Functional Efficacy of Dystrophin Expression from Plasmids Delivered to mdx Mice by Hydrodynamic Limb Vein Injection

Abstract

In these studies we delivered by hydrodynamic limb vein (HLV) injection plasmid DNA (pDNA) expressing the full-length mouse dystrophin gene to skeletal muscles throughout the hind limbs of the mdx mouse model for Duchenne muscular dystrophy (DMD). We evaluated the levels and stability of dystrophin expression and measured the resulting muscle protection, using Evans blue dye (EBD) to mark the damaged myofibers. Plasmid delivery was as efficient in the dystrophic mice as in wild-type mice and equally efficient in young adult and old mice, as long as the dose of pDNA was adjusted for the target muscle weight. The HLV gene delivery procedure was tolerated well by the dystrophic mice and repeat injections could be performed over an extended period of time. Multiple gene deliveries additively increased the amount of dystrophin protein and also increased the percentages of dystrophin-expressing myofibers. Plasmids expressing dystrophin from a cytomegalovirus (CMV) promoter construct containing the HMG1 intron provided stable dystrophin expression for the life of the mouse and provided significant benefit to the limbs. EBD staining showed that dystrophin gene delivery preserved myofibers in the CMV-HMGi-mDys-injected leg by 2.5- to 5-fold in large groups of muscles and by 2.5-fold throughout the injected legs, compared with the contralateral control legs injected with a nonexpressing plasmid. A similar degree of protection was measured in young adult mice evaluated soon after the last gene delivery and in aged mice injected over an extended period of time. This degree of protection resulted from 18 to 20% of the normal level of dystrophin protein, with 11-16% dystrophin-expressing myofibers. These studies show promise for the use of HLV injections to deliver therapeutic doses of full-length dystrophin-expressing plasmids for long-lasting protection of skeletal muscles in patients with DMD.