a time, precisely diagnosing HDCT is still challenging because of the overlapping phenotypes and genotypes. Methods A 67-gene NGS targeted panel or whole-exome sequencing was employed for the diagnosis of HDCT over 4 years. Phenotypes and genotypes of patients were analyzed retrospectively. Results Mutations in 16 genes were discovered in 34 patients with the suspicion of Ehlers-Danlos syndrome (n = 7), Marfan syndrome (n = 2), osteogenesis imperfecta (n = 3), skeletal dysplasia (n = 18), and others (n = 4). Eighteen patients were found to have mutations in collagen genes, three had SERPINF1 mutations, two had TRPV4 mutations, two had FBN1 mutations, two had COMP mutations, and mutations in seven other genes were found in one patient each. The eight patients with COL1A1 mutations had a wide variation in phenotype. Patients with COL3A1 and COL5A1 mutations presented with classic EDS, those with SERPINF1 mutations presented with typical OI type VI, those with TRPV4 mutations presented with severe spinal deformity, and those with COL2A1 mutations presented with syndromic or nonsyndromic bone dysplasia or only short stature. Conclusion A wide diversity in HDCT was observed. Therefore, knowledge about the phenotype-genotype correlation in HDCT is still crucial in the diagnosis of this group of diseases, and an improvement in the screening tool will be needed.

A novel COMP mutation in a Chinese family with multiple epiphyseal dysplasia

BackgroundMultiple epiphyseal dysplasia (MED) is a skeletal disorder characterized by delayed and irregular ossification of the epiphyses and early-onset osteoarthritis. At least 66% of the reported autosomal dominant MED (AD-MED) cases are caused by COMP mutations.MethodsWe recruited a four-generation Chinese family with early-onset hip osteoarthritis, flatfoot, brachydactyly, and mild short stature. An assessment of the family history, detailed physical examinations, and radiographic evaluations were performed on the proband and other family members, followed by the performance of whole-exome sequencing (WES). The pathogenicity of the candidate mutation was also analyzed.ResultsAn AD-MED family with 10 affected members and 17 unaffected members was recruited. The main radiographic findings were symmetrical changes in the dysplastic acetabulum and femoral heads, irregular contours of the epiphyses, a shortened femoral neck, and flatfoot. Lower bone density was also observed in the ankle joints, wrist joints, and knees, as well as irregular vertebral end plates. In the proband, we identified the missense mutation c.1153G > T (p. Asp385Tyr), located in exon 11 of the COMP gene. This mutation was assessed as 'pathogenic' because of its low allele frequency and its high likelihood of co-segregation with disease in the reported family. Sanger sequencing validated the novel heterozygous mutation c.1153G > T (p. Asp385Tyr) in exon 11 of COMP in all affected individuals in the family.ConclusionsOur results underlined a key role of the Asp385 amino acid in the protein function of COMP and confirmed the pathogenicity of the COMP (c.1153G > T; p. Asp385Tyr) mutation in AD-MED disease. We have therefore expanded the known mutational spectrum of COMP and revealed new phenotypic information for AD-MED.

Mutant COMP shapes growth and development of skull and facial structures in mice and humans.

BackgroundCartilage oligomeric matrix protein (COMP) is an important extracellular matrix protein primarily functioning in the musculoskeletal tissues and especially endochondral bone growth. Mutations in COMP cause the skeletal dysplasia pseudoachondroplasia (PSACH) that is characterized by short limbs and fingers, joint laxity, and abnormalities but a striking lack of skull and facial abnormalities.MethodsThis study examined both mice and humans to determine how mutant‐COMP affects face and skull growth.ResultsMutant COMP (MT‐COMP) mice were phenotypically distinct. Snout length and skull height were diminished in MT‐COMP mouse and the face more closely resembled younger controls. Three‐dimensional facial measurements of PSACH faces showed widely spaced eyes, reduced lower facial height, and decreased nasal protrusion, which correlated with a more juvenile appearing face. Neither MT‐COMP mice nor PSACH individuals show midface hypoplasia usually associated with abnormal endochondral bone growth. MT‐COMP mice do show delayed endochondral and membranous skull ossification that normalizes with age.ConclusionTherefore, mutant‐COMP affects both endochondral and intramembranous bones of the skull resulting in a reduction of the nose and lower facial height in mice and humans, in addition to its well‐defined role in the growth plate chondrocytes.

The 2019 FASEB Science Research Conference on Matricellular Proteins in Inflammation and Tissue Remodeling, July 14-19, 2019, Lisbon, Portugal.

