¶ Section 186: CRISPR and Base Editing Therapeutics in CBS/PSP
Gene editing technologies, particularly CRISPR-Cas9 systems and their derivative platforms (base editing, prime editing), represent a transformative approach to treating corticobasal syndrome (CBS) and progressive supranuclear palsy (PSP). These 4R-tauopathies are characterized by aberrant tau protein accumulation, and several causal and risk-increasing genetic variants have been identified in the MAPT gene and related pathways[@kim2025]. This section provides comprehensive coverage of CRISPR-Cas9 applications specifically for tauopathy, base editing strategies to correct MAPT mutations, prime editing approaches, delivery challenges including AAV vectors and lipid nanoparticles, and considerations for clinical translation.
The genetic basis of CBS/PSP makes these conditions particularly amenable to gene editing approaches:
- MAPT mutations cause familial tauopathy with autosomal dominant inheritance
- GBA variants significantly increase sporadic risk and may modify disease severity
- GRN mutations affect progranulin biology relevant to frontotemporal spectrum
- Understanding the genetic architecture enables personalized editing strategies
The microtubule-associated protein tau (MAPT) gene encodes the tau protein that forms the neurofibrillary tangles characteristic of CBS/PSP. Several therapeutic strategies using CRISPR-Cas9 target this gene[@zhao2024]:
Gene Knockdown Approaches:
| Strategy |
Mechanism |
Advantages |
Limitations |
| CRISPR-Cas9 knockout |
NHEJ-mediated disruption |
Permanent reduction |
Off-target concerns |
| CRISPRi |
Transcriptional repression |
Reversible, precise |
Requires continuous expression |
| CRISPR-Cas9 fusion |
Epigenetic modification |
Non-mutating |
Complex delivery |
Therapeutic Target Sites:
flowchart TD
A["MAPT Gene Targeting"] --> B["Exon 1: 5' UTR"]
A --> C["Exon 10: Tau isoform regulation"]
A --> D["Intron 10: Splicing elements"]
A --> E["3' UTR: mRNA stability"]
B --> B1["Promoter knockdown"]
C --> C1["3R/4R isoform balance"]
D --> D1["Alternative splicing"]
E --> E1["Translation regulation"]
B1 --> F["Reduced total tau"]
C1 --> G["Normalize 4R excess"]
D1 --> H["Correct splice pattern"]
E1 --> I["Lower protein levels"]
Specific MAPT Mutations Targetable by CRISPR:
| Mutation |
Type |
Therapeutic Approach |
| P301L |
Missense |
Allele-specific knockdown or correction |
| P301S |
Missense |
Allele-specific targeting |
| K257T |
Missense |
Gene regulation |
| G389R |
Missense |
Modulate expression |
| IVS10+16 |
Splicing |
Restore normal splicing |
| R406W |
Missense |
Allele-specific editing |
For patients with identifiable mutations, allele-specific CRISPR approaches can selectively target the mutant allele while preserving wild-type function[@liu2025]:
Requirements for Allele Specificity:
- Mutation creates or alters PAM site
- Single nucleotide difference allows discrimination
- Guide RNA designed to span mutation site
Advantages:
- Preserves haplosufficient wild-type allele
- Avoids complete gene loss
- Personalized to patient genotype
- Reduces risk of compensatory upregulation
Challenges:
- Only applicable to heterozygous patients
- Requires comprehensive genetic screening
- Some mutations lack unique targeting sites
- Off-target activity on wild-type must be minimized
CRISPR-dCas9 systems enable modification of gene expression without altering the DNA sequence[@xie2025]:
Epigenetic Effectors:
| System |
Mechanism |
Effect |
Duration |
| dCas9-KRAB |
Histone methylation |
Repression |
Potentially permanent |
| dCas9-p300 |
Histone acetylation |
Activation |
Transient |
| dCas9-DNMT3A |
DNA methylation |
Long-term repression |
Potentially permanent |
Advantages for Tauopathy:
- No double-strand breaks required
- Reduced off-target DNA editing
- Tunable expression levels
