Cartilage oligomeric matrix protein (COMP) is a large extracellular matrix protein expressed in cartilage, ligament and tendon. Mutations in the COMP gene cause two dominantly inherited skeletal dysplasias, pseudoachondroplasia (PSACH) and Multiple Epiphyseal Dysplasia (MED/EDMI). We report on a novel point mutation D511Y in the seventh calcium-binding repeat of the COMP gene and the resulting iliac crest growth plate pathology. The PSACH iliac crest growth plate is comprised of a large region of resting chondrocytes above a narrow region composed of clusters of disorganized proliferative and hypertrophic chondrocytes. Chondrocytes in all zones show massive intracellular retention of COMP and the surrounding extracellular matrix is deficient in COMP. Moreover, the 511Y COMP mutation selectively affects type IX collagen as little is found in the growth plate matrix whereas type I1 collagen and aggrecan are abundant in the matrix. Chondrocyte remnants are observed in the chondrocyte clusters and dead cells are found throughout the growth plate. Apoptosis studies demonstrate an unusual pattern of TUNEL staining in the PSACH chondrocytes compared to the control growth plate. These in vivo findings support our previous observation that retention of COMP leads to chondrocyte death. These results also add to the increasing evidence that PSACH and E D M l are rER storage diseases and that impaired linear growth and joint erosion are caused by the disruptive effect of massive amounts of COMP within the chondrocytes.

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.

We have identified haploinsufficiency of the COL5A1 gene that encodes the proalpha1(V) chain of type V collagen in the classical form of the Ehlers-Danlos syndrome (EDS), a heritable connective-tissue disorder that severely alters the collagen-fibrillar structure of the dermis, joints, eyes, and blood vessels. Eight of 28 probands with classical EDS who were heterozygous for expressed polymorphisms in COL5A1 showed complete or nearly complete loss of expression of one COL5A1 allele. Reduced levels of proalpha1(V) mRNA relative to the levels of another type V collagen mRNA, proalpha2(V), were also observed in the cultured fibroblasts from EDS probands. Products of the two COL5A1 alleles were approximately equal after the addition of cycloheximide to the fibroblast cultures. After harvesting of mRNAs from cycloheximide-treated cultured fibroblasts, heteroduplex analysis of overlapping reverse transcriptase-PCR segments spanning the complete proalpha1(V) cDNA showed anomalies in four of the eight probands that led to identification of causative mutations, and, in the remaining four probands, targeting of CGA-->TGA mutations in genomic DNA revealed a premature stop at codon in one of them. We estimate that approximately one-third of individuals with classical EDS have mutations of COL5A1 that result in haploinsufficiency. These findings indicate that the normal formation of the heterotypic collagen fibrils that contain types I, III, and V collagen requires the expression of both COL5A1 alleles.

Ehlers-Danlos syndrome (EDS) is a heterogeneous connective tissue disorder that severely impairs the structure and function of the skin, joints, eyes and blood vessels. We have identified mutations of the COL5A2 gene, which encodes the alpha2(V) chain of type V collagen, in two unrelated patients with the severe type I form of EDS. The first proband was heterozygous for a 7 bp deletion that resulted in skipping of exon 27 while the second proband was heterozygous for a single nucleotide substitution that resulted in skipping of exon 28. Cultured dermal fibroblasts from both probands produced about equal amounts of the normal and mutant alpha2(V) mRNAs and protein chains. The dermis from the first proband contained a sparse collagen fibrillar network with great variability in collagen fibril sizes and shapes. The dermal collagens were also abnormally soluble. Bone cells from the first proband also produced about equal amounts of the normal and mutant alpha2(V) mRNAs. However, the collagen fibrillar architecture and collagen solubility of the bone matrix were normal. Our findings show that heterozygous mutations of the COL5A2 gene can produce the EDS type I phenotype. They also suggest that type V collagen plays a more important role in collagen fibrillogenesis of dermis than that of bone.

Mutations in the diastrophic dysplasia sulfate transporter (DTDST) gene result in a family of skeletal dysplasias, which comprise lethal (achondrogenesis type 1B and atelosteogenesis type 2) and non-lethal conditions (diastrophic dysplasia and recessive multiple epiphyseal dysplasia (rMED)). The most frequent mutation is R279W, which in a homozygous state results in rMED with bilateral clubfoot, MED, and "double layered" patella. We describe three patients with rMED caused by a previously unreported homozygous mutation in the DTDST gene. The three patients (from two families) were born to healthy, non-consanguineous parents. All developed signs of hip dysplasia in early childhood and two had episodes of recurrent patella dislocation. Two underwent bilateral total hip replacements at ages 13 and 14 years. The feet, external ears, and palate were normal. Stature was normal in all cases. Radiographs showed dysplastic femoral heads, mild generalized epiphyseal dysplasia, abnormal patella ossification, and normal hands and feet. Direct sequence analysis of genomic DNA demonstrated a homozygous 1984T > A (C653S) change in the DTDST gene in all patients. The clinically normal parents were heterozygous for the change. This is the first description of a homozygous C653S mutation of the DTDST gene. Hip dysplasia and patella hypermobility dominates the otherwise mild phenotype. These patients further expand the range of causative mutations in the DTD skeletal dysplasia family.