RNA-based therapeutics for neurological diseases

Abstract

RNA-based therapeutics have entered the mainstream with seemingly limitless possibilities to treat all categories of neurological disease. Here, common RNA-based drug modalities such as antisense oligonucleotides, small interfering RNAs, RNA aptamers, RNA-based vaccines and mRNA drugs are reviewed highlighting their current and potential applications. Rapid progress has been made across rare genetic diseases and neurodegenerative disorders, but safe and effective delivery to the brain remains a significant challenge for many applications. The advent of individualized RNA-based therapies for ultra-rare diseases is discussed against the backdrop of the emergence of this field into more common conditions such as Alzheimer's disease and ischaemic stroke. There remains significant untapped potential in the use of RNA-based therapeutics for behavioural disorders and tumours of the central nervous system; coupled with the accelerated development expected over the next decade, the true potential of RNA-based therapeutics to transform the therapeutic landscape in neurology remains to be uncovered.

Keywords: RNA; RNA aptamer; RNA therapeutics; RNA vaccine; antisense oligonucleotide; exon skipping; neurological disease; siRNA.

Figure 1.

A schematic diagram illustrating the three broad categories of RNA-based therapeutics set against the central dogma of molecular biology. Group 1 targets RNA, group 2 uses RNA to target protein and group 3 uses mRNA to make protein. Illustration was created using BioRender.

Figure 2.

Targeting RNA using AONs and siRNA. (a) siRNA. After cellular uptake, a double-stranded siRNA is recruited to the RNA-induced silencing complex (RISC) and the passenger strand is removed. The guide strand then binds to its complementary mRNA before it is converted into protein, the RISC complex together with the siRNA cleaves the target mRNA thus silencing its protein production. (b) ssAONs. ssAONs are targeted to the nucleus where they bind to their target pre-mRNA. This binding sterically blocks the spliceosome and results in splicing modulation. In the example illustrated, the AON targets exon 51 of the dystrophin gene resulting in exon skipping. (c) Gapmer AONs. Gapmers can induce RNAse H-mediated cleavage of a target mRNA in both the nucleus and the cytoplasm. Illustration was adapted from ‘siRNA Nanoparticle Delivery System’, by BioRender.com (2021). Retrieved from https://app.biorender.com/biorender-templates.

Figure 3.

Common oligonucleotide chemistries. An unmodified DNA/RNA nucleotide is shown followed by the phosphorothioate (PS) backbone modification which replaces the original phosphodiester bond. A PS backbone is often used together with modifications to the 2ʹ-O position of the ribose; 2ʹ-O-methyl (2ʹOMe) and 2ʹ-O-methoxyethyl (2ʹMOE) modifications are illustrated. The uncharged phosphorodiamidate morpholino oligonucleotide (PMO) replaces the deoxyribose moiety of DNA with a 6-membered morpholino ring whilst retaining the normal nucleobases. Illustration was created using BioRender.

Figure 4.

RNA-based gene therapy strategies. A) SMaRT is depicted in the context of the MAPT gene. A pre-trans-splicing molecule (PTM) is used to reprogram a mutant MAPT pre-mRNA transcript. The PTM contains the wild-type coding sequence of MAPT exons 10–13. A FLAG sequence at the 3ʹ end allows the detection of trans-spliced products. The PTM binding domain is complementary to the 3ʹ end of MAPT intron 9 and the PTM contains a branch point (BP), a polypyrimidine tract (PPT), an AG dinucleotide acceptor site and a spacer sequence separating the binding domain and branch point. B) U7snRNA-mediated exon skipping of the DMD gene using exon 51 as an example. An AON targeting exon 51 is indicated alongside the structure of the U7 snRNA cassette which is inserted between two inverted terminal repeats (ITRs) encoded by an AAV delivery vector. The U7 snRNA sequence is under the control of the natural U7 promoter (black box); the 3ʹ downstream elements are represented by the white box. Illustration was created using BioRender.

Figure 5.

The SELEX process for the identification of RNA aptamers. The initial RNA library which is typically transcribed from DNA is bound to cells or beads with no target, or with structural analogues of the target, in a pre-clearing or negative selection step. After removing non-specific aptamers, the remaining pool is subjected to binding with the target and unbound aptamers are discarded. RT-PCR is used to amplify bound RNAs which can be identified via sequencing. A new library for the next round is generated using in-vitro transcription and the process repeated up to 40 times. Illustration was created using BioRender.

Figure 6.

RNA-pulsed dendritic cell vaccine therapy for glioblastoma. After resection, total tumour RNA, or RNA encoding a specific tumour antigen, is ‘pulsed’ ex-vivo into dendritic cells derived from patient peripheral blood mononuclear cells (PBMCs). The resulting antigen presenting dendritic cells are administered as a vaccine to enable an antigen-specific immune response against the targeted tumour expressed epitope. Illustration was adapted from ‘Personalized Cell Therapies to Combat COVID-19’, by BioRender.com (2021). Retrieved from https://app.biorender.com/biorender-templates.