Therapeutic Levels of Factor IX Expression Using a Muscle-Specific Promoter and Adeno-Associated Virus Serotype 1 Vector

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Abstract
Extensive studies in animal models of the X-linked bleeding disorder hemophilia B (deficiency in functional coagulation factor IX, F.IX) have shown that muscle-directed adeno-associated (AAV)-mediated F.IX gene transfer can be used to treat this disease. However, large vector doses of AAV-2 vector are required for therapeutic levels of expression, and the number of vector doses that can be injected per intramuscular site is limited. Several studies have shown that some of these limitations can be overcome by use of AAV serotype 1 vector. Here, we demonstrate levels of F.IX transgene expression from a synthetic muscle-specific promoter (C5-12) that were higher than from the cytomegalovirus (CMV) immediate-early enhancer-promoter in cultured muscle cells in vitro and approximately 50% of CMV-driven expression in vivo in murine skeletal muscle after AAV-1 gene transfer. These data show for the first time that a tissue-specific promoter can be used to achieve therapeutic levels of muscle-derived F.IX expression in the context of viral gene transfer. However, use of a muscle-specific promoter did not prevent antibody formation in response to a murine F.IX transgene product in mice with F.IX gene deletion, indicating that the risk of humoral immune responses remains in the context of an immunologically unfavorable mutation.