Truncation of glycoprotein (GP) IIIa (Delta 616-762) prevents complex formation with GPIIb: Novel mutation in exon 11 of GPIIIa associated with thrombasthenia

This work reports the molecular genetic study of a patient who suffered fro m Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (G P) IIb and GPIIIa genes showed the presence of a homozygous G(1846)-->T tra nsversion in exon 11 of GPIIIa that changes Glu(616)-->Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in t he proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the t hrombasthenic phenotype of the proband. (1) We failed to detect mutations o ther than [T-1846]GPIIIa in the coding region of both GPIIb and GPIIIa gene s. (2) The G(1846)-->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 i n cells transiently cotransfected with cDNAs encoding normal GPIIb and [T-1 846]GPIIIa showed neither maturation of GPIIb nor complex formation and sur face exposure of GPIIb-Delta GPIIIa. These observations indicate that the s equence from Glu(616) to Thr(762) in GPIIIa is essential for heterodimeriza tion 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-nuc leotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mut ations, [T-1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon, (C) 1998 by The Amer ican Society of Hematology.