Abstract Body (Do not enter title and authors here): Background: DM1 is the commonest cause of adult-onset muscular dystrophy, affecting 1-in-2100 people in the U.S. DM1 is a (CTG)n trinucleotide repeat expansion disease in the 3’UTR of the DMPK gene. Once expressed, the repeat RNA form toxic hairpins, which sequester the muscle blind-like (MBNL) family of splicing factors. This induces tissue-wide disruption of alternative splicing events, resulting in DM1 symptoms, including muscle weakness, neurological defects, and cardiomyopathy. Recent reports have demonstrated impaired mitochondrial function in the brain, skeletal muscle, and fibroblast of DM1 patients, these have however, not been reported in the heart, nor have their potential contribution to the pathogenesis of DM1 cardiomyopathy been explored.

Methods: Using an inducible, cardiomyocyte-specific DM1 mouse model (aka MHCrtTA:TRE960i), we performed extracellular flux analysis on cardiac mitochondrial fractions obtained from heart tissue of control and DM1 mice. This was accompanied by quantification of ATP production, to determine oxidative phosphorylation (OXPHOS) efficiency. Further, we performed transmission electron microscopy (TEM) and TOM20 immunofluorescence (IF) microscopy to assess the integrity of mitochondrial network in DM1 mice. Finally, we performed gene expression studies to identify mis-splicing events involving mitochondria-related genes and validated these in heart tissues obtained from both DM1 mice and deceased human DM1 subjects.

Results: DM1 induced a multi-state increase in cardiac oxygen consumption rate (OCR) which was however accompanied by decreased ATP levels (2.83µM/10µg vs. 2.45µM/10µg, p=0.0079) indicating reduced cardiac OXPHOS efficiency. DM1 also induced a disruption of mitochondria network, with increased mitochondria fragmentation seen on IF microscopy, and expansion of the intermyofibrillar mitochondria compartment seen on TEM. These findings correlated with the mis-splicing of mitochondrial fission factor (MFF), with an increased skipping of Mff exon 6 in DM1 mice (ΔΨ: -16.8%, p=0.0069) and human DM1 heart tissue (ΔΨ: -19%, p<0.01) compared with controls. Interestingly, this resulted in the increased expression of a shortened 29kDa MFF protein isoform in DM1 mice (Δ%29kDa: 47%, p=0.0003) and human (Δ%29kDa: 21.1%, p<0.0001) heart samples relative to controls.

Conclusion: DM1 impairs cardiac bioenergetic function potentially through the mis-splicing of critical genes regulating mitochondrial fusion-fission dynamics.