Myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults, causes dominantly-inherited muscle weakness, defects of cardiac conduction, variable LV dysfunction, and risk of sudden death. The genetic basis is an expanded CTG repeat in the 3’ untranslated region of DMPK. DM1 patients are functionally hemizygous for DMPK protein, due to nuclear retention of mRNA having expanded repeats. The cardiac aspects are attributed to DMPK loss, toxicity of RNA with expanded repeats, or both. Dmpk heterozygous (+/-) and homozygous knockout (-/-) mice were reported to show AV conduction abnormalities resembling DM1 (Berul et al, JCI, 1999). In an effort to reduce RNA toxicity, antisense oligonucleotides (ASOs) targeting DMPK mRNA have recently entered clinical trials. DM1 phenotypes in skeletal muscle were corrected by ASO knockdown of toxic RNA in mice (Wheeler et al, Nature, 2012). While ASOs may have similar potential to mitigate RNA toxicity in the heart, there is risk of aggravated DMPK deficiency. To reexamine the role of DMPK in the conduction system we studied mice with Dmpk gene deletion or ASO knockdown. We obtained ECGs and echocardiograms on Dmpk -/- and +/- mice, compared to WT littermates. The +/- mice were treated with Dmpk-targeting ASOs or saline. Subcutaneous injection of 50 mg/kg/wk ASO was started at age 2 months, then shifted to biweekly injections after 6 weeks. Dmpk expression in hearts of +/- mice was ~50% of WT, and was further reduced by ASOs (84 ± 3% decrease of mRNA, 93 ± 2% decrease of protein, relative to WT). Surface ECGs and echocardiography at 6 and 10 months showed no differences of heart rate, cardiac conduction, or ejection fraction in WT, saline-treated +/-, ASO-treated +/-, or -/- mice. Conscious, unrestrained ECGs obtained at 11-12 months by radiotelemetry showed no differences among WT, saline-treated +/-, ASO-treated +/-, or -/- mice. We conclude that ASOs can induce posttranscriptional silencing of Dmpk in murine hearts. Constitutive absence of DMPK did not impact cardiac conduction or contractility, and the same was true for ASO knockdown to levels <15% of WT. Our data support the idea that cardiac dysfunction in DM1 results mainly from RNA toxicity, which potentially could be prevented or alleviated by ASOs.
RNA interference mediates RNA toxicity with parent-of-origin effects in C. elegans expressing CTG repeats
