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AAV vectors; Inherited liver disorders; Gene therapy; Hemophilia; Liver gene delivery; Gene editing; Clotting factors; Therapeutic applications; Genetic mutation; Clinical trials

Introduction

Inherited liver disorders represent a broad category of diseases caused by genetic mutations that disrupt the liver’s ability to produce essential proteins or enzymes. Among the most well-known of these are the bleeding disorders hemophilia A and B, which result from deficiencies in clotting factors produced in the liver. In these conditions, the genetic mutations prevent sufficient production of the necessary factors, leading to uncontrolled bleeding, joint damage, and, in severe cases, life-threatening hemorrhages. While the management of hemophilia through regular infusions of clotting factor concentrates has been a cornerstone of treatment, these therapies are often burdensome, costly, and require lifelong administration [1].

In recent years, the development of gene therapy, particularly through the use of adeno-associated virus (AAV) vectors, has opened up exciting new possibilities for treating inherited liver disorders. AAV vectors have demonstrated the ability to deliver therapeutic genes directly to liver cells, potentially correcting the underlying genetic defect and providing long-term benefits with a single treatment. This approach offers the promise of transforming the landscape of inherited liver disease management, particularly for hemophilia [2].

This article provides an overview of the use of AAV vectors in the treatment of inherited liver disorders, focusing on hemophilia as a case study. We examine the mechanisms by which AAV-based gene therapy works, its potential advantages over traditional treatments, the challenges that still need to be overcome, and the future of AAV-based therapies [3].

Description

Adeno-associated virus (AAV) is a small, single-stranded DNA virus that has been developed as a vector for gene therapy. AAVs are non-pathogenic in humans and have a favorable safety profile, making them an attractive choice for gene delivery. The AAV vector is engineered by removing the viral genes and replacing them with the therapeutic gene intended for delivery to the target cells. The virus's natural ability to infect human cells, particularly liver cells, makes it an ideal vehicle for gene therapies aimed at treating inherited liver disorders. Inverted Terminal Repeats (ITRs) These are sequences at both ends of the AAV genome that are necessary for packaging the viral genome into a viral particle. Therapeutic Gene The gene that is delivered to the patient to correct the underlying defect in the target tissue. In the case of hemophilia, this would be the gene encoding the deficient clotting factor (Factor VIII for hemophilia A and Factor IX for hemophilia B) [4].

Helper Genes These are genes that help the virus replicate in the host cell but are not part of the final therapeutic product. AAV vectors are typically administered through intravenous infusion, and they are designed to primarily target liver cells (hepatocytes) due to the liver's ability to uptake viral particles. Once inside the hepatocytes, the viral genome can persist episomally (as non-integrated DNA), leading to long-term expression of the therapeutic gene. Hemophilia is a genetic disorder characterized by an inability to properly form blood clots due to a deficiency in one of the clotting factors produced by the liver. Hemophilia A and B are caused by mutations in the genes encoding Factor VIII (hemophilia A) and Factor IX (hemophilia B), both of which are essential for the clotting cascade. Without these factors, patients experience prolonged bleeding after injury, surgery, or spontaneously, which can lead to severe complications such as joint damage, internal bleeding, and even death [5].

Traditionally, hemophilia has been managed by infusing patients with clotting factor concentrates derived from human plasma or recombinant DNA technology. While effective, these treatments are expensive, require frequent administration, and can cause complications such as the development of inhibitors (antibodies) against the clotting factors, reducing their effectiveness. Gene therapy, particularly using AAV vectors, offers a potential cure by delivering a functional copy of the gene encoding the missing clotting factor directly to the liver, where it can be expressed continuously. This could reduce or eliminate the need for ongoing infusions of clotting factors and provide long-term control of the disease. Administration AAV vectors are typically delivered to patients via intravenous infusion, where they circulate in the bloodstream [6].

Liver Targeting After infusion, AAV vectors predominantly target the liver due to its unique biology. The liver is a highly vascular organ that efficiently takes up viral particles, particularly those with surface proteins that can interact with liver cell receptors. Uptake by Hepatocytes AAV vectors are taken up by hepatocytes (liver cells) through receptor-mediated endocytosis. After entering the cell, the AAV vector is transported to the endosome, where it escapes and delivers the therapeutic gene to the nucleus. Gene Expression Once inside the nucleus, the therapeutic gene is transcribed and translated into the missing protein, such as Factor VIII or IX. The protein is then secreted by the hepatocytes into the bloodstream, where it can contribute to blood clotting [7].

