A divalent siRNA chemical scaffold for potent and sustained modulation of gene expression throughout the central nervous system

Antisense Drugs Make Sense for Neurological Diseases

The genetic basis for most inherited neurodegenerative diseases has been identified, yet there are limited disease-modifying therapies for these patients. A new class of drugs-antisense oligonucleotides (ASOs)-show promise as a therapeutic platform for treating neurological diseases. ASOs are designed to bind to the RNAs either by promoting degradation of the targeted RNA or by elevating expression by RNA splicing. Intrathecal injection into the cerebral spinal fluid results in broad distribution of antisense drugs and long-term effects. Approval of nusinersen in 2016 demonstrated that effective treatments for neurodegenerative diseases can be identified and that treatments not only slow disease progression but also improve some symptoms. Antisense drugs are currently in development for amyotrophic lateral sclerosis, Huntington's disease, Alzheimer's disease, Parkinson's disease, and Angelman syndrome, and several drugs are in late-stage research for additional neurological diseases. This review highlights the advances in antisense technology as potential treatments for neurological diseases.

Emerging Oligonucleotide Therapeutics for Rare Neuromuscular Diseases

Research and drug development concerning rare diseases are at the cutting edge of scientific technology. To date, over 7,000 rare diseases have been identified. Despite their individual rarity, 1 in 10 individuals worldwide is affected by a rare condition. For the majority of these diseases, there is no treatment, much less cure; therefore, there is an urgent need for new therapies to extend and improve quality of life for persons who suffer from them. Here we focus specifically on rare neuromuscular diseases. Currently, genetic medicines using short antisense oligonucleotides (ASO) or small interfering ribonucleic acids that target RNA transcripts are achieving spectacular success in treating these diseases. For Duchenne muscular dystrophy (DMD), the state-of-the-art is an exon skipping therapy using an antisense oligonucleotide, which is prototypical of advanced precision medicines. Very recently, golodirsen and viltolarsen, for treatment of DMD patients amenable to skipping exon 53, have been approved by regulatory agencies in the USA and Japan, respectively. Here, we review scientific and clinical progress in developing new oligonucleotide therapeutics for selected rare neuromuscular diseases, discussing their efficacy and limitations.

Active immunotherapy and alternative therapeutic modalities for Alzheimer's disease

As knowledge of Alzheimer's disease (AD) progression improves, the field has recognized the need to diversify the pipeline, broaden strategies and approaches to therapies, as well as delivery mechanisms. A better understanding of the earliest biological processes of AD/dementia would help inform drug target selection. Currently there are a number of programs exploring these alternate avenues. This meeting will allow experts in the field (academia, industry, government) to provide perspectives and experiences that can help elucidate what the pipeline looks like today and what avenues hold promise in developing new therapies across the stages of AD. The focus here is on Active Immunotherapies and Alternative Therapeutic Modalities. This topic includes active vaccines, antisense oligomers, and cell-based therapy among others, and highlights new clinical developments that utilize these modalities.

Cell Type Impacts Accessibility of mRNA to Silencing by RNA Interference

RNA interference (RNAi) is a potent mechanism that silences mRNA and protein expression in all cells and tissue types. RNAi is known to exert many of its functional effects in the cytoplasm, and thus, the cellular localization of target mRNA may impact observed potency. Here, we demonstrate that cell identity has a profound impact on accessibility of apolipoprotein E (ApoE) mRNA to RNAi. We show that, whereas both neuronal and glial cell lines express detectable ApoE mRNA, in neuronal cells, ApoE mRNA is not targetable by RNAi. Screening of a panel of thirty-five chemically modified small interfering RNAs (siRNAs) did not produce a single hit in a neuronal cell line, whereas up to fifteen compounds showed strong efficacy in glial cells. Further investigation of the cellular localization of ApoE mRNA demonstrates that ApoE mRNA is partially spliced and preferentially localized to the nucleus (∼80%) in neuronal cells, whereas more than 90% of ApoE mRNA is cytoplasmic in glial cells. Such an inconsistency in intracellular localization and splicing might provide an explanation for functional differences in RNAi compounds. Thus, cellular origin might have an impact on accessibility of mRNA to RNAi and should be taken into account during the screening process.

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.

[Gene-selective treatment approaches for Huntington's disease]

In Germany at least 8000 and probably up to ca. 14,000 people currently suffer from clinically manifest Huntington's disease (HD). In addition, an estimated 24,000 Germans carry the HD mutation in the huntingtin (HTT) gene and will develop HD during their lifetime. Although HD is a rare neurodegenerative disease, it is currently in the focus of general medical interest: clinical trials have begun that provide a rational basis for hope to slow down the so far relentless progression of the disease, ultimately resulting in patients becoming entirely dependent on nursing care. If treatment is started early enough it may be possible to mitigate the clinical manifestation of HD. These innovative therapeutic approaches aim at inhibiting the de novo production of mutant HTT gene products. A first clinical drug trial to demonstrate the efficacy (phase III) of intrathecal antisense oligonucleotides (ASO, active substance RG6042) was started in 2019. Additional clinical studies on alternative treatment approaches with allele-selective ASOs as well as gene therapeutic approaches using RNA molecules and zinc finger repressor complexes are imminent. This article gives an overview of the current gene-selective therapeutic approaches in HD under discussion.

