Chondrodysplasia-inducing COL2A1 p.Gly1170Ser causes an ER storage defect without associated unfolded protein response in chondronoids

Collagenopathies are a group of clinically diverse disorders caused by defects in collagen folding and secretion. For example, mutations in the gene encoding collagen type-II (COL2A1), the primary collagen in cartilage, can lead to chondrodysplasias of various severities. One example is the Gly1170Ser substitution in procollagen-II, which causes precocious osteoarthritis and Legg-Calvé-Perthes disease. Here, we develop and characterize a novel induced pluripotent stem cell-based cartilage model of this disease, including both hetero- and homozygous genotypes. Biochemical characterization reveals that Gly1170Ser procollagen-II is notably slow to fold and secrete. Instead, procollagen-II accumulates intracellularly, consistent with an endoplasmic reticulum (ER) storage disorder. Intriguingly, though perhaps due to the pathologic substitution occurring within a triple-helical domain that lacks hydrophobic character, this intra-ER protein accumulation is not recognized by cellular stress responses, such as the unfolded protein response. Interactome studies showed that Gly1170Ser procollagen-II interacts to a greater extent with certain ER chaperones and modifying enzymes, consistent with its slow folding. These findings provide mechanistic elucidation into the etiology of this disease. Moreover, the expandable cartilage model developed here provides a valuable platform to rapidly screen and develop therapeutic strategies that can restore procollagen folding and secretion in this collagenopathy and others.