Alzheimer’s disease (AD) is an incurable neurodegenerative disease and the most common form of dementia.
Research is ongoing to better understand AD pathogenesis and explore potential treatments.
Genome editing tools like CRISPR/Cas9 offer new approaches to study and potentially treat AD.
Key Facts:
- AD pathogenesis likely involves multiple genetic and environmental factors
- Gene mutations linked to early-onset familial AD forms are known (APP, PSEN1, PSEN2)
- Genome editing allows the study of mutations and correction of deficits in cellular/animal models
- Editing mutations in patient-derived neurons may eventually treat early-onset AD
- Editing risk genes like APOE could help understand and prevent late-onset AD
Source: Frontiers in Genome Editing
The Genetic Basis of Alzheimer’s Disease
AD is characterized by progressive cognitive decline along with hallmark brain changes like amyloid-beta plaque deposits and neurofibrillary tangles containing tau protein.
Most AD cases are late-onset, likely caused by a combination of genetic, lifestyle, and environmental factors.
Early-onset familial AD forms involve mutations in the APP, PSEN1, or PSEN2 genes.
These mutations alter amyloid precursor protein processing or the activity of γ-secretase, resulting in increased production of aggregation-prone amyloid-beta.
Genome-wide association studies (GWAS) have uncovered additional AD risk loci in genes related to lipid metabolism and the immune system.
However, much remains unknown about the functional roles of AD-linked mutations and risk alleles.
Modeling mutations in vitro and in vivo using genome editing tools provides a powerful approach to elucidate genetic contributors to AD pathogenesis.
Using Genome Editing to Model AD Mutations
Technologies like zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and the CRISPR/Cas9 system allow precise editing of cellular DNA sequences.
These tools have rapidly advanced AD research by enabling scientists to recreate known mutations in cell lines and animal models.
Several studies have used genome editing to insert AD-causing mutations into induced pluripotent stem cells (iPSCs) or embryonic stem cells and differentiate them into neurons.
Analyzing isogenic mutant and wild-type neurons derived from the same patient sample facilitates the study of mutation-specific effects on amyloid-beta production, tau phosphorylation, gene expression changes and neuronal survival.
In vivo editing has also created animal models for probing AD mutations.
Mice with CRISPR/Cas9-edited mutations in SORL1 and PSEN1 genes exhibit AD-like traits including cognitive deficits and higher brain amyloid levels.
Generating additional rodent models will continue elucidating genetic risk factors that may one day be targeted to prevent or slow human AD.
Developing CRISPR/Cas9 Gene Therapy for Alzheimer’s Disease
Beyond modeling disease, genome editing tools may eventually treat AD patients by correcting mutations that cause familial forms.
As a proof-of-concept, researchers used CRISPR/Cas9 in fibroblasts from an AD patient to successfully disrupt the mutant APP gene responsible for elevated amyloid-beta production.
Gene therapy vectors based on adeno-associated virus (AAV) have also delivered CRISPR/Cas9 components to edit the mutant APP allele and reduce amyloid levels in mouse models.
Additionally, a catalytically inactive Cas9 fused to a cytidine deaminase was able to convert an AD-risk variant of APOE4 into the neutral APOE3 allele in rat astrocytes.
These preliminary gene therapy studies provide hope that directly editing neuron genomes could help AD patients, especially those with known familial mutations.
However, many challenges remain before clinical applications.
Off-target editing and low editing efficiencies must be addressed.
Safe, non-inflammatory delivery methods are also needed to distribute editing tools throughout the aging brain.
Specific Gene Editing Strategies to Treat Alzheimer’s Disease
The application of gene editing in Alzheimer’s disease (AD) treatment focuses on altering specific genes implicated in the disease’s pathology.
Here we delve into the detailed mechanisms through which gene editing, particularly using CRISPR-Cas9 technology, could be employed.
Targeting Amyloid Precursor Protein (APP) Gene
- Background: Mutations in the APP gene can lead to the overproduction of amyloid-beta peptides, forming plaques that are a hallmark of AD.
- Gene Editing Approach: CRISPR-Cas9 can be used to precisely target and modify the sequence of the APP gene. By correcting mutations or altering regulatory regions of the gene, the production of amyloid-beta can be reduced.
