Peptide-conjugated oligonucleotides evoke long-lasting myotonic dystrophy correction in patient-derived cells and mice

Application of CRISPR-Cas9-Mediated Genome Editing for the Treatment of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder, caused by expansion of a CTG microsatellite repeat in the 3' untranslated region of the DMPK (dystrophia myotonica protein kinase) gene. To date, novel therapeutic approaches have focused on transient suppression of the mutant, repeat-expanded RNA. However, recent developments in the field of genome editing have raised the exciting possibility of inducing permanent correction of the DM1 genetic defect. Specifically, repurposing of the prokaryotic CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 (CRISPR-associated protein 9) system has enabled programmable, site-specific, and multiplex genome editing. CRISPR-based strategies for the treatment of DM1 can be applied either directly to patients, or indirectly through the ex vivo modification of patient-derived cells, and they include excision of the repeat expansion, insertion of synthetic polyadenylation signals upstream of the repeat, steric interference with RNA polymerase II procession through the repeat leading to transcriptional downregulation of DMPK, and direct RNA targeting of the mutant RNA species. Potential obstacles to such therapies are discussed, including the major challenge of Cas9 and guide RNA transgene/ribonuclear protein delivery, off-target gene editing, vector genome insertion at cut sites, on-target unintended mutagenesis (e.g., repeat inversion), pre-existing immunity to Cas9 or AAV antigens, immunogenicity, and Cas9 persistence.

Status update in the use of cell-penetrating peptides for the delivery of macromolecular therapeutics

INTRODUCTION

In this review, recent developments and applications with cell-penetrating peptides (CPP) are discussed. CPPs are widely used tools for the delivery of various macromolecular therapeutics, such as proteins and nucleic acids.

AREAS COVERED

The current review focuses on recent important advances and reports that demonstrate high clinical and translational potential. Most important clinical developments have occurred with the CPP-drug conjugate approaches that target various protein-protein interactions, and these have been highlighted subsequently. Most of the applications are targeting cancer, but recently, noteworthy advances have taken place in the field of antisense oligonucleotides and muscular dystrophies, lung targeting, and trans-BBB targeting.

EXPERT OPINION

Successful applications and clinical development with the drug conjugate approaches are discussed. On the other hand, the reasons of why the nanoparticle approaches are not as far in development are analyzed.

Analyzing pre-symptomatic tissue to gain insights into the molecular and mechanistic origins of late-onset degenerative trinucleotide repeat disease

How genetic defects trigger the molecular changes that cause late-onset disease is important for understanding disease progression and therapeutic development. Fuchs' endothelial corneal dystrophy (FECD) is an RNA-mediated disease caused by a trinucleotide CTG expansion in an intron within the TCF4 gene. The mutant intronic CUG RNA is present at one-two copies per cell, posing a challenge to understand how a rare RNA can cause disease. Late-onset FECD is a uniquely advantageous model for studying how RNA triggers disease because: (i) Affected tissue is routinely removed during surgery; (ii) The expanded CTG mutation is one of the most prevalent disease-causing mutations, making it possible to obtain pre-symptomatic tissue from eye bank donors to probe how gene expression changes precede disease; and (iii) The affected tissue is a homogeneous single cell monolayer, facilitating accurate transcriptome analysis. Here, we use RNA sequencing (RNAseq) to compare tissue from individuals who are pre-symptomatic (Pre_S) to tissue from patients with late stage FECD (FECD_REP). The abundance of mutant repeat intronic RNA in Pre_S and FECD_REP tissue is elevated due to increased half-life in a corneal cells. In Pre_S tissue, changes in splicing and extracellular matrix gene expression foreshadow the changes observed in advanced disease and predict the activation of the fibrosis pathway and immune system seen in late-stage patients. The absolute magnitude of splicing changes is similar in pre-symptomatic and late stage tissue. Our data identify gene candidates for early drivers of disease and biomarkers that may represent diagnostic and therapeutic targets for FECD. We conclude that changes in alternative splicing and gene expression are observable decades prior to the diagnosis of late-onset trinucleotide repeat disease.

