03 Understanding Gene Therapy
How is Gene Therapy Delivered?
To understand the role of vectors in delivering a therapeutic gene into cells and learn about the types of vectors used in gene therapy.
For any kind of gene therapy to work, the therapeutic genetic material must be introduced into cells so the usual cellular processes can produce proteins such as enzymes.1,2 A tool called a vector is manufactured to deliver therapeutic genes into cells. Vectors can be built to target certain cells and shuttle therapeutic genetic material directly into those cells. The most commonly used viral vectors in gene therapy are designed to introduce the therapeutic gene into target cells.2,3
Two commonly used viral vectors are lentiviral vectors and adeno-associated viral vectors:
- Lentiviral vectors. These vectors potentially enable the therapeutic gene to be permanently integrated into the hematopoietic stem cells or other target host cells.4,5
- Adeno-associated viral (AAV) vectors. These vectors deliver genetic material to the nucleus of the target cells and that DNA remains independent of the cell’s own DNA. This genetic material will be lost by one of the two daughter cells if the cell divides. AAVs are considered best suited for situations requiring correction in specific tissues or organs where cell turn-over is low or non-existent.1,6,7
The lentiviral vectors are most commonly used ex vivo and the AAV are most commonly used in vivo.
- Ex vivo is a Latin term for “outside the body.”
- One way ex vivo gene therapy can be delivered is through use of a lentiviral vector. Lentiviral vectors enable the therapeutic gene to be permanently integrated into the stem cells or other target host cells.4,5
- This method involves taking blood, bone marrow, or other tissue from a person, and separating out the hematopoietic stem cells or other target cells.2,8
- Next a process called transduction – the transfer of genetic material from the vector to the target cell9 – is used to allow for a viral vector to contact and engage with a target cell to deliver its therapeutic gene (a gene that codes for normal functioning protein).1,2,5
- Once the therapeutic gene is inserted into the hematopoietic stem cells or target cells, the cells are tested for safety and potency, and then delivered back to the person by intravenous (IV) infusion, in whom they can take root (engraft) in the bone marrow, and from there can produce billions of daughter cells that are expected to carry the therapeutic gene.1,2,10
- In vivo is a Latin term for “in the body.”
- This method involves delivering the vector carrying the therapeutic gene into the person’s body by intravenous infusion into the blood stream or direct injection to organs or tissues being targeted (for example the eye, brain, or liver).1,2
- In vivo gene therapy typically employs adeno-associated viral (AAV) vectors. These vectors are not capable of permanently integrating the therapeutic gene into the DNA of the host cells. Instead, they deliver genetic material to the nucleus of the target cells and that DNA remains independent of the cell’s own DNA.
- This genetic material will be lost by one of the two daughter cells if the cell divides. For this reason, AAVs are not considered ideal for delivery to organs or tissues with actively dividing cells, such as the livers of newborns, because of the risk of vector “washout.” Instead, AAVs are considered best suited for situations requiring correction in of specific tissues or organs where cell turn-over is low or non-existent.1,6,7
Ex Vivo and In Vivo Gene Therapy1,2,11
Ex vivo gene therapy
Patient cells modified outside the body and returned
- Patient’s HSCs are collected
- Vector is designed to enable the therapeutic gene to be permanently incorporated into the HSCs DNA.
- Genetically modified HSCs are infused back into the patient.
In vivo gene therapy
Patient cells modified inside the body
- Vector containing the therapeutic gene is delivered to the patient by IV or direct injection into organs or tissues.
- Vector does not integrate into the DNA of the patient’s cell.
