Heterogenous expression of glycoprotein Ib, IX and V in platelets from two patients with Bernard-Soulier syndrome caused by different genetic abnormalities

Bernard-Soulier syndrome (BSS) is a rare inherited bleeding disorder, which is caused by deficiency or decrease of the platelet GPIb/IX/V complex. Analysis of two patients with BSS by flow cytometry of the blood revealed different expression patterns of the components of the GPIb/lX/V complex. In case 1, GPIX was completely absent but residual amounts of GPIb alpha and GPV were detectable; in case 2, GPIb alpha was completely absent. We amplified the coding regions of GPIb alpha, GPIb beta, GPV, and GPIX from the patients' genomic DNA with the polymerase chain reaction (PCR) and sequenced the PCR products. In case 1, we identified a point mutation in the GPIX coding region that changes the codon for tryptophan-126 (TGG) to a nonsense codon (TGA). In case 2, we found a deletion of nucleotide within seven adenine repeats at the position of 1932 to 1938 in the coding region of GPIb alpha, which causes a frame shift that results in 58 altered amino acids and a premature stop codon. These genetic changes alter the transmembrane domain of GPIX or GPIb alpha and, therefore, would prevent proper insertion of the proteins in the plasma membrane. Thus, abnormality of a single component protein (GPIX or GPIb alpha) alters the assembly of the GPIb/IX/V complex and causes heterogenous surface expression of GPIb alpha, GPV and GPIX.