The molecular pathology of a collagen II helix missense mutation

© 2018 Dr Emma SanfordCollagen II (COL2A1) is a major structural component of the extracellular matrix in cartilaginous tissues, and mutations that affect the expression or structure of collagen II result in a range of chondrodysplasias. Many of the known COL2A1 mutations are glycine missense mutations that interfere with the folding and assembly of the critical triple helix structure of the collagen II protein trimer. Recent studies suggests that mutations in the C-terminal trimerisation domain can lead to dysfunctional collagen trimerisation that causes endoplasmic reticulum (ER) stress, which could in turn play a major role in chondrodysplasias. However, the cellular consequences of mutations in the triple helix are currently poorly understood. This study uses a mouse model with a Col2a1 mutation, which causes an arginine to cysteine (Arg1147Cys) substitution in the region that encodes for the triple helix. It presents with a chondrodyplasia that is phenotypically similar to human Spondyloepiphyseal dysplasia congenital (sedc) patients. Characterisation of this mouse model revealed numerous indicators of cellular stress, including distended ER, disorganisation in the extracellular matrix, numerous vacuoles within chondrocytes, and increased cell death compared to wild-type littermates. These attributes suggests that the unfolded protein response (UPR) pathway might be associated with the Col2a1 mutation. However, microarray analysis revealed that the canonical unfolded protein response (UPR) pathway was not activated in this mouse model. However, labelling experiments to investigate the procollagen biosynthesis in chondrocytes revealed that both lysosomal and ERAD-proteasomal pathways are involved in the clearing of aggregated/accumulated mutant type II procollagen in the chondrocytes. Also, the Col2a1 mutation affect molecular transport, cell death and survival, cellular assembly and organisation, cellular development, post-translational modifications, and DNA repair. However, these pathways and mechanisms do not appear to be definitive in causing the pathological changes in the cartilage. Proteomic analysis on cartilage suggests that the Col2a1 mutation causes vast changes to the cartilage protein composition, and revealed that cell death, cellular assembly and organisation, and protein synthesis were all affected by the type II collagen mutation. While no definitive mechanism for the pathology seen in this mouse model was unearthed in this study, it did identify additional extracellular matrix proteins and signalling pathways that could have novel roles in chondrocyte dysfunction. These studies have provided new information about the cellular and extracellular consequences of type II collagen mutation. While the exact molecular mechanism underlying the pathology of this mutation was not identified, the investigations have highlighted the complexity of these types of mutations and provided some potential candidate proteins and pathways that could be investigated further