Bernard-Soulier syndrome: whole blood diagnostic assays of platelets

Approach to the Assessment of Platelet Function: Comparison between Optical-based Platelet-rich Plasma and Impedance-based Whole Blood Platelet Aggregation Methods

Platelet aggregation studies play an important role in the assessment of hereditary and acquired platelet function defects. The first aggregation test introduced into laboratory practice used platelet-rich plasma (PRP) where aggregation was detected by an optical method. The assessment of platelet function using whole blood (WB) aggregation by an impedance method followed up nearly 20 years later. The WB impedance aggregation assay appears to be superior to the optical method because it 1) evaluates platelets in a physiologic milieu in the presence of red and white blood cells, which are known to modulate platelet function; 2) is faster; 3) has higher sensitivity; and 4) does not require centrifugation, thus avoiding injury to platelets and loss of giant thrombocytes. These two assays were compared. Clearly, the WB impedance aggregation methodology has many advantages over the optical PRP assay for the assessment of the hyperactive platelet syndrome and the effects of anti-platelet drugs.

Platelet analysis using flowcytometric procedures

'Ex vivo' testing of functional platelet properties using conventional techniques reflects the overall behaviour of the whole platelet population in the sample under investigation. Since many functional aspects depend on ultrastructural constituents which may vary from one cell to another, multiparameter single cell analysis of platelets may be advantageous in providing direct insight into deviations at the cellular level relevant to the pathophysiology of disease states such as bleeding disorders or thrombophilia. Immunolabelling with monoclonal antibodies against membrane antigens has been combined with flowcytometry to provide a standardized, highly specific and sensitive analytical tool. The assay has been optimized for simultaneous two colour fluorescence staining, and this allows the testing of whole blood to provide a quick monitoring method for the differential diagnosis of thrombasthenic diseases like Bernard Soulier's syndrome or Glanzmann's thrombasthenia in which typical staining patterns lack the specific fluorescence for glycoproteins Ib and IIb/IIIa respectively. Also changes in the antigenicity of the outer membrane of activated platelets are detectable with monoclonal antibodies against specific antigenic epitopes such as thrombospondin (a secretion marker) or α-granule and lysosomal proteins (extrusion markers). However, for detection of activated platelets in diseases associated with a prethrombotic state, the procedures for immunolabelling platelets with monoclonal antibodies and instrumental detection sensitivity remain to be optimized. After further development, flowcytometric assays of the functional status of individual platelets may be superior to the measurement of the indirect plasma markers such as platelet factor 4 or β-thromboglobulin for routine diagnosis of the prethrombotic state.

Flow cytometric analysis of platelet surface glycoproteins in the diagnosis of Bernard-Soulier syndrome

The use of flow cytometry in the diagnosis of Bernard-Soulier syndrome (BSS) in patients with giant platelets and thrombocytopenia was investigated as an adjunct to ristocetin-induced platelet aggregation (RIPA) studies because of problems experienced with aggregation studies, particularly at the time of presentation, in the pediatric age group. Eight patients suspected of having BSS were studied with respect to platelet expression of glycoprotein Ib alpha (CD42b) and glycoprotein IIIa (CD61) using an EPICS Profile II flow cytometer. Twelve patients with normal platelet morphology and platelet counts were used as normal controls. One patient with Alport's syndrome, four patients with immune thrombocytopenic purpura (ITP), and one patient with Glanzmann thrombasthenia were also studied. In all eight patients suspected of having BSS, deficiency of glycoprotein Ib alpha was demonstrated. Normal expression was demonstrated in 12 control patients, in one patient with giant platelets with Alport's syndrome, and in four patients with ITP. Absence of glycoprotein IIIa was demonstrated in Glanzmann thrombasthenia. In the pediatric age group one is able to demonstrate abnormalities of platelet membrane glycoprotien in patients with thrombocytopathias using flow cytometry. This method has the advantage of being rapid and can be performed on small volumes of blood suitable for pediatric practice.

Nonsense mutation in the glycoprotein Ib alpha coding sequence associated with Bernard-Soulier syndrome

Three distinct gene products, the alpha and beta chains of glycoprotein (GP) Ib and GP IX, constitute the platelet membrane GP Ib-IX complex, a receptor for von Willebrand factor and thrombin involved in platelet adhesion and aggregation. Defective function of the GP Ib-IX complex is the hallmark of a rare congenital bleeding disorder of still undefined pathogenesis, the Bernard-Soulier syndrome. We have analyzed the molecular basis of this disease in one patient in whom immunoblotting of solubilized platelets demonstrated absence of normal GP Ib alpha but presence of a smaller immunoreactive species. The truncated polypeptide was also present, along with normal protein, in platelets from the patient's mother and two of his four children. Genetic characterization identified a nucleotide transition changing the Trp-343 codon (TGG) to a nonsense codon (TGA). Such a mutation explains the origin of the smaller GP Ib alpha, which by lacking half of the sequence on the carboxyl-terminal side, including the trans-membrane domain, cannot be properly inserted in the platelet membrane. Both normal and mutant codons were found in the patient, suggesting that he is a compound heterozygote with a still unidentified defect in the other GP Ib alpha allele. Nonsense mutation and truncated GP Ib alpha polypeptide were found to cosegregate in four individuals through three generations and were associated with either Bernard-Soulier syndrome or carrier state phenotype. The molecular abnormality demonstrated in this family provides evidence that defective synthesis of GP Ib alpha alters the membrane expression of the GP Ib-IX complex and may be responsible for Bernard-Soulier syndrome.