Human CalDAG-GEFI deficiency increases bleeding and delays αIIbβ3 activation

Affinity regulation of integrin alpha IIb beta 3 for fibrinogen by inside-out signaling plays a critical role in hemostasis. Calcium and diacylglycerol (DAG)-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) was identified as a Rap1-activating molecule, and its role in inside-out alpha IIb beta 3 activation was established in CalDAG-GEFI-deficient mice. However, little information regarding CalDAG-GEFI in human platelets is available. Here, we report a 16-year-old girl with CalDAG-GEFI deficiency who has been suffering from severe bleeding tendency. Although talin and kindlin-3 were normally detected, CalDAG-GEFI was undetectable in her platelets by western blotting. Genetic analysis revealed compound heterozygous CalDAG-GEFI mutations, Lys309X and Leu360del, which were responsible for CalDAG-GEFI deficiency. The functional analysis demonstrated impaired alpha IIb beta 3 activation by various agonists except for phorbol myristate acetate, normal calcium mobilization, and impaired Rap1 activation, which were consistent with the phenotype of CalDAG-GEFI-deficient mice. Despite substantial aIIbb3 activation at high agonist concentrations, she had severe bleeding tendency. Further functional analysis demonstrated markedly delayed alpha IIb beta 3 activation velocity and decreased shear-induced thrombus formation. Contrary to CalDAG-GEFI-deficient mice, which showed integrin-dependent neutrophil functional abnormality, neutrophil beta 2 integrin activation was not impaired in the patient. Our results demonstrate the critical role of CalDAG-GEFI in rapid alpha IIb beta 3 activation of human platelets.