Single-Stranded Phosphorothioated Regions Enhance Cellular Uptake of Cholesterol-Conjugated siRNA but Not Silencing Efficacy

Small interfering RNAs (siRNAs) have potential to silence virtually any disease-causing gene but require chemical modifications for delivery to the tissue and cell of interest. Previously, we demonstrated that asymmetric, phosphorothioate (PS)-modified, chemically stabilized, cholesterol-conjugated siRNAs, called hsiRNAs, support rapid cellular uptake and efficient mRNA silencing both in cultured cells and in vivo. Here, we systematically evaluated the impact of number, structure, and sequence context of PS-modified backbones on cellular uptake and RNAi-mediated silencing efficacy. We find that PS enhances cellular internalization in a sequence-dependent manner but only when present in a single-stranded but not double-stranded region. Furthermore, the observed increase in cellular internalization did not correlate with functional silencing improvement, indicating that PS-mediated uptake may drive compounds to non-productive sinks. Thus, the primary contributing factor of PS modifications to functional efficacy is likely stabilization rather than enhanced cellular uptake. A better understanding of the relative impact of different chemistries on productive versus non-productive uptake will assist in improved design of therapeutic RNAs.

Artificial miRNAs targeting CAG repeat expansion in ORFs cause rapid deadenylation and translation inhibition of mutant transcripts

Polyglutamine (polyQ) diseases are incurable neurological disorders caused by CAG repeat expansion in the open reading frames (ORFs) of specific genes. This type of mutation in the HTT gene is responsible for Huntington’s disease (HD). CAG repeat-targeting artificial miRNAs (art-miRNAs) were shown as attractive therapeutic approach for polyQ disorders as they caused allele-selective decrease in the level of mutant proteins. Here, using polyQ disease models, we aimed to demonstrate how miRNA-based gene expression regulation is dependent on target sequence features. We show that the silencing efficiency and selectivity of art-miRNAs is influenced by the localization of the CAG repeat tract within transcript and the specific sequence context. Furthermore, we aimed to reveal the events leading to downregulation of mutant polyQ proteins and found very rapid activation of translational repression and HTT transcript deadenylation. Slicer-activity of AGO2 was dispensable in this process, as determined in AGO2 knockout cells generated with CRISPR-Cas9 technology. We also showed highly allele-selective downregulation of huntingtin in human HD neural progenitors (NPs). Taken together, art-miRNA activity may serve as a model of the cooperative activity and targeting of ORF regions by endogenous miRNAs.

Antisense oligonucleotide therapeutics in neurodegenerative diseases: the case of polyglutamine disorders

Polyglutamine (polyQ) disorders are a group of nine neurodegenerative diseases that share a common genetic cause, which is an expansion of CAG repeats in the coding region of the causative genes that are otherwise unrelated. The trinucleotide expansion encodes for an expanded polyQ tract in the respective proteins, resulting in toxic gain-of-function and eventually in neurodegeneration. Currently, no disease-modifying therapies are available for this group of disorders. Nevertheless, given their monogenic nature, polyQ disorders are ideal candidates for therapies that target specifically the gene transcripts. Antisense oligonucleotides (ASOs) have been under intense investigation over recent years as gene silencing tools. ASOs are small synthetic single-stranded chains of nucleic acids that target specific RNA transcripts through several mechanisms. ASOs can reduce the levels of mutant proteins by breaking down the targeted transcript, inhibit mRNA translation or alter the maturation of the pre-mRNA via splicing correction. Over the years, chemical optimization of ASO molecules has allowed significant improvement of their pharmacological properties, which has in turn made this class of therapeutics a very promising strategy to treat a variety of neurodegenerative diseases. Indeed, preclinical and clinical strategies have been developed in recent years for some polyQ disorders using ASO therapeutics. The success of ASOs in several animal models, as well as encouraging results in the clinic for Huntington's disease, points towards a promising future regarding the application of ASO-based therapies for polyQ disorders in humans, offering new opportunities to address unmet medical needs for this class of disorders. This review aims to present a brief overview of key chemical modifications, mechanisms of action and routes of administration that have been described for ASO-based therapies. Moreover, it presents a review of the most recent and relevant preclinical and clinical trials that have tested ASO therapeutics in polyQ disorders.

Nucleic acid therapeutics in neurodevelopmental disease

Nucleic acid therapeutics allow sequence-based targeting of mutation-harboring genes. They can be used to increase the expression and function of disease genes or to decrease the expression of toxic gene products. Antisense oligonucleotides (ASOs), small interfering RNAs (siRNAs), and gene-replacement therapies have received FDA approval, and in vivo gene editing applications are currently under development. Special consideration should be given to target engagement in neurons and amelioration of neurological phenotypes. Here we discuss the uses and limitations of different nucleic acid therapeutics, highlighting examples in the clinical and pre-clinical application of these modalities for the treatment of neurodevelopmental diseases.

