Two different beta3 cysteine substitutions alter alphaIIb beta3 maturation and result in Glanzmann thrombasthenia

We report the defects responsible for Glanzmann thrombasthenia in two patients showing traces of abnormally migrating platelet beta3 in immunoblotting. Using PCR-SSCP and direct sequencing, we identified a novel homozygous mutation in exon 10 of the beta3 gene of patient 1 which gave a C457 to Y amino acid substitution. A C542 to R substitution in beta3 of patient 2 was previously reported by us. These cysteines are present in EGF-domains 1 and 3 respectively of beta3. We therefore constructed mutants carrying substitutions on cysteine residues in each of the first three EGF domains of beta3, C457, C495 and C542 respectively. Transient expression of these mutants in COS-7 cells, including the C542 and C547 double mutant, proved that disulfide disruption directly affects cell surface expression of the integrin. We then showed by metabolic (35S) labeling and Endo-H glycosidase treatment that these substitutions strongly affected complex maturation within the cell.