Study On Pathogenesis Of Apert Syndrome Caused By Fgfr2 Gain-of-function Mutation

Previous studies have shown that FGFR signaling plays an essential role in skeletal development, and FGFR signaling is mediated partially by the extracellular signal-regulated kinase 1/2 (Erk1/2) signaling pathway, which appears to be involved in the regulation of both osteoblast and chondrocyte development. Two kinds of mutations of FGFR2 (S252W and P253R) result in nearly all of cases of Apert syndrome, a severe human congenital skeletal disease. Therefore, the mechanism of Fgfr2 mutation-induced skeletal malformation and the role of the Erk1/2 signaling pathway in it deserve further study. Fgfr2 S252W mutation mouse model has been set up, here we analysed a new Apert syndrome mouse model carrying the Fgfr2 P253R mutation. We found that the mutant mice exhibited craniosynostosis accompanied by short stature, which may be caused by the collective effects of mutant Fgfr2 on the chondrocyte and osteoblast lineages. Our data further indicate that the activated Fgfr2 signaling induces the premature closure of coronal sutures and growth retardation of long bones via the Erk1/2 signaling pathway.Methods: At part 1 of the study, a new knock-in mouse model with the Fgfr2 P253R mutation was analysed. Beside the in vivo examination of skeletal phenotypes, in vitro embryonic organ culture was made to characterize the coronal suture closure and long bone growth. Furthermore, the role of Erk1/2 signaling pathway in the development of skeletal malformation was investigated. At part 2 of the study, a tibia fracture healing mouse model was set up, digital radiographs of fractured limbs were obtained, HE staining and safranin O/fast green staining were used to show the histological structure of the callus. In situ hybridization was performed to detect the expression of Fgfr2, Ihh, collagen 2 and BMP4 in callus.Results: At part 1 of the study, the Fgfr2 mutant mice exhibited craniosynostosis accompanied by short stature. They displayed dysregulated osteoblast and chondrocyte proliferation and differentiation. In vitro organ cultures further revealed that the premature closure of coronal sutures and growth retardation of long bones were the direct effects of the Fgfr2 mutation, and the inhibitor of Erk1/2 signaling pathway alleviated these abnormalities. At part 2 of the study, Fgfr2 S252W mutation caused delayed fracture healing, and the cartilage formation and its replacement by new bone in the fracture location were significantly inhibited.Conclusions: A new Apert syndrome mouse model with Fgfr2 P253R mutation was successfully created. Using this model, we demonstrated that skeleton phenotypes of Apert syndrome, such as craniosynostosis and short stature, may result from the collective effects of mutant Fgfr2 on the chondrocyte and osteoblast lineages. More importantly, the results indicated that these effects are partially mediated by the Erk1/2 signaling pathway. Furthermore, this study indicated that continued enhanced Fgfr2 function may inhibit the Ihh expression in fracture location, thereby suppress the endochondral ossification in fracture callus and delayed healing of fracture.