Gene therapy is the use of DNA as a pharmaceutical agent to treat disease. It derives its name from the idea that DNA can be used to supplement or alter genes within an individual's cells as a therapy to treat disease. The most common form of gene therapy involves using DNA that encodes a functional, therapeutic gene to replace a mutated gene. Other forms involve directly correcting a mutation, or using DNA that encodes a therapeutic protein drug (rather than a natural human gene) to provide treatment. In gene therapy, DNA that encodes a therapeutic protein is packaged within a "vector", which is used to get the DNA inside cells within the body. Once inside, the DNA becomes expressed by the cell machinery, resulting in the production of therapeutic protein, which in turn treats the patient's disease.
Gene therapy was first conceptualized in 1972, with the authors urging caution before commencing gene therapy studies in humans.The first FDA-approved gene therapy experiment in the United States occurred in 1990, when Ashanti DeSilva was treated for ADA-SCID.Since then, over 1,700 clinical trials have been conducted using a number of techniques for gene therapy.
Types of gene therapy Gene therapy may be classified into the two following types:
Somatic gene therapy :In somatic gene therapy, the therapeutic genes are transferred into the somatic cells, or body, of a patient. Any modifications and effects will be restricted to the individual patient only, and will not be inherited by the patient's offspring or later generations. Somatic gene therapy represents the mainstream line of current basic and clinical research, where the therapeutic DNA transgene (either integrated in the genome or as an external episome or plasmid) is used to treat a disease in an individual.
Germ line gene therapy:In germ line gene therapy, germ cells, i.e., sperm or eggs, are modified by the introduction of functional genes, which are integrated into their genomes. This would allow the therapy to be heritable and passed on to later generations. Although this should, in theory, be highly effective in counteracting genetic disorders and hereditary diseases, some jurisdictions, including Australia, Canada, Germany, Israel, Switzerland, and the Netherlands prohibit this for application in human beings, at least for the present, for technical and ethical reasons, including insufficient knowledge about possible risks to future generationsand higher risk than somatic gene therapy (e.g. using non-integrative vectors). The USA has no federal legislation specifically addressing human germ-line or somatic genetic modification (beyond the usual FDA testing regulations for therapies in general)
Preventive gene therapy:Preventive gene therapy is the repair of a gene with a mutation associated with a progressive disease, prior to the expression of a medical condition, to prevent that expression.
One case study of preventive gene therapy: Retinitis Pigmentosa
Blindness can be caused by multiple genetic diseases. Many gene therapy efforts have been focused on treating blindness as a result of moderate success in preventing the loss of vision in multiple animal models. To do this, blindness must be diagnosed early on before the symptoms begin. The retina, which is located in the back of the eyeball, is the first step in processing visual information, accordingly it is a common target in exploration of the genetic issue that leads to blindness. One autosomal genetic disease that has been extensively researched is retinitis pigmentosa (RP) because it has excellent animal models for genetic therapy techniques to treat blindness.
Within a single disease, there can be multiple preventative gene therapy strategies used to combat the progression of the symptoms. Most people who suffer from RP are born with rod cells that are either dead or dysfunctional, so they are effectively blind at nighttime, since these are the cells responsible for vision in low levels of light. What follows often is the death of cone cells, responsible for color vision and acuity, at light levels present during the day. Loss of cones leads to full blindness as early as five years old, but may not onset until many years later. There have been multiple hypotheses about how the lack of rod cells can lead to the death of cone cells. Pinpointing a mechanism for RP is difficult because there are more than 39 genetic loci and genes correlated with this disease. In an effort to find the cause of RP, there have been different gene therapy techniques applied to address each of the hypotheses.
One hypothesis is that when the rod cells die, there is no longer a critical growth factor secreted for proper cone function and survival. Some scientists have experimented with treating this issue by injecting substitute trophic factors into the eye. One group of researchers injected the rod derived cone viability factor (RdCVF) protein into the eye of the most commonly occurring dominant RP mutation rat models. This treatment demonstrated success in promoting the survival of cone activity, but the treatment served even more significantly to prevent progression of the disease by increasing the actual function of the cones. Two of the major issues encountered when trying to inject these desirable proteins, is that they are both difficult to purify and difficult to deliver multiple times as a consistent treatment. Researchers are currently trying to develop an adeno-associated virus (AAV) vector to use for such treatments to address these problems. Injection of an AAV vector encoding this factor might only have to happen once.
Another issue is choosing which protein to inject. Multiple candidates such as brain derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), basic fibroblast growth factor (bFGF), and pigment epithelium derived factor (PEDF) have demonstrated functional success within the eye when they are delivered to the subretinal region. One of the most successful proteins of those aforementioned, is ciliary neurotrophic factor (CNTF). CNTF has shown success of slowing retinal degradation in 13 different animal models. CNTF is currently being used as a treatment in human clinical trials. In Phase I of the clinical trial, there were 10 participants suffering from advanced RP who each had one of their eyes implanted with CNTF encapsulated cell devices for six months, followed by explantation. Five of the participants received higher dose implants of CNTF, and the other five received an approximately five times smaller dose implant. Multiple participants displayed improved acuity both while the implant was in the eye, and one month later when retested. In some of the other animal models, high doses of CNTF have demonstrated increased photoreceptor metabolic activity, which suggests that it is possible for CNTF to increase photoreceptor metabolic activity in human damaged cones as well. CNTF could improve metabolic activity in photoreceptor cones to the extent that it improves visual acuity, as seen in several of the participants who performed significantly better on the letter recognition task compared to their respective pretreatment baseline. Although not all of the participants experienced a significant increase in visual acuity, this method of encapsulated implantations poses an intriguing line of research. It offers the advantage of being reversible, as one can remove the encapsulated cells should anything go wrong. Gene therapy using viral vectors to deliver a therapeutic gene is not obviously reversible.
