Glanzmann's thrombasthenia caused by homozygosity for a splice defect that leads to deletion of the first coding exon of the glycoprotein IIIa mRNA

Glanzmann's thrombasthenia (GT) is the result of the absence or of an altered and dysfunctional expression on the platelet membrane of the fibrinogen receptor (glycoprotein [GP] IIb/IIIa complex). Various molecular genetic mechanisms have been found to be responsible for this inherited disease. In a patient with a severe type of GT, we have found a splice variant in the GP IIIa gene that leads to premature chain termination. Immunoprecipitation experiments, using monoclonal antibodies specific for GP IIb/IIIa, showed that GP IIb/IIIa was not detectable on the platelet membrane. Amplification of reversely transcribed platelet GP IIIa mRNA by the polymerase chain reaction and subsequent sequence analysis showed a 86-bp deletion, which corresponds to exon i of the GP IIIa gene. This deletion results in a shift of the reading frame leading to eight altered amino acids followed by a premature termination codon. Analysis of the corresponding genomic DNA fragments showed three mutations in the exon i-intron i boundary region of the GP IIIa gene. One of these mutations is a G → T transition that eliminates the GT splice donor site in the wild type. This base pair change creates a restriction site for the enzyme Mse I. Allele-specific restriction enzyme analysis (ASRA) with Mse I of amplified genomic DNA of the parents and the proposita showed that both parents (who are first cousins) are heterozygous, whereas the proposita is homozygous for the G → T substitution.