Gene therapy represents one of the most promising therapeutic frontiers for corticobasal syndrome (CBS) and progressive supranuclear palsy (PSP). These approaches target the root causes of neurodegeneration at the genetic level, offering the potential for disease modification rather than merely symptomatic relief. This section covers the major gene therapy modalities—AAV-mediated gene delivery, CRISPR-based gene editing, antisense oligonucleotides (ASOs), and the critical delivery challenges that determine clinical success.
Adeno-associated viruses (AAVs) are the dominant delivery platform for CNS gene therapy due to their favorable safety profile and ability to transduce neurons with long-term expression. Key serotypes for brain delivery include:
| Serotype |
Tropism |
Blood-Brain Barrier |
Clinical Use |
| AAV9 |
Neurons, astrocytes |
Partial crossing |
Clinical trials |
| AAVrh.10 |
Neurons, microglia |
Improved crossing |
Preclinical |
| AAV2 |
Neurons (traditionally) |
Requires direct delivery |
Legacy trials |
| AAV1 |
Motor neurons |
Requires direct delivery |
Clinical trials |
| AAV-PHP.B |
Enhanced CNS |
Crosses BBB |
Research only |
BBB Crossing Strategies: Natural AAV serotypes have limited ability to cross the blood-brain barrier (BBB). Current approaches include:
- Intraparenchymal injection: Direct brain delivery bypassing BBB
- Intrathecal delivery: Injection into cerebrospinal fluid for broader distribution
- Engineered serotypes: AAV-PHP.B and variants with enhanced CNS tropism
- Receptor-mediated transcytosis: Engineering AAV capsids with brain-targeting ligands
The choice of promoter determines which cell types express the therapeutic gene:
| Promoter |
Expression Pattern |
Duration |
| Synapsin |
Neuron-specific |
Long-term |
| GFAP |
Astrocyte-specific |
Long-term |
| CMV |
Broad (ubiquitous) |
Variable |
| Mecp2 |
Neuron-specific |
Moderate |
| hSyn |
Strong neuronal |
Long-term |
Synapsin promoters are preferred for neuronal gene therapy in CBS/PSP because they provide neuron-specific, long-term expression with minimal off-target effects.
Parkinson's Disease (relevant to CBS/PSP):
- VY-AADC (Voyager Therapeutics): AAV-delivered AADC (aromatic L-amino acid decarboxylase) gene to convert levodopa to dopamine directly in the brain. Phase 1b showed sustained motor improvements.
- AAV-GDNF (various): AAV-delivered GDNF to provide trophic support to dopaminergic neurons. Multiple trials completed with mixed results.
- AAV-NTN (Cere-120): AAV-neurturin for trophic support.
Alzheimer's/FTD:
- AAV-GRN (Biogen): AAV-delivered progranulin for FTD-GRN mutations. Relevant to CBS cases with GRN mutations.
Tauopathy programs:
- AAV-anti-tau: Various programs delivering anti-tau shRNA or scFv constructs.
Cerebral Dopamine Neurotrophic Factor (CDNF) is a secreted protein that provides powerful neuroprotection to dopaminergic neurons through multiple mechanisms:
- ER stress reduction: CDNF localizes to the endoplasmic reticulum and reduces protein misfolding and ER stress—a key pathway in tauopathy
- Anti-inflammatory effects: Modulates microglial activation and reduces neuroinflammation
- Anti-apoptotic signaling: Activates PI3K/Akt and MAPK pathways to promote neuronal survival
- Synaptic plasticity: Enhances synaptic function and dopamine release
- Autophagy enhancement: Promotes clearance of misfolded proteins including tau
| Property |
CDNF |
GDNF |
| Primary target |
ER, multi-pathway |
Dopaminergic neurons |
| Distribution |
Better brain distribution |
Requires precise targeting |
| Safety profile |
Favorable |
Challenging in some trials |
| ER stress reduction |
Yes |
Limited |
| Clinical stage |
Phase 1-2 completed |
Phase 2 completed |
| Delivery |
Intraparenchymal |
Intraparenchymal |
NCT01362994 (Herantis Pharma):
- First-in-human study of CDNF in Parkinson's disease
- Results: CDNF was generally well-tolerated with preliminary efficacy signals in motor function
- Status: Phase 1-2 completed; Phase 2b planned
- Relevance to CBS/PSP: CDNF's ER stress reduction mechanism is particularly relevant to 4R-tauopathies like CBS/PSP, where tau pathology induces significant ER stress
CDNF gene therapy for CBS/PSP patients:
- Rationale: Tau pathology causes ER stress in neurons; CDNF directly addresses this mechanism
- Delivery: Requires convection-enhanced delivery (CED) for optimal brain distribution
- Combination potential: Could be combined with anti-tau immunotherapies for multi-target approach
- Patient selection: Alpha-synuclein negative patients (like this patient) may respond better to tau-targeted approaches
ASOs are single-stranded DNA sequences (12-24 nucleotides) that bind complementary mRNA via Watson-Crick base pairing. Upon binding, they recruit RNase H to cleave the hybridized mRNA, leading to reduced protein translation.
