As a consequence of the retention of mutant DMPK transcripts in the nucleus in DM1, patients express baseline levels of DMPK protein that are already half those of unaffected individuals. Since a key therapeutic strategy relies upon degradation of DMPK transcripts using antisense oligonucleotides (ASO), there are concerns as to whether essential functions of DMPK may be comprised and thereby contribute to the pathogenesis in DM1.
A University of Rochester team led by Dr. Charles Thornton has addressed this issue using mouse models with constitutive (genetic deletion) or acquired (ASO reduction) reductions in Dmpk. The function of DMPK is currently unknown. Prior reports in genetic models have shown that mice with heterozygous deletion of Dmpk exhibit cardiac conduction system defects, while those with homozygous deletion show skeletal muscle myopathy and weakness. Thus optimization of the ability of ASOs to target and degrade DMPK transcripts could exacerbate cardiac and skeletal muscle dysfunction in DM1.
Dr. Thornton and colleagues reevaluated the impact of genetic and ASO-induced reductions in Dmpk in two mouse models. They saw no effect of genetic deletion or ASO knockdown on cardiac (heart rate, PR interval, QRS duration, left ventricular contractile parameters) or skeletal (grip strength) functional measures, despite the substantial reductions that were achieved in Dmpk protein levels. Current strategies for ASO knockdown in DM1 utilize an allele-selective approach by targeting and degrading the mutant DMPK transcripts that are retained in the nucleus. However, there is the possibility that wild-type DMPK transcripts that traffic to the cytoplasm may also be degraded, as the next generation ASO chemistries result in more effective delivery to skeletal and cardiac muscles.
Yet despite the concern that substantial reductions in Dmpk protein may impact these muscles, Dr. Thornton’s team did not uncover any pathophysiology associated with Dmpk knockdown, even when genetic and ASO strategies were combined to yield as much as 90% reduction. Differences between the results of the Thornton team and prior investigations may relate to technical differences in the studies, mouse background strain differences, or features of the genetic knockout alleles.
The levels of Dmpk silencing seen in this study most likely would exceed those that could be obtained in DM1 patients, even with a highly effective ASO drug. The level of reduction of Dmpk in the mouse models also would likely exceed the reductions that are necessary to effectively restore the DM1-linked changes in mRNA splicing, and thereby mitigate DM1 signs and symptoms.
Hence these findings support the notion that strategies to increase the effectiveness of ASO candidate therapies can be effective in DM1 without increasing the risk of cardiac and skeletal muscle events.
Publication: "Dmpk gene deletion or antisense knockdown does not compromise cardiac or skeletal muscle function in mice." Carrell ST, Carrell EM, Auerbach D, Pandey SK, Bennett CF, Dirksen RT, Thornton CA. Hum Mol Genet. 2016 Aug 13. pii: ddw266.