Abstract Body (Do not enter title and authors here): Background: DM1 is a (CTG)n trinucleotide repeat expansion disease in the 3’UTR of the DMPK gene. Once expressed, the repeat RNAs form toxic hairpins that sequester the muscle blind-like family of splicing factors. This induces widespread disruptions in tissue alternative splicing landscape, which triggers multisystemic manifestations – myotonia, muscle weakness, cardiac contractile defects, arrhythmia, and neurological or gastrointestinal disturbances. While impaired mitochondrial function has been demonstrated in the brain, skeletal muscle, and fibroblast of DM1 patients, they have not been reported in the heart, nor have their contribution to the DM1 cardiac pathogenesis been explored. Here, we probed the bioenergetics profile of DM1-afflicted heart tissues and explored the mechanistic basis of DM1-induced cardiac bioenergetic defects.

Hypothesis: We hypothesize that DM1 induces cardiac bioenergetics and structural defects through the missplicing of key mitochondria-related genes.

Methods: Using an inducible, cardiac-specific DM1 mouse model, we performed mitochondria extracellular flux analyses, measured total ATP and NAD(H) concentrations, and performed TOM20 immunofluorescence to identify DM1-induced cardiac bioenergetic and structural defects. We performed gene expression analyses to identify mitochondria-related missplicing events, which we validated in human and mouse DM1 heart tissues. We used an antisense oligonucleotide (ASO) strategy to force these missplicing events and tested the recapitulation of DM1-like bioenergetics and structural defects in vitro.

Results: DM1 induced a multi-state decrease in cardiac oxygen consumption rate (OCR) with a corresponding decrease in ATP and NAD(H) concentrations, indicating reduced efficiency of cardiac OXPHOS. DM1 induced significant cardiac mitochondria fragmentation, which correlated with the missplicing of key mitochondrial fission genes, including mitochondria fission factor Mff and dynamin-related protein 1 Dnm1l in both human and mouse DM1 heart tissues (Fig. 1). ASO-mediated modulation of Mff and Dnm1l splicing recapitulated DM1-like impairment in cardiac bioenergetics and mitochondrial dynamics in wild type HL-1 cardiomyocytes.

Conclusion: DM1 impairs cardiac bioenergetic function through the missplicing of critical genes regulating mitochondrial fusion-fission dynamics. These events represent viable novel therapeutic targets for improving cardiac symptoms of DM1 using an RNA therapeutics approach.