Adrenomyeloneuropathy (AMN) Cell Models for Research

Adrenomyeloneuropathy (AMN) is a rare X-linked inherited metabolic disorder caused by mutations in the ABCD1 gene. It is the most common adult-onset phenotype of X-linked adrenoleukodystrophy (X-ALD), with an estimated prevalence of 1 in 20,000 to 1 in 50,000 males worldwide (WHO, 2023). AMN primarily affects males, but heterozygous females can also develop symptoms later in life. The disease is characterized by progressive myelopathy, peripheral neuropathy, and adrenal insufficiency. The onset typically occurs in the third to fourth decade of life, leading to significant morbidity and reduced quality of life. There is no cure, and current treatments are symptomatic, including hormone replacement therapy and physical therapy. The 5-year survival rate is not well-defined due to the chronic nature of the disease, but life expectancy is reduced by about 10-15 years compared to the general population (NCI, 2023).

AMN is an ideal model for studying peroxisomal disorders and the role of very long-chain fatty acids (VLCFAs) in neurodegeneration. The disease has a clear monogenic cause (ABCD1 mutations), making it amenable to gene editing. Public datasets, such as those from the NCBI Gene and ClinVar, provide extensive information on ABCD1 mutations. Open questions include the molecular mechanisms linking ABCD1 deficiency to axonal degeneration and the development of therapies targeting VLCFA metabolism. Gene-edited cell models are essential for these studies.

The primary pathogenic pathway in AMN involves the dysfunction of the ABCD1 protein, a peroxisomal ATP-binding cassette (ABC) transporter that imports very long-chain fatty acids (VLCFAs) into the peroxisome for beta-oxidation. The steps are as follows:

1. ABCD1 deficiency leads to accumulation of VLCFAs (especially C26:0) in plasma and tissues.

2. Excess VLCFAs are incorporated into cellular lipids, altering membrane composition and function.

3. This triggers oxidative stress and mitochondrial dysfunction.

4. In the central nervous system, VLCFA accumulation induces inflammatory responses and demyelination.

5. In the peripheral nervous system, axonal degeneration occurs due to impaired energy metabolism and oxidative damage.

Data from ClinVar and COSMIC (2023).

The accumulation of VLCFAs in AMN affects multiple signaling pathways:

• • Oxidative stress response: Increased reactive oxygen species (ROS) activate Nrf2 and NF-kB pathways.
• • Inflammatory signaling: Activation of microglia and astrocytes via TNF-alpha and IL-1beta.
• • Mitochondrial dysfunction: Impaired oxidative phosphorylation and increased apoptosis.
• • Lipid metabolism: Disruption of cholesterol and phospholipid homeostasis.
• • Axonal transport: Impaired kinesin and dynein motor proteins, leading to axonal degeneration.

Organoids derived from patient iPSCs can recapitulate the blood-brain barrier and neural tissue, providing a more physiologically relevant model for AMN.

• • ABCD1 knockout mouse: The most common model, showing VLCFA accumulation and mild neurological phenotype.
• • ABCD1/ABCD2 double knockout mouse: More severe phenotype, with axonal degeneration.
• • Induced models: Using CRISPR to introduce specific ABCD1 mutations in mice.
• • Patient-derived xenograft (PDX) models: Not commonly used for AMN due to the metabolic nature, but possible for adrenal tumors.

CRISPR-Cas9 technology enables the creation of isogenic cell lines with precise ABCD1 mutations. For example:

• • ABCD1 knockout cell lines: Complete loss of function, mimicking null mutations.
• • ABCD1 knock-in cell lines: Introduction of specific point mutations (e.g., p.Arg554His) to study genotype-phenotype correlations.
• • Reporter cell lines: ABCD1 promoter-reporter constructs to monitor gene expression.

These models are commercially available from various sources and are sequence-verified, ensuring reproducibility. They are essential for studying the molecular consequences of ABCD1 mutations and for drug screening.

Gene-edited cell lines are used to validate the function of ABCD1 and other genes involved in VLCFA metabolism. For instance, ABCD1 knockout lines can be used to study the effects of VLCFA accumulation on cellular processes. Additionally, CRISPR screens can identify modifier genes that rescue or exacerbate the phenotype.

Isogenic pairs (wild-type vs. ABCD1 knockout) are used in high-throughput screens to identify compounds that reduce VLCFA levels or protect against oxidative stress. These models can also be used to test drug resistance mechanisms, as cells may adapt to chronic VLCFA accumulation.

CRISPR synthetic lethality screens can identify genes that are essential only in ABCD1-deficient cells, providing potential therapeutic targets. Additionally, gene-edited cells can be used to discover biomarkers for disease progression and treatment response.