Overview 

Gene therapy has emerged as a promising new direction in the treatment of thalassemias, offering new hope to patients suffering from these inherited blood disorders. Thalassemia is a group of inherited blood disorders characterised by the reduced production of haemoglobin, the protein in red blood cells responsible for oxygen transport. This results in anaemia(low oxygen level in blood), fatigue, and a variety of other serious health complications. The two main types are alpha-thalassemia and beta-thalassemia, both of which can range from mild to severe, with beta-thalassemia major (Cooley’s anaemia) being the most severe form. In recent years, high-impact breakthroughs have been witnessed in this field, with several innovative approaches showing remarkable potential for curing or significantly alleviating the symptoms of thalassemia.

Traditional treatments

Traditionally, thalassemia has been managed with regular blood transfusions and medications to manage symptoms. However, these treatments are not cures and can lead to long-term complications.

Gene therapy: a new hope

Gene therapy offers a potential cure by addressing the root cause of thalassemia (the faulty genes responsible for haemoglobin production). Here are some types of gene therapies:

CRISPR technology

Scientists use a gene-editing tool called CRISPR/Cas9 to precisely change the DNA in a patient's bone marrow stem cells. This technology won the Nobel Prize in 2020 for its inventors.

The CRISPR-Cas9 gene editing technology has revolutionised the treatment of thalassemia. This Nobel Prize-winning technique allows for precise genetic modification, opening up new possibilities for treating genetic disorders.¹

Exagamglogene autotemcel (Casgevy)

One of the most significant advancements is the development of exagamglogene autotemcel, marketed as Casgevy. This CRISPR-Cas9 gene-edited cell therapy has received approval from regulatory bodies in multiple countries to treat transfusion-dependent beta-thalassemia (TDT) in patients aged 12 and older.

Casgevy works by editing the patient's hematopoietic stem and progenitor cells at the erythroid-specific enhancer region of the BCL11A gene. This edit increases the production of fetal haemoglobin in red blood cells, effectively reducing or eliminating the need for blood transfusions in TDT patients.²

Lentiviral vectors

Another significant approach in gene therapy for thalassemia involves using lentiviral vectors. These vectors can efficiently deliver functional genes to haematopoietic stem cells (HSCs), potentially correcting the genetic defect responsible for thalassemia.³

Lentiviral vectors have emerged as a promising approach for gene therapy in β-thalassemia. These vectors introduce functional copies of the β-globin gene into the patient’s HSCs, potentially correcting the underlying genetic defect.³

Mechanism

The lentiviral vectors add functional copies of a modified β-globin gene (βA-T87Q-globin gene) into the patient's HSCs. This allows the body to produce functional adult haemoglobin, potentially eliminating or reducing the need for regular blood transfusions.

Advantages

• One-time treatment approach
• Uses autologous stem cells, reducing risks associated with allogeneic transplants
• No need for long-term immunosuppression
• Potential for long-term cure

The lentiviral approach for gene therapy in β-thalassemia has shown significant promise, with clinical successes leading to the first FDA approval. Ongoing research continues to optimise this treatment strategy for broader application and improved outcomes.³

Editing stem cells

The patient's bone marrow stem cells are removed and edited in a laboratory to correct the genetic mutation. This allows the cells to produce healthy haemoglobin. The edited stem cells are then infused back into the patient's body, where they start making healthy red blood cells.⁴

Autologous stem cell transplantation

Autologous stem cell transplantation involves harvesting a patient's stem cells, editing or modifying them, and reintroducing them into the patient’s body. This avoids the complications associated with allogeneic stem cell transplants, where a donor’s stem cells are used.

Induced pluripotent stem cells (iPSCs)

The generation of induced pluripotent stem cells offers another avenue for thalassemia treatment. These cells can be genetically corrected and then differentiated into functional red blood cells, providing a potential source of healthy blood cells for patients.⁵

This approach has been combined with gene addition (e.g., using viral vectors to add the normal beta-globin gene) and gene editing (e.g., using CRISPR-Cas9) to treat beta-thalassemia. The major advantage of autologous transplants is that the patient does not need immunosuppressive therapy to prevent graft rejection, as the stem cells are derived from the patient’s body.⁴

Other gene addition and editing technologies 

While CRISPR-Cas9 has taken centre stage, other gene editing technologies have also shown promise in thalassemia treatment:

