Gene therapy represents one of the most promising therapeutic frontiers for neurodegenerative diseases, offering the potential to address the underlying genetic and molecular causes of conditions such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), Huntington's disease, Spinal Muscular Atrophy (SMA), and Frontotemporal Dementia (FTD). Unlike traditional pharmacological approaches that manage symptoms, gene therapy aims to correct, replace, or modulate the expression of defective genes, potentially altering the course of disease progression.
The field has seen remarkable advances since the first gene therapy clinical trials in the 1990s. The approval of onasemnogene abeparvovec (Zolgensma) for SMA in 2019 demonstrated that gene therapy could be transformative for neurological conditions. As of 2025, the central nervous system (CNS) accounts for approximately 21% of all adeno-associated virus (AAV) gene therapy clinical trials, making it the second most targeted tissue after the eye [1].
AAV vectors are the dominant platform for CNS gene therapy due to their favorable safety profile, long-term transgene expression, and neurotropism. Key serotypes used in neurological applications include:
- AAV9: Crosses the Blood-Brain Barrier (BBB) after intravenous administration. Used in Zolgensma for SMA and under investigation for multiple neurodegenerative diseases [2]00365-X).
- AAV2: Extensively studied for Parkinson's disease gene therapy, delivered via stereotaxic injection to the putamen or subthalamic nucleus [3]00125-5/abstract).
- AAVrh10: Shows broad CNS transduction and is being explored for lysosomal storage disorders with neurological involvement.
- AAV5: Used by UniQure for AMT-130 in Huntington's disease, delivered via intrastriatal injection.
AAV vectors offer efficient delivery with relatively low immunogenicity, though immune responses to both the capsid and transgene product remain an important consideration. Pre-existing anti-AAV antibodies can limit therapeutic efficacy and exclude some patients from treatment [2]00365-X).
Lentiviral vectors integrate into the host genome, providing potentially permanent transgene expression. ProSavin (Oxford BioMedica), a lentiviral vector delivering three dopamine biosynthesis enzymes, was tested in Phase I/II trials for Parkinson's disease. However, the risk of insertional mutagenesis has limited widespread adoption for CNS applications [4].
Emerging non-viral strategies include lipid nanoparticles (LNPs) for mRNA delivery, exosome-based delivery systems, and polymer-based nanoparticles. These approaches avoid immunogenicity issues associated with viral vectors but typically achieve transient expression and lower transduction efficiency in neurons [5].
The BBB presents a major challenge for CNS gene therapy. Several delivery strategies have been developed:
- Intravenous (IV) administration: Possible with BBB-crossing serotypes (AAV9, AAVrh10). Used for Zolgensma. Requires high vector doses, which increases the risk of hepatotoxicity and immune reactions.
- Intrathecal (IT) injection: Delivers vector into the cerebrospinal fluid (CSF) via lumbar puncture. Provides broader CNS distribution than direct injection with lower peripheral exposure.
- Intracerebroventricular (ICV) injection: Direct delivery into the brain ventricles, providing distribution via CSF circulation.
- Intraparenchymal injection: Stereotaxic injection directly into specific brain regions (e.g., putamen, [substantia nigra). Provides highest local transduction but limited distribution.
- Intracisternal (ICM) delivery: Injection into the cisterna magna, offering improved brainstem and cortical transduction compared to lumbar IT delivery.
Gene replacement therapy delivers a functional copy of a defective gene. This is the most straightforward approach for monogenic disorders:
- SMA: Zolgensma delivers the SMN1 gene via AAV9, restoring survival motor neuron protein production. This has dramatically improved outcomes for infants with Type 1 SMA [6].
- SMA Type 2: Intrathecal onasemnogene abeparvovec is under investigation for older patients.
- Aromatic L-amino acid decarboxylase (AADC) deficiency: Eladocagene exuparvovec (AAV2-AADC) received approval in 2022 for AADC deficiency, demonstrating dramatic improvements in motor function.
- Giant axonal neuropathy: AAV9 delivering gigaxonin is in clinical trials.
Delivering genes encoding neurotrophic factors can protect vulnerable neuronal populations:
- AAV2-BDNF: MRI-guided infusion of AAV2 delivering brain-derived neurotrophic factor (BDNF) is being evaluated in patients with mild Alzheimer's disease and MCI. Six patients with mild AD have been safely treated, with the trial now enrolling MCI patients [7].
- AAV2-GDNF: Delivery of glial cell line-derived neurotrophic factor (GDNF) to the putamen aims to protect dopaminergic neurons in Parkinson's disease [3]00125-5/abstract).
- AAV2-Neurturin (CERE-120): Despite promising preclinical data, Phase II trials for PD failed to meet primary endpoints, highlighting the challenges of neurotrophic factor delivery.
Several gene therapy approaches for Parkinson's disease deliver enzymes involved in dopamine biosynthesis:
- AAV2-GAD (glutamic acid decarboxylase): MeiraGTx's AAV-GAD gene therapy received Regenerative Medicine Advanced Therapy (RMAT) designation from the FDA in 2025, expediting development for PD [3]00125-5/abstract).
