Gail C. Jackson

Pseudoachondroplasia and multiple epiphyseal dysplasia: A 7‐year comprehensive analysis of the known disease genes identify novel and recurrent mutations and provides an accurate assessment of their relative contribution

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are relatively common skeletal dysplasias resulting in short‐limbed dwarfism, joint pain, and stiffness. PSACH and the largest proportion of autosomal dominant MED (AD‐MED) results from mutations in cartilage oligomeric matrix protein (COMP); however, AD‐MED is genetically heterogenous and can also result from mutations in matrilin‐3 (MATN3) and type IX collagen (COL9A1, COL9A2, and COL9A3). In contrast, autosomal recessive MED (rMED) appears to result exclusively from mutations in sulphate transporter solute carrier family 26 (SLC26A2). The diagnosis of PSACH and MED can be difficult for the nonexpert due to various complications and similarities with other related diseases and often mutation analysis is requested to either confirm or exclude the diagnosis. Since 2003, the European Skeletal Dysplasia Network (ESDN) has used an on‐line review system to efficiently diagnose cases referred to the network prior to mutation analysis. In this study, we present the molecular findings in 130 patients referred to ESDN, which includes the identification of novel and recurrent mutations in over 100 patients. Furthermore, this study provides the first indication of the relative contribution of each gene and confirms that they account for the majority of PSACH and MED. Hum Mutat 33:144–157, 2012.

Missense mutations in the beta strands of the single A-domain of matrilin-3 result in multiple epiphyseal dysplasia.

Multiple epiphyseal dysplasia (MED) is a relatively mild and clinically variable osteochondrodysplasia in which the hip and knee joints are most frequently affected. Both autosomal dominant and autosomal recessive forms of MED are recognised. The more severe forms of MED are often described as the “Fairbank type”, whereas the milder cases are known as the “Ribbing type”. However, this classification belies a much greater clinical spectrum in which characteristics such as radiographic features, age of onset, degree of lower limb deformity, stature, and long term morbidity such as osteoarthritis are extremely variable.1–4 It is therefore not surprising that MED is genetically heterogeneous, and to date mutations in six different genes have been shown to cause MED.5,6 Mutations in the genes encoding cartilage oligomeric matrix protein ( COMP ), the α1, α2, and α3 chains of type IX collagen ( COL9A1, COL9A2 , and COL9A3 ) and matrilin-3 ( MATN3 ) all result in autosomal dominant MED,7–11 whereas specific mutations in the sulphate transporter 26A2 ( SLC26A2 /DTDST ) have been shown to result in an autosomal recessive form of MED.12,13 Preliminary genotype–phenotype correlations have suggested that the more severe forms of autosomal dominant MED often result from COMP mutations, while the milder forms are more probably caused by mutations in the genes encoding type IX collagen or matrilin-3.14,15 However, the number of patients available for comparative study, particularly those with type IX collagen and matrilin-3 defects, has remained too limited to date to allow any in depth correlations to be derived. The matrilins are a four member family of extracellular matrix proteins; matrilin-1 and -3 are specifically expressed in cartilaginous tissues, while matrilin-2 and -4 have a wider pattern of expression in a variety of extracellular matrices including non-skeletal tissues.16,17 Each member of the family comprise …

Multiple epiphyseal dysplasia mutations in MATN3 cause misfolding of the A-domain and prevent secretion of mutant matrilin-3

