Cerebrotendinous Xanthomatosis (CTX) Cell Models for Research

Cerebrotendinous Xanthomatosis (CTX) is a rare autosomal recessive disorder of bile acid synthesis caused by mutations in the CYP27A1 gene. The global incidence is estimated at 1 in 50,000 to 1 in 100,000, but it may be underdiagnosed. Clinical manifestations include chronic diarrhea, cataracts, tendon xanthomas, and progressive neurological dysfunction (e.g., cognitive impairment, ataxia, spasticity). Without treatment, the disease leads to severe disability and premature death. Early diagnosis and treatment with chenodeoxycholic acid can reverse many symptoms, highlighting the importance of early detection and research into disease mechanisms.

CTX is an ideal model for studying bile acid metabolism, cholesterol homeostasis, and mitochondrial dysfunction. The disease is monogenic, making it amenable to gene editing. Public datasets, such as those in ClinVar and the Human Gene Mutation Database, provide a wealth of genetic variants. Open questions include the precise pathophysiology of neurological damage and the development of targeted therapies beyond bile acid replacement. Gene-edited cell models enable mechanistic studies and drug screening in a controlled genetic background.

The primary defect in CTX is a deficiency of the mitochondrial enzyme sterol 27-hydroxylase (CYP27A1), which is critical in bile acid synthesis. This leads to:

• • Accumulation of cholestanol and bile acid intermediates (e.g., 7α-hydroxy-4-cholesten-3-one) in tissues, particularly the brain, tendons, and lens.
• • Impaired cholesterol metabolism and altered bile acid synthesis, resulting in reduced chenodeoxycholic acid production.
• • Mitochondrial dysfunction and oxidative stress, contributing to neuronal damage.
• • Disrupted myelin formation and maintenance in the central nervous system.

Data from ClinVar and the Human Gene Mutation Database indicate that over 200 mutations have been reported, with no single mutation predominating.

The metabolic disturbances in CTX affect multiple signaling pathways:

• • Bile acid signaling: Reduced chenodeoxycholic acid leads to altered farnesoid X receptor (FXR) activation, impacting lipid and glucose metabolism.
• • Cholesterol sensing: Accumulation of cholestanol disrupts sterol regulatory element-binding protein (SREBP) pathways, affecting cholesterol homeostasis.
• • Mitochondrial function: CYP27A1 deficiency impairs mitochondrial electron transport, increasing reactive oxygen species (ROS) and triggering apoptosis.
• • Neuroinflammation: Elevated bile acid intermediates activate microglia and promote neuroinflammation, contributing to neurodegeneration.

Organoids derived from patient iPSCs or genetically engineered to carry CYP27A1 mutations offer a more physiologically relevant model for studying tissue-specific effects, especially in the liver and brain.

• • CYP27A1 knockout mice: These mice exhibit biochemical and neurological features of CTX, including elevated cholestanol and impaired bile acid synthesis.
• • Induced models: Treatment with bile acid intermediates can induce CTX-like phenotypes in wild-type animals.
• • Patient-derived xenografts (PDX): Not commonly used for CTX due to the metabolic nature of the disease, but organoid xenografts can be used for drug testing.

CRISPR-based gene editing allows the creation of isogenic cell lines with specific CYP27A1 mutations, such as knockout or point mutations. These models provide a controlled genetic background to study the effects of mutations on cellular function and drug response. For example, a CYP27A1 knockout cell line can be used to screen for compounds that rescue enzyme activity or reduce toxic metabolite accumulation. Sequence-verified, commercially available gene-edited cell lines accelerate research by eliminating the need for time-consuming model generation.

Gene-edited cell lines enable functional validation of CYP27A1 mutations. By comparing wild-type and knockout/knock-in lines, researchers can assess the impact of specific mutations on enzyme activity, bile acid production, and cellular phenotypes. This helps in understanding genotype-phenotype correlations and identifying modifier genes.

Isogenic pairs (wild-type vs. mutant) are invaluable for high-throughput screening of potential therapeutics. For CTX, compounds that upregulate alternative pathways or reduce cholestanol accumulation can be tested. Additionally, gene-edited lines can be used to study resistance mechanisms to existing therapies, such as chenodeoxycholic acid.

CRISPR-engineered cell lines can be used in synthetic lethality screens to identify genes that, when silenced, are lethal only in CYP27A1-deficient cells. This can reveal novel therapeutic targets and biomarkers for early diagnosis or monitoring of disease progression.