Overview

Gene therapy is a medical approach that introduces, modifies or replaces genetic material within a patient’s cells to treat or prevent disease. It plays a central role in modern genomic medicine, enabling targeted correction of genetic defects, regulation of disease pathways and development of advanced therapies for cancer, rare diseases and infectious conditions.

What is Gene Therapy?

Gene therapy is the process of introducing new genetic material into cells to compensate for abnormal genes or to repair damaged genes. This process has the potential to cure many monogenic diseases currently considered incurable, such as cancer, Huntington’s disease, HIV/AIDS and cystic fibrosis.

How Gene Therapy Works

1.      Therapeutic gene design
A functional gene is identified and engineered to replace, repair or regulate a defective gene.

2.      Vector delivery to target cells
The therapeutic gene is delivered into the body using:

• Viral vectors such as recombinant Adeno-associated virus (AAV) or
• Direct injection of genetic material into target cells

3.      Cell entry and transport to the nucleus
The vector carries the genetic payload into the cell and transports it to the cell nucleus.

4.      Persistence of therapeutic DNA
In AAV-based therapies, the delivered DNA typically remains extrachromosomal (non-integrated), reducing the risk of insertional mutations.

5.      Gene expression (transcription and translation)
The therapeutic gene is transcribed into RNA and translated into a functional protein.

6.      Post-translational processing
The newly produced protein undergoes necessary modifications to become fully functional.

7.      Restoration of cellular function
The functional protein performs its intended role, helping correct or compensate for the underlying genetic defect.

Somatic Gene Therapy

Somatic gene therapy involves the delivery of therapeutic nucleic acids into a patient’s non-reproductive (somatic) cells to treat disease. These genetic changes are confined to the individual and are not passed on to future generations. In many cases, the introduced genetic material remains in an extrachromosomal form rather than integrating into the host genome, which is generally preferred as it reduces the risk of insertional mutations. This approach is widely used in current clinical applications due to its more favorable safety and ethical profile.

Germline Gene Therapy

Germline gene therapy involves introducing genetic modifications into germ cells, such as sperm, eggs or early-stage embryos. Unlike somatic therapy, this approach requires the therapeutic gene to be stably integrated into the host genome, ensuring that the modification is inherited by future generations. While it holds the potential to eliminate certain genetic diseases permanently, germline gene therapy raises significant ethical, safety and regulatory concerns and is currently restricted or prohibited in most clinical settings.

Gene Therapy Vectors

There are many different types of vectors, including viruses and nucleic acid carriers such as liposomes and nanoparticles. Each vector and carrier type has its own advantages and disadvantages.

Viral Vectors

• Adeno-associated virus (AAV) is the most used vector for gene therapy, lacking viral genes and efficient at recombinant gene insertions. Constructing vectors involves splicing a new gene with regulatory sequences into restriction enzyme sites on an AAV backbone. An endosome coats the AAV-new gene construct, facilitating its delivery across cell membranes into the nucleus for gene expression.
• Lentiviral vectors are also useful for gene therapy, although integration into host genomes limits their use for recombinant therapeutic proteins. These viral vectors can infect both dividing and non-dividing cells and target specific cell types.
• Retroviruses include RNA viruses that insert a DNA copy into a host’s genome by utilizing viral-encoded reverse transcriptase. Retroviral vectors achieve high titers with recombinant insertions, can transfect a wide variety of host cells and can result in stable insertions of novel gene constructs for gene therapy applications.

Non-Viral Vectors

In addition to vectors that contain recombinant nucleic acid sequences, carriers can deliver genetic payloads into the nuclei of host cells. These non-viral approaches have several advantages over viral delivery, including reduced immunogenicity, the ability to target specific cell types and greater safety. However, non-viral methods are often less efficient than viral delivery and can be more difficult to scale up for large-scale clinical use.

• Liposomes are small lipid (fat) molecules that can carry DNA or RNA into cells. They are less likely to provoke an immune response than viruses, but are not as efficient at delivering genetic material to cells.
• Nanoparticles are very small particles that can carry DNA or RNA into cells. Similarly, nanoparticles do not elicit an immune response as viruses do and they have the advantage of targeting specific cell types. However, nanoparticles are less efficient than AAV vectors.

Ex vivo and in vivo gene therapy

Ex Vivo Gene Therapy

Ex vivo gene therapy involves removing cells from a patient, genetically modifying them outside the body and then reintroducing them to achieve a therapeutic effect. This controlled approach allows precise manipulation of cells before reinfusion.

• Cell-based approach: Cells are harvested from the patient, modified in a laboratory setting and returned to the body

• Genetic modification: Therapeutic genes are introduced using viral or non-viral systems under controlled conditions

• Example application:

  • CAR T cell therapy, where T cells are engineered to recognize and attack cancer cells
  • FDA-approved for leukemia and lymphoma

• Clinical advantage: Enables high efficiency and precise modification of target cells before reinfusion

• Best suited for:

  • Cells that are difficult to target directly in vivo
  • Situations requiring large-scale cell modification

• Broader applications: Used in genetic disorders, cancer and some viral infections

In Vivo Gene Therapy

In vivo gene therapy delivers therapeutic genes directly into a patient’s cells, enabling treatment at the site of disease without removing cells. This approach is commonly used when target tissues, such as the liver, eye or muscle, can be accessed directly through systemic or localized administration.

• Direct delivery approach: Genetic material is introduced inside the body rather than modifying cells externally

• Common delivery method: Typically uses viral vectors to transport genes into target cells

• Administration routes:

  • Intravenous (IV) infusion via the bloodstream
  • Localized injections

• Clinical advantage: Enables treatment of hard-to-isolate or widely distributed tissues

• Considerations: Requires precise targeting and control to ensure efficient delivery and safety

How is Gene Therapy Used in Medicine?

There are several ways gene therapy can be used for therapeutic purposes. Most involve introducing a recombinant gene using a viral vector such as AAV and new gene-editing techniques that offer site-specific gene alterations.

For example, gene therapy can be used for:

Gene Addition

• Replaces missing or defective genes
• Used in inherited disorders

Gene Silencing

• Suppresses harmful gene activity
• Common in oncology applications

Gene Editing

• Precise DNA modification using CRISPR
• Enables targeted correction of mutations

Real-World Applications of Gene Therapy

By adding a fully functional recombinant gene, transcription and translation of a recombinant protein can correct a specific disease-causing genetic defect. For example,

• Pyruvate kinase (PK) deficiency is an inherited disorder causing insufficient enzyme production. It can lead to hemolytic anemia, in which red blood cells are destroyed faster than they are produced. Gene therapy offers a promising treatment by inserting a normal gene copy into cells via a viral vector, enabling proper enzyme production.
• mRNA constructs designed to elicit an immune response to SARS-CoV-2 spike proteins have been successfully used for COVID-19 vaccination.
• Downregulation of harmful gene activity can be accomplished by using anti-sense constructs and has applications with certain cancers where curtailing oncogenic activity is advantageous.
• CRISPR-Cas9 is a powerful genome editing tool that can make precise changes to specific DNA targets. AAV vectors with CRISPR-Cas9 serve as delivery vehicles for in situ gene modification, making them effective in gene therapy. CRISPR is ideal for monogenic diseases, where a single base change can cure the disease. Clinical trials show positive results for sickle-cell disease and beta-thalassemia.