Antisense oligonucleotide (ASO) therapy represents a transformative approach to treating neurodegenerative diseases by enabling the selective targeting of disease-causing genes and proteins at the RNA level. [@crooke2021] ASOs are short, single-stranded DNA or RNA molecules designed to bind to specific messenger RNA (mRNA) sequences through Watson-Crick base pairing, thereby modulating protein expression through various mechanisms. This therapeutic modality has gained significant momentum following the FDA approval of several ASO drugs for neuromuscular disorders, establishing a template for application to neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD).
The fundamental principle of ASO therapy involves the introduction of synthetic oligonucleotides that can hybridize to target mRNA, leading to its degradation, altered splicing, or translational blockade. [@bennett2010] The choice of mechanism depends on the chemical properties of the ASO and the desired therapeutic outcome. ASOs can be designed to reduce the expression of toxic proteins, correct aberrant splicing patterns, or restore the balance of gene isoforms. In the context of neurodegeneration, these capabilities offer the potential to address underlying genetic causes, modulate disease-associated pathways, and potentially slow or halt disease progression.
One of the most extensively utilized ASO mechanisms involves the recruitment of RNase H, an enzyme that specifically cleaves RNA in RNA-DNA hybrids. [@wu2019] Gapmer ASOs are designed with a central DNA "gap" flanked by modified nucleotides, typically 2'-O-methyl or 2'-O-methoxyethyl residues, that protect the molecule from degradation while allowing the central DNA to form a substrate for RNase H. When the gapmer binds to its target mRNA, RNase H recognizes the RNA-DNA hybrid and cleaves the RNA strand, leading to the destruction of the mRNA and subsequent reduction in protein expression.
This mechanism has been successfully employed in several FDA-approved ASO drugs, including nusinersen (Spinraza) for spinal muscular atrophy (SMA) and inotersen (Tegsedi) for hereditary transthyretin amyloidosis. [@corey2017] The success of these drugs has validated RNase H-mediated ASO therapy as a viable approach for central nervous system (CNS) diseases, provided that adequate delivery to target tissues can be achieved.
Alternative ASO mechanisms involve the steric blockade of mRNA without degradation. This approach can prevent the translation of specific mRNAs or modulate their splicing patterns. [@rigo2014] Steric block ASOs are typically fully modified with 2'-O-methyl or phosphorodiamidate morpholino (PMO) chemistries that do not support RNase H activity but instead function by physically blocking the ribosome or spliceosome.
Splicing-modulating ASOs have proven particularly successful in the CNS. nusinersen works by modulating the splicing of the SMN2 gene to increase the production of functional SMN protein, demonstrating that ASOs can correct disease-causing splice defects rather than simply reducing gene expression. [@hua2011] Similar approaches are being explored for neurodegenerative diseases with known splice defects or where modulation of alternative splicing could provide therapeutic benefit.
A particularly promising application of ASO technology is the allele-selective targeting of disease-causing mutations in autosomal dominant genetic disorders. [@carroll2016] In conditions such as Huntington's disease, where a CAG repeat expansion in the huntingtin (HTT) gene causes toxic gain of function, ASOs can be designed to selectively reduce the mutant allele while preserving the wild-type allele. This approach requires careful sequence design to ensure sufficient selectivity, as the mutant and wild-type alleles differ by only a few nucleotides in the expanded region.
The development of single-nucleotide polymorphism (SNP)-linked ASOs provides an alternative approach to allele selectivity. By targeting SNPs that are linked to the disease-causing mutation on the mutant allele, ASOs can achieve allele-selective silencing without requiring perfect sequence complementarity at the mutation site. [@liu2020] This approach has been explored in HD and is currently in clinical development.
Several ASO approaches are under development for AD, targeting various aspects of disease pathogenesis. The amyloid precursor protein (APP) and its processing products represent obvious targets, with ASOs designed to reduce Aβ production by lowering APP expression or modulating the activity of β- and γ-secretase enzymes. [@schoch2017] However, the modest efficacy of Aβ-targeting antibodies in clinical trials has raised questions about the validity of this approach.
Tau-targeting ASOs represent an alternative strategy, with the goal of reducing the expression of tau protein, which forms neurofibrillary tangles in AD brains. Several companies have developed ASOs targeting tau mRNA, with preclinical studies demonstrating reduced tau expression and improved cognitive outcomes in animal models. [@devos2017] The correlation between tau pathology and cognitive decline in AD provides a strong rationale for this approach.
Apolipoprotein E (APOE) represents another ASO target, as the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset AD. ASOs targeting APOE expression in the brain could potentially reduce the risk associated with this allele, though the complex role of ApoE in AD pathogenesis must be carefully considered. [@liu2021]
ASO therapy for PD has focused primarily on α-synuclein, the protein that forms Lewy bodies in PD brains. ASOs targeting the SNCA gene that encodes α-synuclein could reduce the production of this toxic protein, potentially slowing disease progression. [@cole2021] Several preclinical studies have demonstrated that ASO-mediated reduction of α-synuclein can protect dopaminergic neurons in animal models of PD.
