Enhanced antisense oligonucleotide delivery reveals that transcript turnover impacts apparent splicing rescue in myotonic dystrophy

Steric-blocking antisense oligonucleotides rescue myotonic dystrophy type 1 phenotypes in preclinical models and are under evaluation in clinical trials. However, rationale for biomarker selection remains a topic of debate. Here, we show that a cyclic cell-penetrating peptide that escapes endosomes enhances muscle delivery of a phosphorodiamidate morpholino oligonucleotide designed to block pathogenic CUG repeat expansions in HSALR mice. A single systemic administration rescued mis-splicing and eliminated myotonia one-week post-injection, with partial splicing rescue evident after 24 hours. Interestingly, some exons showed more robust rescue than others, but relationships between MBNL concentration ([MBNL]) and Percent Spliced In (Ψ) could not fully explain the extent of rescue. We hypothesized that since pre-existing transcripts must be degraded to reveal full drug effect, rates of transcript replacement might account for these discrepancies. We formulated a mathematical framework and used Bayesian inference to model how apparent Ψ lags behind nascent Ψ as a function of time; faster rates of replacement result in shorter lags. In vivo 5-ethynyl uridine labeling followed by RNAseq validated these predictions. Overall, we show that transcript turnover influences Ψ during periods of dynamically changing [MBNL] and recommend that this be considered when selecting splicing biomarkers and interpreting responses to therapeutic interventions.