The 2019 FASEB Science Research Conference on Matricellular Proteins in Inflammation and Tissue Remodeling, July 14-19, 2019, Lisbon, Portugal.

Age Dependent Progression of Multiple Epiphyseal Dysplasia and Pseudoachondroplasia Due to Heterozygous Mutations in COMP Gene.

Dominantly inherited mutations in COMP gene encoding cartilage oligomeric matrix protein may cause two dwarfing skeletal dysplasias, milder multiple epiphyseal dysplasia (MED) and more severe pseudoachondroplasia (PSACH). We studied the phenotype and X-rays of 11 patients from 5 unrelated families with different COMP mutations. Whole exome and/or Sangers sequencing were used for molecular analyses. Four to ten X-ray images of hands hips, knees or spine were available for each patient for retrospective analyses. Eight patients with MED have mutation c.1220G>A and 3 children with PSACH mutations c.1359C>A, c.1336G>A, or the novel mutation c.1126G>T in COMP. Progressive failure in growth developed in all patients from early childhood and resulted in short stature < 3rd percentile in 7 patients and very short stature < 1st percentile in four. Most patients had joint pain since childhood, severe stiffness in shoulders and elbows but increased mobility in wrists. Six children had bowlegs and two had knock knees. In all patients, X-rays of hands, hips and knees showed progressive, age-dependent skeletal involvement more pronounced in the epiphyses of long rather than short tubular bones. Anterior elongation and biconvex configuration of vertebral bodies were more conspicuous for kids. Six children had correction of knees and two adults had hip replacement. Skeletal and joint impairment in patients with MED and PSACH due to COMP mutation start in early childhood. Although the clinical severity is mutation and age dependent, many symptoms represent a continuous phenotypic spectrum between both diseases. Most patients may benefit from orthopaedic surgeries.

A Chinese Family with Pseudoachondroplasia Caused by <i>COMP</i> Gene Mutation

Pseudoachondroplasia (PSACH; MIM 177170) is a rare disease which was characterized by disproportionate short stature, deformity of the lower limbs, brachydactyly, loose joints, and ligamentous laxity. It is an autosomal dominant osteochondrodysplasia presented in childhood, and usually resolved with age, but osteoarthritis is progressive and severe. Genetic testing using the whole exome sequencing and Sanger sequencing was performed for the patients, a 30-year-old woman and her affected son, who is only 8 years old. A heterozygous mutationin exon 15 of COMP (c.1675G > A, p.Glu559Lys, NM 000095.2) was identified. The Polyphen-2 predicted that the mutation may damage the COMP protein function. This study suggested that the heterozygous mutations in COMP were responsible for PSACH and demonstrated the genotype-phenotype relationship between mutations in COMP and clinical characteristics of PSACH.

MED resulting from recessively inherited mutations in the gene encoding calcium-activated nucleotidase CANT1.

Multiple Epiphyseal Dysplasia (MED) is a relatively mild skeletal dysplasia characterized by mild short stature, joint pain, and early-onset osteoarthropathy. Dominantly inherited mutations in COMP, MATN3, COL9A1, COL9A2, and COL9A3, and recessively inherited mutations in SLC26A2, account for the molecular basis of disease in about 80-85% of the cases. In two families with recurrent MED of an unknown molecular basis, we used exome sequencing and candidate gene analysis to identify homozygosity for recessively inherited missense mutations in CANT1, which encodes calcium-activated nucleotidase 1. The MED phenotype is thus allelic to the more severe Desbuquois dysplasia phenotype and the results identify CANT1 as a second locus for recessively inherited MED.

Comparison of orthopaedic manifestations of multiple epiphyseal dysplasia caused by MATN3 versus COMP mutations: a case control study

BackgroundMultiple epiphyseal dysplasia (MED) is a relatively common skeletal dysplasia mainly involving the epiphyses of the long bones. However, it is a genetically heterogeneous group of diseases sharing certain aspects of the radiologic phenotype. In surveys conducted in East Asia, MATN3 was the most common causative gene, followed by COMP. In this study, the authors compared clinical manifestation of MED patients caused by MATN3 and COMP gene mutations, as well as subsequent orthopaedic interventions.MethodsFifty nine molecularly-confirmed MED patients were subjects of this study. The MATN3 gene mutation group comprised of 37 patients (9 female, 28 male). The COMP gene mutation consisted of 22 cases (15 females, 7 males). Medical records and radiographs were reviewed, and questionnaire surveys or telephone interviews were conducted.ResultsAt the first presentation, the mean age was 8.8 ± 2.8 years (mean ± standard deviation) in the MATN3 group, and 8.5 ± 3.5 years in the COMP group (p = 0.670). The height in the COMP group was significantly shorter than those in the MATN3 group (p < 0.001). Gait abnormality at the first visit (p = 0.041) and the lastest follow-up (p = 0.037) were statistically significant difference. Hip pain (p = 0.084), limitation of daily activity (p = 0.075) at the latest follow-up tended to be more frequent in the COMP group. Hip dysplasia was more common in the COMP group, having significantly larger acetabular angle (p = 0.037), smaller center-edge angle (p = 0.002), severe Stulberg classification (p < 0.001), and smaller femoral head coverage (p < 0.001).ConclusionsClinical manifestations of MED caused by MATN3 were milder than manifestations of the COMP mutation group. These differences in clinical manifestation and prognosis justify molecular differentiation between the two genotypes.

