A Leu7Pro mutation in the signal peptide of platelet glycoprotein (GP)IX in a case of Bernard-Soulier syndrome abolishes surface expression of the GPIb-V-IX complex

This paper describes the molecular defect of the second case of Bernard-Soulier syndrome, initially reported in 1957. Analysis of the patient's platelets by flow cytometry and Western blotting failed to detect surface expression of any of the four subunits of the glycoprotein (GP)Ib-V-IX complex and revealed small amounts of intracellular GPIbalpha, GPIbbeta and GPV but no GPIX. DNA sequencing revealed a novel missense mutation in the GPIX gene which replaced Leu (CTG) by Pro (CCG) at position 7 of the signal peptide. This mutation is, to date, the only known example of a leader sequence defect in Bernard-Soulier syndrome. The change occurred in a prototypic alpha-helical hydrophobic core region, typically enriched in leucine and devoid of proline residues. Co-transfection of GPIXPro7 with normal GPIbalpha and GPIbbeta into Chinese hamster ovary cells reproduced the platelet phenotype, resulting in no detectable GPIX, low intracellular levels of GPIbalpha and GPIbbeta, and an absence of surface expression. This mutation presumably leads to an abnormal conformation and, hence, incorrect insertion of GPIX into the endoplasmic reticulum and/or to defective signal peptide cleavage, both of which are required for correct transport to the cell membrane. This provides further evidence for a critical role of GPIX in controlling biosynthesis of the GPIb-IX complex.