Chapter 3 —— 32 —— Duchenne muscular dystrophy (DMD) (MIM: 310200) is a severe and frequent neuromuscular disorder (incidence of ~1 in 5,500 boys) caused by mutations in the vast X-linked DMD gene (~2.2 Mb), whose largest product, the long rod-shaped 427-kDa dystrophin isoform, is a key structural component of the striated musculature [1]. Contributing to the urgency in the development of currently inexistent DMD therapies is the observation that ~1/3 of cases arise de novo through germline mutations, often intragenic deletions that disrupt the mRNA reading frame. Critically, naturally occurring DMD gene deletions resulting in in-frame transcripts coding for internally truncated, yet partially functional, dystrophins cause the milder Becker muscular dystrophy (MIM: 300376). Hence, DMD gene manipulations yielding Becker-like dystrophins via direct coding sequence reframing or exon skipping have the potential of offering long-lasting therapeutic effects [1]. Toward this end, among other technologies, CRISPR-Cas9 nucleases and adeno-associated viral (AAV) vectors are being investigated for rescuing dystrophin expression upon double-strand DNA break (DSB) formation and ensuing chromosomal end-joining [1]. These experiments demonstrate that AAV/CRISPR-Cas9-based dystrophin restoration can improve striated muscle function in mice; however, a potentially insidious outcome is the identification of prevalent capture of Cas9-encoding AAV at nuclease target sites, including at Dmd exons 51 and 53 [2,3]. Moreover, programmable nucleases can trigger other untoward effects, e.g., locus- or chromosomewide rearrangements [4]. There is, therefore, a pressing need to expand candidate DMD genetic therapies to those based on DSB-independent genome editing systems. In a timely study published in Molecular Therapy – Nucleic Acids, Chai and coworkers [5] identify adenine base editors (ABEs) and guide RNAs (gRNAs) (Figure 1) that, after implementing single basepair substitutions (i.e., A·T-to-G·C transitions) at splicing motifs, a process that they name “single-swap” editing, lead to genotype-specific DMD repair through exon skipping. Next, the authors assemble a dual AAV ABE transsplicing system to demonstrate in dystrophin-defective mice the amelioration
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