Proof of concept of peptide-linked blockmiR-induced MBNL functional rescue in myotonic dystrophy type 1 mouse model

Deletion of miR-146a enhances therapeutic protein restoration in model of dystrophin exon skipping

Duchenne muscular dystrophy (DMD) is a progressive muscle disease caused by the absence of dystrophin protein. One current DMD therapeutic strategy, exon skipping, produces a truncated dystrophin isoform using phosphorodiamidate morpholino oligomers (PMOs). However, the potential of exon skipping therapeutics has not been fully realized as increases in dystrophin protein have been minimal in clinical trials. Here, we investigate how miR-146a-5p, which is highly elevated in dystrophic muscle, impacts dystrophin protein levels. We find inflammation strongly induces miR-146a in dystrophic, but not wild-type myotubes. Bioinformatics analysis reveals that the dystrophin 3' UTR harbors a miR-146a binding site, and subsequent luciferase assays demonstrate miR-146a binding inhibits dystrophin translation. In dystrophin-null mdx52 mice, co-injection of miR-146a reduces dystrophin restoration by an exon 51 skipping PMO. To directly investigate how miR-146a impacts therapeutic dystrophin rescue, we generated mdx52 with body-wide miR-146a deletion (146aX). Administration of an exon skipping PMO via intramuscular or intravenous injection markedly increases dystrophin protein levels in 146aX vs. mdx52 muscles while skipped dystrophin transcript levels are unchanged supporting a post-transcriptional mechanism of action. Together, these data show that miR-146a expression opposes therapeutic dystrophin restoration, suggesting miR-146a inhibition warrants further research as a potential DMD exon skipping co-therapy.

Deletion of miR-146a enhances therapeutic protein restoration in model of dystrophin exon skipping

Duchenne muscular dystrophy (DMD) is a progressive muscle disease caused by the absence of dystrophin protein. One current DMD therapeutic strategy, exon skipping, produces a truncated dystrophin isoform using phosphorodiamidate morpholino oligomers (PMOs). However, the potential of exon skipping therapeutics has not been fully realized as increases in dystrophin protein have been minimal in clinical trials. Here, we investigate how miR-146a-5p, which is highly elevated in dystrophic muscle, impacts dystrophin protein levels. We find inflammation strongly induces miR-146a in dystrophic, but not wild-type myotubes. Bioinformatics analysis reveals that the dystrophin 3'UTR harbors a miR-146a binding site, and subsequent luciferase assays demonstrate miR-146a binding inhibits dystrophin translation. In dystrophin-null mdx52 mice, co-injection of miR-146a reduces dystrophin restoration by an exon 51 skipping PMO. To directly investigate how miR-146a impacts therapeutic dystrophin rescue, we generated mdx52 with body-wide miR-146a deletion (146aX). Administration of an exon skipping PMO via intramuscular or intravenous injection markedly increases dystrophin protein levels in 146aX versus mdx52 muscles; skipped dystrophin transcript levels are unchanged, suggesting a post-transcriptional mechanism-of-action. Together, these data show that miR-146a expression opposes therapeutic dystrophin restoration, suggesting miR-146a inhibition warrants further research as a potential DMD exon skipping co-therapy.

Enhancing the Effectiveness of Oligonucleotide Therapeutics Using Cell-Penetrating Peptide Conjugation, Chemical Modification, and Carrier-Based Delivery Strategies

Oligonucleotide-based therapies are a promising approach for treating a wide range of hard-to-treat diseases, particularly genetic and rare diseases. These therapies involve the use of short synthetic sequences of DNA or RNA that can modulate gene expression or inhibit proteins through various mechanisms. Despite the potential of these therapies, a significant barrier to their widespread use is the difficulty in ensuring their uptake by target cells/tissues. Strategies to overcome this challenge include cell-penetrating peptide conjugation, chemical modification, nanoparticle formulation, and the use of endogenous vesicles, spherical nucleic acids, and smart material-based delivery vehicles. This article provides an overview of these strategies and their potential for the efficient delivery of oligonucleotide drugs, as well as the safety and toxicity considerations, regulatory requirements, and challenges in translating these therapies from the laboratory to the clinic.

The myotonic dystrophy type 1 drug development pipeline: 2022 edition

The beginning of the 20th decade has witnessed an increase in drug development programs for myotonic dystrophy type 1 (DM1). We have collected nearly 20 candidate drugs with accomplished preclinical and clinical phases, updating our previous drug development pipeline review with new entries and relevant milestones for pre-existing candidates. Three interventional first-in-human clinical trials got underway with distinct drug classes, namely AOC 1001 and DYNE-101 nucleic acid-based therapies, and the small molecule pitolisant, which joins the race toward market authorization with other repurposed drugs, including tideglusib, metformin, or mexiletine, already in clinical evaluation. Furthermore, newly disclosed promising preclinical data for several additional nucleic-acid therapeutic candidates and a CRISPR-based approach, as well as the advent into the pipeline of novel therapeutic programs, increase the plausibility of success in the demanding task of providing valid treatments to patients with DM1.