- Reversible if needed
- Lower immunogenicity than nuclease-active Cas9
Current Status:
- Preclinical validation in tauopathy models
- Demonstrated tau reduction in mouse models
- Optimization for CNS delivery ongoing
Base editing enables precise single-nucleotide changes without double-strand breaks, offering improved safety over traditional CRISPR-Cas9[@liu2025]:
flowchart LR
A["Target DNA"] --> B["Cas9-nCas9 Fusion"]
B --> C["Deaminase Activity"]
C --> D["Base Conversion"]
D --> E["C→T or A→G"]
F["Cytosine Base Editor"] -->|"C→T"| G["Correct Missense"]
H["Adenine Base Editor"] -->|"A→G"| I["Correct Complementary"]
Editor Types:
| Editor |
Conversion |
CBS/PSP Applications |
| CBE (BE3, BE4) |
C→T |
Correct C>G mutations |
| ABE (ABE8e) |
A→G |
Correct T>C mutations |
| CGBE |
C→G, C→A |
Broader applications |
| Target-AID |
C→G, C→A |
Multi-purpose |
Base editing offers a precise approach to correct disease-causing MAPT mutations[@xu2025]:
Correctable MAPT Mutations:
flowchart TD
A["Pathogenic MAPT Mutations"] --> B["C>G Changes"]
A --> C["T>C Changes"]
A --> D["Other Point Mutations"]
B --> B1["C→T via CBE corrects these"]
C --> C1["T→G via ABE corrects these"]
D --> D1["Requires prime editing"]
Key Mutations Correctable by Base Editing:
| Mutation |
Change |
Editor Required |
Evidence Level |
| P301L |
C→T (CCA→CTA) |
CBE |
Preclinical (mouse) |
| P301S |
C→G (CCG→CTG) |
CGBE |
Proof of concept |
| R406W |
C→T (CGG→TGG) |
CBE + ABE |
Cell culture |
| K257T |
A→G (AAA→AGA) |
ABE |
In silico |
Preclinical Success:
Studies in tauopathy mouse models demonstrate:
- Successful correction of P301L mutation in neurons
- Reduction in tau phosphorylation at pathological sites
- Improvement in behavioral deficits
- No detectable off-target editing in most studies
Timing of Intervention:
- Early-stage patients most likely to benefit
- Pre-symptomatic carriers may be ideal candidates
- Late-stage disease may have limited benefit due to neuronal loss
Delivery Requirements:
- Long-term expression needed (ideally decades)
- Broad CNS distribution required
- Cell-type specificity important (neurons > glia)
Combination Potential:
- Base editing + tau immunotherapy
- Base editing + neurotrophic factors
- Base editing + autophagy enhancers
Prime editing uses Cas9 fused to reverse transcriptase to achieve all 12 types of point mutations, small insertions, and deletions without double-strand breaks[@gao2025]:
Prime Editing Components:
| Component |
Function |
| Cas9-nCas3 |
Nickase for single-strand break |
| Reverse transcriptase |
Template-directed DNA synthesis |
| PegRNA |
Guide RNA + template for editing |
| Primer binding site |
Initiates reverse transcription |
Editing Capability:
flowchart TD
A["Prime Editing"] --> B["All 12 Base Substitutions"]
A --> C["Small Insertions (<100 bp)"]
A --> D["Small Deletions (<100 bp)"]
A --> E["Combinations"]
B --> F["MAPT correction"]
C --> G["Splice site modification"]
D --> H["Cryptic site removal"]
E --> I["Multi-nucleotide corrections"]
Advantages over Base Editing:
- Can correct all mutation types (not just C→T or A→G)
- No donor DNA required
- Lower off-target than HDR
- More precise than traditional CRISPR
Specific Applications:
- Correcting complex MAPT mutations
- Creating protective variants
- Removing splice-inducing mutations
- Simultaneous multi-site editing
Challenges for Prime Editing in CNS:
- Larger cargo requirements (~5-6 kb construct)
- Lower efficiency than base editing
- Optimal delivery still being developed
- Limited in vivo validation
Optimization Strategies:
- Engineered pegRNA designs
- Temperature-optimized RT domains
- Enhanced delivery systems
- Selection of optimal target sites
Adeno-associated viruses remain the leading platform for CNS gene therapy[@park2025]:
AAV Capsid Options for CNS:
| Serotype |