For tau reduction in CBS/PSP:
- Target: MAPT gene mRNA
- Outcome: Reduced tau protein production
- Delivery: Intrathecal (lumbar puncture) every 3-6 months
| Program |
Company |
Target |
Status |
| BIIB080 (MAPTRx) |
Biogen/Ionis |
MAPT |
Phase 2 (NCT05445964) |
| NIO752 |
Novartis/Ionis |
MAPT |
Phase 1 completed |
| ARO-MAPT |
Arrakis |
MAPT |
Preclinical |
BIIB080 (IONIS-MAPT):
- Most advanced tau-targeting ASO
- Phase 1/2 demonstrated dose-dependent CSF tau biomarker reductions
- Administered via intrathecal injection every 3-6 months
- Currently in Phase 2 trials for PSP and AD
NIO752:
- Second-generation MAPT ASO with improved CNS penetration
- Phase 1 trial completed in 2024 showing target engagement in PSP patients
ASO therapy is particularly relevant for CBS/PSP because:
- 4R-tau: ASOs can be designed to specifically reduce 4R tau isoforms
- Genetic forms: Patients with MAPT mutations could benefit from allele-specific ASOs
- Disease modification: Reduces tau at the source rather than clearing after it's produced
Limitations:
- Requires intrathecal delivery (lumbar puncture)
- Effects are reversible (requires repeated dosing)
- Distribution limited to spinal cord and cortex
CRISPR-Cas9 enables precise genome editing with potential for permanent therapeutic benefit:
Applications in CBS/PSP:
- Tau reduction: Editing the MAPT gene to reduce tau expression
- Gene correction: Correcting pathogenic MAPT or GBA mutations
- Allele-specific editing: Targeting mutant alleles while sparing wild-type
- Regulatory modulation: CRISPRa/CRISPRi to upregulate protective genes
Base editing allows precise single-nucleotide changes without double-strand breaks:
- Cytosine base editors (CBE): Convert C→T or G→A
- Adenine base editors (ABE): Convert A→G or T→C
- Prime editing: More versatile, allows all 12 possible base changes plus insertions/deletions
Advantages over CRISPR-Cas9:
- No double-strand breaks (fewer off-target effects)
- Higher precision for point mutations
- Lower immunogenicity
CRISPR-based approaches for neurodegeneration remain primarily in preclinical development:
- Delivery to the brain remains the major challenge
- AAV-delivered CRISPR components showing promise in animal models
- Clinical trials for other diseases (SCA1, Huntington's) will inform CNS CRISPR delivery
Relevant targets for CBS/PSP:
- MAPT mutations: Some familial CBS/PSP cases have MAPT mutations that could be corrected
- GRN mutations: Progranulin deficiency in some CBS cases
- GBA mutations: Associated with CBS/PSP risk
Direct injection into brain tissue:
- Advantages: Precise targeting, bypasses BBB
- Disadvantages: Invasive, limited distribution
- Use: GDNF, CDNF, AAV vectors
Bulk flow-mediated delivery:
- Advantages: Improved distribution vs. bolus injection
- Disadvantages: Requires specialized equipment
- Use: CDNF, GDNF, large molecules
Injection into cerebrospinal fluid:
- Advantages: Broader CNS distribution than intraparenchymal
- Disadvantages: Limited penetration to deep brain structures
- Use: ASOs, some AAV serotypes
Non-invasive nasal administration:
- Advantages: Non-invasive, potential for repeat dosing
- Disadvantages: Limited CNS penetration
- Use: Proteins, peptides, some viral vectors (research stage)
Techniques to enhance BBB permeability:
- Focused ultrasound (FUS): Temporary BBB opening with microbubbles
- Chemical enhancers: Mannitol, bradykinin analogs
- Receptor-mediated transcytosis: Engineering vectors with transferrin or insulin receptor ligands
| Factor |
Score |
Rationale |
| Mechanism |
9/10 |
Multiple relevant mechanisms (tau reduction, ER stress, neurotrophic support) |
| Clinical readiness |
6/10 |
AAV programs in clinic; CRISPR still preclinical |
| Delivery feasibility |
5/10 |
Invasive delivery required; BBB crossing challenges |
| Safety profile |
7/10 |
AAV generally safe; ASO class effects known |
| CBS/PSP specificity |
8/10 |
Direct targeting of tau pathology; ER stress relevant |
| Combination potential |
8/10 |
Combines well with immunotherapies, small molecules |
| Total |
43/60 |
72% |
Levodopa/Carbidopa:
- No direct interaction with gene therapy mechanisms
- Gene therapy does not affect dopaminergic medication metabolism
- Continue standard levodopa regimen during treatment
Rasagiline (MAO-B inhibitor):
- No direct interaction with gene therapy mechanisms
- Important: Some ASO trials use lithium as a tau phosphorylation inhibitor—lithium is CONTRAINDICATED with MAO-B inhibitors due to serotonin syndrome risk
- Verify ASO trial protocols avoid lithium if considering combination
- Monitor CDNF trials: Track Phase 2b results for CBS/PSP applicability
- Consider ASO therapy: If BIIB080 Phase 2 shows PSP benefit, discuss for this patient
- Genetic testing: WGS to identify potentially targetable mutations (MAPT, GRN, GBA)
- Combination approach: Gene therapy + anti-tau antibody may provide synergistic benefit
- Timing: Gene therapy most likely beneficial early in disease course