crispr-gene-editing

Introduction

Overview

CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats) genome editing technologies have transformed the therapeutic landscape for [neurodegenerative /diseases by enabling precise correction or inactivation of disease-causing genes. Unlike antisense-oligonucleotide-therapy (ASOs), which require repeated dosing and produce temporary knockdown, CRISPR can achieve permanent genetic modification in a single treatment—a transformative prospect for monogenic neurodegenerative disorders such as huntington-pathway (HD), familial alzheimers (fAD), familial parkinsons (fPD), and als (ALS). ^{1Citation2024 · Eur J NeurosciOpen reference}

The CRISPR toolkit has expanded beyond the original Cas9 nuclease to include base editors, prime editors, CRISPRi/CRISPRa for gene regulation, and RNA-targeting Cas13 systems—each offering distinct advantages for neurological applications where off-target DNA cuts in post-mitotic neurons pose irreversible risks (Akyuz et al., 2024; Bhatt et al., 2025). ^{2Citation2025Open reference}

CRISPR-Cas Systems Relevant to Neurodegeneration

CRISPR-Cas9 (Double-Strand Break)

The canonical system uses the Cas9 nuclease guided by a single-guide RNA (sgRNA) to create a double-strand break (DSB) at a specific genomic locus. In neurons, DSBs are repaired primarily by non-homologous end joining (NHEJ), which can inactivate a gene by introducing insertions/deletions (indels). ^{3Citation2025Open reference}

Applications: Disruption of mutant alleles (e.g., mutant htt, sod1-protein, app ^{4CitationPMID 28494701Open reference}

Limitations: Off-target DSBs in post-mitotic neurons are permanent and uncorrectable; risk of large deletions, translocations, or chromothripsis. ^{5CitationPMID 30195778Open reference}

Base Editing

Base editors (cytosine base editors, CBE; adenine base editors, ABE) convert one base pair to another (C→T or A→G) without creating DSBs. This is safer for neurons and ideal for correcting point mutations. ^{6Citation2019 · PMID 30655242Open reference}

Applications: Correcting psen1 and psen2 missense mutations in familial AD; lrrk2 G2019S mutation in PD; specific sod1-protein mutations in ALS. ^{7CitationPMID 34320891Open reference}

Prime Editing

Prime editors use a Cas9 nickase fused to a reverse transcriptase, guided by a prime editing guide RNA (pegRNA) that encodes the desired edit. Prime editing can make all 12 types of point mutations, small insertions, and small deletions without DSBs. ^{8(2018)2018 · neurons · PMID 29809620Open reference}

Applications: Precise correction of expanded CAG repeats in HD; correction of specific mutations in fAD and fPD. ^{9CitationPMID 30030399Open reference}

CRISPRi/CRISPRa (Gene Regulation)

Catalytically dead Cas9 (dCas9) fused to transcriptional repressors (CRISPRi) or activators (CRISPRa) modulates gene expression without altering DNA sequence. ^{10Citation2021 · PMID 34215024Open reference}

• CRISPRi: Silences mutant allele expression (e.g., allele-specific silencing of mutant htt

• CRISPRa: Upregulates neuroprotective genes (e.g., bdnf, gba, trem2

Advantages: No permanent DNA alteration; reversible; lower risk of off-target damage. ^{2Citation2025Open reference}0

CRISPR-Cas13 (RNA Targeting)

Cas13 systems target and degrade specific RNA transcripts, functioning as programmable RNA knockdown tools. Unlike DNA-targeting systems, Cas13 leaves the genome intact. ^{2Citation2025Open reference}1

Applications: Degradation of mutant htt mRNA, c9orf72 repeat RNA, or toxic tdp-43 transcripts. ^{2Citation2025Open reference}2

Disease-Specific Applications

Huntington’s Disease

HD is the most advanced neurodegenerative target for CRISPR, given its monogenic etiology (expanded CAG repeat in [HTT): ^{2Citation2025Open reference}3

Allele-specific silencing: CRISPR-Cas9 can selectively inactivate the mutant htt allele while preserving normal htt by targeting SNPs linked to the disease haplotype. This avoids the problem of total htt loss-of-function, which is developmentally lethal (Monteys et al., 2017). ^{2Citation2025Open reference}4

Repeat excision: Cas9 with two flanking sgRNAs can excise the expanded CAG repeat, replacing it with a normal-length repeat. This has been demonstrated in HD patient-derived iPSC neurons (Dabrowska et al., 2018).

