ALS Gene-Edited Cell Lines for Drug Discovery and Research

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder affecting motor neurons, with an estimated global incidence of 1.5 to 2.7 per 100,000 person-years (WHO, 2023). The disease has a median survival of 2 to 5 years from symptom onset, with only 10% of patients surviving beyond 10 years (NCI SEER data). Risk factors include age (peak onset 55-75 years), male sex, and genetic predisposition. Approximately 10% of cases are familial (fALS), while 90% are sporadic (sALS). The clinical impact is severe, with progressive muscle weakness, paralysis, and respiratory failure.

ALS is ideal for mechanistic studies due to its well-defined genetic subtypes (SOD1, C9orf72, TARDBP, FUS) and the availability of patient-derived iPSC lines and public datasets (e.g., TargetALS, AnswerALS). Key open questions include the role of TDP-43 aggregation, glutamate excitotoxicity, and neuroinflammation. Gene-edited cell models provide a controlled system to dissect these pathways and test therapeutic candidates.

ALS pathogenesis involves multiple interconnected pathways:

1. Protein aggregation and proteostasis dysfunction:

• • Misfolded SOD1 or TDP-43 forms cytoplasmic inclusions.
• • Impaired ubiquitin-proteasome and autophagy-lysosome systems.

2. RNA metabolism defects:

• • C9orf72 repeat expansions produce dipeptide repeat proteins (DPRs) and RNA foci.
• • TDP-43 and FUS mislocalization disrupts splicing and transport.

3. Oxidative stress and mitochondrial dysfunction:

• • SOD1 mutations reduce antioxidant capacity.
• • Mitochondrial fragmentation and impaired ATP production.

4. Glutamate excitotoxicity:

• • Reduced EAAT2 expression on astrocytes leads to excess synaptic glutamate.
• • Calcium overload triggers neuronal death.

Data from ClinVar and COSMIC (v99).

• • Autophagy and lysosomal signaling: Key nodes include SQSTM1/p62, UBQLN2, VCP. Mutations impair clearance of protein aggregates.
• • Stress granule dynamics: TDP-43 and FUS are components of stress granules; mutations promote aberrant phase transitions.
• • Neuroinflammation: Microglial activation via TREM2, CD33, and pro-inflammatory cytokines (TNF-alpha, IL-6).
• • Axonal transport: Defects in dynein/dynactin and kinesin motors (e.g., KIF5A mutations).

Organoid models (e.g., spinal cord organoids) recapitulate 3D cellular interactions and are increasingly used for drug screening.

• • SOD1 G93A transgenic mouse: Most widely used; develops progressive motor deficits.
• • C9orf72 BAC transgenic mice: Express human repeat expansions; show RNA foci and DPRs.
• • TDP-43 A315T transgenic mice: Develop TDP-43 pathology and motor dysfunction.
• • Zebrafish models: Used for high-throughput drug screening and genetic interaction studies.

• • CRISPR/Cas9-engineered isogenic cell lines provide precise genetic control for ALS research. Examples include:
• • TARDBP knockout lines (e.g., in SH-SY5Y or iPSC-derived neurons) to study loss-of-function.
• • SOD1 A4V knock-in lines to model gain-of-toxicity.
• • C9orf72 repeat expansion knock-in lines to study DPR toxicity and RNA foci.

Commercially available, sequence-verified models accelerate research by eliminating the need for de novo editing. These lines are validated by Sanger sequencing and functional assays (e.g., protein expression, aggregation).

• • Knockout and knock-in lines enable validation of ALS-associated genes. For example:
• • TARDBP knockout in iPSC-derived neurons reveals altered splicing and stress granule dynamics.
• • C9orf72 knockout models help distinguish loss-of-function from gain-of-toxicity mechanisms.
• • FUS P525L knock-in lines confirm cytoplasmic mislocalization.

Isogenic pairs (e.g., wild-type vs. SOD1 A4V) are used for high-content screening to identify compounds that reduce aggregation or oxidative stress. Resistance mechanisms can be modeled by chronic drug exposure in mutant lines.

CRISPR screens (e.g., synthetic lethality) identify genes that, when knocked out, selectively kill mutant cells. This approach has revealed potential targets like ATXN2 and KPNB1 in C9orf72 ALS.