- AAV2-AADC: Delivers aromatic L-amino acid decarboxylase to the putamen to enhance conversion of levodopa to dopamine.
For gain-of-function mutations or toxic protein accumulation, gene silencing strategies reduce pathogenic protein expression:
- AMT-130 (UniQure): AAV5 delivering microRNA targeting huntingtin protein for Huntington's disease. Received breakthrough therapy designation in 2025, with Phase I/II data showing 80% slowing of disease progression on the UHDRS at 24 months [8].
- AAV-mediated anti-SOD1: Targets mutant SOD1 in familial ALS.
- SNUG01 (SineuGene): Advanced into global Phase 1 trials for ALS in March 2025 [9].
Next-generation CRISPR tools are emerging for precision treatment of neurodegenerative diseases:
- Base editing: Enables single-nucleotide corrections without double-strand breaks, reducing off-target effects.
- Prime editing: Allows precise insertions, deletions, and all types of point mutations.
- CRISPR interference (CRISPRi): Reversible gene silencing without permanent DNA modification.
CRISPR-based approaches are being explored for correcting PSEN1 and PSEN2 mutations in familial Alzheimer's disease, LRRK2 gain-of-function mutations in Parkinson's disease, and C9orf72 repeat expansions in ALS/FTD [10].
- LX1001 (Lexeo Therapeutics): AAV gene therapy delivering APOE2
| Target Disease |
Therapy |
Vector/Approach |
Phase |
Key Finding |
| Huntington's disease |
AMT-130 |
AAV5-miHTT |
I/II |
80% slowing of progression at 24 months |
| Parkinson's disease |
AAV2-GAD |
AAV2 |
II |
RMAT designation granted 2025 |
| Alzheimer's disease |
AAV2-BDNF |
AAV2 |
I |
6 patients safely treated |
| Alzheimer's disease |
LX1001 (APOE2) |
AAV |
I |
Enrolling APOE4 homozygotes |
| ALS |
SNUG01 |
AAV |
I |
Global trial initiated March 2025 |
| FTD |
GRN gene therapy |
AAV |
I |
First patients dosed 2025 |
| SMA |
Zolgensma IT |
AAV9 |
III |
Intrathecal route for older patients |
The gene therapy landscape for neurodegenerative diseases continues to evolve rapidly:
- Engineered AAV capsids: Directed evolution and rational design approaches are generating new capsid variants with improved CNS tropism, reduced immunogenicity, and the ability to cross the BBB more efficiently.
- Regulatable expression systems: Incorporating drug-responsive promoters (e.g., tetracycline-inducible) to allow dose adjustment post-administration.
- Combinatorial approaches: Targeting multiple pathogenic pathways simultaneously (e.g., reducing tau] and increasing BDNF expression).
- Cell-type-specific promoters: Using neuron-specific (synapsin), astrocyte-specific (GFAP), or microglia-specific promoters for targeted expression.
- In vivo base editing: Direct correction of point mutations without the need for homology-directed repair templates.
The study of Gene Therapy For Neurodegenerative Diseases has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- [Issa SS, Shaimardanova AA, Solovyeva VV, Rizvanov AA. (2024). Emerging Gene Therapies for Alzheimer's and Parkinson's Diseases: An Overview of Clinical Trials and Promising Candidates. Curr Issues Mol Biol. PMC11405083)
- [Mendell JR, et al. (2025). Current clinical applications of AAV-mediated gene therapy. Molecular Therapy. DOI)
- [Axelsen TM, Bhatt D. (2025). Gene therapy for Parkinson's Disease: trials and technical advances. Lancet Neurology. DOI)
- [Aguilar-González A, et al. (2025). Significance of gene therapy in neurodegenerative diseases. Front Neurosci. DOI)
- [Hussain MA, et al. (2025). Viral and non-viral cellular therapies for neurodegeneration. Front Med. DOI)
- [Pena SA, et al. (2025). Current trends in gene therapy to treat inherited disorders of the brain. PubMed. PMID 40181540)
- 2025 NIH Alzheimer's Disease and Related Dementias Research Progress Report. National Institute on Aging. Link
- [PackGene Biotech. (2025). 2025's Most Impactful Cell and Gene Therapy Milestones. Link)
- [PackGene Biotech. (2025). Advances in Cell and Gene Therapy and the Evolving AAV Landscape in 2025 H1. Link)
- [Chen M, et al. (2025). Next-generation CRISPR gene editing tools in the precision treatment of Alzheimer's and Parkinson's Disease. Ageing Res Rev. DOI)
- [Hudry E, Bhatt DK. (2024). Alzheimer's Disease drug development pipeline: 2025. Alzheimers Dement. PMC12131090)
- [CGTlive. (2025). Top FDA Gene and Cell Therapy News: 2025 Year-End Recap. Link)