DMPK is a New Candidate Mediator of Tumor Suppressor p53-Dependent Cell Death

Tumor suppressor p53 plays an integral role in DNA-damage induced apoptosis, a biological process that protects against tumor progression. Cell shape dramatically changes when cells undergo apoptosis, which is associated with actomyosin contraction; however, it remains entirely elusive how p53 regulates actomyosin contraction in response to DNA-damaging agents. To identify a novel p53 regulating gene encoding the modulator of myosin, we conducted DNA microarray analysis. We found that, in response to DNA-damaging agent doxorubicin, expression of myotonic dystrophy protein kinase (DMPK), which is known to upregulate actomyosin contraction, was increased in a p53-dependent manner. The promoter region of DMPK gene contained potential p53-binding sequences and its promoter activity was increased by overexpression of the p53 family protein p73, but, unexpectedly, not of p53. Furthermore, we found that doxorubicin treatment induced p73 expression, which was significantly attenuated by downregulation of p53. These data suggest that p53 induces expression of DMPK through upregulating p73 expression. Overexpression of DMPK promotes contraction of the actomyosin cortex, which leads to formation of membrane blebs, loss of cell adhesion, and concomitant caspase activation. Taken together, our results suggest the existence of p53-p73-DMPK axis which mediates DNA-damage induced actomyosin contraction at the cortex and concomitant cell death.

Abstract 191: Deletion or Knockdown of Myotonic Dystrophy Protein Kinase Does Not Affect Cardiac Conduction or Ejection Fraction in Mice

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.