Absence of Significant Off-Target Splicing Variation with a U7snRNA Vector Targeting Exon 2 Duplications.

Exon skipping therapies for Duchenne muscular dystrophy that restore an open reading frame can be induced by the use of noncoding U7 small nuclear RNA (U7snRNA) modified by an antisense exon-targeting sequence delivered by an adeno-associated virus (AAV) vector. We have developed an AAV vector (AAV9.U7-ACCA) containing four U7snRNAs targeting the splice donor and acceptor sites of dystrophin exon 2, resulting in highly efficient exclusion of exon 2. We assessed the specificity of splice variation induced by AAV9.U7-ACCA delivery in the exon 2 duplication (Dup2) mouse model through an unbiased RNA-seq approach. Treatment-related effects on pre-mRNA splicing were quantified using local splicing variation (LSV) analysis. Filtering the transcriptome for differences in treatment-related splicing resulted in only 16 candidate off-target LSVs. Only a single candidate off-target LSV was found in both skeletal and cardiac muscle tissue and occurred at a known variable cassette exon. In contrast, four LSVs represented significant on-target correction of exon 2 splicing and transcriptome analysis showed correction of known dystrophin-deficient gene dysregulation. We conclude that the absence of off-target splicing induced by treatment with the U7-ACCA vector supports the continued clinical development of this approach.