Mild Myopathy Is Associated with COMP but Not MATN3 Mutations in Mouse Models of Genetic Skeletal Diseases

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are skeletal disorders resulting from mutations in COMP, matrilin-3 or collagen IX and are characterised by short-limbed dwarfism and premature osteoarthritis. Interestingly, recent reports suggest patients can also manifest with muscle weakness. Here we present a detailed analysis of two mouse models of the PSACH/MED disease spectrum; ΔD469 T3-COMP (PSACH) and V194D matrilin-3 (MED). In grip test experiments T3-COMP mice were weaker than wild-type littermates, whereas V194D mice behaved as controls, confirming that short-limbed dwarfism alone does not contribute to PSACH/MED-related muscle weakness. Muscles from T3-COMP mice showed an increase in centronuclear fibers at the myotendinous junction. T3-COMP tendons became more lax in cyclic testing and showed thicker collagen fibers when compared with wild-type tissue; matrilin-3 mutant tissues were indistinguishable from controls. This comprehensive study of the myopathy associated with PSACH/MED mutations enables a better understanding of the disease progression, confirms that it is genotype specific and that the limb weakness originates from muscle and tendon pathology rather than short-limbed dwarfism itself. Since some patients are primarily diagnosed with neuromuscular symptoms, this study will facilitate better awareness of the differential diagnoses that might be associated with the PSACH/MED spectrum and subsequent care of PSACH/MED patients.

Analysis of the cartilage proteome from three different mouse models of genetic skeletal diseases reveals common and discrete disease signatures

SummaryPseudoachondroplasia and multiple epiphyseal dysplasia are genetic skeletal diseases resulting from mutations in cartilage structural proteins. Electron microscopy and immunohistochemistry previously showed that the appearance of the cartilage extracellular matrix (ECM) in targeted mouse models of these diseases is disrupted; however, the precise changes in ECM organization and the pathological consequences remain unknown. Our aim was to determine the effects of matrilin-3 and COMP mutations on the composition and extractability of ECM components to inform how these detrimental changes might influence cartilage organization and degeneration.Cartilage was sequentially extracted using increasing denaturants and the extraction profiles of specific proteins determined using SDS-PAGE/Western blotting. Furthermore, the relative composition of protein pools was determined using mass spectrometry for a non-biased semi-quantitative analysis.Western blotting revealed changes in the extraction of matrilins, COMP and collagen IX in mutant cartilage. Mass spectrometry confirmed quantitative changes in the extraction of structural and non-structural ECM proteins, including proteins with roles in cellular processes such as protein folding and trafficking. In particular, genotype-specific differences in the extraction of collagens XII and XIV and tenascins C and X were identified; interestingly, increased expression of several of these genes has recently been implicated in susceptibility and/or progression of murine osteoarthritis.We demonstrated that mutation of matrilin-3 and COMP caused changes in the extractability of other cartilage proteins and that proteomic analyses of Matn3 V194D, Comp T585M and Comp DelD469 mouse models revealed both common and discrete disease signatures that provide novel insight into skeletal disease mechanisms and cartilage degradation.

