Introduction

Haemophilia is an X-linked recessive monogenic bleeding disorder affecting approximately 1 in 5,000 AMABs(Assigned male at birth) for haemophilia A and 1 in 30,000 for haemophilia B. The disease is characterised by uncontrolled bleeding into joints and soft tissues due to a deficiency of coagulation factors VIII (haemophilia A) or IX (haemophilia B).¹ The current standard of care involves prophylactic intravenous infusions of recombinant or plasma-derived clotting factors. However, this treatment is burdensome and extremely costly. Gene therapy has emerged as a promising approach to provide long-term correction of the bleeding phenotype without regular factor infusions. It aims to offer a one-time treatment that can result in sustained endogenous production of the deficient clotting factor. This review summarises recent clinical advances in gene therapy for haemophilia A and B, highlighting key outcomes, challenges, and future directions.

Gene therapy

Gene therapy is a medical technique that aims to treat, prevent, or cure diseases by modifying genetic material within a patient's cells. It involves introducing, removing, or altering genetic material within cells to address the root cause of a genetic disorder. In the context of haemophilia, it typically aims to–

• Add functional copies of the factor IX gene (for haemophilia B) or factor VIII gene (for haemophilia A)
• Enable the liver to produce the missing clotting factor
• Improve blood clotting ability

Mechanism of gene therapy

Adeno-associated virus (AAV) vectors have emerged as the leading platform for in vivo(in a living organism) gene therapy of haemophilia due to their favourable safety profile and ability to transduce hepatocytes efficiently. Several AAV-based gene therapies for haemophilia A and B have demonstrated promising results in clinical trials. Gene therapy for haemophilia aims to provide a long-term solution by introducing functional genes to produce clotting factors in patients' cells. The most common approach uses adeno-associated viral (AAV) vectors to deliver therapeutic genes to liver cells, enabling them to produce the missing clotting factors.²

Gene therapy for haemophilia involves several key steps, from engineering the therapeutic gene to its expression in the patient's liver cells. Here's an expanded explanation of the process–

1. Engineering the gene cassette

The first step involves creating a bioengineered gene cassette containing–

• The therapeutic gene (factor VIII for haemophilia A or factor IX for haemophilia B)
• A tissue-specific promoter
• Other regulatory elements

This cassette replaces the viral DNA in the adeno-associated virus (AAV) vector.²

2. Packaging into AAV vector

The engineered gene cassette is then packaged into a non-pathogenic and non-replicating recombinant AAV vector. AAV vectors are chosen for several reasons–

• They do not cause illness in humans
• They can efficiently deliver DNA to target cells
• They can be modified to target specific tissues, such as liver cells³

3. Intravenous administration

The AAV vector containing the therapeutic gene is administered to the patient through intravenous infusion. This method allows the vector to circulate throughout the bloodstream and reach the target liver cells.²

4. Transduction and DNA release

Once in the bloodstream, the process continues as follows–

• The AAV vector binds to liver cells
• The vector enters the cell through endocytosis
• It is transported to the cell nucleus
• In the nucleus, the vector releases its genetic material as episomal DNA^2,3

5. Gene Expression and clotting factor production

The final stage of the process involves–

• The therapeutic gene becomes functional in the nucleus(cell organelle where genetic information of cell is stored)
• It instructs the liver cells to produce the missing clotting factor (VIII or IX)
• The cells begin secreting the functional clotting factor proteins into the bloodstream³

Outcomes and considerations

Gene therapy for haemophilia represents a significant advancement in treatment, potentially offering a one-time intervention that could dramatically improve patients' quality of life.⁴

If successful, this process can lead to–

• Sustained production of clotting factors
• Reduction in bleeding episodes
• Potential elimination of the need for regular factor replacement therapy⁴

However, it's important to note that

• The effectiveness can vary between patients
• Long-term follow-up (up to 15 years) is recommended to monitor the safety and duration of factor expression⁴
• Patients may still pass on genetic variations to their children⁴

Cost

Gene therapies typically have high upfront costs, often in the millions of dollars for a single treatment. For example, Hemgenix for haemophilia B is priced at $3.5 million per treatment, making it the most expensive drug in the world.⁵ However, these high upfront costs may be offset by potential long-term savings–

• Traditional treatments like factor replacement therapy can cost $15-$18 million over a patient's lifetime for haemophilia A⁶
• Gene therapy could potentially eliminate the need for ongoing factor replacement, resulting in cost savings over time⁶

The high prices of gene therapies are raising concerns about affordability and access, especially in low- and middle-income countries.⁵

Efficacy

Gene therapy has shown promising results in clinical trials–

• Most patients experienced a significant reduction (53% to 96%) in bleeding episodes compared to previous therapy, even without ongoing prophylactic treatment²
• Persistently elevated factor levels have been observed for up to six years in haemophilia A and up to eight years in haemophilia B²

However, there are some challenges–

• The durability of the effect is still uncertain, with factor levels potentially declining over time, especially for factor VIII in haemophilia A⁷
• Not all patients are eligible for gene therapy due to pre-existing antibodies or other factors⁷

