Deletion of alanine 2201 in the FVIIIC2 domain results in mild hemophilia A by impairing FVIII binding to VWF and phospholipids and destroys a major FVIII antigenic determinant involved in inhibitor development

The C2 domain of factor VIII (FVIII) mediates FVIII binding to von Willebrand factor (VWF) and phospholipids (PLs), thereby determining the stability and the activity of FVIII. A deletion of Ala2201 (Del2201) was identified in the FVIII C2 domain of 2 unrelated patients with mild hemophilia A (FVIII:C 11%-33%). This mutation prevents FVIII binding to a human monoclonal antibody recognizing the C2 domain and inhibiting FVIII binding to VWF and phospholipids. By comparison to healthy FVIII, Del2201 FVIII had a significantly reduced binding to VWF, which likely contributes to reduced FVIII levels in plasma. Del2201 FVIII interaction with phospholipids was evaluated in an FXa generation assay, using various concentrations of synthetic phospholipid vesicles mimicking an activated platelet surface. At the lowest phospholipid concentration allowing FXa generation, De2201 FVIII activity was reduced 3-fold. This is the first report of a mutation altering FVIII binding to phospholipids and occurring in patients with hemophilia A.