Genetic forms of PD caused by mutations in genes such as LRRK2, GBA, and PINK1 represent attractive targets for ASO therapy. For LRRK2-associated PD, ASOs targeting the mutant LRRK2 protein could provide disease-specific treatment for patients with the G2019S mutation, the most common pathogenic LRRK2 variant. [@bartley2023] Similarly, ASOs targeting GBA could benefit patients with Gaucher disease-associated PD risk.
ASO therapy has shown significant promise in ALS, with the FDA approval of tofersen (Qalsody) for ALS caused by SOD1 mutations representing a milestone for the field. [@miller2022] Tofersen is an ASO that targets SOD1 mRNA, reducing the production of toxic SOD1 protein that causes approximately 2% of ALS cases. The Phase III VALOR trial demonstrated that tofersen reduced SOD1 protein levels in CSF and showed a trend toward slower clinical decline, leading to accelerated approval.
The success of tofersen has accelerated the development of ASOs for other genetic forms of ALS, including those caused by mutations in C9orf72, FUS, and TARDBP. [@pathania2020] The C9orf72 hexanucleotide repeat expansion is the most common genetic cause of familial ALS, and ASOs targeting the repeat-containing transcripts have shown promise in preclinical models. Several clinical trials are underway to evaluate these ASOs in ALS patients with C9orf72 mutations.
HD represents a particularly promising indication for ASO therapy due to its monogenic nature and well-characterized pathology. Several ASOs targeting HTT expression have been developed, including the agent tominersen (RG6042) from Roche, which completed a large Phase III clinical trial. [@tabrizi2019] Although tominersen did not meet its primary endpoint in the GENERATION-HD1 trial, the study provided valuable insights into ASO delivery to the CNS and the challenges of targeting genetically determined neurodegenerative diseases.
The experience with tominersen has informed the development of next-generation HTT-targeting ASOs with improved potency and distribution. Additionally, allele-selective approaches that spare the wild-type HTT allele are being explored, as complete reduction of HTT may have unintended consequences given its essential cellular functions. [@kordas2021]
The delivery of ASOs to the brain and spinal cord represents a major challenge for the treatment of neurodegenerative diseases. Unlike peripheral tissues, the CNS is protected by the blood-brain barrier (BBB), which limits the passage of most therapeutic molecules. [@banks2022] Currently, ASOs for CNS diseases are administered directly into the cerebrospinal fluid (CSF) via intrathecal injection, allowing distribution throughout the CNS without crossing the BBB.
The development of ASOs that can be delivered systemically and cross the BBB would significantly improve the feasibility of ASO therapy for neurodegenerative diseases. Several strategies are being explored, including the conjugation of ASOs to brain-penetrant molecules, the use of receptor-mediated transcytosis, and the encapsulation of ASOs in nanoparticles. [@guimond2023] The success of these approaches could enable intravenous administration of ASOs for CNS diseases.
The distribution of ASOs within the CNS following intrathecal administration is not uniform, with higher concentrations in the spinal cord and lower brain regions compared to the cortex. The development of ASOs with improved brain parenchymal penetration, possibly through modifications that enhance cellular uptake, is an active area of research. [@smith2022]
The specificity of ASO therapy depends on the unique complementarity between the ASO sequence and its target mRNA. However, incomplete specificity can lead to off-target effects, where the ASO binds to unintended mRNA sequences and modulates their expression. [@hagedorn2017] Careful sequence design, including the use of bioinformatics tools to predict potential off-target interactions, is essential for minimizing these effects.
Off-target effects can also arise from the hybridization-independent activity of ASOs, including the activation of innate immune pathways by certain ASO chemistries. The selection of ASO sequences that minimize immune activation and the use of modified nucleotides that reduce immunogenicity can help mitigate this risk. [@klein2021]
Neurodegenerative diseases typically require long-term treatment, raising questions about the sustainability of ASO therapy. The repeat administration of ASOs can lead to the development of antibodies against the oligonucleotide or its delivery vehicle, potentially reducing efficacy over time. The use of non-immunogenic ASO chemistries and the careful monitoring of immune responses can address this concern.
The duration of ASO effect following a single administration depends on the rate of ASO clearance and the turnover of target mRNA and protein. [@geary2015] Current ASOs require repeated administrations every few months to maintain target reduction. The development of more potent ASOs with longer duration of effect could reduce treatment frequency and improve patient convenience.
The success of ASO therapy in neuromuscular disorders has established a clear pathway for the development of ASOs for neurodegenerative diseases. The FDA approval of tofersen for SOD1-ALS validates the approach and provides a template for future development. Several ASOs are currently in clinical trials for various neurodegenerative conditions, with results expected in the coming years.
The optimization of ASO delivery, potency, and safety continues to drive innovation in the field. New chemistries, including locked nucleic acids (LNA) and peptide nucleic acids (PNA), offer improved target affinity and specificity. [@khvorova2017] The combination of ASO therapy with other modalities, including small molecules and gene therapy, may provide synergistic benefits.
The identification of new therapeutic targets through advances in disease biology and genetic analysis will expand the range of ASO applications. The development of ASOs for sporadic forms of neurodegenerative diseases, which lack a clear genetic cause, will require the identification of disease-modifying pathways that can be safely modulated.