Huntington disease Cell Models for Research

Huntington disease (HD) is a rare, autosomal dominant neurodegenerative disorder caused by an expanded CAG repeat in the HTT gene. The global prevalence is estimated at 2.7 per 100,000 (WHO, 2023). Onset typically occurs in mid-adulthood (30-50 years), with a median survival of 15-20 years after symptom onset. The disease is characterized by progressive motor dysfunction, cognitive decline, and psychiatric disturbances. There is no cure, and current treatments only manage symptoms. The economic burden is significant, with costs related to long-term care and loss of productivity.

HD is an ideal model for studying neurodegeneration because it is monogenic, with a clear genotype-phenotype correlation. The CAG repeat length inversely correlates with age of onset, providing a natural experiment. Research focuses on understanding the toxic gain-of-function of mutant huntingtin (mHTT), as well as loss of normal HTT function. Public datasets, such as those from the HDinHD (Huntington's Disease in High Definition) project, provide extensive omics data. Open questions include the role of somatic instability, cell-type-specific vulnerability, and the contribution of glial dysfunction.

The primary pathogenic mechanism is the toxic gain-of-function of mutant huntingtin (mHTT) protein, which misfolds and aggregates. Key pathways include:

1. Proteostasis disruption: mHTT impairs the ubiquitin-proteasome system and autophagy, leading to accumulation of damaged proteins.

2. Mitochondrial dysfunction: mHTT interacts with mitochondria, causing impaired electron transport, increased reactive oxygen species, and reduced ATP production.

3. Transcriptional dysregulation: mHTT sequesters transcription factors (e.g., CREB, SP1), altering expression of genes involved in neuronal survival.

4. Excitotoxicity: mHTT sensitizes neurons to glutamate-induced calcium overload, leading to cell death.

5. Axonal transport defects: mHTT disrupts kinesin and dynein motors, impairing trafficking of vesicles and organelles.

Note: HD is monogenic; the HTT CAG repeat expansion is the sole cause. Other genetic modifiers (e.g., in MSH3, FAN1) influence age of onset but are not disease-causing.

mHTT disrupts multiple signaling networks:

• • BDNF/TrkB signaling: Reduced BDNF production and retrograde transport, leading to decreased neuronal survival.
• • MAPK/ERK pathway: Altered phosphorylation and activation, affecting cell survival and stress responses.
• • PI3K/AKT pathway: Impaired signaling, leading to reduced cell survival and increased apoptosis.
• • Wnt signaling: Disrupted, affecting neurogenesis and synaptic plasticity.
• • Calcium signaling: mHTT sensitizes IP3 and ryanodine receptors, causing aberrant calcium release and excitotoxicity.

Organoids: 3D brain organoids derived from HD patient iPSCs recapitulate early neurodevelopmental phenotypes, including impaired neurogenesis and mitochondrial dysfunction. They offer a more physiologically relevant platform than 2D cultures.

Animal models are essential for studying HD pathogenesis and testing therapies.

• • R6/1 and R6/2 mice: Transgenic models expressing exon 1 of human HTT with 115-150 CAG repeats. They exhibit rapid motor deficits and neuropathology.
• • YAC128 and BACHD mice: Yeast/bacterial artificial chromosome models expressing full-length human HTT with 128 or 97 CAG repeats. They show progressive motor and cognitive deficits.
• • zQ175 knock-in mice: Targeted insertion of ~175 CAG repeats into the mouse Htt gene. They display progressive behavioral and neuropathological changes.
• • Non-human primates: Transgenic macaques expressing mHTT have been developed, but their use is limited by cost and ethical concerns.
• • Drosophila and C. elegans: Simple models for genetic screens and high-throughput drug testing.

CRISPR-based gene editing has revolutionized HD research by enabling the creation of isogenic cell lines with precise genetic modifications. These models are essential for dissecting the contribution of specific mutations and for drug discovery.

• • HTT knockout lines: Complete removal of HTT to study loss-of-function effects. These are used to understand the normal function of huntingtin and to identify synthetic lethal interactions.
• • HTT knock-in lines: Introduction of expanded CAG repeats into the endogenous HTT locus, creating physiologically relevant models. For example, a knock-in of 72 CAG repeats in HEK293 cells recapitulates mHTT aggregation and toxicity.
• • Reporter lines: HTT tagged with fluorescent proteins (e.g., GFP) to monitor protein localization and aggregation in real time.
• • Commercially available, sequence-verified models: These are generated using CRISPR and validated by Sanger sequencing and functional assays. They accelerate research by providing reliable, ready-to-use tools. It is important to choose models that are isogenic to avoid confounding effects from genetic background.

Gene-edited cell models are used to validate the function of genes implicated in HD. For example:

• • HTT knockout lines are used to identify genes that become essential when HTT is lost, revealing synthetic lethal interactions that could be targeted therapeutically.
• • Knock-in lines with different CAG repeat lengths are used to study the dose-dependent effects of repeat expansion on cellular phenotypes, such as mitochondrial dysfunction and oxidative stress.
• • CRISPR screens in isogenic backgrounds can identify modifiers of mHTT toxicity, providing novel drug targets.

Isogenic cell line pairs (e.g., HTT knockout vs. wild-type) are powerful tools for drug screening. They allow for the identification of compounds that specifically kill mutant cells while sparing normal cells. This approach is used to discover drugs that target mHTT toxicity or exploit vulnerabilities created by HTT loss. Additionally, gene-edited lines can be used to model resistance to therapies, such as the development of resistance to antisense oligonucleotides (ASOs) that lower HTT levels.

CRISPR-engineered cells are used to identify biomarkers for HD progression and response to therapy. For example, secretome analysis of isogenic lines with different CAG repeats can reveal proteins that are differentially secreted, which may serve as biomarkers in cerebrospinal fluid (CSF) or blood. Additionally, synthetic lethality screens can identify genetic biomarkers that predict sensitivity to specific drugs.