There are also several clinical trials studying the use of lipid nanoparticles as potential vectors in gene therapy. These lab-made fatty particles can carry therapeutic genetic material to the target cells and integrate the therapeutic genetic material into the cells’ DNA.12
Additional Interesting Fact
As of April 2022, the U.S. Food and Drug Administration has approved three lentiviral vector-based T-cell immunotherapies: Abecma® (idecabtagene vicleucel) for the treatment of multiple myeloma; Breyanzi® (lisocabtagene maraleucel) for the treatment of B-cell lymphoma; and Kymriah® (tisagenlecleucel) for the treatment of acute lymphoblastic leukemia and large B-cell lymphoma. The FDA has also approved two AAV-based gene therapies: Luxturna® (voretigene neparvovec-rzyl) for the treatment of retinal dystrophy; and Zolgensma® (onasemnogene abeparvovec-xioi) for the treatment of spinal muscular atrophy.13 Abecma, Kymriah, Luxturna, and Zolgensma are also authorized for use in the European Union (EU), as is the lentiviral-based therapy Libmeldy™ for the treatment of metachromatic leukodystrophy.14
Ex vivo (“outside the body”) gene therapy involves taking blood, bone marrow, or other tissue from a person, and separating out the stem cells. Next, a process called transduction enables a viral vector to deliver the therapeutic gene to the stem cells.
- The cells are then frozen (cryopreserved) and tested for safety and potency before they are thawed and reinfused into the person at a convenient time.
- Once the cells are returned, they can engraft in the bone marrow and produce generations of daughter cells carrying the therapeutic gene.
In vivo (“in the body”) gene therapy generally involves the infusion or injection of a vector (usually an adeno-associated viral [AAV] vector) carrying the therapeutic gene directly to the body part that has defective cells or infuse into the bloodstream to target a specific tissue type or organ with low or non-existent cell turn-over.
Continue learning about gene therapy in the next section
What Are the Potential Risks of Gene Therapy?
To be aware that there are risks, some serious, associated with gene therapy.
- How does gene therapy work? Help Me Understand Genetics: Gene Therapy and Other Medical Advances. MedlinePlus Genetics. U.S. National Library of Medicine, National Institutes of Health, U.S. Department of Health and Human Services; 2021. https://medlineplus.gov/genetics/understanding/therapy/procedures/. Accessed December 22, 2021.
- What Is Gene Therapy? How Does It Work? U.S. Food & Drug Administration; 2017. https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm589197.htm. Accessed December 22, 2021.
- Gene therapy. Mayo Clinic; 2017. https://www.mayoclinic.org/tests-procedures/gene-therapy/about/pac-20384619. Accessed December 22, 2021.
- Lentiviral vectors. Gene Therapy Net.com; 2021. https://www.genetherapynet.com/viral-vector/lentiviruses.html. Accessed December 22, 2021.
- Munis AM. Gene therapy applications of non-human lentiviral vectors. Viruses. 2020;12:1106; doi:10.3390.v121011106. [PubMed]
- Colella P, Ronzitti G, Mingozzi F. Emerging issues in AAV-mediated in vivo gene therapy. Mol Ther Methods Clin Dev. 2018;8:87-104. [PubMed]
- Naso MF, Tomkowicz B, Perry WL, Strohl WR. Adeno-associated virus (AAV) as a vector for gene therapy. BioDrugs. 2017;31:317-334. [PubMed]
- Apheresis fact sheet. Yale Medicine; 2021. https://www.yalemedicine.org/conditions/apheresis. Accessed December 22, 2021.
- Transduction. Farlex Partner Medical Dictionary; 2012. https://medical-dictionary.thefreedictionary.com/transduction. Accessed December 22, 2021.
- Phillips MI, Tang YL. Genetic modification of stem cells for transplantation. Adv Drug Deliv Rev. 2008;60(2):160-172. [PubMed]
- Bulcha JT, Wang Y, Ma H, Tai PWL, Gao G. Viral vector platforms within the gene therapy landscape. Signal Transduct Target Ther. 2021;6(1):53. [PubMed]
- Kulkami JA, Cullis PR, van der Meel R. Lipid nanoparticles enabling gene therapies: from concepts to clinical utility. Nucleic Acid Ther. 2018;28(3):146-157. [PubMed]
- Approved cellular and gene therapy products. U.S. Food and Drug Administration (FDA); 2021. https://www.fda.gov/vaccines-blood-biologics/cellular-gene-therapy-products/approved-cellular-and-gene-therapy-products. Accessed December 22, 2021.
- Gene therapy medicinal products. Paul-Ehrlich-Institut; 2021. https://www.pei.de/EN/medicinal-products/atmp/gene-therapy-medicinal-products/gene-therapy-node.html. Accessed December 22, 2021.