Bone Abstracts

Introduction: Osteogenesis imperfecta (OI) is a genetically heterogeneous disorder characterized by bone fragility. OI type V, with autosomal dominant inheritance, is characterized by ossification of the forearm interosseus membrane, radiodense metaphyseal bands, propensity for hyperplastic callus formation, and mesh-like lamellation on bone histology. Type V OI probands are reported to have white sclerae and normal teeth. Recent reports identified the cause of type V OI as a unique heterozygous mutation in IFITM5 (c.−14C>T), which encodes Bril, a transmembrane protein expressed in osteoblasts. The mutation generates a start codon in the untranslated region, adding five residues at the N-terminus of Bril.

Methods: IFITM5 was sequenced in gDNA from three patients with OI type V and 25 patients with OI of unknown etiology. Mutations were confirmed by BsmAI restriction digest. Cultured osteoblasts from type V OI probands and control were differentiated over 15 days; cells were analyzed by qPCR, western blot and alizarin-red mineralization assay, to compare functional differences.

Results: Three patients with clinical and histological criteria of type V OI were positive for the known IFITM5 mutation. Two patients not previously classified as type V OI, a child with strongly blue sclerae and no dense metaphyseal bands and an adult with progressive deforming OI, were also found to have the same mutation. We verified expression of mutant IFITM5 transcripts in cultured proband osteoblasts. During days 10–15 of the differentiation timecourse, type V OI osteoblasts had less than half the COL1A1 expression of control cells. During the same timeframe, type V osteoblasts displayed increased mineralization and expression of osteocalcin.

Conclusion: Patients without the well-described type V OI phenotype may also have the type V OI IFITM5 mutation. Type V OI osteoblasts demonstrated a collagen-related defect and increased mineralization during differentiation, possibly underlying overactive calcification of interosseus membrane and during callus formation.