Split intein-mediated protein trans-splicing to express large dystrophins

Gene replacement using Adeno-associated viral (AAV) vectors represents a promising approach for many diseases. However, this therapeutic modality is challenged by the modest packaging capacity of AAVs (~4.7 kb), limiting its application for disorders involving large genes, such as Duchenne muscular dystrophy with a coding sequence >11 kb. Here we developed a novel method for expressing large dystrophins by utilizing the protein trans-splicing mechanism mediated by split inteins. We identified several split intein pairs that can efficiently join two or three fragments and generate, respectively, a large midi-dystrophin or the entire full-length dystrophin. We show that the delivery of two or three AAVs into dystrophic mice results in robust expression of large dystrophins with significant functional improvements compared with current micro-dystrophins. Moreover, using the potent myotropic AAVMYO, we demonstrate that low doses (2x10¹³ vg/kg) are sufficient to express large dystrophins in striated muscles bodywide with significant physiological corrections in young or very old dystrophic mice. Our data support the feasibility of this approach in vivo and show a clear superiority of large dystrophins over micro-dystrophins that are being tested in clinical trials. This novel method could benefit any patient with Duchenne or Becker muscular dystrophy regardless of genotype and could be adapted to numerous other disorders caused by mutations in large genes exceeding the carrying capacity of AAV.