Splice-modulating therapies represent a cutting-edge approach in neurodegenerative disease treatment, leveraging antisense oligonucleotides (ASOs) and other nucleic acid-based technologies to modify pre-mRNA splicing patterns. By altering how genes are spliced, these therapies can reduce toxic protein isoforms, restore missing protein function, or shift the balance toward more beneficial protein variants[@hastings2023]. This page provides comprehensive coverage of the scientific basis, therapeutic applications, delivery strategies, and clinical development of splice-modulating approaches for Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), Huntington's disease, and related disorders.
Splice-modulating therapies represent a cutting-edge approach in neurodegenerative disease treatment, leveraging antisense oligonucleotides (ASOs) and other nucleic acid-based technologies to modify pre-mRNA splicing patterns. By altering how genes are spliced, these therapies can reduce toxic protein isoforms, restore missing protein function, or shift the balance toward more beneficial protein variants[@hastings2023]. This page provides comprehensive coverage of the scientific basis, therapeutic applications, delivery strategies, and clinical development of splice-modulating approaches for Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), Huntington's disease, and related disorders.
Pre-mRNA splicing is the process by which introns are removed and exons are joined to produce mature mRNA[@vegf]. This process is mediated by the spliceosome, a large RNA-protein complex composed of five small nuclear ribonucleoproteins (snRNPs) and numerous associated proteins. The spliceosome recognizes specific sequence elements including the 5' splice site, 3' splice site, and branch point sequence to accurately excise introns and join exons.
The precision of splicing is critical for protein function. Alternative splicing, the process by which different combinations of exons are included or excluded from the final mRNA transcript, dramatically increases proteomic diversity. It is estimated that over 95% of human genes undergo alternative splicing, enabling a single gene to produce multiple protein isoforms with distinct functions.
In neurodegenerative diseases, aberrant splicing can lead to multiple pathological outcomes[@transcriptomic]:
Toxic Protein Isoforms: Splice variants that produce harmful proteins. For example, splice variants of [MAPT](/genes/mapt) (tau) can produce isoforms that are more prone to aggregation, forming neurofibrillary tangles characteristic of Alzheimer's disease and tauopathies.
Loss of Protective Isoforms: Failure to produce neuroprotective protein variants. In some cases, disease-causing mutations disrupt normal splicing patterns, leading to reduced levels of beneficial protein isoforms that normally protect neurons.
Cryptic Splice Sites: Activation of alternative splice sites that introduce premature stop codons or produce unstable proteins. These cryptic sites can lead to truncated, non-functional, or dominant-negative protein products.
Exon Skipping: Inclusion or exclusion of exons that alter protein function. The balance between different exon inclusion patterns can shift dramatically in disease states, disrupting normal protein function[@scaffold].
Each neurodegenerative disease exhibits characteristic splicing abnormalities that provide therapeutic targets:
Alzheimer's Disease: Aberrant splicing of [APP](/entities/app-protein), [MAPT](/genes/mapt), and genes involved in tau pathology leads to increased production of amyloidogenic isoforms and disease-promoting tau variants[@smith2024].
Parkinson's Disease: Alternative splicing of [SNCA](/entities/snca), [LRRK2](/entities/lrrk2), and genes involved in mitochondrial function contributes to alpha-synuclein pathology and nigral degeneration.
Amyotrophic Lateral Sclerosis: Mutations in [C9orf72](/entities/c9orf72), [SOD1](/entities/sod1), [FUS](/entities/fus), and [TDP43](/entities/tardbp) disrupt normal RNA processing, leading to toxic gain-of-function products and loss of normal protein function.
Huntington's Disease: Aberrant splicing of [HTT](/entities/huntingtin-gene) produces toxic N-terminal fragments that accumulate in neurons, contributing to striatal and cortical degeneration[@rna].
ASOs are single-stranded DNA analogs that hybridize to specific pre-mRNA sequences[@hastings2023]. Their mechanisms include diverse approaches to modify splicing patterns:
Steric Blockade: Binding to splice sites or regulatory elements blocks spliceosome assembly at specific locations. This prevents the inclusion or exclusion of specific exons, redirecting splicing toward desired patterns.
