Craniosynostosis and FGF signaling: fishing in murky waters – STARFISH

Craniosynostosis (affecting around 1 in 2000 births) results in defective membranous ossification characterized by premature fusion of cranial sutures thus leading to craniofacial malformations affecting brain development. Treatments for craniosynostosis consist of extensive and repeated neurosurgical interventions. The development of new therapeutic strategies is crucial and requires a better understanding of the cellular and molecular mechanisms involved during skull formation.
Fibroblast growth factor (FGF) signaling pathway is one of major signaling pathways disrupted in craniosynostosis. FGFR3 gain-of-function mutations are responsible for the most frequent form of craniosynostoses, Muenke syndrome (MS), and a rare form, Crouzon syndrome with Acanthosis nigricans (CAN). Craniosynostosis was also observed in 60% of X-linked hypophosphatemia (XLH) patients. This disease, XLH, results from loss of function mutations of PHEX gene (phosphate regulating gene with homologies to endopeptidases on the X chromosome) leading to an increase of FGF23 ligand expression and secretion. This craniosynostosis could be due to either the local upregulation of FGF23 which acts on bone cells via FGFR3 or to the direct role of PHEX on osteoblasts. To date the role of FGFR3 and PHEX during cranial vault (CV) development and the aberrant cellular and molecular mechanisms leading to craniosynostosis in MS and XLH disease remains to be understood.
Unfortunately, murine models of MS syndrome or XLH diseases rarely exhibit fusions of cranial sutures. Likewise, mice invalidated for the Fgfr3 gene do not exhibit craniofacial phenotype observed in patients expressing FGFR3 loss-of-function mutations. Given that mouse models do not mimic the human craniofacial phenotype and the relevance of zebrafish model to study skull development, in this proposal we propose an innovative approach to decipher the pathophysiological bases involved in FGF signaling-related-craniosynostosis. The first aim of this proposal will be to identify the role of Fgfr3 during CV development using fgfr3lof/lof fish that we already developed. fgfr3lof/lof fish is the first FGFR3 animal model exhibiting a craniofacial phenotype mimicking human pathology with microcephaly and ectopic bone formation along CV sutures. The second aim will be to understand the pathophysiological mechanisms involved in MS and XLH disease. To achieve this goal, we will generate and study the first zebrafish models of MS and XLH and we will perform in vivo vitro analysis using primary cultures of osteoblasts isolated from patients with MS and XLH disease. Zebrafish has many advantages as a model of human disease, the extra uterine and late development of the zebrafish CV and the existence of transgenic lines expressing fluorescent protein in bone cells will allow to analyze dynamically each step of CV formation at cellular level. Finally, this project should allow to identify the key targets involved during cranial vault development leading to evaluate the efficacy of new drugs in vitro (osteoblasts from patients) and in vivo (MS and XLH disease zebrafish) with the objectives to prevent 1) the premature fusion of sutures when it occurs during childhood or 2) to improve the post-surgical bone repair and to reduce the number of interventions for the severe form of craniosynostoses.