By an analysis of the GHR mRNA transcripts in lymphocytes from a Mediterranean patient with Laron syndrome (262500), Amselem et al. (1989) demonstrated a thymidine-to-cytosine substitution that generated a serine in place of a phenylalanine at position 96 in the extracellular coding domain of the protein. The mutation was not found in 7 unrelated subjects with Laron syndrome who belonged to different population groups; in these families, the GHR markers showed a different haplotype background for the mutation.
Duquesnoy et al. (1991) investigated the effect of the phe96-to-ser mutation on GH-binding activity by expressing the total human GHR cDNA and the mutant form in eukaryotic cells. Cells transfected with the mutant cDNA lacked binding activity. Specific GH-binding activity was found, however, in the lysosomal fraction, and immunofluorescence studies located mutant proteins in the cytosol. The findings suggested that mutant GHRs fail to follow the correct intracellular transport pathway and underscored the potential importance of the phenylalanine residue, which is conserved among the GH, prolactin, and erythropoietin (133171) receptors that belong to the same cytokine receptor superfamily. Edery et al. (1993) introduced the F96S mutation by site-directed mutagenesis into cDNAs encoding the full-length rabbit GHR and the extracellular domain or binding protein of the human and rabbit GHR. All constructs were transiently expressed in COS-7 cells, and expression of the receptors was assessed by Western blot and immunofluorescence studies. Wildtype and mutant full-length GHR had the same cell surface and intracellular distribution and were expressed with comparable intensities. In contrast, all mutant forms completely lost their ability to bind ligand. Thus, this mutation does not modify the synthesis or the intracellular pathway of receptor proteins, but rather abolishes ability of the receptor or binding protein to bind GH and is thereby responsible for the extreme GH resistance in these patients.
