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Constitutive activation of fibroblast growth factor receptor-2 by a point mutation associated with Crouzon syndrome
1Department of Molecular Biology, Holland Laboratory, American Red Cross, Rockville, Maryland 20855, USA.
Abstract:
The fibroblast growth factor receptors (FGFRs) are a family of ligand-activated, membrane-spanning tyrosine kinases. Mutations in several human FGFR genes have been identified as playing a role in certain disorders of bone growth and development. One of these, Crouzon syndrome, an autosomal dominant disorder causing craniosynostosis, has been associated with mutations in the human FGFR-2 gene. We report here that microinjection of Xenopus embryos with RNA encoding an FGFR-2 protein bearing a Cys332-->Tyr mutation (FGFR-2CS) found in Crouzon syndrome results in fibroblast growth factor (FGF)-independent induction of mesoderm in animal pole explants. Wild-type FGFR-2 did not induce mesoderm when injected at similar doses. The effects of the mutant receptor were blocked by co-expression of dominant negative mutants of either Raf or Ras. Analysis of the mutant receptor protein expressed in Xenopus oocytes indicates that it forms covalent homodimers, does not bind radiolabeled FGF, and has increased tyrosine phosphorylation. These results indicate that FGFR-2CS forms an intermolecular disulfide bond resulting in receptor dimerization and ligand-independent activation that may play a role in the etiology of Crouzon syndrome.
Insights
A mutation in fibroblast growth factor receptor 2 (FGFR-2) causes Crouzon syndrome by enabling ligand-independent receptor activation. This mutation leads to abnormal bone development and craniosynostosis.
Area of Science:
- Molecular Biology
- Developmental Biology
- Genetics
Background:
- Fibroblast growth factor receptors (FGFRs) are tyrosine kinases crucial for development.
- Mutations in FGFR genes are linked to bone growth disorders.
- Crouzon syndrome, characterized by craniosynostosis, is associated with FGFR-2 gene mutations.
Purpose of the Study:
- To investigate the functional consequences of an FGFR-2 mutation found in Crouzon syndrome.
- To elucidate the mechanism of ligand-independent receptor activation.
Main Methods:
- Microinjection of Xenopus embryos with RNA encoding wild-type and mutant FGFR-2.
- Analysis of mesoderm induction in animal pole explants.
- Expression and biochemical analysis of mutant FGFR-2 protein in Xenopus oocytes.
Main Results:
- Mutant FGFR-2 (FGFR-2CS) induced mesoderm independently of FGF in Xenopus embryos.
- Wild-type FGFR-2 did not cause mesoderm induction at similar doses.
- FGFR-2CS formed covalent homodimers, did not bind FGF, and exhibited increased tyrosine phosphorylation.
- The effects of FGFR-2CS were blocked by dominant-negative Raf or Ras mutants.
Conclusions:
- The Cys332-->Tyr mutation in FGFR-2 leads to ligand-independent receptor activation through covalent homodimerization.
- This aberrant receptor activity is a potential mechanism underlying Crouzon syndrome.
- FGFR-2 signaling pathways involving Ras and Raf are critical for this process.