Gene therapy, where a defective gene is replaced to restore normal gene function, holds great promise as a treatment for a wide range of diseases. However, early setbacks and unclear regulations meant that many pharma companies paused their development. In recent years, gene therapy has made a comeback and has finally begun to deliver on its potential with the release of several FDA-approved gene therapy treatments for a variety of disorders.
The rapid development in molecular biology over the last few decades has revolutionized how we think of disease. A wide range of disorders have been found to have a genetic basis, and many of the underlying gene mutations have been identified.
Gene therapy holds great promise as an effective treatment for these diseases. Over the coming decades, gene therapy treatment of a wide range of disorders, such as inherited diseases, cancer, and viral infections, may be commonplace.
How it works
The principle of gene therapy is straightforward: deliver a functional gene to target cells in the patient to restore normal gene function. This is most commonly achieved using a recombinant Adeno-Associated Virus (AAV) vector where some of the viral genetic material has been replaced by a therapeutic cargo. Once the virus has infected the target cell, it uses the cell's molecular machinery to produce the therapeutic protein.
Principle of AAV-mediated gene therapy. Recombinant AAV vectors carrying transgenes cross the cell membrane and deliver their cargo into the cell of the nucleus. Here, the transgenes persist in a circular episomal state. Following the transcription of the episome DNA, therapeutic protein is expressed in the cytoplasm of the cell.
In gene therapy, recombinant AAV vectors which lack viral DNA are used. They have been engineered to cross the cell membrane and deliver their DNA cargo into the cell nucleus. The transgenes are flanked by inverted terminal repeats (ITRs) that allow them to form episomes in the nucleus of the cells. Since the episomes do not integrate into the cell genome and have no way of self-replicating, they will be diluted to a point where they are eventually lost over multiple rounds of cell replication.
A focus on the eye
Gene therapy can be used to treat diseases in many different tissues. However, most clinical trials focus on retinal diseases, which is not surprising considering the number of advantages the eye offers:
- First, the enclosed nature of the eye and the fact that it is relatively immune-privileged mean that the immune response against the viral vector will be weaker, increasing the probability of success.
- Second, retinal cells do not proliferate after birth. This is important as a single injection could offer life-long expression of the therapeutic protein.
- Third, several animal models are available for inherited retinal diseases, which is instrumental for safe and efficient drug development.
There are over 200 genes known to be involved in retinal diseases, some of which are tempting targets for gene therapy. Most retinal gene therapy development has focused on single-gene rare diseases with clear genomic targets and few effective traditional treatments.
The importance of immunogenicity testing
Since most gene therapy projects use a recombinant AAV vector, where some of the viral genome has been replaced by a therapeutic cargo, immunogenicity testing is essential. Patients previously infected by naturally occurring AAVs may carry neutralizing antibodies (NAbs) that prevent the gene therapy vector from transfecting the host cell and delivering its cargo.
This can eliminate the therapeutic effect and potentially lead to other more severe cases of unwanted immunogenicity effects.
Neutralizing antibodies (NAbs) can block AAVs from entering the cell. Patients who have been infected by naturally occurring AAVs in the past may have developed NAbs against the capsid, which may render the treatment ineffective.
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