Inherited bleeding disorders: disorders of platelet adhesion and aggregation

Platelet aggregation at sites of vascular injury is essential for the formation of the primary haemostatic plug. The mechanism of platelet aggregation under conditions of physiological flow is a complex multistep process, which requires the synergistic action of several different platelet receptors. Platelet interaction with collagen at sites of damage to the vascular endothelium involves adhesion, activation, secretion of platelet granular contents and finally aggregation. Other agonists other than collagen, such as fibrinogen, vWF and soluble agonists released from activated platelets (thromboxane A2 (TXA2) and ADP) are involved in platelet aggregation. Platelets express a variety of receptors including GP Ib-IX-V, GP VI, GP Ia-IIa and GP IIb-IIIa. One aspect of this complexity of function is the variety of inherited defects of platelet function. Hereditary disorders of platelet adhesion are Bernard-Soulier syndrome and von Willebrand disease. Glanzmann thrombasthenia is an inherited disorder of platelet aggregation. The application of molecular biology to the study of platelet disorders has identified defects in other collagen receptors, ADP receptors and TXA2 receptors. Defects affecting TXA2 production, the generation of procoagulant activity and secretion from dense bodies and alpha-granules are also encountered. Other rare diseases, Chediak-Higashi, Hermansky-Pudlak and Wiskott-Aldrich syndrome also affect platelet storage granules. In this article, recent advances in the understanding of platelet function and knowledge of inherited disorders that affect platelet adhesion and aggregation is reviewed. As progress advances towards individualisation of therapy the phenotypic bleeding tendency of each patient becomes relevant.