A Gln747-->Pro substitution in the IIb subunit is responsible for a moderate IIbbeta3 deficiency in Glanzmann...

To clarify a molecular defect responsible for moderate alphaIIb beta3 deficiency, we examined two unrelated patients, MT and MS, suffering from type II and type I Glanzmann thrombasthenia (GT), respectively. Sequence analysis of polymerase chain reaction (PCR) fragments derived from platelet mRNA showed a single A-->C substitution at nucleotide (nt) 2334 leading to a Gln747--> Pro in alphaIIb in both patients. Allele-specific restriction enzyme analysis (ASRA) of genomic DNA demonstrated that patient MT was homozygous for the Gln747-->Pro substitution and patient MS was compound heterozygous for this substitution and for an RNA splice mutation at the consensus sequence of the splice acceptor site of exon 18 (AG-->AA). Furthermore, ASRA showed that, among 17 unrelated Japanese GT patients, this Gln747-->Pro substitution was detected in 4 patients, including MT and MS (homozygous, 2 patients; heterozygous, 2 patients). Cotransfection of Pro747alphaIIb and beta3 constructs into 293 cells resulted in moderate reduction in the amount of alphaIIb beta3 within the transfected cells as well as on the cell surface. However, Pro747alphaIIb beta3 bound the ligand mimetic monoclonal antibody (MoAb) PAC-1 after activation of alphaIIb beta3 by the MoAb PT25-2, suggesting that the mutant alphaIIb beta3 possesses the ligand-binding function. The association between the mutant proalphaIIb and beta3 was not disturbed. Surface labeling and pulse chase study showed that the Gln747-->Pro substitution moderately impaired both intracellular transport of the alphaIIb beta3 heterodimers to the Golgi apparatus and endoproteolytic cleavage of proalphaIIb into heavy and light chains. By contrast, replacement of Gln747 with Ala by mutagenesis did not impair alphaIIbbeta3 expression on the cell surface. These results suggest that the presence of Pro, rather than the absence of Gln, at amino acid residue 747 on alphaIIb is responsible for moderate alphaIIbbeta3 deficiency.