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Gene Therapy

We’re Not Sure What Gene Therapy Is – Here’s Why That’s A Problem

Posted on 7 January 2021

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Gene therapy is an approach that uses genes to treat or prevent disease. It is a revolutionary set of technologies that offers the potential to cure previously incurable genetic conditions as well as some non-genetic ones. For example, gene therapy can be used to replace a mutated, disease-causing gene with a healthy version.

Fluorescence Guided Suicide Gene delivery
Gene therapy can fix genetic conditions by introducing a health version of the defective gene.

Despite the concept of gene therapy existing since the 1960s, science has not yet agreed on a clear definition of what counts as gene therapy. The original definition proposed 50 years ago described gene therapy as the replacement of ”bad” DNA with ”good” DNA. However, the field of gene therapy has rapidly evolved to make this definition inadequate. New genetic techniques that don’t involve modifying genes within living humans have nevertheless been branded as gene therapy. For example, it is possible to introduce molecules that will inactivate or interfere with the expression of a disease gene without altering the gene itself. It is also possible to introduce new genes to cells that have been removed from the body, then introduce them back into the patient (ex vivo gene therapy).

This graphic depicts the two major types of gene therapy.  The illustration on the left shows ex vivo gene therapy, where the cells are modified outside the body and then are delivered back to the patient.  The illustration on the right shows in vivo gene therapy, where the genetic modification of the cell takes place inside the body.
Gene therapy techniques: in vivo vs ex vivo.
FDA: What Is Gene Therapy
Gene Therapy Products: Separate Regulations? - BioProcess  InternationalBioProcess International
CAR T-cell therapy is a promising ex-vivo gene therapy in which the patients immune cells are modified to attack cancer cells.
BioProcess International

Comparing Pfizer-BioNTech and Moderna’s mRNA vaccines to the Oxford-AstraZeneca adenovirus vaccine for COVID-19 is an appropriate and topical way to demonstrate the difficulties in defining gene therapy today. The Oxford-AstraZeneca vaccine uses a harmless, non-replicating adenovirus to deliver viral DNA to the nuclei of human cells, where the human DNA is stored. The viral DNA includes the gene for the coronavirus spike protein. This gene is used to make a template called mRNA, which is then used by the cell to make the spike protein.

An adenovirus is used to introduce the spike protein gene into the nucleus, which is then copied into mRNA.
The New York Times

mRNA vaccines skip most of this process by directly introducing mRNA that can be used to produce the spike protein. These vaccine strategies therefore share the same end point – the production of spike protein from mRNA. However, adenovirus vaccines are generally considered to be a form of gene therapy, while mRNA vaccines are not, as they do not involve the introduction of a new gene. Some might argue that even adenovirus-delivered genes are not technically gene therapy, because they don’t correct defects in the recipient’s genetic makeup.

At this point, you may be asking yourself why any of this matters. After all, vaccines will work regardless of whether they are considered a gene therapy or not. However, this does matter in practice, as it affects regulation by health authorities, support for streamlining development, manufacturing procedures, and pricing. As the field continues to move forward and the boundaries of what is considered to be gene therapy become blurred, we could all therefore benefit from a clearer definition of the term.

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