Biochemical and molecular basis of Bernard-Soulier syndrome: a review

Bernard-Soulier syndrome (BSS) is a rare hereditary recessive autosomal bleeding disorder characterized by a prolonged bleeding time, giant platelets, thrombocytopenia, normal platelet aggregation in response to ADP and no agglutination in response to ristocetin. This disease is due to absence or abnormality of the platelet membrane glycoprotein GPIb-IX-V, the receptor for von Willebrand factor. All four genes encoding the complex have been cloned and 17 forms of BSS have to date been characterized at the functional, immunological and molecular levels. The mutations can be divided into two main groups. Firstly, mutations located in leucine rich repeats (LRR), responsible for conformational modifications of the molecule, in some cases higher sensitivity to proteases and loss of adhesive function of the receptor, which is expressed at lower than normal levels at the platelet membrane. When mutations affect the LRR of GPIbalpha, the presence of the other chains varies from normal to residual amounts. When mutations affect the LRR of GPIX, expression of the other chains is strongly diminished, suggesting that GPIX plays a major role in the stability of the complex. A second type of mutations leads to synthesis of a truncated molecule lacking the transmembrane domain and absence of its expression at the platelet surface, while the other chains are present in residual amounts. Expression of recombinant proteins in eukaryotic cells has recently confirmed the results derived from studies of natural mutations. Separate expression of each chain can be obtained, although the presence of all subunits is required for full expression.