AntimiR blocking of miR-23b and -218 corrects myotonic dystrophy primary cell defects over a wide range of CTG repeat expansions.

This study tested candidate therapeutic antimiRs in a comprehensive new collection of primary myoblasts derived from myotonic dystrophy type 1 (DM1) patients. DM1 is caused by genetically unstable CTG repeat expansions in the DMPK gene and has multiple clinical presentations, likely because of variable repeat sizes at the tissue and organ levels. Expanded DMPK transcripts deplete Muscleblind-Like Splicing Regulator protein MBNL1, which in turn causes DM1 symptoms. A promising therapeutic approach increases the expression of MBNL1 by blocking natural repressors miR-23b and miR-218 employing antimiRs with chemical modifications designed for a better pharmacological profile in humans. In untreated primary cells, we demonstrate that upregulated therapeutic targets miR-23b and miR-218 and reduced MBNL1 protein levels inversely correlate with CTG repeat size, supporting that active MBNL1 protein repression synergizes with the sequestration by CUG repeats in DM1. Upon treatment with novel antimiRs, multiple readouts of RNA toxicity associated with typical DM1 alterations improved. MBNL1-dependent exon inclusions, and myoblast fusion and myotube area defects, showed robust correction over a wide range of CTG expansions. Unexpectedly, antimiR treatment also reduced DMPK transcript levels and the number of ribonuclear aggregates. Based on these data, we selected a leading antimiR for further transcriptomics characterization and demonstrated that it reversed up to 68% of dysregulated genes. The study revealed that the candidate antimiRs could provide therapy to clinically relevant DM1 forms over a wide range of repeat sizes and genetic backgrounds by simultaneously reducing MBNL1 sequestration and derepressing protein synthesis.