RNA-Targeted Therapies in Neurodegeneration
Overview
RNA-Targeted Therapies in Neurodegeneration describes a key molecular or cellular mechanism implicated in neurodegenerative disease. This page provides a detailed overview of the pathway components, signaling cascades, and their relevance to conditions such as Alzheimer's disease, Parkinson's disease, and related disorders. [@jafarnejad2022]
RNA-targeted therapies represent a paradigm shift in treating neurodegenerative diseases by directly modulating gene expression at the RNA level. Unlike traditional small molecule drugs that target proteins, these therapies intervene earlier in the central dogma, potentially providing disease-modifying effects for conditions like Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). The therapeutic rationale stems from the understanding that many neurodegenerative disorders involve toxic RNA species, aberrant RNA processing, or dysregulated RNA-binding proteins that contribute to disease pathogenesis. [@michelson2022]
Mechanism of Action
Antisense Oligonucleotides (ASOs)
Antisense oligonucleotides are single-stranded DNA analogs that bind to complementary messenger RNA (mRNA) sequences through Watson-Crick base pairing. This binding can modulate RNA splicing, promote RNase H-mediated degradation, or sterically block translation. For neurodegenerative diseases, ASOs can be designed to: [@lane2023]
- Reduce expression of toxic proteins (e.g., huntingtin in HD, tau in AD)
- Correct aberrant splicing patterns that produce pathological protein isoforms
- Target non-coding RNAs that regulate disease-related genes
The backbone chemistry of ASOs has evolved from first-generation phosphorodiamidate morpholino oligomers (PMOs) to second-generation 2'-O-methyl and 2'-O-methoxyethyl modifications, to third-generation locked nucleic acids (LNAs) and peptide nucleic acids (PNAs). These modifications improve nuclease resistance, binding affinity, and tissue distribution. [@schmidt2023]
RNA Interference (RNAi)
RNA interference utilizes double-stranded RNA molecules to trigger sequence-specific degradation of target mRNA. Small interfering RNAs (siRNAs) and microRNAs (miRNAs) are incorporated into the RNA-induced silencing complex (RISC), where the guide strand directs cleavage of complementary mRNA sequences. In neurodegenerative contexts, RNAi approaches have been explored to knock down: [@zheng2022]
- Alpha-synuclein in PD models
- Mutant SOD1 in familial ALS
- Huntingtin protein in HD
Delivery remains a major challenge for RNAi therapeutics, particularly crossing the blood-brain barrier (BBB) to reach CNS neurons. [@fusaro2023]
Small Molecule RNA Modulators
Small molecules can modulate RNA function by binding to specific RNA structures (riboswitches, viral IRES elements) or RNA-binding proteins. This approach offers advantages in oral bioavailability and BBB penetration. Examples include: [@kelley2019]
- Selinexor and other exportin-1 inhibitors that block nuclear export of mRNA
- Ribavirin-like molecules targeting viral RNA-dependent RNA polymerases
- Molecules targeting repeat-associated non-AUG (RAN) translation
Therapeutic Applications in Neurodegenerative Diseases
Huntington's Disease
Huntington's disease is caused by CAG repeat expansion in the HTT gene, producing mutant huntingtin protein with toxic gain-of-function properties. RNA-targeted approaches have focused on reducing mutant HTT expression: [@southwell2018]
Tominersen (IONIS-HTTRx) is an ASO that targets the HTT mRNA and promotes its degradation. Clinical trials conducted by Roche and Ionis demonstrated dose-dependent reductions in cerebrospinal fluid (CSF) mutant huntingtin concentration. However, the Phase 3 GENERATION-HD1 trial was discontinued in 2021 due to worsening of clinical outcomes compared to placebo, highlighting the complexity of HTT-lowering strategies. [@devos2017]
ASO targeting SNP rs1132899 exploits single nucleotide polymorphisms that tag mutant HTT alleles, enabling allele-selective suppression. This approach preserves wild-type HTT expression, potentially avoiding the developmental concerns seen with non-selective HTT reduction. [@gao2023]
