Overview 

Tangier disease is a rare autosomal recessive genetic disease, caused due to a reduction or absence of high-density lipoprotein (HDL) in the blood – also known as ‘severe HDL deficiency’. This condition was first found in individuals from Tangier Island, Virginia. Since then, approximately 100 cases have been identified worldwide.^1,2 

Lipoproteins transport lipids and cholesterol between cells. HDL is a ‘good cholesterol’ that removes cholesterol from the tissues via blood, to the liver. The cholesterol is broken down in the liver, to form bile. Due to the accumulation of cholesterol, patients with Tangier disease are at a high risk for various cardiovascular diseases.²

There isn't a cure yet for this disease, and current therapeutic approaches aren't capable of treating it. Pharmacological treatments, such as raising HDL levels using lipid-lowering agents like niacin, have become ineffective for Tangier disease due to the dysfunctional ABC transporter A1.³ 

Considering all these issues, genetic correction emerges as a promising therapeutic solution. By targeting and rectifying the ABCA1 transporter, efflux of the cholesterol can be normalised. This can enhance the cholesterol absorption within the cells. Thus, the lipid accumulation in the body can be reduced. This can be achieved by either:³

• Increasing cholesterol uptake by the liver
• Reducing HDL metabolism

Research in this area enhances the understanding of lipid metabolism and the role of ABCA1 gene in cholesterol exchange. Additionally, success in treating Tangier disease can lead to treatment of other lipid-related genetic disorders.³

Understanding Tangier disease

HDL helps in reverse cholesterol transport (RCT) by removing cholesterol from tissues and vessel walls. This ensures proper excretion of cholesterol. HDL contains cholesterol and phospholipids with apolipoprotein A-1 (apoA-I) as the core protein. The cholesterol is primarily formed through the secretion of lipid-poor apoA-I in:⁴

• The liver (70%)
• The intestines (30%)

Apo-A1 interacts with the ATP-binding cassette (ABCA1) transporter to acquire phospholipids and cholesterol. It finally forms a nascent pre-beta (β) HDL. Dysfunctional ABCA1, as seen in Tangier disease, leads to cholesterol buildup in macrophages which causes diseases like atherosclerosis, cancer, and Alzheimer's disease.^5,6,7

The symptoms and complications related to this disease include:³

• Oral cavity: The first noticeable symptom is the cholesterol disposition in the tonsils, causing tonsil enlargement (in children and young adults) and colour change (they appear yellow or orange)
• Cardiovascular system: Affected individuals develop premature atherosclerosis. This increases their risk of coronary artery disease, early strokes, and sometimes heart valve diseases
• Nervous system: Cholesterol buildup in Schwann cells leads to nerve demyelination. This can cause diseases like:
  • Syringomyelia-like neuropathy
  • Multifocal neuropathy
  • Distal symmetric polyneuropathy
• Cornea: Mild corneal clouding without vision loss
• Reticuloendothelial system: Cholesterol accumulation in organs, such as liver and spleen causes:
  • Hepatosplenomegaly
  • Thrombocytopenia
  • Lymph node involvement
• Pancreas: Cholesterol deposits in pancreatic Alpha cells have been linked to diabetes
• Hematologic manifestations: Patients may experience:
  • Reticulocytosis
  • Haemolytic anaemia
  • Stomatocytosis

Gene therapy as a potential cure

Gene therapy cures genetic disorders by replacing or modifying defective genes. The gene can be edited by using a molecular vehicle called a vector. These vectors can be of two types: 

• Viral 
• Non-viral 

Viral vectors are more effective in transferring the genetic material. In contrast, non-viral vectors may cause lower immune response and larger gene capacity. Recent advancements on gene editing like CRISPR-Cas9 and CAR T-cell therapy have shown enhanced potential in gene therapy.⁸ 

Several methods can be employed for gene correction in Tangier disease. Some of the methods are listed below:

Gene replacement therapy 

ABCA1 gene is mutated in case of Tangier disease. So, introducing a functional ABCA1 gene into the liver cells (hepatocytes) can improve the HDL cholesterol (HDL-C) production. This process is carried out using viral vectors like adeno-associated viruses (AAVs). 

Preclinical studies have shown that by using this strategy, HDL-C levels are increased by implementing ApoA-I (primary protein part of HDL-C) lipid loading in the liver. Though it is an effective method in treating this disease, it may not resolve cholesterol deposition in macrophages, and peripheral tissues.⁹

Gene editing

CRISPR-based editing can repair the DNA defects by restoring the ABCA1 protein function. In this technique, two important components are used - a guide RNA that complements with the target sequence and an endonuclease called Cas9 that forms double stranded breaks at the area of interest.

This genome editing technique is used for rapid editing of human pluripotent stem cell (hPSC)-derived macrophages to study Reverse Cholesterol Transport (RCT). CRISPR technology is used to induce frameshift mutations in the macrophages. 