Multiple epiphyseal dysplasia (MED) is a relatively common skeletal dysplasia that can present in childhood with a variable phenotype of short stature and pain and stiffness in the large joints, and often progresses to early‐onset osteoarthritis in adulthood. Mutations in the matrilin‐3 gene (MATN3) have recently been shown to underlie some forms of autosomal dominant MED. To date all MED mutations in matrilin‐3 cluster in the single A‐domain, suggesting that they may disrupt the structure and/or function of this important domain. To determine the effects of MATN3 mutations on the structure and function of matrilin‐3 we expressed both normal and mutant matrilin‐3 in mammalian cells. Wild‐type (wt) matrilin‐3 was efficiently secreted into conditioned medium, whereas mutant matrilin‐3 was retained and accumulated within the cell. Furthermore, when the mutant A‐domains were examined individually, they existed primarily in an unfolded conformation. Co‐immunoprecipitation experiments demonstrated that the mutant A‐domains were specifically associated with ERp72, a chaperone protein known to be involved in mediating disulfide bond formation. Light microscopy of cartilage from an MED patient with a MATN3 mutation showed the presence of intracellular material within the chondrocytes, whilst the overall matrix appeared normal. On electron micrographs, the inclusions noted at the light microscopy level appeared to be dilated cisternae of rough endoplasmic reticulum and immunohistochemical analysis confirmed that the retained protein was matrilin‐3. In summary, the data presented in this paper suggest that MED caused by MATN3 mutations is the result of an intracellular retention of the mutant protein. Hum Mutat 26(6), 557–565, 2005.

Clinical and radiographic findings in multiple epiphyseal dysplasia caused by MATN3 mutations: Description of 12 patients

Multiple epiphyseal dysplasia (MED) is characterized by pain and stiffness in joints and delayed and irregular ossification of epiphyses. Causative mutations have been recognized in six different genes. We have identified disease‐causing mutations in the gene encoding matrilin‐3, an extracellular matrix protein, in seven families with autosomal dominant MED. Review of the clinical and radiographic features in 12 of the affected family members shows a uniform pattern of skeletal anomalies in all patients with considerable degree of variability in severity, both between and within families. The characteristic clinical findings are onset of symptoms in early childhood with predominance of knee and hip related complaints, normal stature, and early‐onset osteoarthritis. Radiographs show small and irregular epiphyses and mild metaphyseal irregularities and striations, especially at the knees and hips and mild spinal changes. Despite overlap, both clinically and radiographically, with other forms of MED, the described features may help to differentiate this particular form from other entities within the MED spectrum.

COMP mutation screening as an aid for the clinical diagnosis and counselling of patients with a suspected diagnosis of pseudoachondroplasia or multiple epiphyseal dysplasia.

The skeletal dysplasias are a clinically and genetically heterogeneous group of conditions affecting the development of the osseous skeleton and fall into the category of rare genetic diseases in which the diagnosis can be difficult for the nonexpert. Two such diseases are pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED), which result in varying degrees of short stature, joint pain and stiffness and often resulting in early onset osteoarthritis. PSACH and some forms of MED result from mutations in the cartilage oligomeric matrix protein (COMP) gene and to aid the clinical diagnosis and counselling of patients with a suspected diagnosis of PSACH or MED, we developed an efficient and accurate molecular diagnostic service for the COMP gene. In a 36-month period, 100 families were screened for a mutation in COMP and we identified disease-causing mutations in 78% of PSACH families and 36% of MED families. Furthermore, in several of these families, the identification of a disease-causing mutation provided information that was immediately used to direct reproductive decision-making.

Mutations in the known genes are not the major cause of MED; distinctive phenotypic entities among patients with no identified mutations

Multiple epiphyseal dysplasia (MED) is a clinically and genetically heterogeneous chondrodysplasia. Mutations in six genes (COMP, COL9A1, COL9A2, COL9A3, MATN3 and DTDST) have been reported, but the genotype–phenotype correlations and the proportions of cases due to mutations in these genes are still not well characterized. We performed a clinical, radiological and molecular analysis of known MED genes on 29 consecutive MED patients. The mutation analysis resulted in identification of the DTDST mutation in four patients (14%), the COMP mutation in three (10%) and the MATN3 mutation in three (10%). Thus, a disease-causing mutation was identified in 10 patients altogether (34%). The phenotypic features observed in the patients with mutations were in accordance with previously described phenotypes, but two new distinct phenotypic entities were identified in patients in whom no mutation was found. One of them was characterized by severe, early-onset dysplasia of the proximal femurs with almost complete absence of the secondary ossification centres and abnormal development of the femoral necks. The other phenotype was characterized by ‘mini-epiphyses’, resulting in severe dysplasia of the proximal femoral heads. The findings suggest that mutations in the known genes are not the major cause of MED and are responsible for less than half of the cases. The existence of additional MED loci is supported by the exclusion of known loci by mutation analysis and finding of specific subgroups among these patients.