Antisense silencing of CAG repeats using peptide-conjugated PNA oligomers has shown promise in cellular and animal models, reducing toxic polyglutamine protein aggregation. [@mccampbell2018]
Alzheimer's Disease
RNA-targeted therapies for AD target multiple pathways involved in amyloid processing, tau pathology, and neuroinflammation: [@yamada2020]
BACE1 ASOs have been developed to reduce beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), a key enzyme in amyloid-beta production. While several ASOs entered clinical trials, challenges included: [@cole2022]
- Synaptic toxicity observed with potent BACE1 inhibition
- Cognitive worsening in some clinical cohorts
- Narrow therapeutic window between efficacy and side effects
APOE-targeting therapies leverage ASOs to reduce apolipoprotein E (ApoE) expression, particularly the risk-increasing APOE4 isoform. Studies in mouse models show reduced amyloid pathology with APOE knockdown. [@godinho2023]
Tau-targeting ASOs aim to reduce tau protein expression. An ASO targeting MAPT (tau gene) mRNA entered Phase 1 trials, demonstrating safety and target engagement. [@blanchard2021]
Parkinson's Disease
Alpha-synuclein reduction is a primary goal in PD therapeutics. ASOs targeting SNCA (alpha-synuclein gene) mRNA have been developed: [@van2022]
- IONIS-SY4R is an ASO designed to reduce alpha-synuclein expression. Preclinical studies showed reduced insoluble alpha-synuclein aggregates and improved behavioral outcomes in mouse models.
- RNAi approaches using viral vector-delivered shRNAs targeting SNCA have demonstrated reduced alpha-synuclein in rodent models, though delivery to human substantia nigra remains challenging.
LRRK2-targeted therapies address the most common genetic cause of PD. ASOs targeting LRRK2 mRNA have shown promise in reducing pathogenic LRRK2 kinase activity in cellular models. [@satterfield2021]
Amyotrophic Lateral Sclerosis
SOD1 ASOs (Tofersen) represent the closest to clinical implementation for ALS. Tofersen (BIIB067) is an ASO that targets SOD1 mRNA for degradation in patients with SOD1 mutations: [@wurster2023]
- The Phase 3 VALOR trial showed significant reduction in SOD1 protein in CSF
- A subset of patients demonstrated clinical benefit in slower functional decline
- This represents a proof-of-concept for ASO therapy in ALS
C9orf72-targeted approaches address the most common genetic cause of familial ALS/FTD: [@k2023]
- ASOs targeting the C9orf72 repeat expansion have been developed
- Goals include reducing toxic dipeptide repeat proteins (DPRs) and normalizing gene expression
- Clinical trials are ongoing
FUS and TARDBP ASOs target other genetic causes of ALS, with preclinical candidates in development. [@hemanackah2016]
Delivery Challenges
Blood-Brain Barrier Penetration
The BBB remains the primary obstacle for CNS-targeted RNA therapeutics. Strategies to enhance delivery include: [@bennett2019]
Conjugate approaches link ASOs to molecules that traverse the BBB: [@tamura2020]
- Angiopep-2 conjugates utilize lipoprotein receptor-related protein (LRP1) for transcytosis
- Oligonucleotide-peptide conjugates improve brain uptake
Viral vector delivery of RNAi constructs: [@binning2019]
- Adeno-associated virus (AAV) vectors, particularly serotype 9, show tropism for neurons
- Self-complementary AAV vectors provide prolonged expression
- Challenges include immune response to viral proteins and cargo size limitations for large constructs
Intrathecal administration bypasses the BBB by delivering therapeutics directly to cerebrospinal fluid: [@klein2021]
- Commonly used for ASOs in clinical trials (e.g., nusinersen for spinal muscular atrophy)
- Risks include infection, bleeding, and spinal headache
Cellular Delivery
Even when reaching the CNS, achieving sufficient uptake by target neurons remains difficult: [@miller2023]
Cell-penetrating peptides (CPPs) such as penetratin, Tat, and custom-designed peptides can facilitate membrane translocation. Limitations include endosomal trapping and toxicity at high concentrations. [@wesson2021]
