The role of FGF signalling in the pathogenesis of craniofacial birth defects

Supervisors: Dr Erwin Pauws and Dr Philip Stanier

Project summary: The care of children with craniofacial birth defects is an important clinical activity at Great Ormond Street Hospital. Diagnosis and treatment of these disorders is a multi-disciplinary activity and typically involves each child seeing a number of consultant specialists. Collectively, up to 35% of all birth defects are craniofacial, with cleft lip and/or palate, and craniosynostosis being the most common. A subgroup of these patients have syndromic craniosynostosis which is often accompanied by midfacial hypoplasia and cleft palate. Although mutations in the FGFR2 gene have been identified in many of these patients, major questions remain unanswered about the molecular and developmental pathogenesis. FGFR2 mutations have been shown to be gain-of-function at the biochemical level by either increased affinity of ligands for the receptor-binding site or cross-linking of Cysteine residues causing constitutive activation[1]. Paradoxically, at the cellular level, some studies provide evidence that loss-of-function of FGF signalling downstream of FGFR that is the cause of the developmental defect[2,3]. An improved understanding of the effects of FGFR activation on the cellular level will allow the development of RNAi compounds or small molecules to correct impaired FGF signalling at the RNA or protein level, respectively.

In this study we aim to determine whether FGFR2 ‘activating’ mutations cause a gain- or loss-of-function of FGF signalling at the cellular level. This will be achieved by investigating two Fgfr2 transgenic mouse models, one that has a missense mutation found in human Crouzon syndrome[4], and a second that conditionally expresses an Fgfr2 transgene. The main aims of the project are to:

  • Characterise the phenotypic spectrum resulting from overexpression of FGFR2. This will be achieved through tissue-specific expression of the Fgfr2 transgene. We will use Wnt1- and Mesp1-Cre drivers to determine whether the observed phenotypic features originate from the neural crest or mesodermal cell lineages. Both these cell types contribute to the formation of the palate as well as the coronal sutures. Experimental analysis will focus on morphological changes at different embryonic stages during the formation of the coronal sutures and palatal shelves.
  • Identify and/or validate downstream targets of FGFR2 in relevant tissues. we will be able to use Fgfr2 overexpression mice to perform global gene expression analysis using microarray or RNAseq to identify novel FGFR2 targets in embryonic tissues that show phenotypic differences at later stages.

Knowledge acquired during this project will have the potential to be translated into novel, relevant, clinical treatment strategies to enhance or replace current approaches for the treatment of FGF related birth defects.

References:
1. Wilkie AO. Craniosynostosis: genes and mechanisms. Hum Mol Genet 1997;6(10):1647-1656.
2. Lee KM, Santos-Ruiz L, Ferretti P. A single-point mutation in FGFR2 affects cell cycle and Tgfbeta signalling in osteoblasts. Biochim Biophys Acta 2010;1802(3):347-355.
3. Snyder-Warwick AK, Perlyn CA, Pan J, Yu K, Zhang L, Ornitz DM. Analysis of a gain-of-function FGFR2 Crouzon mutation provides evidence of loss of function activity in the etiology of cleft palate. Proc Natl Acad Sci U S A 2010;107(6):2515-2520.
4. Eswarakumar VP, Horowitz MC, Locklin R, Morriss-Kay GM, Lonai P. A gain-of-function mutation of Fgfr2c demonstrates the roles of this receptor variant in osteogenesis. Proc Natl Acad Sci U S A 2004;101(34):12555-12560.