Comparative Proteomic Analysis of Normal and Collagen IX Null Mouse Cartilage Reveals Altered Extracellular Matrix Composition and Novel Components of the Collagen IX Interactome

Collagen IX is an integral cartilage extracellular matrix component important in skeletal development and joint function. Proteomic analysis and validation studies revealed novel alterations in collagen IX null cartilage. Matrilin-4, collagen XII, thrombospondin-4, fibronectin, βig-h3, and epiphycan are components of the in vivo collagen IX interactome. We applied a proteomics approach to advance our understanding of collagen IX ablation in cartilage. The cartilage extracellular matrix is essential for endochondral bone development and joint function. In addition to the major aggrecan/collagen II framework, the interacting complex of collagen IX, matrilin-3, and cartilage oligomeric matrix protein (COMP) is essential for cartilage matrix stability, as mutations in Col9a1, Col9a2, Col9a3, Comp, and Matn3 genes cause multiple epiphyseal dysplasia, in which patients develop early onset osteoarthritis. In mice, collagen IX ablation results in severely disturbed growth plate organization, hypocellular regions, and abnormal chondrocyte shape. This abnormal differentiation is likely to involve altered cell-matrix interactions but the mechanism is not known. To investigate the molecular basis of the collagen IX null phenotype we analyzed global differences in protein abundance between wild-type and knock-out femoral head cartilage by capillary HPLC tandem mass spectrometry. We identified 297 proteins in 3-day cartilage and 397 proteins in 21-day cartilage. Components that were differentially abundant between wild-type and collagen IX-deficient cartilage included 15 extracellular matrix proteins. Collagen IX ablation was associated with dramatically reduced COMP and matrilin-3, consistent with known interactions. Matrilin-1, matrilin-4, epiphycan, and thrombospondin-4 levels were reduced in collagen IX null cartilage, providing the first in vivo evidence for these proteins belonging to the collagen IX interactome. Thrombospondin-4 expression was reduced at the mRNA level, whereas matrilin-4 was verified as a novel collagen IX-binding protein. Furthermore, changes in TGFβ-induced protein βig-h3 and fibronectin abundance were found in the collagen IX knock-out but not associated with COMP ablation, indicating specific involvement in the abnormal collagen IX null cartilage. In addition, the more widespread expression of collagen XII in the collagen IX-deficient cartilage suggests an attempted compensatory response to the absence of collagen IX. Our differential proteomic analysis of cartilage is a novel approach to identify candidate matrix protein interactions in vivo, underpinning further analysis of mutant cartilage lacking other matrix components or harboring disease-causing mutations.

A novel COMP mutation in a Chinese patient with pseudoachondroplasia

A 2.75-year-old Chinese boy presented with typical clinical features of pseudoachondroplasia, including disproportionate short-limb short stature, brachydactyly, genu varus and waddling gait. Radiologically, tubular bones were short with widened metaphyses, irregular and small epiphyses; anterior tonguing or beaking of vertebral bodies were characteristic. DNA sequencing analysis of the COMP gene revealed a heterozygous mutation (c.1511G>A, p.Cys504Tyr) in the patient but his parents were unaffected without this genetic change. The missense mutation (c.1511G>A) was not found in 100 healthy controls and has not been reported previously. Our findings expand the spectrum of known mutations in COMP leading to pseudoachondroplasia.

A novel form of chondrocyte stress is triggered by a COMP mutation causing pseudoachondroplasia

Pseudoachondroplasia (PSACH) results from mutations in cartilage oligomeric matrix protein (COMP) and the p.D469del mutation within the type III repeats of COMP accounts for approximately 30% of PSACH. To determine disease mechanisms of PSACH in vivo, we introduced the Comp D469del mutation into the mouse genome. Mutant animals were normal at birth but grew slower than their wild-type littermates and developed short-limb dwarfism. In the growth plates of mutant mice chondrocyte columns were reduced in number and poorly organized, while mutant COMP was retained within the endoplasmic reticulum (ER) of cells. Chondrocyte proliferation was reduced and apoptosis was both increased and spatially dysregulated. Previous studies on COMP mutations have shown mutant COMP is co-localized with chaperone proteins, and we have reported an unfolded protein response (UPR) in mouse models of PSACH-MED (multiple epiphyseal dysplasia) harboring mutations in Comp (T585M) and Matn3, Comp etc (V194D). However, we found no evidence of UPR in this mouse model of PSACH. In contrast, microarray analysis identified expression changes in groups of genes implicated in oxidative stress, cell cycle regulation, and apoptosis, which is consistent with the chondrocyte pathology. Overall, these data suggest that a novel form of chondrocyte stress triggered by the expression of mutant COMP is central to the pathogenesis of PSACH. Hum Mutat 33:218–231, 2012. © 2011 Wiley Periodicals, Inc.