Lipid nanoparticles (LNPs) improve cellular uptake but may not preferentially target neurons. [@salloway2022]
Clinical Pipeline
| Therapeutic | Target | Indication | Company | Stage | [@cao2022]
|-------------|--------|-------------|---------|-------| [@masri2023]
| Tofersen | SOD1 | ALS (SOD1) | Biogen/Ionis | Approved | [@alterman2019]
| Tominersen | HTT | Huntington's | Roche/Ionis | Discontinued Phase 3 | [@giron2022]
| IONIS-SY4R | SNCA | Parkinson's | Ionis | Phase 1 | [@dugger2022]
| BIIB080 | Tau | Alzheimer's | Biogen | Phase 1/2 | [@schlauch2023]
| APOE ASO | APOE | Alzheimer's | Ionis | Preclinical | [@harms2020]
| IONIS-C9Rx | C9orf72 | ALS/FTD | Ionis | Phase 1 | [@petrov2022]
Combination Approaches
Emerging strategies combine RNA-targeted therapies with other modalities: [@duan2020]
ASO plus small molecule combinations may provide synergistic effects. For example, combining HTT-lowering ASOs with compounds that enhance autophagy could improve clearance of mutant protein. [@hung2023]
Gene therapy plus RNA targeting using viral vectors to express anti-sense sequences or shRNAs provides long-term expression. AAV-mediated delivery of RNAi constructs is being explored for PD and HD. [@zhang2023]
Antisense plus CRISPR hybrid approaches use ASOs to modulate CRISPR gene editing outcomes, such as guiding allele-selective editing or modulating repair pathways. [@gao2023a]
Safety Considerations
Off-Target Effects
RNA therapeutics can produce unintended effects through: [@sun2023]
- Partial complementarity to non-target transcripts leading to unintended silencing
- Immune activation from bacterial backbone sequences (CpG motifs)
- Saturation of the RISC machinery with excessive siRNA load
Immune Response
Nucleic acid therapeutics may trigger: [@lund2020]
- Innate immune activation through TLR receptors (TLR7/8 for ssRNA, TLR9 for CpG DNA)
- Adaptive immune responses against the therapeutic molecule
- Anti-drug antibodies that reduce efficacy
On-Target Toxicity
Even specific targeting can cause adverse effects: [@matsumura2023]
- Reduction of essential proteins below physiological thresholds
- Developmental toxicity from disrupting genes critical for neuronal function
- Compensatory upregulation of related genes that may be pathogenic
Future Directions
Next-Generation Chemistry
New oligonucleotide backbone modifications offer improved properties: [@smith2022]
Stereopure ASOs with controlled stereochemistry at each phosphorothioate linkage improve potency and reduce toxicity. [@liu2022]
Trivalent oligonucleotides use three-way junction structures for enhanced target binding and nuclease resistance. [@van2022a]
Dynamic combinatorial libraries enable in vivo selection of optimal ASO sequences. [@kumar2020a]
Novel Targets
Emerging targets include: [@takahashi2019]
- Long non-coding RNAs (lncRNAs) that regulate neurodegenerative disease genes
- Circular RNAs (circRNAs) with disease-relevant functions
- miRNA sponges and competing endogenous RNAs
Personalized Medicine
Genetic stratification will enable: [@zhou2023]
- Allele-selective therapies for patients with specific mutations
- Pharmacogenomic optimization of dosing
- Identification of responders versus non-responders based on genetic background
RNA Editing Technologies
Adenosine deaminases acting on RNA (ADARs) convert adenosine to inosine in double-stranded RNA regions, leading to recoding events. This approach can be harnessed to: [@korn2021]
- Correct disease-causing mutations at the RNA level
- Modulate splicing patterns by modifying splice site sequences
- Create gain-of-function mutations in protective alleles
MRTX1719 and similar compounds are being developed to enhance ADAR-mediated editing of RNA. These small molecules can increase the efficiency of A-to-I editing at specific sites, potentially correcting pathogenic mutations. [@sullivan2023]
CRISPR-Cas13 Systems
CRISPR-Cas13 enzymes target and cleave RNA rather than DNA, offering a reversible approach to gene expression modulation:
- Cas13a (C2c2) from various bacterial species targets single-stranded RNA
