Mutations causing achondroplasia and thanatophoric dysplasia alter bFGF- induced calcium signals in human diploid fibroblasts

Mutations in the fibroblast growth factor receptor (FGFR) gene familyrecently have been shown to underlie several hereditary disorders of bonedevelopment, with specific FGFR3 mutations causing achondroplasia (Ach) andthanatophoric dysplasia (TD). However, for none of these mutations has thedefect in receptor function been demonstrated directly and, therefore, fornone has the pathophysiological mechanism of the disease been defined.Using our established techniques for single-cell ratiometric real-timecalcium image analysis, we defined the nature of the basic fibroblastgrowth factor (bFGF)-induced calcium signal in human diploid fibroblasts,and, in blinded studies, have analyzed the bFGF-induced signals from 18independent fibroblast cell lines, including multiple lines from patientswith known mutant alleles of FGFR3 and syndromes of Ach or TD. Controlcells responded with transient increases in intracellular calcium, withmany cells showing oscillatory calcium waves. Homozygous Ach cell linesfailed to signal, whereas heterozygous Ach lines responded nearly normally.We observed heterogeneous signals in TD heterozygotes: the unresponsivelines all turned out to carry TD1 alleles, whereas all responsive lines hadTD2 alleles. Since FGFR1, 2 and 3 receptors are known to be expressed infibroblasts, our results suggest that specific mutant FGFR3 alleles canfunction in a dosage-dependent dominant-negative fashion to inactivate FGFRsignaling.