Benefits of gene therapy

Gene therapy offers several potential advantages over traditional factor replacement therapy:

• Long-term factor production: Sustained clotting factor levels can be achieved for years after a single treatment²
• Reduced bleeding: Most patients experience a significant reduction (53-96%) in bleeding episodes²
• Improved quality of life: Patients may be freed from the burden of regular factor infusions⁸
• Cost-effectiveness: Gene therapy may result in long-term cost savings compared to lifelong factor replacement¹

Challenges and risks

Despite its promise, gene therapy for haemophilia faces several challenges–

• Immune responses: The most common side effect is an inflammatory response with elevated liver enzymes, occurring in 17-89% of patients depending on the study²
• Vector immunogenicity: Pre-existing neutralising antibodies to AAV vectors can exclude some patients from treatment⁸
• Durability: Factor levels may decline over time, particularly for factor VIII in haemophilia A⁹
• Safety concerns: Long-term safety profiles are still being established⁸
• Limited eligibility: Current trials exclude children, patients with liver dysfunction, and those with a history of inhibitors⁸

Recent clinical advances

Haemophilia B

The first successful AAV gene therapy trial for haemophilia B was reported in 2011, demonstrating dose-dependent factor IX expression of 2-11% in 6 patients. Long-term follow-up showed sustained FIX activity of 2-5% for up to 8 years post-treatment. Subsequent trials have achieved higher FIX levels using optimised AAV vectors and transgenes. A phase 3 trial of etranacogene dezaparvovec (AMT-061) reported mean FIX activity of 37% at 18 months post-treatment, with a 64% reduction in bleeding rate and a 97% reduction in factor use. This therapy was recently approved in the EU for treatment of adults with severe haemophilia B.¹⁰

Haemophilia A

Gene therapy for haemophilia A has been more challenging due to the large size of the factor VIII gene. However, recent trials have shown promising results. A phase 1/2 study of valoctocogene roxaparvovec reported median FVIII activity of 20% at 2 years post-treatment. A phase 3 trial demonstrated sustained FVIII expression (mean 42.9 IU/dL at 2 years), with an 84% reduction in annualised bleeding rate. However, a gradual decline in FVIII levels was observed over time, highlighting the need for longer follow-up.¹¹

Conclusion

Gene therapy for haemophilia has made remarkable progress, with the first approved therapy now available for haemophilia B. Ongoing research aims to optimise vectors, transgenes, and treatment protocols to achieve durable, safe, and effective gene therapy for all patients with haemophilia. While challenges remain, gene therapy holds great promise to dramatically improve the lives of individuals with this chronic disease.. Gene therapy for haemophilia has shown significant promise in clinical trials, offering the potential for long-term factor production and reduced bleeding episodes after a single treatment. While challenges remain, particularly regarding immune responses and long-term durability, ongoing research and optimisation efforts are addressing these issues. As the field progresses, gene therapy may revolutionise haemophilia treatment, providing a functional cure for many patients and dramatically improving their quality of life.

FAQs

What is haemophilia?

Haemophilia is a lifelong genetic condition with insufficient clotting factor VIII (8) or IX (9) in the blood. This results in prolonged bleeding after injuries or surgeries, and can sometimes cause spontaneous internal bleeding, particularly in joints and muscles. There are two main types of haemophilia–

• Haemophilia A: Deficiency in Factor VIII
• Haemophilia B: Deficiency in Factor IX

The severity is classified based on the amount of clotting factor present–

• Mild: 5-40% factor activity
• Moderate: 1-5% factor activity  
• Severe: Less than 1% factor activity

Is haemophilia inherited?

Haemophilia is an X-linked genetic disorder, meaning–

• Males with the gene alteration will have haemophilia
• Females can be carriers of the gene alteration
• About 1/3 of cases occur spontaneously in families with no previous history

Treatment for haemophilia?

While there is no cure, haemophilia can be effectively managed with–

• Clotting factor replacement therapy 
• Prophylactic treatment to prevent bleeds
• On-demand treatment for acute bleeding episodes

Modern treatments allow most people with haemophilia to self-administer factor concentrates at home.

How is the lifestyle for people with haemophilia?

• Exercise and sports– Regular exercise is encouraged to strengthen muscles and protect joints. Low-impact activities like swimming, cycling, and walking are recommended. Contact sports should generally be avoided
• School– Children with haemophilia can attend regular schools. It's important to inform school staff about the condition and establish an emergency plan
• Safety– Those caring for someone with haemophilia should be informed about the condition and know what to do in case of bleeding

Future Directions

Ongoing research aims to address current limitations and expand the applicability of gene therapy for haemophilia–

• Optimising vector design to reduce immunogenicity and improve targeting¹
• Enhancing transgene expression and efficacy¹
• Developing strategies to overcome pre-existing AAV antibodies²
• Exploring alternative delivery methods, such as lentiviral vectors¹
• Investigating gene editing approaches to correct mutations directly²