Efficient and Stable Adeno-Associated Virus-Mediated Transduction in the Skeletal Muscle of Adult Immunocompetent Mice

Abstract

Recombinant adeno-associated virus (rAAV) vectors were evaluated for gene transfer into the skeletal muscle of adult immunocompetent mice. A study using a vector encoding nuclear localized β-galactosidase (rAAV-nls-lacZ) examined: (i) the efficiency and duration of transgene expression; (ii) the status of the AAV genome in the transduced fibers; and (iii) the possibility of improving gene transfer by inducing muscle regeneration. In the absence of regeneration, the injection of 1.7 × 10⁷ particles in the quadriceps resulted in gene transfer to 10–70% of myofibers. Histological analysis indicated that the vector was able to reach myofiber nuclei distant from the injection point. Cellular infiltrates were absent at early time points but became conspicuous in the vicinity of some positive fibers at 4–8 weeks and subsided by 26 weeks. Southern analysis indicated that one to three copies of the vector genome were present per cell genome equivalent. They were associated with high-molecular-weight DNA in the form of tandem oligomers or interlocked circles. Gene transfer was not facilitated in the regenerating muscle. Rather, an early inflammatory response resulted in the elimination of most positive fibers after 8 weeks. The presence of regenerated fibers with β-galactosidase-positive nuclei suggested that myoblasts had been transduced and were able to fuse to form new fibers. Gene transfer in the absence of immune reactions against the transgene product was studied by injecting mice with a rAAV carrying the murine erythropoietin (mEpo) cDNA. Dose-dependent elevation in the hematocrit was measured for over 200 days and corresponded to 5- to 20-fold increases in plasma Epo levels. We conclude that AAV vectors efficiently and stably transduce post-mitotic muscle fibers and myoblasts in vivo. Noninvasive administration of vectors used in gene therapy is conveniently performed by injection into the skeletal muscle. The tissue is easily accessible, densely vascularized, and has been shown to be capable of post-translationally modifying proteins expressed following transduction using a variety of vectors. In this study, recombinant adeno-associated virus (rAAV) vectors were delivered to the quadriceps of immunocompetent mice, and prolonged expression of β-galactosidase and murine erythropoietin were demonstrated following a single injection. By comparing the normal post-mitotic tissue to the regenerating muscle where cell division has been induced, we observe that gene transfer is more efficient in the quiescent state. This further supports the utility of rAAV vectors for direct gene transfer in vivo.