All of the mutations identified in FGFRs are autosomal dominant and frequently arise sporadically. The great majority of these disorders result from point mutations in the coding sequence of the FGFR that result in a single amino acid substitution.
Biochemical studies have shown that mutations causing ACH and TD are gain-of-function mutations resulting in increased receptor tyrosine kinase activity (Naski et al 1996, Webster et al 1996, Webster & Donoghue 1996, Li et al 1997) (Fig. 2). The G380R mutation in the transmembrane domain of FGFR3 (responsible for most cases of ACH) partially activates receptor signalling (Naski et al 1996). The basal mitogenic activity of this receptor (assayed as a chimeric receptor containing the tyrosine kinase domain from FGFR1) could be augmented by the addition of ligand. The dose response curve suggested that this receptor has a similar ligand binding affinity to that of the wild-type receptor. Additionally, studies of receptor tyrosine phosphorylation showed ligand independent receptor autophosphorylation. The K650E and R248C mutations of TD are also activating mutations. These mutations result in ligand independent receptor activation as evidenced by ligand independent cell proliferation and receptor tyrosine phosphorylation. Significantly, the mutations causing TD were more strongly activating than the mutation causing ACH. This suggested a correlation between the degree of receptor activation and the severity of the dwarfing chondrodysplasia. This study also demonstrated that the FGFR3 R248C mutation constitutively activated the receptor by forming a disulfide-linked receptor homodimer in which an unpaired cysteine residue in the extracellular domain of the receptor forms an intermolecular disulfide linkage (Naski et al 1996). The FGFR3 K650E mutation occurs in a highly conserved lysine residue in the activation loop of the receptor (Hanks et al 1988, Mohammadi et al 1996). This mutation results in a constitutively active tyrosine kinase, presumably by altering the structure of the activation loop. Unlike the R248C mutation which showed constitutive activation matching that of maximally stimulated wild-type receptor, the K650E mutant receptor can be activated by ligand to a level greater than that of the wild-type receptor. These observations are consistent with the observed covalent homodimerization consequence of the R248C mutation and the deregulation of the kinase domain by the K650E mutation.
Functional and structural studies on the FGFR have localized a ligand binding domain to the second and third Ig domains and the intervening linker (Johnson et al 1990, Wang et al 1995, Chellaiah et al 1999, Plotnikov et al 1999). Interestingly, many of the mutations that affect FGFR activity (such as FGFR3:R248C) are localized to three highly conserved amino acid residues (RSP) within the linker sequence between Ig domain II and III (Fig. 3). The conserved RSP motif is itself embedded within a highly conserved sequence that is thought to function as a receptor dimerization domain (Wang et al 1997) or ligand binding surface (Plotnikov et al 1999). The biochemical and structural data suggest that mutations in this sequence either affect ligand binding or receptor dimerization or both.
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