Neurological Disorder CRISPR Knockout Models for Drug Discovery

Neurological disorders are the leading cause of disability and the second leading cause of death globally, according to the World Health Organization (WHO). In 2021, neurological conditions affected over 3.4 billion people worldwide. Alzheimer disease and other dementias affect approximately 55 million people, with nearly 10 million new cases each year. Parkinson disease affects over 8.5 million individuals. Stroke remains a major contributor, with 12.2 million new cases annually. The National Cancer Institute (NCI) reports that primary brain tumors (e.g., glioblastoma) have a 5-year survival rate of only 6.9% for glioblastoma multiforme. Key risk factors include aging, genetic predisposition (e.g., APOE4 for Alzheimer, GBA1 mutations for Parkinson), environmental toxins, and traumatic brain injury.

Neurological disorders are ideal for mechanistic studies due to their complex genetic heterogeneity and well-characterized subtypes. For example, Alzheimer disease is classified into early-onset familial (caused by mutations in APP, PSEN1, PSEN2) and late-onset sporadic (associated with APOE, TREM2, CLU). Parkinson disease includes familial forms (SNCA, LRRK2, PRKN, PINK1) and sporadic cases. Large public datasets such as the Alzheimer Disease Neuroimaging Initiative (ADNI), the Parkinson Progression Markers Initiative (PPMI), and the Allen Brain Atlas provide rich molecular and clinical data. Open questions include the role of neuroinflammation, protein aggregation mechanisms, and the contribution of non-coding genetic variants.

• • Amyloid-beta (A-beta) cascade in Alzheimer disease:

1. Sequential cleavage of amyloid precursor protein (APP) by beta-secretase (BACE1) and gamma-secretase (PSEN1/PSEN2) generates A-beta peptides.

2. A-beta monomers aggregate into oligomers and fibrils, forming senile plaques.

3. Plaques trigger microglial activation, oxidative stress, and tau hyperphosphorylation.

• • Tau pathology in tauopathies:

1. Hyperphosphorylation of tau protein (MAPT) leads to detachment from microtubules.

2. Misfolded tau aggregates into neurofibrillary tangles.

3. Tangles disrupt axonal transport and cause synaptic loss.

• • Alpha-synuclein aggregation in Parkinson disease:

1. Misfolding of alpha-synuclein (SNCA) forms Lewy bodies.

2. Aggregates impair proteasomal and autophagic pathways.

3. Mitochondrial dysfunction and oxidative stress amplify neuronal death.

Data from ClinVar, NCBI Gene, and published literature (e.g., Alzheimer Disease & Frontotemporal Dementia Mutation Database, PDGene).

• • Wnt/beta-catenin signaling: Dysregulated in Alzheimer disease; reduced Wnt signaling increases tau phosphorylation and A-beta production. Key nodes: beta-catenin, GSK3beta, LRP6.
• • MAPK/ERK pathway: Hyperactivated in tauopathies; ERK phosphorylates tau at disease-relevant sites. Key nodes: RAS, RAF, MEK, ERK.
• • PI3K/AKT/mTOR pathway: Impaired in Parkinson disease; reduced AKT activity leads to neuronal apoptosis. Key nodes: PTEN, AKT, mTOR, S6K.
• • Autophagy-lysosome pathway: Defective in GBA1-associated Parkinson and Huntington disease. Key nodes: Beclin1, LC3, p62, LAMP1.
• • Neuroinflammation signaling: Chronic microglial activation via TLR4, NLRP3 inflammasome, and NF-kB contributes to neurodegeneration. Key nodes: TLR4, MyD88, NLRP3, IL-1beta, TNF-alpha.

Organoids offer advantages: they model 3D tissue architecture, cell-cell interactions, and can be derived from patient iPSCs to study genetic variants in a human context.

• • Transgenic mouse models:
• • APP/PS1 mice (Alzheimer): Express human APP with Swedish mutation and PSEN1 with deltaE9 mutation; develop A-beta plaques and cognitive deficits.
• • LRRK2 G2019S transgenic mice (Parkinson): Show progressive motor deficits and dopaminergic neuron loss.
• • R6/2 mice (Huntington): Express exon 1 of human HTT with ~150 CAG repeats; exhibit motor dysfunction and striatal atrophy.
• • Induced models:
• • MPTP-treated mice (Parkinson): Toxin induces dopaminergic neuron death.
• • Kainic acid-induced seizure models (epilepsy).
• • PDX models: Rare for neurological disorders due to blood-brain barrier; used for glioblastoma (e.g., patient-derived xenografts in immunodeficient mice).

• • CRISPR-Cas9 technology enables the creation of isogenic cell lines that differ only in a specific genetic alteration, providing clean controls for functional studies. Examples include:
• • TP53 knockout in SH-SY5Y cells to study p53-dependent neurodegeneration.
• • APP Swedish knock-in in iPSC-derived neurons to model familial Alzheimer disease.
• • LRRK2 G2019S knock-in in LUHMES cells to study kinase-dependent toxicity.
• • HTT CAG repeat knock-in in HEK293T cells to model polyglutamine aggregation.
• • MAPT P301L knock-in in iPSC-derived neurons to study tau pathology.

Commercially available, sequence-verified gene-edited cell models accelerate research by eliminating the need for in-house editing and validation. These models are available from commercial sources and can be customized for specific mutations or reporter constructs (e.g., GFP-tagged SNCA).

• • CRISPR knockout and knock-in lines are used to validate the functional impact of genetic variants identified in GWAS and sequencing studies. For example:
• • Knockout of TREM2 in iPSC-derived microglia reduces phagocytosis and increases inflammatory cytokine release, confirming its role in Alzheimer risk.
• • Knock-in of LRRK2 G2019S in SH-SY5Y cells increases neurite shortening and autophagic defects, validating the mutation's pathogenicity.
• • Knockout of C9orf72 in iPSC-derived neurons leads to RNA foci formation and reduced survival, confirming haploinsufficiency as a disease mechanism.

• • Isogenic pairs (wild-type vs. mutant) enable high-throughput screening for compounds that selectively target mutant cells. Examples:
• • Screening for compounds that reduce A-beta production in APP Swedish knock-in neurons.
• • Identifying kinase inhibitors that rescue LRRK2 G2019S toxicity in isogenic LUHMES cells.
• • Testing antisense oligonucleotides (ASOs) that reduce HTT expression in Huntington knock-in cell lines.
• • Resistance modeling: Chronic treatment of isogenic lines with candidate drugs can identify resistance mechanisms (e.g., upregulation of efflux transporters).

• • CRISPR-engineered cells are used in synthetic lethality screens to identify genes that, when knocked out, selectively kill cells with a disease-associated mutation. For example:
• • In LRRK2 G2019S cells, knockout of GAK or DNAJC12 reduces cell viability, suggesting these as therapeutic targets.
• • In HTT CAG repeat cells, knockout of MSH3 or FAN1 modifies somatic instability, identifying potential biomarkers for Huntington progression.
• • Secretome analysis of isogenic APP mutant neurons identifies novel A-beta species and other secreted proteins as potential CSF biomarkers.