Truncation of Glycoprotein (GP) IIIa (▵ 616-762) Prevents Complex Formation With GPIIb: Novel Mutation in Exon 11 of GPIIIa Associated With Thrombasthenia

Abstract This work reports the molecular genetic study of a patient who suffered from Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (GP) IIb and GPIIIa genes showed the presence of a homozygous G1846→T transversion in exon 11 of GPIIIa that changes Glu616→Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in the proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the thrombasthenic phenotype of the proband. (1) We failed to detect mutations other than [T1846]GPIIIa in the coding region of both GPIIb and GPIIIa genes. (2) The G1846→T mutation was observed in either parent and a brother of the proband, but none of 100 unrelated individuals carried this defect. (3) Pulse-chase and immunoprecipitation analysis of GPIIb-IIIa complexes in cells transiently cotransfected with cDNAs encoding normal GPIIb and [T1846]GPIIIa showed neither maturation of GPIIb nor complex formation and surface exposure of GPIIb-▵GPIIIa. These observations indicate that the sequence from Glu616 to Thr762 in GPIIIa is essential for heterodimerization with GPIIb. Polymerase chain reaction-based analysis demonstrated the presence of normal levels of full-length GPIIIa-mRNA in the proband and in heterozygous relatives. In addition, a shortened transcript, with a 324-nucleotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mutations, [T1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon.