Gene therapy is an experimental technique to treat disease or disorders using genes. It is predicted that in the near future instead of drugs or surgery doctors will use gene therapy to treat a disorder. Various approaches are being tested by researchers. Important among them are replacing a mutated gene with a healthy copy, inactivating a mutated gene, or introducing a new gene to treat a disease. Though gene therapy is a promising option to treat a number of diseases, the technique is risky and under study to ensure that it is safe and efficacious. Gene therapy is currently tested only for diseases without any cure. This article reviews how gene therapy works, applications of gene therapy and future prospects.
How gene therapy works
Gene therapy is the technique where genetic material is introduced into affected cells to compensate for the mutated gene. The mutated gene causes some essential proteins to behave abnormally. In these cases a copy of the gene that is normal can be introduced specifically to restore the function of this protein. The genetically engineered gene is not inserted into a cell directly; instead a carrier called a vector is used to deliver the gene. The most common vectors are viruses due to their ability to deliver new genes to the cells by infecting them. The viruses do not cause disease when introduced in human cells as they are modified for this purpose. There are two major ways to deliver new genes:
In vivo technique: The in vivo technique does not involve any surgery. In this technique, viruses are used as vectors for the therapeutic DNA and directly injected into the body cells. Most commonly either retroviruses or adenoviruses are used as the vector. Mann et al. have synthetically synthesised retroviruses without a reproduction sequence at MIT (Mann et al., 1983). This virus is used as the vector to deliver therapeutic DNA that is spliced into it during gene therapy. This virus does not have viral DNA that would damage the cell or cause disease. Gene therapy using retroviruses is considered safe and providing long-lasting effects. However, the new DNA will not help the already existing cells but only newly formed daughter cells. Adenoviruses used as vectors are derived from the common cold virus (Vorburger and Hunt, 2002). After injecting the spliced therapeutic DNA this virus also dies. However, the immune system attacks the virus leading to sore throat with runny nose in patients receiving the therapy (Fujiwara et al., 2000). Though both the adenovirus and retrovirus work in a similar way, the adenovirus acts very quickly and the effects can be observed within 48 hours. Another advantage is that a few millilitres of altered adenovirus is enough to cure the patient compared to litres of retrovirus.
Ex vivo technique: This involves culturing of the cells removed from the affected tissue area after splicing the new DNA into the cells. These new tissues are used to replace the affected area of the patient. Most commonly only the patient’s bone marrow is cultured as it produces the blood which will eventually travel throughout the body. However, this procedure involves a surgical process which is painful. Moreover, as the culturing requires hours to complete, the patients have to undergo the process twice, once to extract the bone marrow and a second time to replace it back after culturing.
Miscellaneous techniques: There are various other emerging methods. However these are not commonly used and are under investigation. One such method involves culturing endothelium tissue after inserting the therapeutic DNA followed by grafting in the patient. The other technique involves submerging the patients in a therapeutic DNA solution and subjecting them to electric shock. The electric shock opens pores in the skin and allows entry of the DNA. There are many other options under investigation like skin grafting, connective tissue grafting, etc.
Safety of gene therapy
There is a big question around the safety of gene therapy. Several reports have indicated serious health risks like cancer, toxicity and inflammation. Some risks are unpredictable and regulatory bodies demand exhaustive safety studies before they approve their use. Gene therapy is currently under investigation for safety. Future studies would indicate the efficacy of the technique. Since the technical procedures are new medical research, regulatory agencies and research institutions are working rigorously to ensure the safety of gene therapy. In the US the volunteers for clinical trials are protected by federal laws, guidelines and regulations. Every research protocol involving gene therapy needs to be approved by the regulatory agencies. The agencies reserve rights to reject or suspend the trials if suspected to be unsafe.
Applications of gene therapy
Gene therapy can be a better approach for disorders resulting from single gene mutation. Though there are thousands of single gene disorders which can be addressed with relative ease, most of the genetic disorders are caused by multiple genes. There are numerous challenges facing scientists researching gene therapy, which has limited the applications. For these reasons the procedure cannot be optimized and approved for routine use immediately. Further clinical trials in the United Kingdom and France have shown various concerns which prevent gene therapy from becoming a routine practice. Though the few trials have shown that transfer of the new gene into the target cell works, its effect is transient requiring multiple treatments for the patients. There is always a risk of severe immune response with gene therapy, as the immune cells have the tendency to attack any foreign molecule entering the body. The use of viral vectors is challenging because the body immediately attacks common viruses. These complications led to research being focused on potential non-virus vectors.
Treatment of type I diabetes using gene therapy is very promising (Halban et al., 2001). In one report, an adenovirus is used as a vector to deliver the genes to pancreatic cells removed from rats. The new genes are delivered for hepatocyte growth factor (HGF). The altered cells were injected into diabetic rats. These rats were found to have better control over blood glucose levels compared to the untreated group (Nyanguile et al., 2003).
