Gene therapy is one of the key techniques associated with biotechnology, involving the insertion and introduction of foreign DNA into the genome of some other organism, in order to cure, treat or prevent diseases caused by genetic factors. This approach involves introducing a functional gene through a vector which may replace the problem gene in the genome and starts work in a proper way in order to rectify the disease.

Genes consist of codons that code for the formation of various proteins in the body. If the sequence of genes is altered due to mutation; the specific protein fails to form mostly resulting in the disorder. This disorder might be uprooted by replacing the problem genes by new functional genes, through a vector (mostly viral DNA, RNA or bacterial plasmids) which integrates the gene of interest in the genome of the patient. This gene of interest starts forming normal proteins and required products to cure and stop the disease.

This field of biotechnology has not been explored at a large scale and its applications have not been exploited widely due to many ethical, social and medical view points. The use of gene therapy is mainly restricted to cure the diseases controlled by a single gene and concentrated on a specific part of the body e. g. sickle cell anemia, cystic fibrosis, haemophilia and muscular dystrophy.

Gene therapy has following main types:
• Germ line gene therapy: Germ line gene therapy is related to the introduction of genes in the germ cells i.e. sperm or egg cells', thus curing the inheritable diseases and the functional gene is also inherited and works in the next generation. Germ line gene therapy appears to be very effective and helpful in controlling diseases but due to many expected alarming consequences it has not got popularity in applied treatment. There are many ethical and technical reasons for which the germ line therapy has not been applied to humans on clinical trials.

• Somatic gene therapy: somatic gene therapy involves the insertion and integration of foreign gene in the somatic cells of the patient. In this way the specific cells undergo change and show their results and its consequences are restricted to the transgenic individual and are not transferred to the filial generations. This appears to be safer and more applicable approach as it may not involve some unseen and unexpected consequences resulting from the new genetic make up. This approach may also be tested on clinical trials easily as its all beneficial as well as harmful effects (if any) are confined to that particular person.

• Preventive gene therapy: Preventive gene therapy involves repair of mutated gene by integration of some other functional gene in the genome prior to the impression of the disease in the individual. This approach may have some considerations regarding to the approval of health department for the application of particular therapy, expected mutation and tumor induction in the cells. Moreover, the long term and short term impacts of preventive gene therapy are difficult to evaluate and measure the exact results. The effects of therapy on disease reduction remains unknown as the effects of disease would have spread due to mutation.

Limitations of gene therapy
Human immune system prevents the invaders by attacking them. Thus the inserting genes may not be able to reach the target loci or lose their potential to perform appropriately. The opposite probability is also there that the viral vectors may also induce a problem by regaining the ability to induce disease in human body. It may also cause toxicity, inflammation, take over the control of gene action.

Most common diseases of our societies are diabetes, Alzheimer's disease, arthritis, high blood pressure and cardiac problems, which all are multigene disorders and gene therapy is unable to present the best solution for the multigene disorders. This technology is the best one for the disorders which are controlled by a single gene so that they can be easily controlled.
It is difficult to have long term results of the gene therapy as most of the gene cannot remain functional for long term due to promptly dividing nature of cells. Thus the patient may have to undergo several therapies at intervals.

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