Emerging siRNA Design Principles and Consequences for Biotransformation and Disposition in Drug Development

After two decades teetering at the intersection of laboratory tool and therapeutic reality, with two siRNA drugs now clinically approved, this modality has finally come into fruition. Consistent with other emerging modalities, initial proof-of-concept efforts concentrated on coupling pharmacologic efficacy with desirable safety profiles. Consequently, thorough investigations of siRNA absorption, distribution, metabolism, and excretion (ADME) properties are lacking. Advancing ADME knowledge will aid establishment of in vitro-in vivo correlations and pharmacokinetic-pharmacodynamic relationships to optimize candidate selection through discovery and translation. Here, we outline the emerging siRNA design principles and discuss the consequences for siRNA disposition and biotransformation. We propose a conceptual framework for siRNA ADME evaluation, contextualizing the site of biotransformation product formation with PK-PD modulation, and end with a discussion around safety and regulatory considerations and future directions for this modality.

Huntingtin Lowering Strategies

Trials using antisense oligonucleotide technology to lower Huntingtin levels in Huntington’s disease (HD) are currently ongoing. This progress, taking place only 27 years after the identification of the Huntingtin gene (HTT) in 1993 reflects the enormous development in genetic engineering in the last decades. It is also the result of passionate basic scientific work and large worldwide registry studies that have advanced the understanding of HD. Increased knowledge of the pathophysiology of this autosomal dominantly inherited CAG-repeat expansion mediated neurodegenerative disease has led to the development of several putative treatment strategies, currently under investigation. These strategies span the whole spectrum of potential targets from genome editing via RNA interference to promoting protein degradation. Yet, recent studies revealed the importance of huntingtin RNA in the pathogenesis of the disease. Therefore, huntingtin-lowering by means of RNA interference appears to be a particular promising strategy. As a matter of fact, these approaches have entered, or are on the verge of entering, the clinical trial period. Here, we provide an overview of huntingtin-lowering approaches via DNA or RNA interference in present clinical trials as well as strategies subject to upcoming therapeutic options. We furthermore discuss putative implications for future treatment of HD patients.

This was the year that was: brain barriers and brain fluid research in 2019

This editorial highlights advances in brain barrier and brain fluid research published in 2019, as well as addressing current controversies and pressing needs. Topics include recent advances related to: the cerebral endothelium and the neurovascular unit; the choroid plexus, arachnoid membrane; cerebrospinal fluid and the glymphatic hypothesis; the impact of disease states on brain barriers and brain fluids; drug delivery to the brain; and translation of preclinical data to the clinic. This editorial also mourns the loss of two important figures in the field, Malcolm B. Segal and Edward G. Stopa.

Recent Advances in the Treatment of Huntington's Disease: Targeting DNA and RNA

Huntington's disease is a dominantly inherited neurodegenerative disease caused by an unstable expanded trinucleotide repeat at the short end of the fourth chromosome. Central nervous system pathology begins in the striatum, eventually affecting the entire brain and occurs consequent to multiple intracellular derangements. The proximate cause is a mutant protein with an elongated polyglutamine tract. Pharmacological approaches targeting multiple domains of intracellular functions have universally been disappointing. However, recent developments in gene therapy, including antisense oligonucleotides, small interfering RNAs, and gene editing are bringing new hope to the Huntington's community. This review discusses the promises and challenges of these new potential treatments.

Highly efficient gene silencing in mouse brain by overhanging-duplex oligonucleotides via intraventricular route

Gapmer-type antisense oligonucleotides have not yet been approved for the treatment of central nervous system diseases, whereas steric-blocking-type antisense oligonucleotides have been well-developed for clinical use. We here characterize a new type of double-stranded oligonucleotides, overhanging-duplex oligonucleotides, which are composed of the parent gapmer and its extended complementary RNA. By intracerebroventricular injection, overhanging oligonucleotides show greater silencing potency with more efficient delivery into mouse brains than the parent single-stranded gapmer. Structure-activity relationship analyses reveal that the potency enhancement requires 13-mer or more overhanging oligonucleotides with a phosphorothioate backbone. Overhanging oligonucleotides provide a new platform of therapeutic oligonucleotides for gene modulation in the central nervous system.

Universal RNAi Triggers for the Specific Inhibition of Mutant Huntingtin, Atrophin-1, Ataxin-3, and Ataxin-7 Expression

The expansion of CAG repeats within the coding region of associated genes is responsible for nine inherited neurodegenerative disorders including Huntington’s disease (HD), spinocerebellar ataxias (SCAs), and dentatorubral-pallidoluysian atrophy (DRPLA). Despite years of research aimed at developing an effective method of treatment, these diseases remain incurable and only their symptoms are controlled. The purpose of this study was to develop effective and allele-selective genetic tools for silencing the expression of mutated genes containing expanded CAG repeats. Here we show that repeat-targeting short hairpin RNAs preferentially reduce the levels of mutant huntingtin, atrophin-1, ataxin-3, and ataxin-7 proteins in patient-derived fibroblasts and may serve as universal allele-selective reagents for polyglutamine (polyQ) diseases.