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 functio nal 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 he mophilia B dogs, an important preclinical model of the disease. While previ ously published data demonstrated sustained expression of canine F9 in dogs with a missense mutation in the gene F9, we show here that AAV-mediated ca nine F9 gene transfer to skeletal muscle of hemophilia B dogs carrying a nu ll mutation of F9 (causing an early stop codon and an unstable mRNA) result s in induction of inhibitory anticanine 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 underly ing mutation in F9. Transient immune suppression with cyclophosphamide at t he time of vector administration blocked formation of anti-canine F9 antibo dies in the one animal treated with this approach. Treatment with this comb ination of gene transfer and transient immune modulation has resulted in su stained expression (> 8 months) of canine F9 at levels sufficient for parti al correction of coagulation parameters.