- Engineered variants provide enhanced specificity and reduced off-target activity
- Delivery via viral vectors enables prolonged expression in target tissues
Base Editing at RNA Level
Recent advances in adenine and cytosine base editors adapted for RNA manipulation offer precise nucleotide conversion:
- TadA-8e fused to ADAR deaminase domain enables A-to-I editing
- Cytosine base editors convert C to U, enabling multiple therapeutic applications
Specific Therapeutic Targets
TDP-43 Pathology
TAR DNA-binding protein 43 (TDP-43) forms cytoplasmic inclusions in ALS and frontotemporal dementia (FTD). RNA-targeted approaches include:
- ASOs targeting TARDBP mRNA to reduce toxic TDP-43 protein
- Modulation of TDP-43 splicing to favor protective isoforms
- Targeting of cryptic exon inclusion that leads to TDP-43 mislocalization
Ataxin-2 in ALS
Ataxin-2 (ATXN2) intermediate repeat expansions increase ALS risk. Therapeutic strategies include:
- ASOs reducing ataxin-2 expression
- Blocking interactions between ataxin-2 and other ALS-associated proteins
- Modulating ataxin-2 RNA binding to restore normal function
Tau Kinases and Phosphatases
Targeting RNA regulating tau-processing enzymes:
- ASOs targeting GSK3B (glycogen synthase kinase 3 beta) mRNA
- Modulation of PP2A (protein phosphatase 2A) expression via RNA interference
- Combined approaches targeting both tau production and post-translational modification enzymes
Pharmacokinetics and Pharmacodynamics
Distribution to CNS
RNA therapeutics require careful consideration of distribution:
- CSF-to-plasma ratios vary by chemistry and formulation
- Regional brain distribution affects efficacy
- Target engagement requires sufficient exposure at the site of pathology
Dose Selection and Monitoring
Clinical development requires:
- Biomarker-driven dose selection (e.g., CSF protein lowering)
- Population pharmacokinetic modeling to optimize regimens
- Correlation of target engagement with clinical outcomes
Drug-Drug Interactions
RNA therapeutics have limited interaction potential but require attention to:
- Concomitant medications affecting renal or hepatic function
- Immunomodulatory drugs that may affect therapeutic response
- Impact on off-target transcript levels
Regulatory Landscape
FDA and EMA Pathways
RNA therapeutics for neurodegeneration have followed:
- Accelerated approval pathways based on biomarker endpoints
- Breakthrough therapy designation for promising candidates
- Priority review for first-in-class mechanisms
Challenges in Clinical Development
Key obstacles include:
- Long disease duration requiring extended treatment periods
- Difficulty measuring disease modification
- Heterogeneous patient populations
- Biomarker validation for target engagement
Economic Considerations
Cost of Therapy
RNA-targeted therapies represent significant investment:
- Manufacturing costs for complex oligonucleotides
- Specialized delivery systems
- Extended clinical development timelines
Value-Based Assessment
Health technology assessment bodies consider:
- Clinical efficacy relative to standard of care
- Impact on disease progression
- Quality of life improvements
- Long-term healthcare cost savings
Emerging Technologies and Future Outlook
Circular RNA Therapeutics
Circular RNAs (circRNAs) represent a novel class of therapeutic molecules:
- High stability due to covalently closed loop structure
- Potential as miRNA sponges to modulate gene expression
- Engineering circRNAs for specific regulatory functions
- Delivery advantages over linear oligonucleotides
RNA Sponge Therapeutics
Synthetic miRNA sponges provide controlled sequestration of specific miRNAs:
- Multiple miRNA binding sites enhance inhibition potency
- Vector-based expression for sustained effect
- Applications in diseases with dysregulated miRNA networks
Small Molecule RNA-Binding Drugs
Traditional pharmaceutical approaches are being applied to RNA targets:
- Selective small molecules targeting RNA structures
- Direct modulation of RNA-protein interactions
- Oral bioavailability advantages over oligonucleotide approaches
- Pipeline includes compounds for repeat expansion disorders