RNase H-Mediated Cleavage: ASOs targeting the pre-mRNA trigger RNase H degradation of the RNA strand while leaving the DNA ASO intact. This mechanism is commonly used for gene knockdown but can also be employed to block splice sites[@voltagated].
Splice-Switching Oligonucleotides (SSOs): These ASOs modulate splicing without RNase H recruitment. They bind to pre-mRNA and sterically block access to regulatory sequences, allowing desired splice site selection.
RNase P-Mediated cleavage: Some ASOs are designed to cleave at specific sites using RNase P, providing an alternative mechanism for targeted RNA degradation.
| Target | Disease | Mechanism | Development Status |
|--------|---------|-----------|-------------------|
| [SMN2](/entities/smn2) | Spinal muscular atrophy | Enhance exon 7 inclusion to increase functional SMN protein[@transcriptomic] | FDA approved (nusinersen, risdiplam) |
| [APP](/entities/app-protein) | Alzheimer's disease | Redirect splicing away from amyloidogenic isoforms[@scaffold] | Preclinical |
| [MAPT](/genes/mapt) | Tauopathies | Modify tau isoform expression | Preclinical |
| [HTT](/entities/huntingtin-gene) | Huntington's disease | Reduce toxic HTT isoforms[@rna] | Phase 1/2 |
| [SOD1](/entities/sod1) | ALS | Modulate SOD1 splicing variants | FDA approved (tofersen) |
| [C9orf72](/entities/c9orf72) | ALS/FTD | Target repeat-containing transcripts | Phase 1/2 |
| [SNCA](/entities/snca) | Parkinson's disease | Reduce alpha-synuclein expression | Preclinical |
| [TREM2](/proteins/trem2) | Alzheimer's disease | Modulate TREM2 splicing | Preclinical |
The specific mechanisms by which splice-switching ASOs alter splicing patterns include[@chen2023]:
Exon Inclusion: ASOs binding to intronic or exonic silencing sequences can block negative regulatory elements, promoting inclusion of specific exons.
Exon Skipping: ASOs targeting splice sites or exonic splicing enhancers can promote exclusion of disease-causing exons.
Intron Retention: Modulating splicing to promote retention of specific introns can alter protein coding or stability.
Alternative 5' or 3' Splice Site Selection: ASOs can shift usage toward alternative splice sites that produce beneficial protein isoforms.
The success of splice-modulating ASOs in SMA provides a template for neurodegeneration[@martinez2024]. This autosomal recessive neuromuscular disorder results from deletion or mutation of SMN1, with SMN2 as a backup gene that produces only small amounts of functional protein due to exon 7 skipping.
Nusinersen (Spinraza): The first FDA-approved ASO for SMA, nusinersen modifies SMN2 splicing to increase functional SMN protein production. Administered intrathecally, it has demonstrated dramatic improvements in motor function and survival in infants and children with SMA[@lewis2022].
Risdiplam: This small molecule splicing modifier promotes exon 7 inclusion in SMN2 mRNA and has received FDA approval for oral treatment of SMA. It represents an alternative approach to splice modulation using drug-like molecules.
Onasemnogene abeparvovec (Zolgensma): While not an ASO, this gene therapy delivers a functional SMN1 gene, providing another approach to addressing the underlying genetic deficit.
ALS presents several promising targets for splice-modulating therapies[@schoch2022]:
Tofersen: This ASO targets SOD1 mutations, which account for approximately 2% of ALS cases. By reducing SOD1 protein production, tofersen addresses the toxic gain-of-function properties of mutant SOD1[@robinson2024]. After successful clinical trials, tofersen received FDA approval for treatment of SOD1-associated ALS.
ASOs Targeting C9orf72: The most common genetic cause of familial ALS involves hexanucleotide repeat expansions in C9orf72. Multiple approaches are under development to reduce toxic repeat-containing transcripts:
FUS and TDP-43 Targeting: ASOs targeting genes involved in RNA metabolism are being developed for ALS cases with mutations in FUS or TARDBP (encoding TDP-43)[@transcriptomic].
