Gene Therapy: A new approach to treating diseases
What is gene therapy?
The average human body consists of trillions of small building blocks called cells. Within each of these cells, there are numerous genes. Genes contain the necessary genetic information to govern our bodily functions; As an example, a particular gene carries this unique “code” that is needed for the synthesis of this specific protein, which in turn exerts its defined biological role. The “code” that is the specific DNA sequence of the gene, can undergo alterations as a result of exposure to mutagens or due to the occurrence of spontaneous errors during replication. These alterations, also known as mutations, will often lead to cancer and various other genetic disorders that possibly harm the health of the individual.
Recognizing this, scientists have been working on ways to modify and replace the defective genes to prevent, treat, and cure certain genetic diseases instead of using conventional methods such as molecular therapeutics or surgery.
The approach to gene therapy can differ according to the genetic problem that is present. A mutated gene can either be replaced with a healthy copy of the mutated gene or its expression can be suppressed by employing RNA interference or genome editing tools. Although not yet proven in clinical trials, it is theoretically possible to correct a mutated gene in its precise location using genome editing.
Vectors as gene delivery vehicles
Inserting the modified or novel gene directly into the cell usually does not work. Therefore, a carrier called a “vector” is used to carry the gene. An ideal vector should be able to transfer the specific genetic material to the target cell and allow the expression of the gene product without causing toxicity. Vectors can be classified as viral and non-viral.
Viral vectors are viruses that are specifically modified prior to insertion so that they will not instigate the disease. Viral vectors can be further divided into two types: Integrating and non-integrating viral vectors. Integrating vectors, such as, retroviral, lentiviral, and adeno-associated viral vectors, are designed to integrate at one or more loci in the chromosomes and are used when introducing genetic material into stem cells. It ensures that the transferred DNA will be incorporated into the genome of the stem cell, and then will be passed on to the daughter cells in subsequent cell divisions. Non-integrating vectors, such as adenoviral vectors, are used to deliver genes into long-lived postmitotic cells and slowly dividing cells. Since these cells, in general, are no longer dividing, prolonged expression of the gene can be achieved as long as the DNA is stabilized extrachromosomally.
Non-viral vectors, such as naked DNA and liposomes, are simple and safer alternatives to viral vectors. Their relatively simple quantitative production and low host immunogenicity make them excellent vectors. Naked DNA vectors are usually in the form of a plasmid, which is a small, circular, double-stranded DNA molecule. The desired gene can be inserted into the plasmid, and the recombinant plasmid can be inserted into the target cell in a variety of ways. Plasmid DNA can also be coated with lipids to construct structures such as liposomes or micelles.
The use of “gene guns” is another novel technique for the transfer of a gene. This technique does not require the presence of complicated and potentially harmful delivery systems. Therefore it is much safer than any other kind of vector. In this technique, microprojectiles (e.g., gold or tungsten) coated with DNA are shot into the target cell under high pressure and speed to ensure that it passes through the membrane barrier.
The latest studies in gene therapy have proven that bacterial systems are capable of transferring genes to tumor cells. This technique relies on the ability of certain anaerobic bacteria to colonize and replicate in hypoxic areas (tumors), and thus achieve expression of the desired gene in that specific area.
In vivo gene delivery and ex vivo gene delivery
The major difference between ex vivo and in vivo gene therapy lies in the choice of the vector and the way it is inserted into the patient. In the ex vivo method, the cells from the patient are transduced with the gene of interest and are subsequently transplanted back. This process requires an integrating vector. On the other hand, the in vivo method is similar to the insertion of other pharmaceutical agents. Once the insertion is completed successfully, the transgene (or its protein product) will be stably expressed.
Safety aspects of gene therapy
Although gene therapy is a promising treatment option, several hurdles regarding its safety remain to be overcome before it becomes more common in clinical use. Most of the risks of integrating vectors arise due to their potential for insertional mutagenesis, where it disrupts a functional DNA element in the cell, such as a gene. On the other hand, for vectors administered in vivo, the risks arise from immune response to vectors. Scientists have managed to reduce the effects of insertional mutagenesis by the production of safer (lentiviral) vectors and the effects of the immune response have been reduced through the use of adjuvant immunomodulatory drugs.
Possible therapy areas
Clinical trials of gene therapy have proven that it is capable of treating diseases such as hemophilia, leukemia, severe combined immune deficiency, and blindness caused by retinitis pigmentosa. More than 800 research studies are still underway to test gene therapy as a treatment for previously untreatable diseases such as Duchenne’s muscular dystrophy and Huntington’s disease.
Currently, gene therapy is available only as part of a clinical trial. Gene therapy continues to be an active area of research as it holds the promise to revolutionize medicine and create more options for patients suffering from incurable diseases.
References:
Anguela, X. M., & High, K. A. (2019). Entering the Modern Era of Gene Therapy. Annual Review of Medicine, 70(1), 273–288. https://doi.org/10.1146/annurev-med-012017-043332
Dunbar, C. E., High, K. A., Joung, J. K., Kohn, D. B., Ozawa, K., & Sadelain, M. (2018). Gene therapy comes of age. Science, 359(6372), eaan4672. https://doi.org/10.1126/science.aan4672
High, K. A., & Roncarolo, M. G. (2019). Gene Therapy. New England Journal of Medicine, 381(5), 455–464. https://doi.org/10.1056/NEJMra1706910
MedlinePlus — Health Information from the National Library of Medicine. Retrieved November 6, 2020, from https://medlineplus.gov/
Mali S. (2013). Delivery systems for gene therapy. Indian journal of human genetics, 19(1), 3–8. https://doi.org/10.4103/0971-6866.112870
Tomatsu, Shunji & Sawamoto, Kazuki & Chen, Hui-Hsuan & Mason, Robert. (2018). Gene therapy for mucopolysaccharidoses. Molecular Genetics and Metabolism. 123. S140. 10.1016/j.ymgme.2017.12.385. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3722627/
