Abetalipoproteinemia (ABL) Cell Models for Research

Abetalipoproteinemia (ABL) is an extremely rare autosomal recessive disorder, with an estimated prevalence of less than 1 in 1,000,000 individuals worldwide. The exact incidence is unknown due to underdiagnosis, but it is more common in populations with consanguinity. The disease is caused by mutations in the MTTP gene, leading to a deficiency in microsomal triglyceride transfer protein, which is essential for the assembly and secretion of apolipoprotein B-containing lipoproteins (chylomicrons, VLDL, LDL). Clinically, patients present in infancy with fat malabsorption, failure to thrive, and steatorrhea. Later, they develop progressive neurological complications (ataxia, peripheral neuropathy), retinitis pigmentosa, and acanthocytosis. Without treatment, the disease leads to severe morbidity and early mortality. However, with early diagnosis and dietary management (low-fat diet, fat-soluble vitamin supplementation), many complications can be prevented, and life expectancy can be near-normal. The disease is a model for understanding lipid metabolism and the role of lipoproteins in health and disease.

ABL is an ideal model for studying lipid metabolism, lipoprotein assembly, and the molecular mechanisms underlying fat-soluble vitamin transport. The disease is monogenic, making it amenable to gene editing and mechanistic studies. Key research questions include: How does MTTP deficiency affect cellular lipid homeostasis? What are the downstream effects on signaling pathways? How can we develop therapies to restore MTTP function or bypass its deficiency? Gene-edited cell models, such as MTTP knockout cell lines, provide a valuable tool to address these questions. Additionally, the disease's neurological and retinal manifestations offer a window into the role of lipoproteins in neuronal and retinal health. Public datasets, such as those in ClinVar and gnomAD, provide mutation information, while DepMap offers functional data on MTTP dependency in various cell lines.

Although ABL is not a cancer, its molecular pathways are relevant to metabolic diseases and potential cancer links. The primary pathway is the MTTP-mediated assembly of apolipoprotein B-containing lipoproteins. Steps include:

1. MTTP transfers lipids (triglycerides and cholesteryl esters) to apolipoprotein B (APOB) in the endoplasmic reticulum.

2. This lipidation is essential for the proper folding and secretion of APOB.

3. In the absence of MTTP, APOB is degraded, leading to a lack of VLDL and chylomicron secretion.

Additionally, MTTP is involved in the regulation of cellular lipid homeostasis, including the activation of the unfolded protein response (UPR) and the modulation of lipid droplet formation. These pathways may intersect with oncogenic signaling, as lipid metabolism is increasingly recognized as a driver of cancer progression.

Data from ClinVar and gnomAD indicate that MTTP mutations are highly heterogeneous, with no common founder mutation. The functional effect is typically a complete loss of MTTP activity, leading to the clinical phenotype.

MTTP deficiency affects multiple signaling networks:

• • Lipid metabolism: Impaired assembly and secretion of lipoproteins, leading to intracellular lipid accumulation and altered lipid signaling.
• • Unfolded protein response (UPR): Accumulation of unfolded APOB in the ER triggers ER stress and UPR activation, which can lead to apoptosis.
• • Inflammatory signaling: Chronic ER stress can activate NF-κB and other inflammatory pathways.
• • Retinoic acid signaling: Vitamin A deficiency due to impaired transport may affect retinoic acid signaling, which is crucial for neuronal and retinal function.

Key nodes include: MTTP, APOB, SREBP1/2, PPARα, and LXR.

Organoids derived from patient-derived induced pluripotent stem cells (iPSCs) can recapitulate intestinal and hepatic lipid metabolism, providing a more physiologically relevant model. However, they are more complex and less scalable than cell lines.

• • Mttp knockout mice: Global knockout is embryonic lethal, but conditional knockouts in liver or intestine have been generated to study tissue-specific effects.
• • Mttp hypomorphic mice: These mice have reduced MTTP activity and exhibit some features of ABL, such as hypolipidemia and fat malabsorption.
• • ApoB knockout mice: These mice lack APOB and have a similar phenotype to ABL.
• • Zebrafish models: Morpholino or CRISPR-mediated knockdown of mttp has been used to study lipid metabolism and development.

These models are valuable for studying the systemic effects of MTTP deficiency, but they are not suitable for high-throughput screening.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with precise mutations in MTTP or other genes. For example:

• • MTTP knockout cell lines: These lines have a complete loss of MTTP function, mimicking the genetic defect in ABL. They are useful for studying the cellular consequences of MTTP deficiency and for screening compounds that might bypass the defect.
• • MTTP point mutation knock-in lines: These lines carry specific patient-derived mutations, allowing for the study of genotype-phenotype correlations.
• • APOB knockout or mutant lines: These lines can be used to study the interaction between MTTP and APOB.

Commercially available, sequence-verified gene-edited cell models accelerate research by providing consistent and validated tools. They are essential for functional genomics, drug screening, and biomarker discovery.

Gene-edited cell lines are used to validate the function of genes involved in lipid metabolism. For example, MTTP knockout lines can be used to confirm the role of MTTP in lipoprotein secretion and to identify downstream effectors. Additionally, they can be used in CRISPR screens to identify genes that modify the phenotype, such as genes that rescue or exacerbate the defect.

Isogenic pairs (wild-type vs. MTTP knockout) are ideal for drug screening. They allow for the identification of compounds that specifically target MTTP-deficient cells, which could be developed as therapies for ABL. Additionally, they can be used to study drug resistance mechanisms, as the knockout cells may exhibit altered sensitivity to lipid-lowering drugs.

CRISPR synthetic lethality screens can identify genes that are essential for the survival of MTTP-deficient cells. These genes could serve as biomarkers or therapeutic targets. For example, if a gene is specifically required for the survival of MTTP knockout cells, inhibiting that gene might selectively kill the defective cells.