Long-Term Expression One of the key advantages of AAV vectors is their ability to provide long-term expression of the therapeutic gene. While AAV vectors do not integrate into the host genome (unlike some other viral vectors), they maintain an episomal form of the therapeutic gene in the hepatocyte nucleus, allowing for prolonged gene expression over time. Liver Tropism AAV vectors naturally target the liver, making them ideal for treating liver-based disorders like hemophilia. The liver’s high vascularity and ability to efficiently uptake viral vectors help ensure that a sufficient number of hepatocytes are transduced with the therapeutic gene. Safety AAV vectors are non-pathogenic in humans, and they generally do not elicit strong immune responses. This makes them a safer alternative to other viral vectors, such as adenoviruses, which can provoke significant immune reactions [8].

Long-Term Expression Once delivered, AAV vectors can provide long-term expression of the therapeutic gene without integrating into the host genome, reducing the risk of insertional mutagenesis (genetic changes that could lead to cancer). Low Risk of Immune Reactions The immune response to AAV vectors is generally low compared to other viral vectors, although pre-existing immunity to certain AAV serotypes can complicate therapy [9,10].

Discussion

One of the primary concerns when using AAV vectors for gene therapy is the possibility of pre-existing immunity. Many individuals have been naturally exposed to AAVs and may have developed antibodies against them. These antibodies can neutralize the virus, preventing it from effectively transducing the liver cells and reducing the efficacy of the therapy. To overcome this, researchers are exploring alternative AAV serotypes and immune-suppressive strategies to mitigate the impact of pre-existing immunity.

AAV vectors have a relatively small genome (approximately 4.7 kilobases), which limits the size of the therapeutic gene they can carry. While the genes encoding Factor VIII and IX fit within this capacity, larger genes may require the use of modified AAV vectors or combination strategies to enable successful delivery. Advances in vector engineering are ongoing to expand the capacity of AAVs and allow them to deliver larger genes. While early results with AAV-based gene therapies have been promising, the long-term efficacy and safety of these treatments remain uncertain. Gene therapy using AAV vectors can lead to stable expression of therapeutic proteins, but the immune system may still develop responses to the transduced cells or to the delivered protein. Additionally, the long-term durability of the gene expression remains to be fully established, especially in patients with varying genetic backgrounds or in the presence of immune responses.

The production of AAV vectors for clinical use is complex and costly. The large-scale manufacturing of these viral vectors requires careful control of vector quality and purity to ensure that patients receive a safe, effective product. The costs associated with vector production, clinical trials, and the final treatment can be prohibitively high, making access to these therapies challenging for some patients. Efforts are underway to reduce production costs and improve the scalability of AAV vector production. Despite these challenges, there have been significant advances in the development of AAV-based gene therapies for hemophilia. Several clinical trials have shown encouraging results in both hemophilia A and B patients, with some patients achieving stable, long-term expression of Factor VIII or Factor IX following a single infusion of AAV vectors.

For example, in 2020, the U.S. Food and Drug Administration (FDA) approved Hemgenix, an AAV-based gene therapy for haemophilia B, marking a significant milestone in the use of AAV vectors for genetic liver disorders. Clinical trials have demonstrated that patients who received the therapy had significant reductions in bleeding episodes and clotting factor requirements, with some achieving near-normal levels of Factor IX. Similarly, several ongoing trials for hemophilia A are showing promising results, with some patients experiencing sustained levels of Factor VIII expression and reduced bleeding episodes, which may eventually eliminate the need for regular infusions of clotting factor concentrates.

Conclusion

AAV-based gene therapy represents a groundbreaking approach for the treatment of inherited liver disorders, including hemophilia. By delivering therapeutic genes directly to the liver, AAV vectors can potentially provide long-lasting, life-changing benefits for patients suffering from these genetic diseases. Although challenges remain-such as pre-existing immunity to AAV, manufacturing complexities and the need for long-term safety data-the progress made thus far has been remarkable. With continued research, innovation, and clinical testing, AAV vectors hold the promise of transforming the treatment landscape for haemophilia and other inherited liver disorders, offering hope for more effective, durable, and accessible therapies for patients worldwide.

Acknowledgement

None

Conflict of Interest

None

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