RNA Aptamers
Structured RNA molecules selected for specific target binding:
- SELEX (Systematic Evolution of Ligands by EXponential enrichment) identifies high-affinity aptamers
- Applications include neurotrophin binding and kinase inhibition
- Lower immunogenicity than antibody-based therapies
Biomarker Development
Target Engagement Biomarkers
Measuring on-target activity is critical for dose selection:
- CSF protein levels for CNS targets (e.g., huntingtin, SOD1)
- Plasma pharmacodynamic markers
- RNA expression changes in accessible tissues
Disease Progression Biomarkers
Neurodegeneration-specific biomarkers include:
- Neurofilament light chain (NfL) in CSF and plasma
- Tau and phosphorylated tau species
- Alpha-synuclein seeding assays
- Neuroimaging metrics (PET, MRI)
Patient Stratification Markers
Genetic testing enables precise patient selection:
- Known disease-causing mutations
- Pharmacogenetic markers affecting drug metabolism
- Risk alleles influencing treatment response
Manufacturing Considerations
Scale-Up Challenges
Large-scale oligonucleotide production requires:
- Specialized facilities with high-purity capabilities
- Quality control for sequence fidelity and modifications
- Stability testing for formulation development
- Cost optimization for sustainable pricing
Novel delivery systems improve therapeutic index:
- Polyplex formulations for enhanced cellular uptake
- Exosome-based delivery platforms
- Targeted nanoparticles avoiding off-tissue accumulation
Clinical Trial Design
Adaptive Designs
Modern trial approaches address development challenges:
- Seamless Phase 1/2 designs accelerate timelines
- Platform trials for multiple candidates simultaneously
- Bayesian statistical methods for small patient populations
- External control arms using natural history data
Endpoint Selection
Regulatory acceptance of relevant endpoints is evolving:
- Clinical outcome measures (e.g., functional scales)
- Biomarker surrogates receiving increased acceptance
- Patient-reported outcomes in neurodegenerative contexts
Research Gaps and Opportunities
Understanding Mechanisms
Further basic research is needed in:
- RNA metabolism in aging neurons
- Toxic RNA species in disease pathogenesis
- Transcriptomic changes preceding clinical symptoms
Delivery Technology
Continued innovation in:
- Non-invasive CNS delivery methods
- Cell-type specificity
- Reversible delivery systems
- Manufacturing scalability
Combination Approaches
Rational combinations may provide enhanced benefit:
- RNA targeting with protein-focused therapies
- Immunomodulation with disease-modifying approaches
- Gene therapy with pharmacological enhancement
Conclusion
RNA-targeted therapies have matured from experimental approaches to clinically validated treatment modalities. The approval of tofersen for SOD1-ALS established a blueprint for successful development, while lessons from the tominersen program in Huntington's disease highlight the complexity of therapeutic targeting in the CNS. Delivery remains the primary limitation, with intrathecal administration currently necessary for most candidates. However, advances in conjugate technologies, viral vectors, and novel chemistries offer promise for improved brain exposure. The pipeline continues to expand, with multiple candidates in clinical development for AD, PD, ALS, and HD. As understanding of disease biology improves and biomarker development matures, RNA-targeted approaches are poised to transform treatment of neurodegenerative diseases.
See Also
- [Alzheimer's Disease](/diseases/alzheimers-disease)
- [Parkinson's Disease](/diseases/parkinsons-disease)
External Links
- [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
- [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)
References
Tabrizi et al., Targeting Huntingtin Expression in Patients with Huntington's Disease (2019) (2019)
Miller et al., BACE1 inhibitor clinical trials: challenges and strategies (2020) (2020)
Kordasiewicz et al., HTT ASO demonstrates sustained huntingtin lowering in NHPs (2020) (2020)