Multiple splice-modulating approaches are being explored for AD[@smith2024]:
ASOs Targeting APP: By modulating APP splicing, these ASOs aim to reduce production of amyloidogenic Aβ42 while preserving beneficial APP functions. Several approaches target different splice events in the APP transcript.
[BACE1](/entities/bace1) Splice Modulators: BACE1 (beta-secretase) is essential for amyloid-beta production. ASOs targeting BACE1 can reduce its expression, though safety concerns have limited clinical development.
Tau Splice Modifiers: Targeting the 3R vs 4R tau isoform ratios represents a promising approach for tauopathies. Several ASOs are being developed to shift splicing toward more beneficial tau isoforms.
TREM2-Targeting ASOs: TREM2 genetic variants are strong risk factors for AD. ASOs are being developed to modulate TREM2 expression and potentially enhance microglial function[@thomas2022].
Huntington's disease is particularly well-suited for splice-modulating approaches due to the clear toxic effects of mutant HTT[@johnson2023]:
Allele-Selective ASOs: These ASOs specifically target the mutant HTT allele while sparing the wild-type allele, potentially avoiding the side effects associated with complete HTT suppression.
Non-Selective ASOs: These reduce expression of both mutant and wild-type HTT. Early clinical trials demonstrated safety, though optimal dosing is still being determined.
Splice-Modulating ASOs: Rather than simply reducing HTT expression, these ASOs modify HTT splicing to reduce production of toxic N-terminal fragments that are particularly harmful to neurons.
Targeting HTT Exon 1: The N-terminal fragment containing the polyglutamine expansion is particularly toxic. ASOs targeting exon 1 can reduce production of these harmful fragments.
Emerging splice-modulating approaches for PD target multiple disease-relevant genes[@hall2023]:
Alpha-Synuclein ASOs: Multiple ASOs are being developed to reduce SNCA expression. Given the central role of alpha-synuclein aggregation in PD pathogenesis, reducing its expression could slow disease progression.
LRRK2-Targeting ASOs: LRRK2 mutations are a common genetic cause of PD. ASOs targeting LRRK2 could benefit patients with both sporadic and familial PD associated with LRRK2 mutations.
GBA-Targeting ASOs: Mutations in GBA (glucocerebrosidase) are significant risk factors for PD. Modulating GBA expression or splicing could potentially reduce PD risk in carriers.
The delivery of ASOs to the central nervous system remains a significant challenge[@williams2022]. Several routes are available:
Intrathecal Delivery: Direct injection into the cerebrospinal fluid (CSF) provides direct access to the spinal cord and brain surface. This approach is used for nusinersen and tofersen. Advantages include:
Systemic Delivery with BBB-Penetrant Formulations: Many ASOs cannot cross the BBB when administered systemically. However, novel formulations are being developed to enable intravenous delivery:
Lipid Nanoparticles (LNPs): Encapsulation of ASOs in lipid nanoparticles can improve delivery to the brain when formulated with brain-penetrant lipids[@chen2023]. LNPs can be surface-modified with targeting ligands to enhance specific uptake.
Conjugates: Various ligand-ASO conjugates enable receptor-mediated transport across the BBB:
Focused Ultrasound: Combining ASO delivery with focused ultrasound-mediated blood-brain barrier opening can enhance CNS uptake of systemically administered ASOs.
Intranasal Delivery: Direct nose-to-brain delivery using specialized formulations may enable non-invasive CNS targeting.
implantable Pumps: For chronic delivery, implantable pumps can provide continuous intrathecal infusion of ASOs.