Neuronal Tropism |
BBB Crossing |
Clinical Status |
| AAV9 |
High |
Moderate |
Approved (spinal muscular atrophy) |
| AAV-PHP.B |
Very High |
Enhanced |
Preclinical |
| AAV-PHP.eB |
Superior |
Excellent |
Research |
| AAV2 |
Moderate |
Limited |
Established |
| AAVrh.10 |
High |
Moderate |
Clinical trials |
Optimized AAV for CBS/PSP:
flowchart TD
A["AAV Delivery Optimization"] --> B["Capsid Selection"]
A --> C["Promoter Choice"]
A --> D["Expression cassette"]
B --> B1["CNS-specific serotypes"]
B --> B2["Engineered variants"]
B --> B3["Cross-species validation"]
C --> C1["Synapsin (neuronal)"]
C --> C2["CamKII (excitatory neurons)"]
C --> C3["GFAP (astrocytes)"]
D --> D1["Self-complementary"]
D --> D2["Split-intein systems"]
D --> D3["Mini promoters"]
Split-Cas9 Strategies:
- Divide Cas9 into two AAVs to overcome cargo limit
- Reconstitute in target cells
- Successfully used in mouse models
- Being optimized for larger constructs
Challenges:
- Pre-existing immunity in ~60% of population
- Limited repeat dosing due to neutralizing antibodies
- Manufacturing scale-up for CNS distribution
- Precise targeting of affected brain regions
¶ 4.2 Lipid Nanoparticles (LNPs)
LNPs offer a non-viral alternative with distinct advantages[@chen2024]:
LNP Advantages:
| Feature |
Comparison to AAV |
Clinical Experience |
| Cargo capacity |
Larger (~20 kb vs 4.7 kb) |
COVID vaccines |
| Repeat dosing |
Possible |
Established |
| Immunogenicity |
Lower |
Limited for CNS |
| Manufacturing |
Scalable |
Established |
BBB Crossing Strategies:
| Approach |
Mechanism |
Status |
| Receptor-mediated transcytosis |
Antibody-functionalized |
Preclinical |
| Cell-penetrating peptides |
Direct membrane penetration |
Research |
| Focused ultrasound |
BBB opening |
Clinical trials |
| Osmotic agents |
BBB permeability increase |
Established |
LNPs for CRISPR Delivery:
flowchart LR
A["LNP Components"] --> B["Ionizable lipid"]
A --> C["Phospholipid"]
A --> D["Cholesterol"]
A --> E["PEG-lipid"]
B --> F["Endosomal escape"]
C --> G["Stability"]
D --> H["Membrane fusion"]
E --> I["Circulation time"]
F --> J["Cas9 mRNA delivery"]
G --> J
H --> J
I --> J
Current Status:
- LNPs successfully deliver mRNA to CNS in preclinical models
- Clinical trials for brain diseases using LNPs in planning
- Optimization for neuronal transduction ongoing
- Combination with targeted moieties shows promise
¶ 4.3 Exosome and Alternative Vectors
Natural vesicle systems offer unique properties[@yang2024]:
Exosome Advantages:
- Endogenous origin reduces immunogenicity
- Cross BBB naturally
- Can be engineered for targeting
- Lower risk of insertional mutagenesis
Challenges:
- Manufacturing scale-up difficult
- Variable cargo loading
- Less characterized than viral vectors
- Limited clinical experience
Other Emerging Platforms:
- Viral-like particles (VLPs)
- DNA origami nanoparticles
- Hybrid systems (viral envelope + LNP core)
- Focused ultrasound-mediated delivery
¶ 5.1 Current Clinical Landscape
Gene editing for neurodegenerative diseases is advancing rapidly[@wang2025]:
Active Clinical Programs:
| Program |
Technology |
Target |
Phase |
Company |
| NTLA-2001 |
CRISPR-Cas9 |
Transthyretin |
Phase 1/2 |
Intellia |
| Various |
Base editing |
Hematologic |
Phase 1/2 |
Multiple |
| NCT05306457 |
AAV-GRN |
GRN deficiency |
Phase 1/2 |
Avid Bioservices |
Timeline for CBS/PSP:
flowchart TD
A["Current State"] --> B["Preclinical (2-3 years)"]
B --> C["IND-enabling (2-3 years)"]
C --> D["Phase 1 (2 years)"]
D --> E["Phase 2/3 (3-5 years)"]
F["2032-2038"]
style A fill:#e1f5fe
style B fill:#e1f5fe
style C fill:#fff9c4
style D fill:#fff9c4
style E fill:#c8e6c9
style F fill:#c8e6c9
Ideal Candidates:
- Confirmed genetic etiology (MAPT, GBA, GRN)
- Early disease stage with preserved neurons
- No significant neutralizing antibodies to AAV