Zinc finger nucleases (ZFNs)

ZFNs have been used to reactivate γ-globin gene expression, resulting in increased foetal haemoglobin levels and were the first nucleases used in clinical experiments for thalassemia.¹

Transcription activator-like effector nucleases (TALENs) 

TALENs have successfully corrected β-globin gene mutations in induced pluripotent stem cells derived from thalassemia patients.¹

Clinical trial results and successes

Clinical trials using CRISPR technology

CRISPR-based approaches

Early trials using CRISPR-Cas9 to edit the BCL11A gene or directly modify the beta-globin gene have successfully increased foetal haemoglobin levels in patients with beta-thalassemia.¹ The results suggest that gene editing has the potential to cure or significantly reduce the severity of the disease in some patients. 

CT103A (CRISPR-Cas9 by city of hope)

In a recent clinical trial, patients with beta-thalassemia treated with CRISPR-based gene editing showed impressive outcomes, with some becoming transfusion-independent after treatment. This suggests that gene editing has the potential to offer long-term, durable benefits to patients with thalassemia.

Clinical trials using Lentivirus therapy

Several clinical trials have shown promising results. The TIGET-BTHAL trial used the GLOBE lentiviral vector to modify autologous HSCs. Phase I/II and III trials using the BB305 vector led to transfusion independence in 90% of treated patients.

In 2022, the FDA approved betibeglogene autotemcel (beti-cel), marketed as Zynteglo, as the first lentiviral vector gene therapy for β-thalassemia in the US.³

LentiGlobin (Bluebird Bio)

This lentiviral gene therapy has shown long-term success in clinical trials. Several patients who were transfusion-dependent achieved transfusion independence after receiving the therapy. The results have been promising, with patients experiencing sustained haemoglobin levels and improved quality of life.⁶

Clinical trials using stem cell therapy

The field of gene therapy for thalassemia has progressed rapidly from bench to bedside(from research laboratories to hospital clinics). For gene therapy clinical trials in the UK, applications must be submitted to the Gene Therapy Advisory Committee (GTAC) for ethical approval. Several clinical trials have demonstrated the safety and efficacy of these new treatments. 

Recent trials have shown promising outcomes

Global approval

Casgevy received regulatory approval in the UK in late 2023, marking it the first CRISPR-based therapy authorised for clinical use. The remaining patients experienced more than a 70% reduction in blood transfusion needs

Ongoing research

Multiple clinical trials are exploring various gene therapy approaches for beta-thalassemia, including:¹

• Direct repair of the defective beta-globin gene using technologies like CRISPR-Cas9 
• Activation of other genes that can replace beta-globin function, such as gamma-globin 

Future directions and challenges

Long-term safety

While early results are promising, the long-term safety of gene therapies is a major source of worry. Two potential dangers are off-target consequences (unintended genomic alterations) and vector-related problems.

Cost and accessibility

Gene therapy remains costly, and availability is limited worldwide. Furthermore, obtaining stem cells, modifying them, and reinfusing them can be time-consuming and need highly specialised infrastructure.

Immune response

The body's immune system may react against the viral vectors employed to deliver gene therapy. This may reduce the therapy's efficacy or create negative effects.

Ethical considerations

Using genome editing technologies, such as CRISPR, raises ethical considerations regarding potential long-term repercussions, accidental mutations, and the larger societal impact.

Researchers are working on improving gene editing technologies and exploring the use of hematopoietic stem and progenitor cells (HSPCs) for more effective treatments.¹ Zynteglo (betibeglogene autotemcel or beti-cel), approved in August 2022 for patients of all ages.

Summary 

Recent technical developments have accelerated the area of gene therapy for thalassemias. Advanced treatments have emerged with the approval of CRISPR-based therapeutics, such as casgevy and lentiviral gene insertion. Early clinical data indicate that gene therapy may cure thalassemia, particularly severe variants such as beta-thalassemia major, by lowering or eliminating the requirement for lifelong blood transfusions. However, problems persist, including questions about the long-term safety, cost, and availability of these cutting-edge therapies. As research and clinical trials progress, gene therapy for thalassemia has a bright future. This treatment has the potential to transform the lives of patients suffering from this genetic disorder, as these therapies evolve and become more accessible, they have the potential to dramatically improve the lives of thalassemia patients all over the world, potentially offering a cure for a condition that has long been considered lifelong.