To enhance stability, delivery, and target specificity, ASOs are chemically modified[@white2024]:
| Modification | Benefit | Application |
|-------------|---------|-------------|
| Phosphorothioate backbone | Nuclease resistance, protein binding | All ASOs |
| 2'-O-methyl | Reduced immunogenicity, improved binding | Standard modification |
| 2'-O-methoxyethyl | Enhanced binding, nuclease resistance | Clinical ASOs |
| Locked nucleic acid (LNA) | High-affinity binding | Gapmer designs |
| Morpholino | Stable, neutral backbone | Splice-switching |
| 2'-fluoro | Improved stability | Various applications |
| Constraint ethyl (cEt) | Very high affinity | Advanced designs |
| Phosphorodiamidate morpholino (PMO) | Excellent stability | Exon skipping |
The phosphorothioate (PS) backbone, where a non-bridging oxygen is replaced with sulfur, provides nuclease resistance and enables binding to plasma proteins, prolonging circulation time. This modification is used in most clinical ASOs.
2'-O-methyl (2'-OMe) and 2'-O-methoxyethyl (2'-MOE) modifications at the ribose sugar enhance nuclease resistance and binding affinity while reducing immunogenicity.
Locked nucleic acid (LNA) modifications create a locked conformation that dramatically increases binding affinity. Gapmer designs use LNA at the ends with DNA in the middle to recruit RNase H.
Modified nucleobases can enhance target specificity and reduce off-target effects. 5-methylcytosine is commonly used to reduce immune activation.
Problem: ASOs may bind to unintended RNAs, leading to unexpected splicing changes or toxicity.
Solutions[@hastings2023]:
Problem: Achieving sufficient ASO concentrations in the brain remains challenging.
Solutions:
Problem: ASO effects are transient, requiring repeated dosing.
Solutions:
Problem: ASOs may trigger immune responses against the oligonucleotide or delivery vehicle.
Solutions:
Problem: Long-term safety of chronic ASO treatment is not fully characterized.
Solutions:
Small molecules that modulate splicing offer advantages of oral bioavailability and easier manufacturing[@adams2024]:
Risdiplam: FDA-approved for SMA, risdiplam demonstrates that small molecules can modify splicing. This provides proof-of-concept for similar approaches in neurodegenerative diseases.
PTC518: Under development for Huntington's disease, this small molecule splicing modifier promotes inclusion of exon 1 in HTT mRNA, potentially reducing toxic fragment production.
Alternative Splicing Modulators: Various small molecules are being developed to target splicing in AD, PD, and ALS.
CRISPR-Cas13: Direct editing of pre-mRNA splicing using CRISPR-Cas13 systems enables precise modification of splice sites[@patel2022]. While still preclinical, this approach could provide permanent correction of splicing defects.
Base Editing: Precise modification of splice sites using base editors can correct disease-causing mutations that disrupt normal splicing[@taylor2023].
Splice-Correcting CRISPR: Engineered CRISPR systems can be directed to specific splice sites to either block or enhance splicing of particular exons.
Splice-Switching Proteins: Engineered proteins that modify splicing patterns by interfering with or enhancing spliceosome function at specific sites.
Antisense Peptide-PNA Conjugates: Enhanced delivery and specificity using peptide conjugates with peptide nucleic acids.
RNA-Based Scaffolds: Programmable splicing control using engineered RNA scaffolds that recruit specific splicing factors.
| Drug | Target | Disease | Phase | Status |
|------|--------|---------|-------|--------|
| Tofersen | SOD1 | ALS | Phase 3 | FDA approved |
| Nusinersen | SMN2 | SMA | Approved | Marketed |
| ASO-C9orf72 | C9orf72 | ALS/FTD | Phase 1/2 | Recruiting |
| ASO-HTT | Huntingtin | Huntington's | Phase 1/2 | Completed |
| ASO-SNCA | Alpha-synuclein | Parkinson's | Preclinical | - |
The future of splice-modulating therapies likely involves combinations[@smith2024]:
Genetic Testing: Identification of specific mutations enables selection of appropriate splice-modulating approaches.
Biomarker-Guided Patient Selection: Splicing biomarkers may help identify patients most likely to benefit from specific therapies.
Allele-Specific Approaches: For diseases like Huntington's where mutant alleles can be targeted selectively, patient-specific design may be possible.
Splice-Switching Ribozymes: Catalytic RNA-based splice modification offers potential for enhanced potency.
Aptamer-ASO Conjugates: Targeted delivery to specific cell types using aptamer-guided ASO conjugates.
