Recombinant adeno-associated virus (rAAV) vectors have been shown to be useful for efficient gene delivery to a variety of dividing and nondividing cells. Mechanisms responsible for the long-term, persistent expression of the rAAV transgene are not well understood. In this study we investigated the kinetics of rAAV-mediated human factor IX (hFIX) gene transfer into human primary myoblasts and myotubes. Transduction of both myoblasts and myotubes occured with a similar and high efficiency. After 3 to 4 weeks of transduction, rAAV with a cytomegalovirus (CMV) promoter showed 10- to 15-fold higher expression than that with a muscle-specific creatine kinase enhancer linked to β-actin promoter. Factor IX expression from transduced myoblasts as well as myotubes reached levels as high as approximately 2 μg of hFIX/106 cells/day. Southern blot analyses of high-molecular-weight (HMW) cellular genomic and Hirt DNAs isolated from rAAV/CMVhFIXm1-transduced cells showed that the conversion of single-stranded vector genomes to double-stranded DNA forms, but not the level of the integrated forms in HMW DNA, correlated with increasing expression of the transgene. Together, these results indicate that rAAV can transduce both proliferating and terminally differentiated muscle cells at about the same efficiency, that expression of transgenes increases linearly over their lifetime with no initial lag phase, and that increasing expression correlates with the appearance of double-stranded episomal rAAV genomes. Evidence showing that the rAAV virions can copackage hFIX, presumably nonspecifically, was also obtained.
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