Type V OI Primary Osteoblasts Display Increased Mineralization Despite Decreased COL1A1 Expression

Context: Patients with type V osteogenesis imperfecta (OI) are heterozygous for a dominant IFITM5c.-14C>T mutation, which adds five residues to the N terminus of bone-restricted inter-feron-induced transmembrane-like protein (BRIL), a transmembrane protein expressed in osteoblasts. Type VOI skeletal findings include hyperplastic callus formation, ossification of the forearm interosseous membrane, and dense metaphyseal bands. Objective: The objective of this study was to examine the role of osteoblasts in the active mineralization traits of type V OI and the effect of the IFITM5 mutation on type I collagen. Methods: We identified eight patients with the IFITM5c.-14C>T mutation. Cultured osteoblasts from type V OI patients were used to study osteoblast differentiation and mineralization. Results: We verified the expression and stability of mutant IFITM5 transcripts. In differentiated type V OI primary osteoblasts in culture, the IFITM5 expression and BRIL level is comparable with control. Both early and late markers of osteoblast differentiation are increased in type V OI osteoblasts. Mineralization, assayed by alizarin red staining, was increased in type V OI osteoblasts compared with control. However, type V OI osteoblasts have significantly decreased COL1A1 transcripts in mid-to late differentiation. Type I collagen protein is concomitantly decreased, with decreased cross-linked collagen in matrix and altered appearance of fibrils deposited in culture. Conclusions: This study demonstrates that type V OI mineralization has a gain-of-function mechanism at the osteoblast level, which likely underlies the overactive tissue mineralization seen in patients. Decreased type I collagen expression, secretion, and matrix incorporation establish type V OI as a collagen-related defect.