Factor VIII C2 domain missense mutations exhibit defective trafficking of biologically functional proteins

The half-life of coagulation factor VIII (FVIII) in plasma is prolonged by noncovalent interaction with von Willebrand factor (vWF). Antibody inhibition data indicate that epitopes within the carboxyl terminus of the FVIII light chain play a role in vWF binding. Analysis of hemophilia A patient DNA samples have identified missense mutations within this carboxyl terminus of the FVIII light chain at amino acid 2307 in which arginine is replaced with either glutamine or leucine. Patients with these mutations have reduced FVIII activity proportional to reduced cross-reacting material in their plasma. It was hypothesized that the reduced levels of FVIII in plasma due to these mutations may be related to a defect in vWF binding with resultant plasma instability. Wild-type and mutant FVIII cDNA expression vectors were prepared and expressed in COS-1 monkey cells by transient DNA transfection. FVIII mutants R2307Q and R2307L were synthesized at equal rates compared to FVIII wild-type but had greater than 10-fold reduced accumulation of antigen and activity levels in the conditioned medium. An additional mutation, Y2305F, also displayed a similar defect in protein accumulation, whereas Y2332F was secreted similarly to wild-type. The specific activity of immunoaffinity purified R2307Q was mildly reduced compared to FVIII wild-type, whereas vWF binding properties were retained. Inhibition of intracellular cysteine proteases resulted in intracellular accumulation of R2307Q protein, suggesting that the mechanism leading to hemophilia A is related to a block in secretion and subsequent degradation within the secretory pathway rather than extracellular instability.