CRTAP is required for collagen 3-prolyl hydroxylation and loss of its function causes recessive Osteogenesis Imperfecta
Osteogenesis Imperfecta (OI) is a major cause of pediatric brittle bone disease and although its clinical spectrum of severe to mild OI (represented by clinical types II, III, IV, and I) has been well described, new types of OI (types V, VI, and VII), defined either genetically or histologically, have emerged. We hypothesized that this group of OI phenotypes may include recessive forms arising from dysregulation of other components of the collagen processing machinery. We previously identified a novel protein, CRTAP, that was differentially expressed during chondrogenesis. We generated Crtap null mice and found that they exhibit osteochondrodysplasia characterized by rhizomelia, kyphosis, and severe osteopenia. The in vivo histomorphometric analyses showed significantly decreased bone mass and decreased bone formation rate, but normal numbers of osteoblasts and osteoclasts. Interestingly, the mice have very little osteoid and a decreased mineralization lag time. Together these data show that the low bone mass is due to decreased osteoid formation, but with an accelerated mineralization of available osteoid. We demonstrate that CRTAP exists in a protein complex with the recently identified prolyl-3-hydroxylase-1 protein. At a molecular level, Crtap-/- mice lack fibrillar collagen prolyl 3-hydroxylation by tandem mass spectrometric peptide mapping of cyanogen derived peptides. On EM, collagen fibrils are thickened suggesting a defect in fibrillogenesis. Interestingly, CRTAP maps within the OI type VII critical region, and we found down regulation of CRTAP expression on both RNA and protein levels in patients with OI type VII. Our data demonstrate the biological importance of prolyl 3-hydroxylation, a poorly understood collagen post-translational modification, for bone formation and that loss of function of CRTAP causes a recessive form of OI.