Clinical and genetic aspects of Bernard-Soulier syndrome: searching for genotype/phenotype correlations

Abstract

Background: Bernard-Soulier syndrome is a severe bleeding disease due to a defect of GPIb/IX/V, a platelet complex that binds the von Willebrand factor. Due to the rarity of the disease, there are reports only on a few cases compromising any attempt to establish correlations between genotype and phenotype. In order to identify any associations, we describe the largest case series ever reported, which was evaluated systematically at the same center.

Design and methods: Thirteen patients with the disease and seven obligate carriers were enrolled. We collected clinical aspects and determined platelet features, including number and size, expression of membrane glycoproteins, and ristocetin induced platelet aggregation. Mutations were identified by direct sequencing of the GP1BA, GP1BB, and GP9 genes and their effect was shown by molecular modeling analyses.

Results: Patients all had a moderate thrombocytopenia with giant platelets and a bleeding tendency whose severity varied among individuals. Consistent with expression levels of GPIbα always lower than 10% of control values, platelet aggregation was absent or severely reduced. Homozygous mutations were identified in the GP1BA, GP1BB and GP9 genes; six were novel alterations expected to destabilize the conformation of the respective protein. Except for obligate carriers of a GP9 mutation with a reduced GPIb/IX/V expression and defective aggregation, all the other carriers had no obvious anomalies.

Conclusions: Regardless of mutations identified, the patients' bleeding diathesis did not correlate with thrombocytopenia, which was always moderate, and platelet GPIbα expression, which was always severely impaired. Obligate carriers had features similar to controls though their GPIb/IX/V expression showed discrepancies. Aware of the limitations of our cohort, we cannot define any correlations. However, further investigations should be encouraged to better understand the causes of this rare and underestimated disease.

Figure 1.

Effect of missense mutations on protein folding. (A, B) Ribbon representation of GPIbα (entry name 1GWB). (A) The side chains of Asn126 and Cys225 are indicated, as well as the positions of the N- and C-termini. (B) Close-up of the region around Asn126 to show the environment and its involvement in hydrogen bonds. A linear fog effect is applied to give a 3D impression. (C, D) Modeling of GPIX based on the coordinates of 2O6Q. (C) Alignment of the relevant region of the template with the sequence of GPIX. The positions that carry the observed mutations are marked in red. (D) Mapping the mutations into the structure using a thicker bond radius for the positions mutated.