Revisit of multiple epiphyseal dysplasia: Ethnic difference in genotypes and comparison of radiographic features linked to the COMP and MATN3 genes

Multiple epiphyseal dysplasia (MED) is a genetically heterogeneous group of diseases characterized by variable degrees of epiphyseal abnormality primarily involving the hip and knee joints. The purpose of this study was to investigate the frequency of mutations in individuals with a clinical and radiographic diagnosis of MED and to test the hypothesis that characteristic radiological findings may be helpful in predicting the gene responsible. The radiographs of 74 Korean patients were evaluated by a panel of skeletal dysplasia experts. Six genes known to be associated with MED (COMP, MATN3, COL9A1, COL9A2, COL9A3, and DTDST) were screened by sequencing. Mutations were found in 55 of the 63 patients (87%). MATN3 mutations were found in 30 patients (55%), followed by COMP mutations in 23 (41%), and COL9A2 and DTDST mutations in one patient (2%) each. Comparisons of radiographic findings in patients with COMP and MATN3 mutations showed that albeit marked abnormalities in hip and knee joints were observed in both groups, the degree of involvement and the morphology of dysplastic epiphyses differed markedly. The contour of the pelvic acetabulum, the presence of metaphyseal vertical striations, and/or the brachydactyly of the hand were also found to be highly correlated with the genotypes. The study confirms that MATN3 and COMP are the genes most frequently responsible for MED and that subtle radiographic signs may give precious indications on which gene(s) should be prioritized for mutational screening in a given individual.

Case Report A novel COMP mutation in an Inuit patient with pseudoachondroplasia and severe short stature

Pseudoachondroplasia (PSACH) is an autosomal dominant skeletal dysplasia, generally identified clinically at two years of age due to decreased linear growth and a waddling gait. Radiographic features include small and irregular epiphyses, with metaphyseal changes of the long bones and characteristic vertebral changes. Mutations in the COMP gene cause PSACH and some cases of multiple epiphyseal dysplasia. Mutations generally cluster in the calmodulin-like repeat regions of the gene. Mutations in exon 13 (encoding the seventh calmodulin-like repeat) have been associated with severe short stature (-6 SD) in PSACH. We examined an Inuit boy with PSACH and severe short stature. Height essentially remained at -1 SD on the PSACH growth curve (-7.5 SD on a normal growth curve at 10.5 years). Analysis of COMP in our patient revealed a previously undescribed heterozygous A>T substitution in exon 8, at nucleotide 812. This change in the sequence resulted in replacement of a highly conserved and negatively charged aspartic acid with an uncharged, hydrophobic valine at amino acid position 271. Both unaffected parents were negative for this genetic change. This exon encodes the first calmodulin-like repeat, which has not been previously implicated in severe short stature. We propose that this novel missense substitution is responsible for the phenotype of this patient.

Multilayered patella: Similar radiographic findings in pseudoachondroplasia and recessive multiple epiphyseal dysplasia

A multilayered patella is a characteristic radiographic finding of recessive multiple epiphyseal dysplasia (rMED) caused by DTDST mutations. However it has been recently reported in a dominant MED case with a COL9A2 mutation. We report on a new radiographic patellar finding in a patient with pseudoachondroplasia and a heterozygous COMP mutation. It is similar to the radiographic appearance of fusing multilayered patellae in rMED cases. This led us to search the International Skeletal Dysplasia Registry for similar abnormalities. We did not observe this finding in other skeletal dysplasias or other pseudoachondroplasia cases. However we found an accessory ossification center of the patella in another pseudoachondroplasia case. Thus, we hypothesize that variable defects of cartilage extracellular matrix can result in similar abnormal patellar ossifications, and emphasize the importance of a lateral knee radiograph in patients with the pseudoachondroplasia-MED bone dysplasia group of disorders.

Model systems for studying skeletal dysplasias caused by TSP-5/COMP mutations.

Cartilage oligomeric matrix protein, also known as thrombospondin-5 (TSP-5), is an extracellular matrix protein found primarily in cartilage and musculoskeletal tissues. TSP-5 is of interest because mutations in the gene cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED/EDM1). Both PSACH and EDM1 have a characteristic chondrocyte phenotype distinguished by giant rough endoplasmic reticulum (rER) cisternae containing TSP-5 and other extracellular matrix proteins such as type IX collagen and matrilin-3. The accumulation of proteinaceous material in the rER compromises cellular function and leads to premature chondrocyte death. Both in vitro and in vivo models have been generated with varying degrees of success to study the cellular mechanisms of the disease process. Here we review and discuss in vitro and in vivo PSACH and MED model systems and describe two transgenic mouse lines expressing human mutant TSP-5 protein. These model systems have revealed several important features of the PSACH cellular pathology: unfolded protein response activation, upregulation of apoptosis and inappropriate assembly of matrix network in the rER. Some of these models are valuable reagents that may be of use in testing therapeutic interventions. (Part of a Multiauthor Review).