A Leu117 → Trp Mutation Within the RGD-Peptide Cross-Linking Region of β3 Results in Glanzmann Thrombasthenia by Preventing αIIbβ3 Export to the Platelet Surface

Abstract

AbstractWe report a case of Glanzmann thrombasthenia in a Pakistani child whose platelets express less than 10% of the normal amount of αIIbβ3 on their surface. Single-stranded conformation polymorphism analysis of the exons of the patient's αIIb and β3 genes showed an abnormality in exon 4 of the β3 gene. Direct sequence analysis showed that the patient was homozygous for a T → G nucleotide substitution in this exon, resulting in the replacement of a highly conserved Leu at position 117 with Trp. Heterologous expression of αIIbβ3 containing the β3 mutation in COS-1 cells confirmed the pathogenicity of the Leu117 → Trp substitution and showed that it resulted in the intracellular retention of malfolded αIIbβ3 heterodimers. Additional site-directed mutagenesis at position 117 indicated that, although the smaller hydrophobic amino acid Val could be substituted for the wild-type Leu, the larger hydrophobic amino acids Trp and Phe or the charged amino acids Asp and Lys were not tolerated. These studies indicate that Leu117 in β3 plays a critical role in attaining the correct folded conformation of αIIbβ3. These studies also suggest that the hydrophobic side chain of Leu117 is likely folded into the interior of β3, where it serves to stabilize internal packing of the protein and determines its overall shape.