Antisense Oligonucleotides and DM1
Targeting antisense oligonucleotides (ASOs) to expanded CUG repeats has received increasing interest as a means to suppress the RNA-mediated toxicity that is mechanistic in DM1. While the strategy initially focused on using ASOs to sterically block the binding of MBNL protein to structural hairpins formed by the expanded triplet repeats in DMPK (Wheeler et al., 2009), there appears to now be a consensus toward instead triggering RNase H-mediated degradation of the toxic RNA (Lee et al., 2012; Wheeler et al., 2012). Clinical trials based upon this strategy (IONIS-DMPK-2.5 Rx) did not achieve adequate bioavailability at the skeletal muscle target, but there appear to be multiple alternatives to correct this delivery problem. Strategies range from further optimizing ASO backbone chemistry to conjugation with any of a variety of agents capable of mediating translocation across the sarcolemma.
Evolving a Promising Strategy
Penetration of ASO drugs into muscle fibers for exon skipping in Duchenne muscular dystrophy is thought to be mediated, at least in part, by the frank sarcolemmal breaks present in this disease. Sarcolemmal disruption, however, is not a component of the pathogenesis of DM1. Linking large molecule drugs, such as ASOs, to cell-penetrating peptides represents one strategy to augment the often slow and limited transition of these drugs across an intact sarcolemma. A new paper in Journal of Clinical Investigation from Drs. Matthew Wood (Oxford University) and Denis Furling (Institute of Myology) and their colleagues explore the potential of an arginine-rich Pip6a cell-penetrating peptide to enhance bioavailability and efficacy of PMO-based ASOs in a mouse model of DM1 (Klein et al., 2019).
Low dose treatment of mice with the Pip6a-PMO-CAG conjugate proved superior to both naked PMO and other previously published ASO chemistries in reversing mis-splicing defects and myotonia in HSALR mice. Systemically delivered Pip6a-PMO-CAG (2-3 iv injections at 12.5 mg/kg each) produced complete splice correction of three test transcripts, reduced expanded repeat RNA and nuclear foci, and abolished myotonia. Splicing correction duration was reported as extending as long as six months.
Efficacy with the Pip6a conjugate tested here was achieved at doses 5-10x less than those reported in previous publications with other ASO formulations. Transcriptome analysis further established that broad splicing correction was obtained. These findings contrasted with little or no effect obtained with naked PMO. The research team also demonstrated biodistribution to cardiac muscle.
Finally, the research team showed that the Pip6a-PMO-CAG conjugate displaced MBNL from nuclear foci, reduced toxic RNA levels, and corrected splicing in four test transcripts in treated DM1 patient muscle cell cultures with large expansions (2600 and >1300 repeats). Again, effects were superior to naked PMO.
Going Forward
The authors have tested one candidate cell-penetrating peptide as a delivery vehicle for ASOs based upon an RNase H strategy to mitigate DM1. While efforts to select the optimal cell-penetrating peptide are still warranted (since no head-to-head tests of alternative peptides were done here), these data provide additional proof of principle that enhanced delivery may help overcome the hurdles encountered in DM1 clinical trials to date. Taken together, these findings support the renewed efforts to improve the delivery of ASOs to DM1 patient skeletal muscle and thereby address what is most likely the primary barrier to the efficacy of this therapeutic strategy.
References:
Reversal of RNA dominance by displacement of protein sequestered on triplet repeat RNA.
Wheeler TM, Sobczak K, Lueck JD, Osborne RJ, Lin X, Dirksen RT, Thornton CA.
Science. 2009 Jul 17;325(5938):336-9. doi: 10.1126/science.1173110.
RNase H-mediated degradation of toxic RNA in myotonic dystrophy type 1.
Lee JE, Bennett CF, Cooper TA.
Proc Natl Acad Sci U S A. 2012 Mar 13;109(11):4221-6. doi: 10.1073/pnas.1117019109. Epub 2012 Feb 27.
Targeting nuclear RNA for in vivo correction of myotonic dystrophy.
Wheeler TM, Leger AJ, Pandey SK, MacLeod AR, Nakamori M, Cheng SH, Wentworth BM, Bennett CF, Thornton CA.
Nature. 2012 Aug 2;488(7409):111-5. doi: 10.1038/nature11362.
Peptide-conjugated oligonucleotides evoke long-lasting myotonic dystrophy correction in patient-derived cells and mice.
Klein AF, Varela MA, Arandel L, Holland A, Naouar N, Arzumanov A, Seoane D, Revillod L, Bassez G, Ferry A, Jauvin D, Gourdon G, Puymirat J, Gait MJ, Furling D, Wood MJA.
J Clin Invest. 2019 Sep 3. pii: 128205. doi: 10.1172/JCI128205. [Epub ahead of print]