Altered Cytoskeleton Organization in Platelets from Patients with MYH9-related Disease

MYH9-related disease (MYH9-RD) is an autosomal dominant disorder deriving from mutations in the MYH9 gene encoding for the heavy chain of non-muscle myosin IIA, and characterized by thrombocytopenia and giant platelets. Isoform IIA of myosin is the only one expressed in platelets, but the possibility that MYH9 mutations affect the organization of contractile structures in these blood elements has never been investigated. In this work we have analyzed the composition and the agonist-induced reorganization of the platelet cytoskeleton from seven MYH9-RD patients belonging to four different families. We found that an increased amount of myosin was constitutively associated with actin in the cytoskeleton of resting MYH9-RD platelets. Upon platelet stimulation, an impaired increase in the total cytoskeletal proteins was observed. Moreover, selected membrane glycoproteins, tyrosine kinases, and small GTPases failed to interact with the cytoskeleton in agonist-stimulated MYH9-RD platelets. These results demonstrate for the first time that mutations of MYH9 result in an alteration of the composition and agonist-induced reorganization of the platelet cytoskeleton. We suggest that these abnormalities may represent the biochemical basis for the previously reported functional alterations of MYH9-RD platelets, and for the abnormal platelet formation from megakaryocytes, resulting in thrombocytopenia and giant platelets.