RNA-Based Scaffolds: Programmable splicing control using engineered RNA scaffolds provides flexible targeting.
Splice-modulating therapies represent a paradigm shift in neurodegenerative disease treatment. By directly targeting the splicing machinery, these therapies can modify disease pathogenesis at its source[@hastings2023]. The success in SMA and ongoing trials in ALS, AD, and HD demonstrate the potential of this approach. As delivery technologies and chemical modifications improve, splice-modulating therapies may become a cornerstone of precision neurology.
Key success factors include continued advancement in delivery technology to achieve adequate brain concentrations, careful optimization of dosing regimens to balance efficacy and safety, and development of biomarkers to guide patient selection and treatment monitoring. The pipeline of ASOs targeting neurodegenerative diseases continues to expand, with multiple candidates expected to enter clinical trials in the coming years.
The APOE gene represents a critical therapeutic target in Alzheimer's disease, with splice-modulating approaches showing promise for reducing pathological APOE isoforms[@kim2024].
APOE4's Role in AD: APOE4 carriage significantly increases AD risk and promotes amyloid deposition, tau pathology, and neuroinflammation. Approximately 20-25% of the population carries at least one APOE4 allele, making it one of the most important genetic risk factors for late-onset AD.
ASO Approaches: Antisense oligonucleotides targeting APOE splicing aim to shift expression toward the less pathogenic APOE2 or APOE3 isoforms. These approaches include:
TAR DNA-binding protein 43 (TDP-43) is central to ALS pathogenesis, and splice-modulating approaches aim to restore normal TDP-43 function[@nguyen2023].
TDP-43 Pathology: In approximately 95% of ALS cases, TDP-43 aggregates in cytoplasmic inclusions within motor neurons. This aggregation leads to loss of nuclear TDP-43 function and aberrant splicing of target transcripts.
Splicing Targets: Several key transcripts are mis-spliced in TDP-43-deficient neurons:
Ataxin-2 (ATXN2) has emerged as a significant therapeutic target in ALS, with ASO therapy showing promise in preclinical and early clinical studies[@yang2024].
Genetic Link: Intermediate polyglutamine expansions in ATXN2 (27-33 repeats) increase ALS risk approximately 3-fold. ATXN2 is involved in RNA metabolism, stress granule formation, and translation control.
Mechanism of Action: ATXN2 ASOs reduce ATXN2 protein expression, potentially:
Glucocerebrosidase (GBA) mutations represent the most significant genetic risk factor for Parkinson's disease, with splice-modulating approaches under development[@petrov2023].
GBA and PD Risk: Heterozygous GBA mutations increase PD risk 5-10 fold. These mutations lead to reduced glucocerebrosidase enzyme activity, impaired lysosomal function, and alpha-synuclein accumulation.
Splice-Switching Strategies: ASOs targeting GBA splicing aim to:
LRRK2 mutations account for approximately 5-10% of familial PD and 1-3% of sporadic PD, with splice-modulating approaches under investigation[@kumar2024].
LRRK2 Splice Variants: Multiple LRRK2 splice variants have been identified, some of which may be pathogenic. Aberrant splicing can produce:
Huntington's disease involves complex splicing abnormalities that provide multiple therapeutic targets[@lin2023].
Splicing Dysregulation: HD brains show widespread splicing changes, including:
Focused ultrasound combined with microbubbles enables temporary blood-brain barrier opening, dramatically enhancing ASO delivery to the brain[@wang2024].
Mechanism: Microbubble oscillation under ultrasound creates transient pores in BBB endothelium, allowing ASOs to cross into brain tissue.
Advantages:
Cell-derived extracellular vesicles (exosomes) provide natural carriers for ASO delivery across biological barriers[@park2023].
Advantages:
Splice-modulating ASOs follow established regulatory pathways:
Key biomarkers for ASO development include:
Regulatory frameworks for combination therapies (ASO + small molecule) require:
ASO therapies are among the most expensive drugs globally:
Value considerations include:
Challenges include:
Emerging technologies include:
Combinations include:
Future directions include: