Ex vivo gene therapy is a medical approach that addresses diseases at their genetic origin. It involves modifying a patient’s cells outside the body to correct or introduce genetic material, treating or potentially curing conditions. The name “ex vivo” means “outside the living,” distinguishing it from “in vivo” gene therapy, which modifies cells directly inside the patient. Modified cells are then returned to the patient to restore proper cellular function.
The Ex Vivo Gene Therapy Process
Ex vivo gene therapy begins with cell collection from the patient. These are often blood-forming (hematopoietic) stem cells, gathered from bone marrow or peripheral blood via apheresis. The choice of cells depends on the disease targeted, as these cells will carry the corrected genetic information.
Collected cells are transported to a laboratory under controlled conditions. This step maintains cell viability and integrity before modification. Cells are prepared for the next stage: introducing new genetic material or editing existing genes to address the disease’s underlying cause.
In the lab, genetic modification introduces a functional gene or corrects a faulty one within patient cells. This process involves molecular tools that alter the cells’ genetic makeup. After modification, engineered cells multiply in culture, expanding for reinfusion. This expansion phase can take several days to weeks, depending on the cell type and the specific therapy.
Finally, modified and expanded cells are prepared for reinfusion. Patients often undergo a conditioning regimen, like chemotherapy, to make space in bone marrow for new cells to establish. The modified cells are then administered intravenously, similar to a blood transfusion, allowing them to travel to appropriate locations and begin their therapeutic work.
Tools of Genetic Modification
Ex vivo gene alteration relies on tools: viral vectors and gene-editing technologies. Viral vectors are modified viruses repurposed to deliver genetic material into cells. These viruses are engineered to be harmless, stripped of their disease-causing components, while retaining their natural ability to efficiently enter cells and carry a therapeutic gene.
Retroviruses, including lentiviruses, are used in ex vivo gene therapy for their ability to integrate genetic material directly into host cell DNA. This integration allows for stable and long-lasting expression of the new gene, which is particularly beneficial for therapies requiring a permanent genetic correction. Lentiviral vectors are valuable because they can infect both dividing and non-dividing cells, expanding the range of target cell types.
Adeno-associated viruses (AAVs) are another class of viral vectors. AAVs can be used ex vivo, but carry smaller genetic payloads than lentiviruses. Their safety profile and tendency to remain as extrachromosomal elements, rather than integrating into the host genome, are considerations in their application.
Beyond viral vectors, modern gene-editing tools like CRISPR/Cas9, Zinc Finger Nucleases (ZFNs), and Transcription Activator-Like Effector Nucleases (TALENs) offer precise ways to modify genes. CRISPR/Cas9 uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence, making a cut. This targeted cut allows for the removal of faulty genes, the insertion of new genetic material, or the correction of single base pairs.
ZFNs and TALENs operate similarly, using engineered proteins to recognize and bind specific DNA sequences, creating precise cuts. These tools enable scientists to correct disease-causing mutations with accuracy, offering an alternative to simply adding a new gene. The choice of gene-editing tool depends on the specific genetic modification needed and the characteristics of the target cells.
Diseases Treated by Ex Vivo Gene Therapy
Ex vivo gene therapy holds promise for diseases affecting blood cells, immune function, and inherited disorders. One area of application is in inherited immunodeficiencies, such as Severe Combined Immunodeficiency (SCID), often referred to as “bubble baby disease”. In these conditions, faulty genes prevent the development of a functional immune system, leaving individuals vulnerable to infections. Ex vivo therapy allows for the correction of these genetic defects in a patient’s immune cells outside the body, then reintroducing them to rebuild a healthy immune system.
The approach has also shown success in treating certain cancers, notably through Chimeric Antigen Receptor (CAR) T-cell therapy. In this therapy, patient T-cells (a type of immune cell) are harvested and genetically modified to express a CAR, enabling them to recognize and attack cancer cells more effectively. These enhanced T-cells are then expanded and reinfused, offering a personalized anti-cancer treatment. This has been approved for specific types of leukemia and lymphoma.
Inherited blood disorders like beta-thalassemia and sickle cell disease are also targets for ex vivo gene therapy. In beta-thalassemia, the body cannot produce enough hemoglobin, the oxygen-carrying protein in red blood cells. For sickle cell disease, red blood cells become abnormally shaped, leading to blockages and reduced oxygen delivery. Ex vivo gene therapy aims to correct the genetic errors in hematopoietic stem cells, allowing the body to produce healthy red blood cells and alleviate disease symptoms.
Beyond blood and immune disorders, ex vivo gene therapy is being explored for other conditions, including certain neurological disorders like Parkinson’s disease, Huntington’s disease, and Alzheimer’s disease, as well as some genetic skin diseases. While in earlier stages for some conditions, modifying and reintroducing cells outside the body offers a targeted approach for diseases where accessible cells can be manipulated for therapeutic effects.