Introduction Platelets may undergo a process of activation by which they are enabled to bind with high affinity to either adhesive immobi- lized ligand from the extracellular matrix or to soluble proteins. The αIIbβ3 complex is a non-covalent, calcium-dependent het- erodimer, expressed almost exclusively in platelets (1), that works as a surface receptor for fibrinogen (Fg) and other adhe- sive ligands (2). Since Fg is a multivalent ligand, the platelets aggregate once they bind to it. In order to prevent spontaneous cell aggregation, an efficient system of control must help main- tain the cells in a basal, inactive state. The molecular mecha- nism(s) controlling the platelet state of activation remains elu- sive (3-5). The occupation of platelet receptors for physiologi- cal agonists turns the αIIbβ3 complex into a high affinity recep- tor through the activation of signaling pathways arising from inside the cell (“inside-out”, cellulifugal, signaling) (5). It has been postulated that the integrin activation is the result of con- © 2004 Schattauer GmbH, Stuttgart A variant thrombasthenic phenotype associated with compound heterozygosity of integrin β 3 -subunit: (Met124Val)β3 alters the subunit dimerization rendering a decreased number of constitutive active αIIbβ3 receptors Consuelo González-Manchón, Nora Butta, Susana Larrucea, Elena G. Arias-Salgado, Sonia Alonso, Angela López, Roberto Parrilla Department of Physiopathology and Human Molecular Genetics, Centro de Investigaciones Biológicas (CSIC), Madrid, Spain Thromb Haemost 2004; 92: 1377–86 minute expression (≤ 5%) of αIIb(124Val)β3 receptors. CHO cells expressing (124Val)β3 showed a diminished surface expression of αIIbβ3, enhanced adhesion to immobilized Fg, and spontaneous aggregation in the presence of soluble Fg, sug- gesting that (124Val)β3 may confer constitutive activity to the αIIb(124Val)β3 receptors. A distinct feature of these cells is the failure of DTT to enhance the binding to soluble Fg and the for- mation of cell aggregates. The substitution of (124Met)β3 by either a polar or a positively charged amino acid restored the surface exposure and function of the αIIbβ3 receptors where- as a negatively charged residue did not. Keywords Platelets, αIIbβ3, thrombasthenia, β3 gain function mutation Summary We report the analysis of a variant case of thrombasthenic phe- notype that is a compound heterozygote for two mutations located within the metal ion dependent adhesion site (MIDAS) of the β3 subunit.The patient inherited a maternal allele carry- ing the Met124Val substitution and a paternal allele that changes Asp119 to Tyr.Phenotyping of the human platelet anti- gen 1 (HPA-1) showed that the platelet αIIbβ3 complex in the patient was mostly accounted for by the Asp 119Tyr allele that does not bind to fibrinogen (Fg). The patient showed agonist- induced binding of platelets to Fg but neither binding to PAC-1 nor cell aggregation could be detected, most likely due to the Platelets and Blood Cells 1377 Correspondence to: Roberto Parrilla, M.D. Centro de Investigaciones Biológicas Ramiro de Maeztu, 9 28040-Madrid, Spain Tel.:/Fax: +34 91 5349154 E-mail: rparrilla@cib.csic.es Received June 17, 2004 Accepted after revision September 20, 2004 Financial support: This work has been supported in part by grants from the Direccion General de Investigacion (SAF 2000-0127, BMC2002-01053 and BMC2003-01409), Fondo de Investigaciones Sanitarias (FIS-PI021263). N. Butta is recipient of a tenure track grant Ramon y Cajal from the Spanish Ministry of Science. Susana Larrucea was supported by a postdoctoral fellowships from the Comunidad de Madrid (08.4/0015.1/2001) and Sonia Alonso by a predoctoral fellowship from the Gabierno Vasco (BF201-40). Prepublished online November 8, 2004 DOI: 10.1160/TH04-06-0380