Procollagen C-propeptide Processing Impacts Bone Matrix

Classical Osteogenesis imperfecta (OI), characterized by short stature, fragile bones and low bone mass, is predominantly caused by genetic defects in the structure or synthesis of type I collagen. Unexpectedly, there is a rare high bone mass form of OI (HBM OI) with extremely high bone mineralization by quantitative backscattered electron imaging (qBEI) and a moderately severe skeletal phenotype. HBM OI is caused by dominant missense mutations at residues comprising the type I procollagen C-propeptide cleavage site, preventing processing of the C-propeptide. To elucidate the role of C-propeptide processing in bone mineralization and development, we generated heterozygous HBM OI mice, with substitutions in the Col1a1 C-propeptide cleavage site. Femoral extracts from HBM OI mice contain increased monomeric proα1(I) C-propeptide and pC-collagen, resulting in a “barbed-wire” appearance to bone collagen fibrils. The decrease in type I procollagen cleavage has a profound impact on bone, leading to diminished bone strength and integrity despite normal collagen helical primary sequence. HBM OI femora have thin cortices with decreased BV/TV leading to decreased stiffness, yield and fracture load, which does not improve with age. Their femora are also extremely brittle, with increased mineral content and HP crosslinks. Similar to the high to normal DXA bone density of HBM OI patients, µCT bone density is normal in femora. However, murine HBM OI cortical bone is hypermineralized in excess of other OI mouse models, with thinner, more disorganized mineral particles. Increased expression of ossification genes is detected in both murine and human HBM OI osteoblasts, with enhanced mineral deposition in culture. There is decreased expression of genes involved with extracellular matrix organization, correlating with changes in actin and vinculin staining when osteoblasts are grown on mutant matrix. Murine HBM OI bone mineralization is increased throughout life and increases with age, raising concerns for long-term management of HBM OI patients.