Pathomechanism of x-linked osteogenesis imperfecta

Abstract

Osteogenesis imperfecta (OI) or brittle bone disease is a genetic disorder characterized by deformities of bones and multiple bone fractures. We identified a new causative gene, MBTPS2, in two unrelated patients with OI inherited in X-linked pattern (X-OI). MBTPS2 or site-2 protease (S2P) is a protease that cleaves the transmembrane proteins to activate them acting as transcription factors. To determine pathomechanism of X-OI caused by MBTPS2 mutations, S2P functions were investigated. RNA and protein of S2P were stable. Instead, the defective S2P decreases its cleavage activities. ATF6 and CREB3L1 are target proteins of S2P. We demonstrated decreased a transcriptional activity of ATF6 and a decreased transcription factor form of CREB3L1. Both of them were resulted from defects in the cleavage activity of S2P. In addition, we generated iPSCs from patients’ skin fibroblasts and induced them to osteoblasts to elucidate the involving pathomechanism. During osteoblast differentiation, we found the delay or absence of calcium deposit and abnormal patterns of calcium binding in X-OI patients. Consistently, the studies of osteogenic gene expression revealed the defect of osteoblast differentiation and mineralization supporting the defects of calcium deposit. Interestingly, COL1A1, a downstream gene of CREB3L1, and SPARC, SEC23A, SEC24D and LOX, downstream genes of CREB3L3, were downregulated. These genes have functions involving in osteoblastogenesis. Taken together, we concluded that X-OI phenotype is possibly caused by the defects of osteoblast differentiation and mineralization resulting from the decreased transcriptional activities of CREB3L1 and CREB3L3 that downregulate COL1A1, SPARC, SEC23A, SEC24D and LOX.