Gene therapy is a method for curing disease by changing a person's DNA. Gene therapies can function in a variety of ways: Replacing a disease-causing gene with a healthy copy of the gene, inactivating a disease-causing gene that is not functioning properly. Gene therapy consists of 3 components: gene silencing using siRNA, shRNA, and miRNA, gene replacement in which the desired gene is directly administered in the form of plasmids and viral vectors, and gene editing based therapy in which mutations are modified using specific nucleases such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regulatory interspaced short tandem repeats (CRISPR-Cas9). Gene therapy has already been used clinically for correcting the genetic alteration in some inherited disorders like, haemophilia, familial cystic fibrosis, thalassemia, hypercholesterolemia, RPE65 gene-related retinal disease like Leber's congenital amaurosis, retinal dystrophy and is also used to tackle acquired diseases like AIDS, Parkinson's disease, cancer and Alzheimer's disease. Gene therapy is less prone to face the problem of being resistant as we see in the case of chemotherapeutics. In addition, this approach can be a permanent substitute for patients born along with any genetic disorder to survive a healthy life. Despite its numerous therapeutic benefits, it has some side effects and certain disadvantages as well. Most commonly used viruses like the retrovirus, adenovirus for gene therapy carry the risk of mutagenesis as they can integrate into non-target cells and in vivo efficiency of gene delivery is quite challenging as the delivery vector either could not enter inside the host cells or may become unstable before reaching the target cells.

Fig. Gene therapy using viral vectors. The steps involved in gene therapy utilizing viral and non-viral vectors; out of which, in viral vectors, the gene coding for pathogenicity and replication is removed and replaced by the transgene cassette. This viral construct is cloned in a plasmid and the viral packaging plasmids are transfected inside the packaging cell line that eventually produces the viruses containing the transgene and is released in the supernatant. These viruses are used for in vivo as well as for ex vivo gene therapy. Similarly, in gene therapy using non-viral vectors, the transgene is inserted in plasmid DNA, which is complexed with the non-viral delivery system and directly administered to the patient.
Dr. Md. Monirul Islam
Senior Scientist

References: Nilofer et al. 2022. Gene therapy: Comprehensive overview and therapeutic applications. Life Sciences. Volume 294, 120375.