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 α(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.

Preselection of cases through expert clinical and radiological review significantly increases mutation detection rate in multiple epiphyseal dysplasia

Skeletal dysplasias are difficult to diagnose for the nonexpert. In a previous study of patients with multiple epiphyseal dysplasia (MED), we identified cartilage oligomeric matrix protein (COMP) mutations in only 36% of cases and suspected that the low-mutation detection rate was partially due to misdiagnosis. We therefore instituted a clinical–radiographic review system, whereby all cases were evaluated by a panel of skeletal dysplasia experts (European Skeletal Dysplasia Network). Only those patients in whom the diagnosis of MED was confirmed by the panel were screened for mutations. Under this regimen the mutation detection rate increased to 81%. When clinical–radiological diagnostic criteria were relaxed the mutation rate dropped to 67%. We conclude that expert clinical–radiological review can significantly enhance mutation detection rates and should be part of any diagnostic mutation screening protocol for skeletal dysplasias.

Novel mutations in exon 2 of MATN3 affect residues within the alpha-helices of the A-domain and can result in the intracellular retention of mutant matrilin-3.

Multiple epiphyseal dysplasia (MED) is a clinically variable and genetically heterogeneous chondrodysplasia characterized by mild to moderate short stature and early onset osteoarthritis. Some forms of MED result from mutations in the gene encoding the cartilage structural protein matrilin‐3 (MATN3). The majority of MATN3 mutations affect conserved residues within the β‐sheet of the single A‐domain of matrilin‐3. These mutations cause the protein to misfold and prevent its secretion from the rER, both in vitro and in vivo. More recently a single mutation (p.Phe105Ser) has been identified within the α1‐helix of the A‐domain, but its affect on the structure and/or function of matrilin‐3 is unknown. In this paper we describe the characterization of two additional α‐helical mutations (p.Ala173Asp and p.Lys231Asn) and show that both p.Phe105Ser and pAla173Asp prevent the secretion of A‐domain in vitro. In contrast, p.Lys231Asn does not prevent the secretion of matrilin‐3 A‐domain, nor does it disrupt the structure of this domain or inhibit its binding to type II or type IX collagen. Therefore, despite extensive biochemical analysis the disease mechanism of p.Lys231Asn remains unresolved and care should be taken in counseling for these types of mutation in MATN3.

Serum or plasma cartilage oligomeric matrix protein concentration as a diagnostic marker in pseudoachondroplasia: differential diagnosis of a family

Pseudoachondroplasia (PSACH) is an autosomal-dominant osteochondrodysplasia due to mutations in the gene encoding cartilage oligomeric matrix protein (COMP). Clinical diagnosis of PSACH is based primarily on family history, physical examination, and radiographic evaluation, and is sometimes extremely difficult, particularly in adult patients. Genetic diagnosis based on DNA sequencing, on the other hand, can be expensive, time-consuming, and intensive because COMP mutations may be scattered throughout the gene. However, there is evidence that decreased plasma COMP concentration may serve as a diagnostic marker in PSACH, particularly in adult patients. Here, we report the serum and/or plasma COMP concentration-based differential diagnosis of a family with affected adult members. The mean serum and/or plasma COMP concentrations of the three affected family members alive (0.69±0.15 and/or 0.81±0.08 μg/ml, respectively) were significantly lower than those of an age-compatible control group of 21 adults (1.52±0.37 and/or 1.37±0.36 μg/ml, respectively; P<0.0001). Bidirectional fluorescent DNA sequencing-based genetic diagnosis of these patients revealed a heterozygous mutation for the nucleotide change 1532A>G in exon 14 of the COMP gene, resulting in a substitution of amino acid 511 from aspartic acid to glycine in COMP. Thus, serum and/or plasma COMP concentration may be suggested as an additional diagnostic marker to aid clinical and radiographic findings in suspected cases of PSACH.