Recent advances in base editing have created an exciting opportunity to precisely correct disease-causing mutations. However, the large size of base editors and their inherited off-target activities pose challenges for in vivo base editing. Moreover, the requirement of a protospacer adjacent motif (PAM) nearby the mutation site further limits the targeting feasibility. Here we modify the NG-targeting adenine base editor (iABE-NGA) to overcome these challenges and demonstrate the high efficiency to precisely edit a Duchenne muscular dystrophy (DMD) mutation in adult mice. Systemic delivery of AAV9-iABE-NGA results in dystrophin restoration and functional improvement. At 10 months after AAV9-iABE-NGA treatment, a near complete rescue of dystrophin is measured in mdx4cv mouse hearts with up to 15% rescue in skeletal muscle fibers. The off-target activities remains low and no obvious toxicity is detected. This study highlights the promise of permanent base editing using iABE-NGA for the treatment of monogenic diseases.