Non-Lethal Type VIII Osteogenesis Imperfecta Has Elevated Bone Matrix Mineralization

Context: Type VIII osteogenesis imperfecta (OI; OMIM 601915) is a recessive form of lethal or severe OI caused by null mutations in P3H1, which encodes prolyl 3-hydroxylase 1. Objectives: Clinical and bone material description of non-lethal type VIII OI. Design: Natural history study of type VIII OI. Setting: Pediatric academic research centers. Patients: Five patients with non-lethal type VIII OI, and one patient with lethal type VIII OI. Interventions: None. Main Outcome Measures: Clinical examinations included bone mineral density, radiographs, and serum and urinary metabolites. Bone biopsy samples were analyzed for histomorphometry and bone mineral density distribution by quantitative backscattered electron imaging microscopy. Collagen biochemistry was examined by mass spectrometry, and collagen fibrils were examined by transmission electron microscopy. Results: Type VIII OI patients have extreme growth deficiency, an L1–L4 areal bone mineral density Z-score of −5 to −6, and normal bone formation markers. Collagen from bone and skin tissue and cultured osteoblasts and fibroblasts have nearly absent 3-hydroxylation (1–4%). Collagen fibrils showed abnormal diameters and irregular borders. Bone histomorphometry revealed decreased cortical width and very thin trabeculae with patches of increased osteoid, although the overall osteoid surface was normal. Quantitative backscattered electron imaging showed increased matrix mineralization of cortical and trabecular bone, typical of other OI types. However, the proportion of bone with low mineralization was increased in type VIII OI bone, compared to type VII OI. Conclusions: P3H1 is the unique enzyme responsible for collagen 3-hydroxylation in skin and bone. Bone from non-lethal type VIII OI children is similar to type VII, especially bone matrix hypermineralization, but it has distinctive features including extremely thin trabeculae, focal osteoid accumulation, and an increased proportion of low mineralized bone.