Human cartilage model of the precocious osteoarthritis-inducing p.Arg719Cys reveals pathology-driving matrix defects and a failure of the ER proteostasis network to recognize the defective procollagen-II.

OBJECTIVES: Mutations in the procollagen-II gene () often cause chondrodysplasias, including the precocious osteoarthritis-inducing p.Arg719Cys. Understanding the molecular basis of such diseases has long been challenging, owing to a lack of models accurately reflecting disease genotypes and phenotypes. To address this challenge, we develop and characterize human cartilage derived from wild-type and disease-causing Arg719Cys isogenic induced pluripotent stem cell (iPSC) lines.METHODS: Using directed differentiation of iPSCs to chondrocytes, we generated cartilage from wild-type and Arg719Cys lines. We compared the resulting protein, cell, and tissue properties using immunohistochemistry, electron microscopy, SDS-PAGE, RNA-sequencing, and quantitative interactomics.RESULTS: While both wild-type and disease lines deposited a cartilage matrix, the Arg719Cys matrix was deficient. Arg719Cys collagen-II was excessively post-translationally modified and modestly intracellularly retained, leading to endoplasmic reticulum (ER) distention suggestive of an ER storage defect. Interactomic studies indicated that Arg719Cys procollagen-II was not differentially engaged by the ER proteostasis network. RNA-sequencing showed that the ER storage defect engendered by Arg719Cys procollagen-II also did not activate cellular stress responses, including the unfolded protein response. These data suggest that cells fail to properly recognize Arg719Cys-associated procollagen-II defects.CONCLUSIONS: A failure to identify and rectify defective procollagen-II folding in cells expressing Arg719Cys procollagen-II leads to the deposition of a sparse and defective collagen-II matrix, culminating in pathology. Combined with the highly expandable human cartilage disease model reported here, this work provides motivation and a platform to discover therapeutic strategies targeting procollagen folding, quality control, and secretion in this collagenopathy and others.