AAV-based neonatal gene therapy for hemophilia A: long-term correction and avoidance of immune responses in mice

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Hemophilia A gene therapy has been hampered by immune responses to vector-associated antigens and by neutralizing antibodies or inhibitors to the factor VIII (FVIII) protein; these ‘inhibitors’ more commonly effect hemophilia A patients than those with hemophilia B. A gene replacement strategy beginning in the neonatal period may avoid the development of these immune responses and lead to prolonged expression with correction of phenotype thereby avoiding long-term consequences. Serotype rh10 AAV was developed splitting the FVIII coding sequence into heavy and light chains with the chicken β-actin promoter/CMV enhancer for dual recombinant AAV vector delivery. Coinjection of virions of each FVIII chain intravenously to mice on the second day of life was performed. Mice express sustained FVIII antigen levels of ≥5% to 22 months of life without the development of antibodies to FVIII. Phenotypic correction was manifest in all AAV-FVIII-treated mice as demonstrated by functional assay and reduction in bleeding time. This study demonstrates the use of AAV in a gene replacement strategy in neonatal mice that establishes both long-term phenotypic correction of hemophilia A and lack of antibody development to FVIII in this disease model where AAV is administered shortly after birth. These studies support consideration of gene replacement therapy for diseases that are diagnosed in utero or in the early neonatal period.