Cancer is a genetic disease where a group of cells show uncontrolled growth destroying adjacent tissues. There are huge efforts to develop specific non-toxic cancer therapies; however, the clinical progress in this area is very slow. Oncology is an area with enormous potential for success and hence a highly targeted area for gene therapy. At least 500 gene therapy clinical trials have been registered with the FDA (Dachs et al., 1997). There are numerous strategies to treat cancer using gene therapy, including enhancing immunological rejection of the tumour by the host, decreasing tumour cell proliferation, poisoning the tumour cells specifically, and specifically lysing defective tumour cells using oncolytic viruses. The key genetic difference between the cancer and healthy cells is the relative deficiency of a gene called p53. One report claims positive results in a study on patients suffering from head and neck injury. These patients were treated with a modified virus called ONYX-015 which specifically destroys only those cells lacking the p53 gene (Nielsen and Maneval, 1998)
Gene therapy can be potentially used to treat X-linked severe combined immunodeficiency (X-SCID). X-SCID is disease that develops in babies when their immune system lacks T and B cells and become vulnerable to infections. Currently these conditions are treated by transplanting the bone marrow from a sibling. In the long term this is not regularly possible or even effective. Fourteen children were treated using the ex vivo technique of replacing faulty genes by researchers in UK and France. The immune system was found to be greatly improved after receiving the altered genes. However, two children developed leukaemia after several years of treatment. Detailed study was continued on these children and the results showed that the T cells had grown in an uncontrolled fashion as the vector had inserted the gene in the proximity of a proto-oncogene. The clinical trials were not safe and hence put on hold. Efforts to develop safe methods of delivering the genes are being investigated and form the topic of current research.
Sjogren’s syndrome affects millions of people, especially female. This is the most common autoimmune disease. In this syndrome focal lymphoid cell infiltration is observed in the salivary and lacrimal glands (Pillemer et al., 2001). Since there is no suitable treatment available for these patients, gene therapy may be beneficial. Kok et al. (2003) have hypothesized that increased salivation and symptomatic relief may be obtained by transferring immunomodulatory genes into SGs which may reduce the autoimmune sialadenitis. They have reported prevention of salivary flow in a female non-obese diabetic mouse after delivering altered DNA.
Apart from these examples various diseases are being tested in vitro, in vivo and clinically using gene therapy. The most common diseases are cystic fibrosis, haemophilia A, haemophilia B, atherosclerosis and many more.
Gene therapy is absolutely a new hope for many genetic disorders, but still it has not reached the final stage. Gene therapy’s advantages and disadvantages are not clearly defined. The main negative aspect of gene therapy is that it is based on theoretical science rather than solid facts. Although gene therapy has many benefits, there have been some cases of death. The development of gene therapy began in the 1970s, and much more time will be required to understand and explore the science behind it. Though the current situation of gene therapy is under process in future it will be one of the obtainable options for curing and preventing genetic disorders.
Dachs G.U., Dougherty G.J., Stratford I.J. and Chaplin D.J. (1997). Targeting gene therapy to cancer: a review. Oncol Res, 9, pp.313–25.
Fujiwara T., Kataoka M. and Tanaka N. (2000). Adenovirus-mediated p53 gene therapy for human cancer. Mol Urol, 4, pp.51–54.
Halban P.A., Kahn S.E., Lernmark A. and Rhodes C.J. (2001). Gene and cell-replacement therapy in the treatment of type 1 diabetes. How high must the standards be set? Diabetes 50, pp.2181–2191.
Kok M.R., Yamano S., Lodde B.M., Wang J., Couwenhoven R.I., Yakar S., Voutetakis A., Leroith D., Schmidt M., Afione S., Pillemer S.R., Tsutsui M.T., Tak P.P., Chiorini J.A. and Baum J. (2003). Local adeno-associated virus-mediated interleukin 10 gene transfer has disease-modifying effects in a murine model of Sjogren’s syndrome. Hum Gene Ther, 14, pp.1605–18.
Mann R., Mulligan R.C. and Baltimore D. (1983). Construction of a retrovirus packaging mutant and its use to produce helper-free defective retrovirus. Cell, 33, pp.153–159.
Nielsen L.L. and Maneval D.C. (1998). P53 tumor suppressor gene therapy for cancer. Cancer Gene Therapy. 5, pp.52–63.
Nyanguile O., Dancik C., Blakemore J., Mulgrew K., Kaleko M. and Stevenson S.C. (2003). Synthesis of adenoviral targeting molecules by intein-mediated protein ligation. Gene Therapy, 10, pp.1362–1369.
Pillemer S.R., Matteson E.L., Jacobsson L.T., Martens P.B., Melton L.J., O’Fallon W.M. and Fox P.C. (2001). Incidence of physician-diagnosed primary Sjögren syndrome in residents of Olmsted County, Minnesota. Mayo Clin Proc, 76, pp.593–99.
Vorburger S.A. and Hunt K.K. (2002). Adenoviral gene therapy, Oncologist, 7, pp.46–59.