Molecular pathogenesis of Bernard-Soulier syndrome

Bernard-Soulier syndrome (BSS), a hereditary qualitative platelet disorder, is now proved to be caused by a qualitative or quantitative abnormality of the platelet glycoprotein (GP) Ib/IX/V complex. We investigated the genetic background of two Japanese females with BSS and identified the molecular defects underlying this disease. The first case was a single base pair deletion within seven adenine repeats at position 4464-4470 (EMBL, no. M22403) of the GPIb alpha gene resulting in a frameshift and a premature stop codon, The second case was a single T-->C substitution at nucleotide 1856 (EMBL, no. M80478) of the GPIX gene resulting in Phe(55)(TTT)-->Ser(TCT) substitution. The latter case is of interest in considering the pathogenesis of BSS, because all four GPs consisting of the GPIb/IX/V complex have the same structural unit called leucine-rich repeat (LRR), Because this mutation is located within the LRR of the GPIX polypeptide, Phe(55)-->Ser substitution may result in an alteration of the LRR that leads to impaired surface expression of the GPI/IX/V complex, To clarify the effect of this mutation on surface expression of the GPIb/IX complex, we performed transfection studies with a plasmid having this mutation. Mutant GPIX could not increase surface expression of the GPIb/IX complex, thus demonstrating an important role of the LRR of the GPIX polypeptide during biosynthesis.