A Secreted Variant of Cartilage Oligomeric Matrix Protein Carrying a Chondrodysplasia-Causing Mutation (p.H587R) Disrupts Collagen Fibrillogenesis

Objective. Mutations in human cartilage oligomeric matrix protein (COMP) cause multiple epiphyseal dysplasia or pseudoachondroplasia. Electron microscopic analyses of patient biopsy tissue have shown that, in most cases, mutated COMP is retained in granular or lamellar inclusions in the endoplasmic reticulum of chondrocytes. However, some mutations that do not interfere with protein trafficking, resulting in normal secretion of the mutated protein, have been identified. These mutations are likely to cause the chondrodysplasia phenotype, via events that occur after secretion. The aim of the present study was to identify such extracellular mechanisms associated with the pathogenesis of chondrodysplasias. Methods. A mutated but secreted COMP variant, p.H587R, as well as wild-type COMP were recombinantly expressed and purified from cell culture supernatants. Since recent studies have shown that COMP can facilitate collagen fibrillogenesis in vitro, the effect of the p.H587R mutation on this process was determined by analyzing the kinetics of fibrillogenesis in vitro and determining the structure of the collagen fibrils formed by immunogold electron microscopy. Results. Mutated p.H587R COMP accelerated fibril formation by type I collagen in vitro to a slightly greater extent than that with wild-type COMP. However, p.H587R COMP induced aggregation and disorganization of fibril intermediates and end products. Mixtures of cartilage collagens or of type XI collagen alone produced similar results. The addition of p.H587R COMP to preformed fibrils induced aggregation and fusion of the fibrils, whereas wild-type COMP had little effect. Conclusion. The mutant COMP variant p.H587R generally interferes with normal collagen organization during fibrillogenesis. This constitutes a novel pathogenetic mechanism of COMP-associated chondrodysplasias.