CRISPRi approach: dCas9-KRAB targeted to the htt promoter region suppresses mutant htt expression by 60-80% in mouse striatal neurons without DNA cleavage.

In vivo delivery: AAV-packaged CRISPR targeting mutant htt reduced huntingtin aggregates and improved motor function in HD mouse models (Ekman et al., 2019).

Alzheimer’s Disease

CRISPR approaches in AD target multiple pathogenic genes:

app editing: CRISPR-mediated introduction of the protective A673T (Icelandic) mutation in app reduces amyloid-beta production by ~40%. This mutation decreases bace1.

psen1 correction: Base editing can correct specific presenilin mutations that cause familial Alzheimer’s Disease. Over 300 psen1 mutations are known, many of which are single nucleotide changes amenable to base editing.

**apoe using base editors has been demonstrated in human iPSC-derived astrocytes and neurons, reducing amyloid-beta production and tau] hyperphosphorylation (Lin et al., 2018).

trem2 activation: CRISPRa to upregulate trem2 expression in microglia/entities/microglia. Adenine base editors can revert the pathogenic G→A mutation with high efficiency in patient iPSC-derived dopaminergic neurons.

alpha-synuclein (SNCA) reduction: CRISPRi-mediated downregulation of SNCA expression reduces alpha-synuclein aggregation. SNCA gene duplication/triplication causes familial PD, and even partial reduction of wild-type SNCA may be therapeutic.

gba correction: GBA1 mutations (the most common genetic risk factor for PD) can be corrected by base or prime editing, restoring glucocerebrosidase activity and improving lysosomal function.

pink1/prkn enhancement: CRISPRa to upregulate mitophagy genes, enhancing clearance of damaged mitochondrial-dynamics.

Amyotrophic Lateral Sclerosis

sod1-protein silencing: CRISPR-Cas9 disruption of mutant SOD1 in the SOD1-G93A mouse model reduced mutant SOD1 protein levels by ~50% and extended survival. This approach complements the tofersen strategy.

c9orf72 repeat excision: Cas9 with flanking guides can excise the hexanucleotide repeat expansion, eliminating both RNA foci and dipeptide repeat protein production (Selvaraj et al., 2018).

fus correction: Base editing of FUS mutations that disrupt nuclear localization (affecting the PY-NLS recognized by transportin-1), restoring proper nucleocytoplasmic transport.

Other Neurodegenerative Diseases

Delivery to the Central Nervous System

The greatest challenge for CRISPR-based neurotherapeutics is delivering editing machinery across the blood-brain-barrier to target cells in the CNS.

Viral Vectors

Adeno-associated virus (AAV):

• AAV9 and AAVrh10 cross the blood-brain-barrier with moderate efficiency

• AAV-PHP.eB provides enhanced blood-brain-barrier penetration in mice (but not primates)

• Limitations: ~4.7 kb packaging capacity (Cas9 alone is ~4.2 kb); split-intein strategies or smaller Cas variants (CjCas9, Cas12f) help address this

• Single-dose intrathecal or intravenous administration

Lentiviral vectors:

• Larger cargo capacity (~8 kb)

• Integrate into genome (risk of insertional mutagenesis)

• Used primarily in ex vivo applications

Non-Viral Delivery

Lipid nanoparticles (LNPs):

• Can deliver Cas9 mRNA + sgRNA without permanent vector integration

• Transient expression reduces off-target editing risk

• blood-brain-barrier penetration remains limited; enhanced by focused-ultrasound blood-brain-barrier opening

Extracellular vesicles/exosomes:

• Exosome-mediated delivery of CRISPR RNPs; natural blood-brain-barrier-crossing ability

• Under development; limited cargo loading efficiency

Polymer nanoparticles:

• PEG-PLGA and other polymeric systems

• Surface modification with targeting ligands for neuron-specific delivery

Direct CNS Administration

• Intrathecal injection: Into cerebrospinal fluid; distributed along spinal cord and brain surfaces

• Intraparenchymal injection: Stereotactic injection directly into target brain regions (e.g., striatum for HD, substantia-nigra for PD)

• Convection-enhanced delivery (CED): Pressure-driven infusion for larger volume distribution

Safety Considerations

Off-Target Editing

CRISPR-Cas9 can cut at genomic sites with partial sgRNA complementarity. In post-mitotic neurons, off-target DSBs are irreversible and could activate oncogenes, inactivate tumor suppressors, or disrupt essential genes.

