Elimination of myotonia improves myopathy in a muscleblind knockout model of myotonic dystrophy: longread sequencing of Clcn1 transcripts in muscle from Mbnl1 knockout, Exon 7a deletion, and ADR mice.

A cardinal sign of myotonic dystrophy type 1 (DM1) is myotonia, slow muscle relaxation after voluntary contraction. Myotonia results from mis-regulated splicing of chloride channel 1 (ClC-1), leading to loss of channel function and runs of involuntary action potentials in muscle fibers. Heralding the onset of weakness, myotonia is often the first symptom of DM1, and thus this raises the possibility that muscle hyperexcitability promotes the subsequent development of myopathy. We used genome editing to test this possibility by deleting the alternatively spliced and frameshift inducing ClC-1 exon 7a (E7a) in the Mbnl1 knockout model of DM1. Although several ClC-1 exons exhibit mis-regulated splicing in DM1, deletion of this single cryptic exon was sufficient to restore ClC-1 function and eliminate myotonia systemically and permanently. We utilized long-read sequencing to assess transcript isoform utilization in mice with Mbnl1 knockout, deletion of E7a reduced the frequency of other splicing defects in ClC-1 transcripts, likely as a passive consequence of restoring reading frame and nonsense surveillance.