Glanzmann thrombasthenia associated with a 21-amino acid deletion (Leu817-Gln837) in glycoprotein IIb due to abnormal splicing in exon 25.

We report a novel genetic defect in a Japanese patient with type I Glanzmann thrombasthenia. The glycoprotein (GP) Ilb complementary DNA (cDNA) from platelet messenger RNA had a 63-base pair deletion in the 5' boundary of exon 25, resulting in an in-frame deletion of 21 amino acid residues (Leu817-Gln837) in the calf-2 domain. The deleted region was present in the genomic DNA, but the splice acceptor site (AG) of exon 25 was mutated to AC, leading to the use of an AG sequence in the middle of exon 25 as an abnormal cryptic splice acceptor site. The effect of this deletion on protein synthesis was further analyzed. Mutant GPIIb-IIIa complexes were not detected on the surfaces of cells cotransfected with cDNAs of mutant GPIIb and normal GPIIIa. Mutant pro-GPIIb was detected in cell lysates and was coimmunoprecipitated with an anti-GPIIb-IIIa complex antibody. Immunostaining demonstrated that the mutant pro-GPIIb colocalized with an endoplasmic reticulum protein, calnexin, within the cells. These results indicate that complex formation was not completely prevented and that impairment of the subsequent transport was the major reason for the defect in cell surface expression. The data suggest that the GPIIb calf-2 domain is important for intracellular transport of GPIIIb-IIIa complexes.