Alternative splicing mediates the compensatory upregulation of MBNL2 upon MBNL1 loss-of-function

Loss of gene function can be compensated by paralogs with redundant functions. An example of such compensation are the paralogs of the Muscleblind-Like (MBNL) family of RNA-binding proteins that are sequestered and lose their function in Myotonic Dystrophy Type 1 (DM1). Loss of MBNL1 increases the levels of its paralog MBNL2 in tissues where Mbnl2 expression is low, allowing MBNL2 to functionally compensate for MBNL1 loss. Here, we show that loss of MBNL1 increases the inclusion of Mbnl2 exon 6 and exon 9. We find that inclusion of Mbnl2 exon 6 increases the translocation of MBNL2 to the nucleus, while the inclusion of Mbnl2 exon 9 shifts the reading frame to an alternative C-terminus. We show that the C-terminus lacking exon 9 contains a PEST domain which causes proteasomal degradation. Loss of MBNL1 increases the inclusion of exon 9, resulting in an alternative C-terminus lacking the PEST domain and the increase of MBNL2. We further find that the compensatory mechanism is active in a mouse DM1 model. Together, this study uncovers the compensatory mechanism by which loss of MBNL1 upregulates its paralog MBNL2 and highlights a potential role of the compensatory mechanism in DM1.

Application of Antisense Conjugates for the Treatment of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is one of the most common muscular dystrophies and can be potentially treated with antisense therapy decreasing mutant DMPK, targeting miRNAs or their binding sites or via a blocking mechanism for MBNL1 displacement from the repeats. Unconjugated antisense molecules are able to correct the disease phenotype in mouse models, but they show poor muscle penetration upon systemic delivery in DM1 patients. In order to overcome this challenge, research has focused on the improvement of the therapeutic window and biodistribution of antisense therapy using bioconjugation to lipids, cell penetrating peptides or antibodies. Antisense conjugates are able to induce the long-lasting correction of DM1 pathology at both molecular and functional levels and also efficiently penetrate hard-to-reach tissues such as cardiac muscle. Delivery to the CNS at clinically relevant levels remains challenging and the use of alternative administration routes may be necessary to ameliorate some of the symptoms experienced by DM1 patients. With several antisense therapies currently in clinical trials, the outlook for achieving a clinically approved treatment for patients has never looked more promising.

Recent Progress and Challenges in the Development of Antisense Therapies for Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a dominant genetic disease in which the expansion of long CTG trinucleotides in the 3' UTR of the myotonic dystrophy protein kinase (DMPK) gene results in toxic RNA gain-of-function and gene mis-splicing affecting mainly the muscles, the heart, and the brain. The CUG-expanded transcripts are a suitable target for the development of antisense oligonucleotide (ASO) therapies. Various chemical modifications of the sugar-phosphate backbone have been reported to significantly enhance the affinity of ASOs for RNA and their resistance to nucleases, making it possible to reverse DM1-like symptoms following systemic administration in different transgenic mouse models. However, specific tissue delivery remains to be improved to achieve significant clinical outcomes in humans. Several strategies, including ASO conjugation to cell-penetrating peptides, fatty acids, or monoclonal antibodies, have recently been shown to improve potency in muscle and cardiac tissues in mice. Moreover, intrathecal administration of ASOs may be an advantageous complementary administration route to bypass the blood-brain barrier and correct defects of the central nervous system in DM1. This review describes the evolution of the chemical design of antisense oligonucleotides targeting CUG-expanded mRNAs and how recent advances in the field may be game-changing by forwarding laboratory findings into clinical research and treatments for DM1 and other microsatellite diseases.

RNA solutions to treat inborn errors of metabolism

RNA-based therapies are a new, rapidly growing class of drugs that until a few years ago were being used mainly in research in rare diseases. However, the clinical efficacy of recently approved oligonucleotide drugs and the massive success of COVID-19 RNA vaccines has boosted the interest in this type of molecules of both scientists and industry, as wells as of the lay public. RNA drugs are easy to design and cost effective, with greatly improved pharmacokinetic properties thanks to progress in oligonucleotide chemistry over the years. Depending on the type of strategy employed, RNA therapies offer the versatility to replace, supplement, correct, suppress, or eliminate the expression of a targeted gene. Currently, there are more than a dozen RNA-based drugs approved for clinical use, including some for specific inborn errors of metabolism (IEM), and many other in different stages of development. New initiatives in n-of-1 RNA drug development offer new hope for patients with rare diseases and/or ultra-rare mutations. RNA-based therapeutics include antisense oligonucleotides, aptamers, small interfering RNAs, small activating RNAs, microRNAs, lncRNAs and messenger RNAs. Further research and collaborations in the fields of chemistry, biology and medicine will help to overcome major challenges in their delivery to target tissues. Herein, we review the mechanism of action of the different therapeutic approaches using RNA drugs, focusing on those approved or in clinical trials to treat IEM.

Molecular Therapies for Myotonic Dystrophy Type 1: From Small Drugs to Gene Editing

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy affecting many different body tissues, predominantly skeletal and cardiac muscles and the central nervous system. The expansion of CTG repeats in the DM1 protein-kinase (DMPK) gene is the genetic cause of the disease. The pathogenetic mechanisms are mainly mediated by the production of a toxic expanded CUG transcript from the DMPK gene. With the availability of new knowledge, disease models, and technical tools, much progress has been made in the discovery of altered pathways and in the potential of therapeutic intervention, making the path to the clinic a closer reality. In this review, we describe and discuss the molecular therapeutic strategies for DM1, which are designed to directly target the CTG genomic tract, the expanded CUG transcript or downstream signaling molecules.