Cancer, as today, is projected as one of the fatal diseases amongst all the dreadful diseases of this modern world. Cancer cells are different from other neighbouring cells in having phenotypic changes in the manner of a rapid division rate, a high metabolic rate, and alteration in their shapes. Some of the mutations may get transmitted from parents through the germ line, while others may arise de novo in the somatic cell lineage of a particular cell. Oral cancer, namely, Oral Squamous Cell Carcinoma (OSCC) is one of the commonly seen malignant lesions in the oral cavity, which is seen globally  and it is about the 6th most common cancer world-wide . Oral cancer is associated with genetic mutations which occur due to the exposure to tobacco, alcohol, betel quid, etc . It occurs in people who are aged 50 years or over. However, about 6% of the cases occur in young people who are under the age of 45 years [4,5]. It is a malignant disorder in which the genes that control cell growth and apoptosis are mutated and this results in an uncontrolled proliferation of the cells in the tumour . With an increase in the total cell mass in the tumour, it has the ability to invade and undergo metastasis. The patients with cancer usually remain resistant to the standard therapies which are used readily. But, there may be chances of acute and chronic toxicities, as well as secondary malignancies. Hence, to improve the treatment modality and the over- all survival rates, gene therapy has emerged in the field of bio-medicine, which replaces the defective gene and this is repaired by a therapeutic gene . The applications of gene therapy in cancer are associated with bio-engineering and clinical development . In the dental field, it is also applied in bone repair, auto immune diseases, pain, caries and periodontal diseases .
THE HISTORY OF GENE THERAPY
Gene amplification, which is used in the treatment of various human diseases, was put forward by Cusack and Tanabe in 1998 . Gene therapy is defined as gene transfer for the purpose of treating human diseases effectively (Cusack and Tanabe, 1998) [9,10], which includes both the transfer of new genetic material and manipulation of the existing genetic material . The first successful treatment was of X-linked Severe Combined Immunodeficiency (X-SCID) by ex vivo gene replacement therapy .
The Types of Gene Therapies
Somatic gene therapy
Germ line gene therapy
In somatic gene therapy, the therapeutic genes are introduced into somatic cells, which restricts the effects of the individual and are not passed on to the next generation. In germ line gene therapy, either the sperm or egg can be altered by introducing the therapeutic gene, which gets integrated into the genome .
The other various gene therapies are suicide gene therapy, immunologic gene therapy, excision gene therapy, etc. Suicide gene therapy is the introduction of a gene into the cell, which converts a non-toxic pro-drug into a toxic substance [9,12] and it is also called genetic pro-drug activation therapy [9,13]. Immunologic gene therapy aims at increasing the immunologic potential of the tumour cells, thereby increasing the patient’s immune response to the tumour. Excision gene therapy is the removal of oncogenes, which inhibits the growth of the tumour cells .
The Vectors in Gene Therapy
Genetic material is delivered into the host cells through viruses or bacteria . Gene therapy is concerned with DNA which can be delivered into cells by various methods. All viruses such as retroviruses, adenoviruses, lentiviruses, herpes simplex virus, vaccinia, pox virus, and adeno-associated virus bind to their hosts by introducing their genetic materials into the host cells. In gene therapy, the viral DNA can be removed, while the viruses can act as vehicles to deliver the therapeutic DNA into the host cells. The viruses which are used as vectors in gene therapy include retroviruses, adenoviruses, adenoassociated viruses and Herpes simplex virus . The methods of non-viral gene therapy include the injection of naked DNA, electroporation, the gene gun and the use of oligonucleotides, dendrimers and inorganic nanoparticles. However, the non-viral vectors which are inhibited by the serum components, limit the efficiency of the gene delivery in vivo . Despite the use of several non-viral methods, viruses provide a more efficient mode in gene therapy .
