Engineered factor IX variants bypass FVIII and correct hemophilia A phenotype in mice

The complex of the serine protease factor IX (FIX) and its cofactor, factor VIII (FVIII), is crucial for propagation of the intrinsic coagulation cascade. Absence of either factor leads to hemophilia, a disabling disorder marked by excessive hemorrhage after minor trauma. FVIII is the more commonly affected protein, either by X-chromosomal gene mutations or in autoimmune-mediated acquired hemophilia. Whereas substitution of FVIII is the mainstay of hemophilia A therapy, treatment of patients with inhibitory Abs remains challenging. In the present study, we report the development of FIX variants that can propagate the intrinsic coagulation cascade in the absence of FVIII. FIX variants were expressed in FVIII-knockout (FVIII-KO) mice using a nonviral gene-transfer system. Expression of the variants shortened clotting times, reduced blood loss after tail-clip assay, and reinstalled clot formation, as tested by in vivo imaging of laser-induced vessel injury. In addition, we confirmed the therapeutic efficacy of FIX variants in mice with inhibitory Abs against FVIII. Further, mice tolerant to wild-type human FIX did not develop immune responses against the protein variants. Our results therefore indicate the feasibility of using variants of FIX to bypass FVIII as a novel treatment approach in hemophilia with and without neutralizing FVIII Abs.