Bernard-Soulier Syndrome

[The molecular mechanisms of platelets activation in patients with cerebrovascular disease]

Cerebrovascular disease is a main cause of mortality and one of the big medical problems. After the vascular wall's damage the endothelial cells secrete the von Willebrand factor which then connects with its platelet's receptor GP Ib-V-IX. There are two polymorphisms Thr145Met and T(-5)C of the GP Iba gene associated with arterial thrombosis development. Also the difference in platelets' genes expressions was shown in patients with various clinical course of ischemic heart disease. The aim of this study was to investigate the role of platelet's receptor for von Willebrand factor in platelets' activation in patients with cerebrovascular disease. 123 patients with cerebrovascular disease and 97 healthy donors were included into the study. We analyzed the level of receptor for von Willebrand factor on platelet's membrane by flow cytometry, Thr145Met and T(-5)C GP Iba polymorphiams by PCR-RFLP, the GP Iba gene expression by RT-PCR and ADP-induced platelet aggregation by Born method. We have shown: 1) the 145Met GP Iba allele prevalence in patients with atherotrombotic stroke development due to macroangiopathy; 2) the pre-mRNA transform into the mature mRNA in activated platelets and this process may be stopped by the antiplatelet therapy by acetylsalicylic acid.

Menorrhagia and bleeding disorders in adolescent females

In women, von Willebrand disease (VWD) is the most common inherited bleeding disorder. Since VWD and other inherited bleeding disorders are autosomal disorders, they affect women and men. Menorrhagia, or heavy menstrual bleeding (HMB), is the most common symptom of women with bleeding disorder experience. Objectively, it is defined as bleeding that lasts for more than seven days or results in the loss of more than 80 ml of blood per menstrual cycle. The prevalence of menorrhagia in a woman with a bleeding disorder ranges from 32 to 100% in patients with VWD, from 5 to 98% in patients with a platelet dysfunction and from 35 to 70% in women with a rare factor deficiency. A detailed history and a careful physical exam are the first steps towards a diagnosis in adolescents, adding a PBAC>100 increased the sensitivity of the screening tool further to 95%. Laboratory testing should be made at the time of menstrual bleeding in an effort to capture the lowest level of VWF:Ag and FVIII:C. Treatment options for menorrhagia in VWD: (1) antifibrinolytic therapy with tranexamic acid, (2) the non-transfusional agent desmopressin (DDAVP), (3) purified blood products that contain factor VIII and VWF concentrated from plasma and (4) hormonal preparations.

Surface expression of glycoprotein Ibα is dependent on glycoprotein Ibβ: evidence from a novel mutation causing Bernard-Soulier syndrome

Bernard-Soulier syndrome is a rare bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP) Ib-IX-V complex. The complex, which serves as a platelet receptor for von Willebrand factor, is composed of 4 subunits: GPIb, GPIbβ, GPIX, and GPV. We here describe the molecular basis of a novel form of Bernard-Soulier syndrome in a patient in whom the components of the GPIb-IX-V complex were undetectable on the platelet surface. Although confocal imaging confirmed that GPIb was not present on the platelet surface, GPIb was readily detectable in the patient's platelets. Moreover, immunoprecipitation of plasma with specific monoclonal antibodies identified circulating, soluble GPIb. DNA-sequence analysis revealed normal sequences for GPIb and GPIX. There was a G to A substitution at position 159 of the gene encoding GPIbβ, resulting in a premature termination of translation at amino acid 21. Studies of transient coexpression of this mutant, W21stop-GPIbβ, together with wild-type GPIb and GPIX, demonstrated a failure of GPIX expression on the surface of HEK 293T cells. Similar results were obtained with Chinese hamster ovary  IX cells, a stable cell line expressing GPIb that retains the capacity to re-express GPIX. Thus, we found that GPIbβ affects the surface expression of the GPIb-IX complex by failing to support the insertion of GPIb and GPIX into the platelet membrane.

The Glycoprotein Ib/IX Complex Regulates Cell Proliferation

The glycoprotein (Gp) Ib/IX complex contains three transmembranous leucine-rich repeat polypeptides (GpIb, GpIbβ, and GpIX) that form the platelet von Willebrand factor (vWF) receptor. GpIb/IX functions to effect platelet adhesion, activation, and aggregation under conditions of high shear stress. GpIb/IX is expressed late in the ontogeny of megakaryocytes, the precursor cell that releases platelets when it reaches its terminal stage of differentiation. Because signal pathways can be reused at different stages of development by integration with different effector pathways and because cellular adhesion through other receptor families often modulates cell growth, the hypothesis that GpIb/IX regulates cell growth was investigated. The surface expression of recombinant GpIb decreases the proliferation of transduced CHO cells. GpIb causes growth arrest in the G1 phase of the cell cycle associated with the induction of the cyclin-dependent kinase inhibitor p21. G1 arrest induced by recombinant GpIb in heterologous cells requires signaling through the 14-3-3ζ binding domain of GpIb and is partially dependent on its engagement by the extracellular ligand vWF. Growth arrest induced by the expression of recombinant GpIb/IX is followed by apoptosis of the transduced cells. The endogenous expression of GpIb in human hematopoietic cells is associated with decreased proliferation. These results suggest that the expression of the GpIb/IX complex regulates megakaryocyte growth.

Identification and Characterization of Endothelial Glycoprotein Ib Using Viper Venom Proteins Modulating Cell Adhesion

The expression and function of a glycoprotein Ib (GPIb) complex on human umbilical vein endothelial cells (HUVECs) is still a matter of controversy. We characterized HUVEC GPIb using viper venom proteins: alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin is an antagonist, and alboaggregins act as agonists of the platelet GPIb complex. Botrocetin is a venom protein that alters von Willebrand factor (vWF) conformation and increases its binding affinity for the GPIb complex. Echistatin is a disintegrin that blocks vβ3. Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese hamster ovary (CHO) cells transfected with vβ3. We found the following: (1) Binding of monoclonal antibodies against GPIb to HUVECs was moderately increased after stimulation with cytokines and phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both echicetin and echistatin, an vβ3 antagonist, inhibited the adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The inhibitory effect was additive when both proteins were used together. (3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this effect was completely abolished by echicetin, but not by echistatin. (4) CHO cells expressing GPIbβ/IX adhered to vWF (in the presence of botrocetin) and to alboaggregins; GPIb was required for this reaction. Echicetin, but not echistatin, inhibited the adhesion of cells transfected with GPIbβ/IX to immobilized vWF. (5) HUVECs adhered strongly to immobilized vWF and alboaggregins with extensive spreading, which was inhibited by LJ1b1, a monoclonal antibody against GPIb. The purified vβ3 receptor did not interact with the alboaggregins, thereby excluding the contribution of vβ3 in inducing HUVEC spreading on alboaggregins. In conclusion, our data confirm the presence of a functional GPIb complex expressed on HUVECs in low density. This complex may mediate HUVEC adhesion and spreading on immobilized vWF and alboaggregins.