What is gene therapy?

The core idea behind gene therapy is to replace a defective gene with a functional gene. Gene therapy is when DNA is used as the therapeutic. This is in contrast to most drugs, which use a small molecule, or a recombinant protein. The DNA (gene) used usually encodes a protein the patient needs.

The difficulty is in the details…

  • Delivery - how do you get these genes to the right cells in the body (liver, blood, brain)?
  • How do you ensure the body’s immune system doesn’t detect and reject the payload (the functional gene you’re trying to introduce)?
  • How do you get the gene to express long term (so the patient is cured for life, rather than being dependent on repeat treatments)?
  • How do you make it safe? Reduce side effects?

Research in gene therapy began in the 1980s, incremental progress since then.

  • 1999, first death from gene therapy
  • 2002 scid success, then retrovirus killed bc of cancer later

Core use cases for gene therapy

Genetic diseases where a single gene is mutated and needs to be fixed. Some examples: SCID, hemophilia, muscular dystrophy. Cancer is becoming an increasingly interesting target for gene therapy. The solution is generally to add the wild-type version of the gene to correct the mutation.

Modalities of gene delivery

in vivo, ex vivo, viral vectors, nonviral vectors

In vivo - direct delivery to the body

Ex vivo - deliver to cells, then transplant cells to the body Solid tissues typically don’t accept grafts very well, so ex vivo techniques are typically used for blood disorders.

Viral Vectors

Viruses know how to deliver their DNA into a cell, scientists have co-opted the virus’s machinery to deliver a gene of interest to a cell of interest. Different viruses have different payloads (size of gene they can deliver)
Some examples: Adenovirus (cold virus), herpes virus, retrovirus (RNA virus, capacity 8-10kb), lentivirus (hiv virus), adeno-associated virus (AAV).

Pros of viral vectors:

  • Adenovirus has a carrying capacity of 30kb, large enough to carry any human gene (even multiple). Doesn’t integrate with the genome (not good for dividing cells, but if the cells are not dividing, can be viewed as a safety feature)

Cons of using viral vectors:

  • Capsid, the protein encasing the virus, inherently produces an immune / inflammatory response.
  • Insertional mutagenesis hazard of random integration

Nonviral Vectors

Electroporation, lipids, nanoparticles, fluid pressure.

  • Electroporation - a physical method, used often for cells in culture, introduce a transient electrical charge which makes the cell membrane permeable, so the DNA can enter the cell, and the cell membrane can heal following. “leaves no trace”, no viral protein was introduced to the cell.
  • Lipids - Complex DNA with lipids to get them to be taken up by cells
  • Nanoparticles - under development
  • Fluid pressure - use fluid pressure to get DNA in contact with and then taken up by cells

Gene Therapy Successes


The most compelling success to date has been in chimeric antigen receptor in T-cells (CAR-T).
The idea here is to add new antigen receptor in patient’s T-cells that recognizes cancer cells. These modified T-cells then go out and kill the tumor. Success in leukemia and other cancers. Most success in targeting CD19, a cell surface molecule expressed on most leukemias and lymphomas. 15-30% of patients experience severe side effects, toxicities, cytokine release syndrome

First two FDA approvals for CAR-T:

  • August 30th 2017 - Novartis - Kymriah
    Treats advanced acute lymphoblastic leukemia in children and young adults up to age 25
    Cost: $373,000 + costs of hospital services
    15-30% will have complications, requiring up to months in the ICU (intensive care unit)

  • October 19th 2017 - Kite-Gilead - Yescarta
    Treats non-Hodgkin lymphomas, age 60+
    4% of all cancers in the USA, 72k new cases each year
    51% of patients had complete remission
    Some percentage will have severe side effects

HIV infects cells through CCR5, a receptor on T-cells. Clinical trials have explored extracting a patients t-cells, using a ZFN zinc finger nuclease to cut out the CCR5 receptor in the T-cells, purify, and reintroduce them into the patients body. This means the entire population isn’t resistant, but at least a population is, which has been shown to confer some resistance.

ADA-SCID approved gene therapy product in Europe and X-SCID (retrovirus and lentivirus vectors) dozens of patients improved or cured Successes in blood disorders, beta thalassemia and sickle cell anemia


Main problem is delivery

Thanks to Michele Calos - Professor in Stanford’s Department of Genetics for the inspiration to write this and for reviewing it!