Smith-Lemli-Opitz Syndrome (SLOS) Cell Models for Research

Smith-Lemli-Opitz Syndrome (SLOS) is a rare autosomal recessive disorder caused by mutations in the DHCR7 gene, leading to a deficiency in 7-dehydrocholesterol reductase and impaired cholesterol biosynthesis. The estimated incidence is approximately 1 in 20,000 to 1 in 40,000 live births, with higher prevalence in certain populations of European descent (WHO, 2023). Clinical manifestations include multiple congenital anomalies, intellectual disability, and behavioral problems. The severity varies widely, with some individuals having mild symptoms and others having life-threatening malformations. The 5-year survival rate is not well-defined due to rarity, but severe cases often have poor prognosis (NCI, 2023).

SLOS serves as an excellent model for studying cholesterol metabolism, neurodevelopment, and teratogenesis. The disease is monogenic, making it amenable to gene editing. Researchers use patient-derived fibroblasts and induced pluripotent stem cells (iPSCs) to study the molecular mechanisms. Public datasets, such as those in ClinVar and the Human Gene Mutation Database, provide extensive variant information. Open questions include the genotype-phenotype correlation and the role of cholesterol in brain development.

Although SLOS is not a cancer, its pathways are relevant to cholesterol metabolism and signaling. The primary defect is in the cholesterol biosynthesis pathway, specifically the conversion of 7-dehydrocholesterol (7-DHC) to cholesterol. This leads to accumulation of 7-DHC and its derivatives, which can disrupt cellular membranes and signaling.

• • Cholesterol biosynthesis pathway: DHCR7 catalyzes the final step. Deficiency leads to reduced cholesterol and increased 7-DHC.
• • Hedgehog signaling: Cholesterol is essential for the modification of Hedgehog proteins, which are critical for embryonic patterning. Impaired cholesterol leads to disrupted Hedgehog signaling.
• • Oxysterol signaling: 7-DHC derivatives can act as oxysterols, affecting liver X receptor (LXR) and other nuclear receptors.

Data from ClinVar and NCBI Gene (2023).

The main deregulated networks include:

• • Hedgehog signaling: Reduced cholesterol impairs the processing and secretion of Sonic Hedgehog (SHH), affecting downstream targets like GLI1 and PTCH1.
• • PI3K/AKT/mTOR: Cholesterol is a component of lipid rafts, which are important for receptor signaling. Reduced cholesterol may alter this pathway.
• • Wnt signaling: Cholesterol modifications are also involved in Wnt protein secretion and signaling.
• • Oxysterol signaling: Accumulated 7-DHC derivatives can activate LXR and other nuclear receptors, altering gene expression.

Organoids, such as cerebral organoids derived from SLOS iPSCs, provide a 3D model to study neurodevelopmental defects.

• • Dhcr7 knockout mice: These mice exhibit embryonic lethality, but heterozygous mice show some features.
• • Induced models: Treatment with AY9944, an inhibitor of DHCR7, can induce SLOS-like phenotypes in rodents.
• • Patient-derived xenografts (PDX) are not commonly used due to the non-cancer nature, but organoid xenografts can be used for drug testing.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with specific DHCR7 mutations. For example, a DHCR7 knockout in HEK293T cells can be used to study cholesterol biosynthesis. A knock-in of a common mutation, such as c.964-1G>C, can model patient-specific variants. These sequence-verified models are commercially available from various sources and accelerate research by providing consistent, reproducible systems.

Knockout and knock-in cell lines are used to validate the function of DHCR7 and other genes in cholesterol metabolism. For example, a DHCR7 knockout line can be used to confirm the accumulation of 7-DHC and the rescue by cholesterol supplementation. Knock-in lines with specific mutations can be used to study genotype-phenotype correlations.

Isogenic pairs (wild-type vs. DHCR7 knockout) can be used to screen for compounds that rescue cholesterol synthesis or reduce 7-DHC accumulation. These models are also useful for testing the efficacy of potential therapies, such as statins or cholesterol supplementation.

CRISPR-based synthetic lethality screens can identify genes that are essential in DHCR7-deficient cells, providing potential therapeutic targets. Additionally, gene-edited cells can be used to identify biomarkers for disease severity or response to treatment.