Muscle-directed gene transfer and transient immune suppression result in sustained partial correction of canine hemophilia B caused by a null mutation

The X-linked bleeding disorder hemophilia B is caused by absence of functional blood coagulation factor IX (F9) and can be treated by adeno-associated viral (AAV) mediated gene transfer to skeletal muscle. The safety of this approach is currently being evaluated in a phase I clinical trial. Efficacy of this and several other gene therapy strategies has been addressed in hemophilia B dogs, an important preclinical model of the disease. While previously published data demonstrated sustained expression of canine F9 in dogs with a missense mutation in the gene F9, we show here that AAV-mediated canine F9 gene transfer to skeletal muscle of hemophilia B dogs carrying a null mutation of F9 (causing an early stop codon and an unstable mRNA) results in induction of inhibitory anti-canine F9 at comparable vector doses (1 x 10(12) vector genomes/kg). Thus, the risk of inhibitor formation following AAV-mediated F9 gene therapy may be influenced by the nature of the underlying mutation in F9. Transient immune suppression with cyclophosphamide at the time of vector administration blocked formation of anti-canine F9 antibodies in the one animal treated with this approach. Treatment with this combination of gene transfer and transient immune modulation has resulted in sustained expression (>8 months) of canine F9 at levels sufficient for partial correction of coagulation parameters.