Murine model of high bone mass osteogenesis imperfecta exhibits bone matrix hyper-mineralization, misaligned mineral...

Osteogenesis imperfecta (OI), characterized by bone fragility and low bone mass, is predominantly caused by mutations in type I collagen. High bone mass OI (HBM OI) is a rare form caused by heterozygous missense mutations at the type I procollagen C-propeptide cleavage site. Knock-in HBM OI mice were generated to elucidate the effect of this mutation on cells and bone. HBM OI murine femora contain increased monomeric pro-α1(I)C-propeptide and pC-collagen; their bone collagen fibrils have a "barbed-wire" appearance. Decreased C-propeptide cleavage diminishes bone strength. HBM OI femora are extremely brittle, with thin cortices, decreased BV/TV, and fracture load. The cortical bone has increased mineral content, with thinner, more disorganized mineral particles. Increased expression of ossification genes in both murine and human HBM OI osteoblasts during in vitro differentiation and increased mineral deposition in culture indicate impaired C-propeptide processing affects cellular processes related to mineralization, rather than being a passive matrix process. Gene ontology analysis of RNA-seq data from differentiating HBM OI osteoblasts revealed top upregulated pathways for ossification, mineralization, and osteoblast differentiation (5-25×) while top-downregulated pathways involved cellular adhesion, migration, and angiogenesis (5-10×), all related to cell-matrix interactions. Moreover, the HBM matrix affects osteoblast function. WT osteoblasts plated on HBM OI decellularized matrix in vitro showed less punctate vinculin, increased peripheral actin staining, and the presence of lamellipodia, suggesting a decrease in cellular adhesion. Insights into the mechanism of HBM OI mineralization may lead to improved therapies for HBM OI and low bone mass conditions.