Aberrant Splicing of DNM1L Impairs Cardiac Bioenergetics and Mitochondrial Dynamics in Myotonic Dystrophy Type I...

Myotonic dystrophy type 1 (DM1) is caused by a (CTG)n trinucleotide repeat expansion in the 3'UTR of the DMPK gene. Once expressed, repeat RNA forms toxic hairpins that sequester the MBNL (muscle blind-like) family of splicing factors. This disrupts the tissue alternative splicing landscape, triggering multisystemic manifestations-myotonia, muscle weakness, cardiac contractile defects, arrhythmia, and neurological disturbances. Although impaired mitochondrial function has been reported in the brain, skeletal muscle, and fibroblasts of patients with DM1, they have not been reported in the heart, nor have their contribution to the DM1 cardiac pathogenesis been explored. Here, we probed the bioenergetic profile of DM1-afflicted heart tissues and explored the mechanistic basis of DM1-induced cardiac bioenergetic defects.

Using an inducible, heart-specific DM1 mouse model, we performed extracellular flux analyses, measured total ATP and NAD(H) concentrations, and performed immunofluorescence staining and transmission electron microscopy to characterize DM1-induced cardiac bioenergetics and mitochondrial structural defects. We analyzed eCLIP-Seq data to identify mitochondria-related missplicing events, which we validated in human and mouse DM1 heart tissues. Finally, we used antisense oligonucleotides to replicate these events and to test the recapitulation of DM1-like bioenergetic and structural defects in vitro.

DM1 induced a multistate decrease in oxygen consumption rate with a corresponding reduction in ATP and NAD(H) concentrations, indicating impaired oxidative phosphorylation in DM1-afflicted mouse hearts. We also found significant cardiac mitochondria fragmentation, which correlated with the missplicing of transcripts encoding mitochondria fission factor (Mff, encodes MFF protein) and dynamin related protein 1 (Dnm1l, encodes DRP1 protein) in DM1-afflicted human and mouse hearts. Antisense oligonucleotides-mediated redirection of Dnm1l alternative splicing reproduced DM1-like impairment in cardiac bioenergetics and mitochondrial dynamics in wild-type HL-1 cardiomyocytes.

Together, these findings reveal that expanded (CUG)n RNA toxicity in DM1 disrupts cardiac bioenergetics through the missplicing of critical mitochondrial fission transcripts. These misspliced transcripts represent potential therapeutic targets for improving mitochondrial function and cardiac symptoms of DM1.