Advances in oligonucleotide drug delivery

Oligonucleotides can be used to modulate gene expression via a range of processes including RNAi, target degradation by RNase H-mediated cleavage, splicing modulation, non-coding RNA inhibition, gene activation and programmed gene editing. As such, these molecules have potential therapeutic applications for myriad indications, with several oligonucleotide drugs recently gaining approval. However, despite recent technological advances, achieving efficient oligonucleotide delivery, particularly to extrahepatic tissues, remains a major translational limitation. Here, we provide an overview of oligonucleotide-based drug platforms, focusing on key approaches - including chemical modification, bioconjugation and the use of nanocarriers - which aim to address the delivery challenge.

Advances in the diagnosis of inherited neuromuscular diseases and implications for therapy development

Advances in DNA sequencing technologies have resulted in a near doubling, in under 10 years, of the number of causal genes identified for inherited neuromuscular disorders. However, around half of patients, whether children or adults, do not receive a molecular diagnosis after initial diagnostic workup. Massively parallel technologies targeting RNA, proteins, and metabolites are being increasingly used to diagnose these unsolved cases. The use of these technologies to delineate pathways, biomarkers, and therapeutic targets has led to new approaches entering the drug development pipeline. However, these technologies might give rise to misleading conclusions if used in isolation, and traditional techniques including comprehensive neurological evaluation, histopathology, and biochemistry continue to have a crucial role in diagnostics. For optimal diagnosis, prognosis, and precision medicine, no single ruling technology exists. Instead, an interdisciplinary approach combining novel and traditional neurological techniques with computer-aided analysis and international data sharing is needed to advance the diagnosis and treatment of neuromuscular disorders.

Systemic Evaluation of Chimeric LNA/2'-O-Methyl Steric Blockers for Myotonic Dystrophy Type 1 Therapy

Myotonic dystrophy type 1 (DM1) is a dominantly inherited, multisystemic disorder characterized clinically by delayed muscle relaxation and weakness. The disease is caused by a CTG repeat expansion in the 3′ untranslated region (3′ UTR) of the DMPK gene, which leads to the expression of a toxic gain-of-function mRNA. The expanded CUG repeat mRNA sequesters the MBNL1 splicing regulator in nuclear-retained foci structures, resulting in loss of protein function and disruption of alternative splicing homeostasis. In this study, we used CAG repeat antisense oligonucleotides (ASOs), composed of locked nucleic acid (LNA)- and 2′-O-methyl (2′OMe)-modified bases in a chimeric design, to alleviate CUG^expanded-mediated toxicity. Chimeric 14–18mer LNA/2′OMe oligonucleotides, exhibiting an LNA incorporation of ∼33%, significantly ameliorated the misregulated alternative splicing of Mbnl1-dependent exons in primary DM1 mouse myoblasts and tibialis anterior muscles of DM1 mice. Subcutaneous delivery of 14mer and 18mer LNA/2′OMe chimeras in DM1 mice resulted in high levels of accumulation in all tested skeletal muscles, as well as in the diaphragm and heart tissue. Despite the efficient delivery, chimeric LNA/2′OMe oligonucleotides were not able, even at a high-dosage regimen (400 mg/kg/week), to correct the misregulated splicing of Serca1 exon 22 in skeletal muscles. Nevertheless, oligonucleotide doses were well-tolerated as determined by histological and plasma biochemistry analyses. Our results provide proof of concept that inhibition of MBNL1 sequestration by systemic delivery of a steric-blocking ASO is extremely challenging, considering the large number of target sites that need to be occupied per RNA molecule. Although not suitable for DM1 therapy, chimeric LNA/2′OMe oligonucleotides could prove to be highly beneficial for other diseases, such as Duchenne muscular dystrophy, that require inhibition of a single target site per RNA molecule.

Better living through peptide-conjugated chemistry: next-generation antisense oligonucleotides

Two different antisense oligonucleotide-based (ASO-based) therapies are currently in clinical use to treat neuromuscular diseases. This success, for Duchenne muscular dystrophy and spinal muscular atrophy, offers hope not only for additional neuromuscular diseases, but also for other disorders that could benefit from RNA-targeted therapies. A major limitation for more widespread application of ASOs relates to relatively poor tissue penetration. In this issue of the JCI, Klein et al. showed that conjugating an ASO with an arginine-rich cell-penetrating peptide, Pip6a, enhanced delivery, resulting in corrective outcome for a mouse model of myotonic dystrophy. Linking ASOs to cell-penetrating peptides, or even other moieties, is an approach currently under development with treatment potential to expand to other disorders.