Human CalDAG-GEFI deficiency confers severe bleeding tendency and delayed αIIbβ3 activation velocity.

Affinity regulation of integrin αIIbβ3 for fibrinogen by inside-out signaling plays a critical role in hemostasis. Calcium and DAG-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) was identified as a Rap1 activating molecule and its role in inside-out αIIbβ3 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 αIIbβ3 activation by various agonists except for PMA, normal calcium mobilization, and impaired Rap1 activation, which were consistent with the phenotype of CalDAG-GEFI deficient mice. Despite of substantial αIIbβ3 activation at high agonist concentrations, she has severe bleeding tendency. Further functional analysis demonstrated markedly delayed αIIbβ3 activation velocity and decreased shear-induced thrombus formation. Contrary to CalDAG-GEFI deficient mice which showed integrin dependent neutrophil functional abnormality, neutrophil β2 integrin activation was not impaired in the patient. Our results demonstrate the critical role of CalDAG-GEFI in rapid αIIbβ3 activation of human platelets.