Mutations in Cartilage Oligomeric Matrix Protein Causing Pseudoachondroplasia and Multiple Epiphyseal Dysplasia Affect Binding of Calcium and Collagen I, II, and IX

Abstract

Mutations in type 3 repeats of cartilage oligomeric matrix protein (COMP) cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED). We expressed recombinant wildtype COMP that showed structural and functional properties identical to COMP isolated from cartilage. A fragment encompassing the eight type 3 repeats binds 14 calcium ions with moderate affinity and high cooperativity and presumably forms one large disulfidebonded folding unit. A recombinant PSACH mutant COMP in which Asp-469 was deleted (D469⌬) and a MED mutant COMP in which Asp-361 was substituted by Tyr (D361Y) were both secreted into the cell culture medium of human cells. Circular dichroism spectroscopy revealed only small changes in the secondary structures of D469⌬ and D361Y, demonstrating that the mutations do not dramatically affect the folding and stability of COMP. However, the local conformations of the type 3 repeats were disturbed, and the number of bound calcium ions was reduced to 10 and 8, respectively. In addition to collagen I and II, collagen IX also binds to COMP with high affinity. The PSACH and MED mutations reduce the binding to collagens I, II, and IX and result in an altered zinc dependence. These interactions may contribute to the development of the patient phenotypes and may explain why MED can also be caused by mutations in collagen IX genes.

Antisense Reduction of Mutant COMP Reduces Growth Plate Chondrocyte Pathology

“…11,[58][59][60][61][62][63] Tolerability and toxicity must be carefully considered with long-term administration, whereas these factors are less important with short-duration therapies. Collectively, the observations reported here demonstrate that early ASO1 systemic administration dampens the MT-COMP growth plate chondrocyte phenotype.…”

“…4,5 Functionally, COMP facilitates type II collagen fibril assembly, enhances chondrocyte attachment in vitro, and interacts with other ECM proteins, including type II and IX collagens, matrilin 3, and SPARC. [6][7][8][9][10][11][12] In contrast, mutations in COMP cause misfolding of the protein, preventing export from the chondrocyte and resulting in massive retention within the endoplasmic reticulum (ER). 10,[13][14][15] Although this finding has long been appreciated, there was little understanding of the pathologic molecular mechanisms, which are critical to develop mechanism-driven therapeutics.…”

Thrombospondins: from structure to therapeutics

“…Twenty-two additional disease-linked mutations result in insertions or deletions of residues in the wire. Several examples of THBS-5 with mutations in the wire have been found to bind less calcium and to differ from wild-type THBS-5 in their stability and binding properties [47][48][49][50][51]. Thus, the sites of disease-causing mutations in THBS-5 target residues that appear to be critical for either the stability of the calcium-sensitive structure of the wire or the ability of the wire to stabilize the structure of the lectin-like module.…”

“…These results suggested that calcium keeps the signature domains of THBSs in a more compact conformation that unravels when calcium is lost. In addition, the circular dichoic spectra changed with the removal or addition of calcium, indicating again that calcium causes a conformational change [44,48,60]. Interestingly, the circular dichroic spectrum of a THBS-2 construct comprising solely its wire repeats displays a calcium-dependent ellipticity minimum at 203 nm that is characteristic of the circular dichroic spectra of all THBSs studied to date [44].…”

Abnormal bone quality in cartilage oligomeric matrix protein and matrilin 3 double‐deficient mice caused by increased tissue inhibitor of metalloproteinases 3 deposition and delayed aggrecan degradation

“…After the collagen fibrils have formed, COMP dissociates from the mature fibril (9,10). It also interacts with a number of other extracellular matrix proteins, including matrilins (3), type IX collagen (11)(12)(13), and aggrecan (14).…”