- Mechanism: Guide RNA (gRNA) designed to match the APP gene sequence directs the Cas9 enzyme to the specific DNA location. Cas9 induces a double-strand break, triggering the cell’s repair mechanisms. By providing a template DNA, researchers can encourage the cell to repair the break with a corrected sequence, effectively reducing harmful amyloid-beta production.
Modifying Presenilin Genes (PSEN1 and PSEN2)
- Background: Mutations in PSEN1 and PSEN2 are associated with early-onset familial Alzheimer’s. These genes encode subunits of the gamma-secretase complex, which cleaves the APP, leading to amyloid-beta formation.
- Gene Editing Approach: CRISPR-Cas9 can be applied to edit PSEN1 and PSEN2 genes, aiming to reduce their pathological activity without disrupting their normal function (which is crucial for cell signaling).
- Mechanism: Similar to APP, gRNAs target the specific mutations in PSEN1 and PSEN2. The Cas9 enzyme then creates a break, which is repaired using a donor template carrying the normal gene sequence. This approach aims to restore normal gamma-secretase function, reducing abnormal amyloid-beta production.
Tau Protein Modification
- Background: Abnormal accumulation of tau proteins leads to neurofibrillary tangles, another key feature of AD.
- Gene Editing Approach: Gene editing could be used to modify the MAPT gene, which encodes the tau protein, to prevent or reduce the formation of these tangles.
- Mechanism: CRISPR-Cas9 targets the MAPT gene, introducing mutations that reduce the propensity of tau to form pathological aggregates. This could involve altering phosphorylation sites on the tau protein, which are critical for its aggregation.
Enhancing Cellular Clearance Mechanisms
- Background: Impaired clearance of amyloid-beta and tau aggregates contributes to AD progression.
- Gene Editing Approach: Genes involved in autophagy and proteasomal degradation pathways could be targeted to enhance the cell’s ability to clear these aggregates.
- Mechanism: By editing genes that encode key proteins in these pathways, their activity or expression levels can be increased, potentially enhancing the clearance of pathological proteins.
Challenges and Considerations
- Delivery: Efficient and safe delivery of CRISPR-Cas9 components to the brain remains a major challenge. Potential strategies include viral vectors, nanoparticles, or direct injection.
- Specificity and Off-target Effects: Ensuring that CRISPR-Cas9 edits only the intended genes without affecting others is crucial to prevent unintended consequences.
- Ethical and Regulatory Issues: Gene editing in humans, especially in the brain, raises significant ethical questions and requires rigorous regulatory scrutiny.
Using CRISPR Screens to Clarify AD Mechanisms
In addition to modeling specific mutations, unbiased CRISPR interference and activation screens assess gene functions genome-wide.
By individually inhibiting or overexpressing thousands of genes, researchers can determine which ones affect key AD phenotypes.
For example, one study identified genes that control tau aggregate propagation by screening a human tauopathy model.
Multiple hits converged on the endolysosomal pathway, suggesting it facilitates release of tau aggregates that spread disease between cells.
A separate screen highlighted genes influencing amyloid-beta uptake through phagocytosis.
Network analysis of screen hits then connected risk genes like INPP5D and CD2AP to the endocytic machinery disturbances underlying amyloid accumulation.
These experiments demonstrate how CRISPR screens provide unsupervised clues about disease mechanisms.
Future screens assessing neuron health, neuroinflammation, and other facets of AD pathology will uncover more potential drug targets.
Final Thoughts: Gene Editing for Alzheimer’s Disease
Despite rapid progress studying and modeling AD with genome editing tools, many hurdles remain translating these findings into clinical treatments.
Ethical considerations limit editing human embryos to prevent heritable AD mutations.
The blood-brain barrier complicates delivery of editing components to the brain.
Off-target effects and low editing efficiency in post-mitotic neurons also pose challenges.
Nonetheless, combining precise genome surgery with stem cell models and unbiased CRISPR screens provides an exciting multi-pronged attack against AD’s genetic underpinnings.
Optimizing editing tools to safely disrupt AD mutations holds clinical promise, while screens identify novel drug targets by linking risk genes to disease pathways.
Though no magic bullet for this complex disorder yet exists, editing technologies move us toward demystifying AD genetics and developing genetics-based treatments.
References
- Paper: Gene Editing and Alzheimer’s Disease: Is there Light at the end of the Tunnel? (2020)
- Authors: Mikhail Stepanichev