Single novel mutation in transmembrane region of glycoprotein (GP) IX affects GP Ib- IX complex expression and causes Bernard-Soulier syndrome

A Novel Homozygous c.800C>G Substitution in GP1BA Exon 2 in a Kuwaiti Family with Bernard-Soulier Syndrome

BACKGROUND

Bernard-Soulier syndrome (BSS) is a congenital bleeding disorder characterised by thrombocytopenia, giant platelets and decreased platelet adhesion resulting from genetic alterations of the glycoprotein (GP) Ib/IX/V complex.

OBJECTIVES

Three sisters with a lifelong bleeding history and a provisional diagnosis of BSS were referred for further characterisation of their bleeding diathesis. The siblings' symptoms varied in severity from skin and gum bleeding to menorrhagia associated with iron-deficiency anaemia requiring regular transfusion of red cells and platelets. The parents were consanguineous but did not demonstrate any bleeding disorder.

METHODS

The family were investigated using standard haematological techniques, platelet aggregometry, platelet membrane GP analysis and DNA sequencing of the genes encoding the GPIb/IX complex.

RESULTS

All 3 sisters had thrombocytopenia and giant platelets. Platelet aggregation and flow cytometry studies confirmed the lack of aggregation with ristocetin and a markedly reduced GPIb/IX surface expression. Molecular analysis demonstrated a novel homozygous c.800C>G substitution in GP1BA exon 2 leading to a serine 267 Ter stop codon in all 3 siblings.

CONCLUSIONS

A novel, nonsense mutation was identified as the cause of the bleeding disorder in this family. This is the first reported BSS mutation identified in a family from Kuwait.

Spectrum of the mutations in Bernard-Soulier syndrome

Bernard-Soulier syndrome (BSS) is a rare autosomal recessive bleeding disorder characterized by defects of the GPIb-IX-V complex, a platelet receptor for von Willebrand factor (VWF). Most of the mutations identified in the genes encoding for the GP1BA (GPIbα), GP1BB (GPIbβ), and GP9 (GPIX) subunits prevent expression of the complex at the platelet membrane or more rarely its interaction with VWF. As a consequence, platelets are unable to adhere to the vascular subendothelium and agglutinate in response to ristocetin. In order to collect information on BSS patients, we established an International Consortium for the study of BSS, allowing us to enrol and genotype 132 families (56 previously unreported). With 79 additional families for which molecular data were gleaned from the literature, the 211 families characterized so far have mutations in the GP1BA (28%), GP1BB (28%), or GP9 (44%) genes. There is a wide spectrum of mutations with 112 different variants, including 22 novel alterations. Consistent with the rarity of the disease, 85% of the probands carry homozygous mutations with evidence of founder effects in some geographical areas. This overview provides the first global picture of the molecular basis of BSS and will lead to improve patient diagnosis and management.

Novel Mutation in Bernard-Soulier Syndrome

BACKGROUND: Bernard-Soulier syndrome (BSS) is a severe congenital bleeding disorder characterized by thrombocytopenia, thrombocytopathy and decreased platelet adhesion. BSS results from genetic alterations of the glycoprotein (GP) Ib/IX/V complex. METHODS: We report on a patient demonstrating typical BSS phenotype (thrombocytopenia with giant platelets, bleeding symptoms). However, BSS was not diagnosed until he reached the age of 39 years. RESULTS: Flow cytometry of the patient's platelets revealed absence of GPIb/IX/V receptor surface expression. In addition, immunofluorescence analysis of patient's platelets demonstrated very faint staining of GPIX. A novel homozygous deletion comprising 11 nucleotides starting at position 1644 of the GPIX gene was identified using molecular genetic analysis. CONCLUSIONS: The novel 11-nucleotide deletion (g.1644_1654del11) was identified as causing the bleeding disorder in the BSS patient. This homozygous deletion includes the last 4 nucleotides of the Kozak sequence as well as the start codon and the following 4 nucleotides of the coding sequence. The Kozak sequence is a region indispensable for the initiation of the protein translation process, thus preventing synthesis of functional GPIX protein in the case of deletion.