Mitigation strategies:

• High-fidelity Cas9 variants (eSpCas9, HiFi Cas9)

• Base and prime editors (no DSBs)

• Cas13 RNA targeting (no DNA modification)

• Transient delivery (mRNA/RNP instead of DNA)

• Genome-wide off-target profiling (GUIDE-seq, CIRCLE-seq)

Immunogenicity

• Cas9 protein (derived from S. pyogenes or S. aureus) can elicit pre-existing immune responses in ~50% of humans

• AAV capsid proteins trigger humoral and cellular immune responses

• Strategies: immunosuppression, transient expression, engineered non-immunogenic Cas variants

Mosaicism and Incomplete Editing

• Not all target neurons will be edited, particularly with systemic delivery

• Partial editing may be therapeutically sufficient for dominant-negative diseases (e.g., reducing mutant htt by 50% is therapeutic)

• Allele-specific approaches must avoid editing the normal allele

Ethical Considerations

• Germline editing is not being pursued for neurodegeneration (somatic editing only)

• Equitable access to potentially curative but expensive gene editing therapies

• Informed consent for irreversible genetic modifications

• Long-term monitoring requirements for edited patients

Clinical Translation Status

The Intellia Therapeutics NTLA-2001 trial for transthyretin amyloidosis—using LNP-delivered Cas9 to inactivate TTR in the liver—demonstrated >90% reduction in serum transthyretin after a single dose, providing the first clinical proof that in vivo CRISPR editing is feasible and effective (Gillmore et al., 2021).

Current Research Directions

1. Compact Cas systems: CasΦ, Cas12f, and Un1Cas12f for packaging in single AAV vectors

2. Epigenome editing: dCas9 fused to DNA methyltransferases or demethylases for durable gene silencing without DNA cuts

3. Multiplexed editing: Simultaneously targeting multiple pathogenic variants or pathways

4. Cell-type-specific editing: neurons-specific or glia-specific promoters driving Cas expression

5. In situ base editing: Correcting mutations in patient neurons without the need for cell transplantation

6. RNA editing: CRISPR-Cas13 and ADAR-based systems for reversible transcriptome modification

See Also

• APOE, Focused Ultrasound

• gene-therapy

• immunotherapy

Background

The study of Crispr Gene Editing 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.

External Links

• PubMed - Biomedical literature

• Alzheimer’s Disease Neuroimaging Initiative - Research data

• Allen Brain Atlas - Brain gene expression data

Gene Editing Mechanism

flowchart TD
    A["CRISPR-Cas9<br/>System"]  -->  B["Guide RNA<br/>Design"]
    B  -->  C["gRNA + Cas9<br/>Complex"]
    C  -->  D["Target Gene<br/>Recognition"]
    D  -->  E{"PAM Site<br/>Present?"}
    E  -->|"Yes"| F["DNA Double<br/>Strand Break"]
    E  -->|"No"| G["No Cutting"]
    F  -->  H{"Repair<br/>Mechanism"}
    H  -->|"NHEJ"| I["Knockout<br/>Gene Disruption"]
    H  -->|"HDR"| J["Precise<br/>Correction"]
    I  -->  K["Frameshift/<br/>Premature Stop"]
    J  -->  L["Wild-type<br/>Sequence"]
    K  -->  M["Reduced Toxic<br/>Protein"]
    L  -->  N["Restored<br/>Protein Function"]
    M  -->  O["Therapeutic<br/>Effect"]
    N  -->  O
    
    style A fill:#0a1929
    style O fill:#0e2e10

Delivery Methods for CNS

Therapeutic Targets in Neurodegeneration