Co-opting MBNL-dependent alternative splicing cassette exons to control gene therapy in myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is a multisystemic genetic disorder caused by a CTG repeat expansion that accumulate as toxic CUG repeat RNA. Functional sequestration of muscleblind-like (MBNL) proteins by CUG repeat RNA leads to deleterious, yet predictable changes in alternative splicing in DM. Genetic medicines for DM that reduce CUG repeat RNA or increase MBNL are advancing, but application of a viral-based approach must contend with high phenotypic and molecular variability. To address this, we have repurposed well-described cassette exons that are excluded by MBNL action to tune translational output of a therapeutic protein. We show that these splicing events can be taken out of genetic context and respond to changes in MBNL concentration or accumulation of toxic CUG repeat RNA, can deliver therapeutic MBNL1 protein to improve skeletal muscle myotonia or prevent cardiac toxicity associated with MBNL1 overexpression in mice, and distinguish DM patient-derived skeletal muscle myotubes from isogenic controls. Further work to fine-tune events to specific tissue targets and therapeutic cargos is needed, but these events can increase the therapeutic window for viral-based approaches for DM1.