The Technique of Gene Therapy
The main objective of gene therapy is to introduce new genetic material into the target cells without causing any sort of damage to the surrounding normal cells. The therapies that express gene products, which result in the death of cancer cells, include, gene addition therapy, gene excision therapy, antisense RNA therapy, immunotherapy, suicide gene therapy, gene therapy with the use of oncolytic viruses, the introduction of genes to inhibit tumour angiogenesis and the delivery of drug resistance genes into normal tissues for protection against chemotherapy . According to the results of animal studies which were done in mice, combination gene therapy which uses several genes, showed significant tumour regression in mice [17,18].
Gene Addition Therapy
Genetic alterations include mutations of p53, the Retinoblastoma Gene (RB1), p16 and p21. Among which, the tumour suppressor gene which is commonly used in gene therapy is the p53 gene  and about 60% of the human tumours are associated with mutations at the p53 locus . In this technique, the tumour growth is controlled by the introduction of tumour suppressor genes which inactivate the carcinogenic cells .
Gene Excision Therapy
In this technique, the defective oncogenes are removed, as a result of which, there is an inhibition in the growth of the tumour cells .
Antisense RNA Therapy
The Antisense RNA checks the tumour growth by inhibiting the RNA which is complementary to the strands of the DNA which expresses that particular gene .
Oral cancer patients usually show defects or deficient immune cell functions of the natural killer cells, lymphocytes, cytokines, etc. This technique increases the immune response of the patients to the tumour .
Suicide Gene Therapy
Many studies have been done on the gene delivery system with retrovirusor adenovirus vectors [21–24]. This therapy involves enzymes, the expression of which transforms the non-toxic producing drug into an active cytotoxic substance . It is the most commonly used gene therapy which uses thimidine kinase or other chemosensitizing genes .
Gene Therapy with the Use of Oncolytic Viruses
In this therapy, a vector (virus) is genetically modified, which replicates and lyses the tumour cells . For example, adenovirus-mediated gene therapy is used for advanced cancers than traditional therapies .
The Delivery of Drug Resistance Gene(s) to Normal Tissues for Protection from Chemotherapy
The drug resistance genes protect the normal tissues which are vulnerable to destruction. The drug resistance gene in humans is the Multidrug Resistance-1 (MDR-1) gene. The other drug resistance genes include the bacterial nitroreductase gene and the dihydrofolatereductase mutants which protect against methotrexate .
Some Disadvantages of Gene Therapy
Even after the therapeutic DNA is integrated into the genome, some cells prevent the gene therapy for long-term effects, for which patients may undergo multiple rounds of the gene therapy.
There is a possibility that the host’s immune system and its response may reduce the effectiveness of the gene therapy.
The viral vectors can present a variety of potential problems to the patient, such as toxicity and immune and inflammatory responses.
Single gene disorders are the best candidates for gene therapy. But, some of the most commonly occurring diseases, such as heart disease, high blood pressure, Alzheimer’s disease, arthritis, and diabetes are multi-gene or multi-factorial disorders which are difficult to treat effectively with the use of gene therapy.
If the DNA is introduced into a wrong place in the genome, for example, into a tumour suppressor gene, it can induce a tumour.
Despite of the disadvantages of gene therapy, it has advantages as well;
A functional gene has the ability to replace a defective gene.
Gene therapy aids in the prevention against the potentially toxic effects in the body, which can be caused by other therapies.
It decreases the cost of various therapies and improves the patient’s life style for a longer period .
Gene therapy is an attractive tool in the treatment of oral squamous cell carcinoma and pre-cancer, because it targets cancer cells only. Today, the research on gene therapy in oral cancer is increasing day by day, both in the laboratory and the clinical settings. In future, it can be a forerunner as a definitive treatment option for oral cancer and pre-cancer, which can offer better effectiveness as compared to that of the current therapies, by reducing the high mortality which is associated with these lesions. At present, the use of adenoviruses altered gene therapy technique with chemotherapy or immunotherapy appears to be the most promising approach in the management of oral cancer and pre-cancer.
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