Frameshift mutations further eliminate the cholesterol efflux into the macrophages, reducing its deposition. In a study, wild-type cells showed cholesterol efflux into the macrophage while the mutated ones showed impaired efflux. However, this approach needs more detailed research on cholesterol metabolism and macrophage function.^10,11

RNA-based therapies

RNA-based therapies, such as the use of small interfering RNAs (siRNAs) and microRNAs (miRNAs), can help treat defective ABCA1 genes. MicroRNAs silence the gene expression that helps in regulating the transcription mechanism. A study found that sterol regulatory element-binding protein (SREBP) regulates ABCA1 in these therapies. 

SREBP transcribes microRNAs like miR-33a and miR-33b which inhibits ABCA1 expression. This process reduces circulating HDL-C levels, and HDL-C expression is increased in mouse models.¹² 

Further research is needed to find the efficacy of this technique as the results might not be similar in different organisms.

Recent research and advances

Animal models are used in understanding the pathology of Tangier disease and testing the therapeutic strategies. Some of the animal models used for this purpose are:

Murine (mouse) models 

ABCA1 knockout reduces HDL cholesterol storage in macrophages. For example, apoA-I knockout mice have low HDL. Introducing the human apoA-I adenovirus in knockout mice can restore the HDL levels. Though gene transfer could potentially help apoA-I-deficient patients by increasing HDL and reducing atherosclerosis risk, the rarity of such patients makes it a less attractive target for gene therapy development.¹³

Avian (bird) models 

A naturally occurring model of Tangier disease has been observed in chickens. Heterozygous ABCA1 mutation carriers (people who have just one copy of the ABCA1 mutation) have very low HDL cholesterol levels and a higher risk of atherosclerosis. Macrophage-directed gene therapy with ABC1 could be a potential therapeutic approach for these patients.^13,14 

Researchers found that WHAM chickens exhibit characteristics similar to human Tangier disease that includes low HDL cholesterol levels and cholesterol accumulation in tissues. This avian model offers an alternative system to study the disease and assess potential gene therapy approaches.¹⁵ 

Challenges and ethical considerations

Common challenges and ethical considerations related to gene therapy:

• Viral methods of delivery can trigger immune responses in the body, while non-viral methods might not be so efficient in delivering genes into the cells
• Vectors can trigger non-specific immune reactions that reduce the transfer efficiency and pose safety issues
• Gene therapy can cause ethical issues in which it is necessary for the people to have informed decisions on its risks and benefits for long-term applications
• Gene therapies can be extremely expensive, which limits their accessibility for many patients

Future prospects and clinical applications

Future prospects and clinical applications should include the initiation of clinical trials and the adoption of personalised medicine approaches for the treatment of Tangier disease. With advanced technologies like artificial intelligence (AI) in biotechnology, gene therapy strategies can be improved and made faster. 

The transition from preclinical studies to human clinical trials is quite sensitive. The reason involves the requirement for the efficacy and safety of gene therapies. For Tangier disease, characterised by mutations in the ABCA1 gene leading to defective cholesterol transport, targeted gene therapy could rectify the underlying genetic defect. Specific clinical trials are currently limited for successful gene therapies in this disease. However, therapies used for similar lipid metabolism disorders can provide foundation for future investigations.¹⁶ 

For instance, the development of olezarsen, an antisense oligonucleotide (ASO) technique that targets apo-CIII, has reduced triglyceride levels in patients with familial chylomicronemia syndrome, another lipid metabolism disorder. This approach can be studied further to use gene therapy strategies in Tangier disease.^17,18

The use of AI and biotechnology is evolving gene therapy by enhancing the design, development, and delivery of therapeutic interventions:

• Protein structure prediction: AI tools have achieved remarkable accuracy in predicting gene regulation and protein structure. This advancement accelerates the drug discovery and the understanding of disease mechanisms¹⁹
• Data analysis and interpretation: AI helps in managing and interpreting vast genomic datasets. Use of AI helps in:²⁰
  • Improving efficiency of gene therapy
  • Identifying specific patterns and correlations like mutations and biomarkers
  • Fastening the process in therapeutic development

The combination of AI and gene therapy has the potential to develop treatments for genetic disorders such as Tangier disease. However, to ensure the responsible application of these technologies, the challenges such as data privacy concerns, ethical considerations, and the need for robust regulatory frameworks must be addressed.

Summary

Gene therapy for the ABCA1 gene holds promise for treating Tangier disease. Restoring ABCA1 expression in the liver may increase HDL-C levels but might not fully restore cholesterol efflux or prevent lipid buildup in atherogenic cells. 

However, developing a successful treatment for Tangier disease remains challenging. Continued research and collaboration across disciplines are crucial to translate these prospects into effective clinical applications.