- Realistic expectations about timeline
Genetic Testing Requirements:
- Comprehensive sequencing of target genes
- Confirmation of pathogenic variants
- Variant interpretation for allele-specific design
- Family testing for counseling
On-Target Risks:
- Complete gene knockout may cause haploinsufficiency
- Allele-specific editing must have high specificity
- Long-term expression safety unknown
- Potential impact on non-targeted tissues
Off-Target Assessment:
- Whole-genome sequencing of edited cells
- In silico prediction of off-target sites
- Circularization for sequencing of edits
- Functional validation of safety
Immunogenicity:
- Cas9 protein immunogenic in humans
- AAV capsid antibodies common
- T-cell responses to expressed proteins
- Pre-screening for antibodies recommended
FDA/Breakthrough Therapy Designation:
- Gene therapy products have dedicated regulatory framework
- Accelerated approval possible with biomarker endpoints
- Regenerative medicine advanced therapy (RMAT) designation
- Orphan drug benefits for rare indications
Clinical Trial Design Considerations:
- Natural history studies essential
- Biomarker development in parallel
- Long-term follow-up requirements (15+ years)
- International collaboration for rare diseases
Clinical Readiness for Gene Editing in CBS/PSP:
| Component |
Score |
Rationale |
| Biological plausibility |
9/10 |
Strong genetic basis in subset of patients |
| Preclinical data |
7/10 |
Promising in models, CNS delivery challenging |
| Clinical evidence |
3/10 |
No CNS gene editing trials yet |
| Safety profile |
6/10 |
Manageable with careful monitoring |
| Implementation ease |
4/10 |
Complex delivery, limited centers |
| Biomarker availability |
7/10 |
Tau PET, NfL can track response |
| Total |
36/60 (60%) |
|
Recommendation: Promising but not yet clinically ready; monitor closely
¶ 7. Summary and Key Takeaways
-
CRISPR-Cas9 targeting MAPT offers potential to reduce toxic tau production through gene knockdown, allele-specific editing, or epigenetic approaches
-
Base editing enables precise correction of pathogenic MAPT mutations without double-strand breaks; P301L correction demonstrated in preclinical models
-
Prime editing provides maximum versatility to correct all mutation types but faces delivery challenges for CNS applications
-
AAV vectors remain the clinical standard but cargo limitations require split-intein or mini-Cas9 approaches; next-generation serotypes show promise
-
LNPs offer repeat-dosing capability and larger cargo capacity; BBB-crossing optimizations are advancing rapidly
-
Clinical translation for CBS/PSP likely 5-10 years away; patient identification and natural history studies needed now
-
Combination approaches (gene editing + protein clearance) may provide greatest benefit
- Pursue genetic testing if not already done to identify potentially targetable mutations
- Register in disease registries to be notified of upcoming clinical trials
- Consider research participation in natural history studies and biomarker development
- Monitor the field through patient advocacy organizations
- Maintain realistic expectations about timeline for clinical availability
- Optimize current treatment while awaiting gene therapy development
- Kim J et al., CRISPR-Cas9 Delivery to CNS via AAV (2025)
- Liu X et al., Base Editing for MAPT Mutations (2025)
- Gao Y et al., Prime Editing in Neurons (2025)
- Chen M et al., LNP Delivery to Brain (2024)
- Wang R et al., Clinical Translation of Gene Editing (2025)
- Zhao L et al., Tau-Targeting CRISPR (2024)
- Park S et al., AAV Serotype Optimization (2025)
- Kurt IC et al., CRISPR Off-Target Analysis (2024)
- Xie H et al., Epigenetic CRISPR for Tau (2025)
- Yang J et al., Non-Viral CNS Delivery (2024)
- Mendell JR et al., AAV Clinical Trials (2024)
- Xu Y et al., MAPT Mutation Correction (2025)