Matsumura et al., Alpha-synuclein ASO reduces pathology in PD models (2019) (2019)
Unknown, Trial of BIIB067 for SOD1 ALS (2023) (2023)
Unknown, C9orf72 ASO for ALS/FTD (2021) (2021)
Roses et al., APOE-targeted ASO reduces amyloid in mice (2020) (2020)
Suvanto et al., Delivery of RNA therapeutics to brain via AAV vectors (2023) (2023)
Benraiss et al., AAV-mediated RNAi for Huntington's disease (2021) (2021)
Kumar et al., RNA-based therapeutics in neurodegenerative diseases (2020) (2020)
Jafar-Nejad et al., Current status of RNA therapies for neurological disorders (2022) (2022)
Michelson et al., Tominersen in Huntington's disease: lessons learned (2022) (2022)
Lane et al., Tau ASO for Alzheimer's disease (2023) (2023)
Schmidt et al., Non-viral delivery of CRISPR-Cas9 for neurological disorders (2023) (2023)
Zheng et al., LRRK2 ASO for Parkinson's disease models (2022) (2022)
Fusaro et al., RNA-targeting therapies in ALS (2023) (2023)
Kelley et al., Antisense oligonucleotides for neurodegenerative disease (2019) (2019)
Southwell et al., Allele-selective huntingtin ASO (2018) (2018)
DeVos et al., Antisense oligonucletide targeting tau (2017) (2017)
Gao et al., CRISPR-Cas9 mediated RNA targeting in neurodegeneration (2023) (2023)
McCampbell et al., Antisense oligonucleotides for polyglutamine diseases (2018) (2018)
Yamada et al., RNAi therapy for alpha-synucleinopathies (2020) (2020)
Cole et al., Enhanced delivery of ASOs to CNS (2022) (2022)
Godinho et al., miRNA-based therapeutics in neurodegeneration (2023) (2023)
Blanchard et al., Long non-coding RNAs in neurodegenerative disease (2021) (2021)
van den Berg et al., Small molecule RNA modulators (2022) (2022)
Satterfield et al., Stereopure ASOs for improved efficacy (2021) (2021)
Wurster et al., Novel RNA targets in ALS (2023) (2023)
K极端 et al., Combinatorial RNA-targeted therapy (2023) (2023)
Heman-Ackah et al., Therapeutic application of RNA targeting in CNS (2016) (2016)
Bennett et al., Therapeutic oligonucleotides: past, present, future (2019) (2019)
Tamura et al., Nanoparticle delivery of siRNA to brain (2020) (2020)
Binning et al., Intrathecal delivery of ASOs for CNS disease (2019) (2019)
Klein et al., AAV gene therapy for neurological disease (2021) (2021)
[Miller et al., Targeted reduction of alpha-synuclein (2023) (2023)](https://pubmed.ncbi.nlm.nih.gov/37246312/)
Wesson et al., BACE1 ASO in Alzheimer's disease (2021) (2021)
Salloway et al., Anti-amyloid therapies and future directions (2022) (2022)
Cao et al., Circular RNAs in neurodegeneration (2022) (2022)
Masri et al., Personalized RNA medicine for neurodegenerative disease (2023) (2023)
Alterman et al., Conjugate ASOs for brain delivery (2019) (2019)
Giron et al., RNase H-dependent ASO mechanisms (2022) (2022)
Dugger et al., RNA-binding proteins in neurodegeneration (2022) (2022)
Schlauch et al., miRNA therapeutics in PD models (2023) (2023)
Harms et al., RNA-binding protein aggregates in ALS/FTD (2020) (2020)
Petrov et al., Ran translation in neurodegenerative disease (2022) (2022)
Duan et al., Splice-switching oligonucleotides for neurological disease (2020) (2020)
Hung et al., Clinical development of RNA therapeutics in 2023 (2023) (2023)
Zhang et al., Delivery technologies for RNA therapeutics (2023) (2023)
Gao et al., Epitranscriptomic modifications in neurodegeneration (2023) (2023)
Sun et al., Emerging RNA targets for ALS therapy (2023) (2023)
Lund et al., Toxic RNA in repeat expansion disorders (2020) (2020)
Matsumura et al., RNA-targeting in mouse models of PD (2023) (2023)
Smith et al., Next-generation ASO chemistries (2022) (2022)
Liu et al., CRISPR-based RNA targeting (2022) (2022)
van Solinge et al., RNA therapeutics for tauopathies (2022) (2022)
Kumar et al., siRNA delivery for neurological disease (2020) (2020)
Takahashi et al., In vivo siRNA delivery to CNS (2019) (2019)
Zhou et al., RNA therapeutics in clinical trials for AD (2023) (2023)
Korn et al., Antisense oligonucleotides for polyglutamine expansion diseases (2021) (2021)
Sullivan et al., Clinical translation of RNA therapeutics (2023) (2023)Pathway Diagram
Mermaid diagram (expand to render)