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REVIEW ARTICLE Table of Contents   
Year : 2007  |  Volume : 18  |  Issue : 3  |  Page : 120-123
Gene therapy for oral squamous cell carcinoma: An overview


1 Department of Oral and Maxillo-Facial Pathology, Meenakshi Ammal Dental College and Hospitals, Chennai - 600 095, Tamilnadu, India
2 Department of Microbiology, Meenakshi Ammal Dental College and Hospitals, Chennai - 600 095, Tamilnadu, India

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Date of Submission23-Jan-2007
Date of Decision06-Feb-2007
Date of Acceptance07-Feb-2007
 

   Abstract 

A potential approach to the treatment of genetic disorders is gene therapy. The goal of gene therapy is to introduce therapeutic genetic material into the target cell to exert the intended therapeutic effect. Gene therapy has already shown promising results for the treatment of monogenic disorders such as severe combined immunodeficiency and haemophilia. Now the procedure has been extended to the level of treating malignant conditions such as cancer of the lungs, breast, colon etc. The prevalence of tumours of the larynx and oral cavity has increased in both developed and developing countries. This increase underscores the need for a novel therapeutic modality that would decrease or completely terminate the proliferation of malignant cells. This review highlights various types of gene therapy procedures with respect to oral squamous cell carcinoma.

Keywords: Adjuvant therapy, gene therapy, oral squamous cell carcinoma, viral vectors

How to cite this article:
Saraswathi T R, Kavitha B, Vijayashree Priyadharsini J. Gene therapy for oral squamous cell carcinoma: An overview. Indian J Dent Res 2007;18:120-3

How to cite this URL:
Saraswathi T R, Kavitha B, Vijayashree Priyadharsini J. Gene therapy for oral squamous cell carcinoma: An overview. Indian J Dent Res [serial online] 2007 [cited 2020 Feb 24];18:120-3. Available from: http://www.ijdr.in/text.asp?2007/18/3/120/33787
The current treatment strategies for oral squamous cell carcinoma (OSCC) include a combination of surgery, radiation therapy and chemotherapy. However, surgical resection of tumours frequently causes profound defects in oral functions such as speech and swallowing as well as in cosmetic aspects. [1] Chemotherapy is associated with well-known toxicity and has demonstrated no clear impact on the survival of patients with recurrent oral cancer. Recurrence develops in approximately one third of the patients despite definitive treatment. [2] Two thirds of the patients dying of this disease have no evidence of symptomatic distant metastasis, therefore, local and regional disease control is paramount, underscoring an urgent need for more effective therapy.

Several reports have indicated that the combination of radiation and gene therapies has synergistic suppressive effects on various cancer cells, including colorectal, ovarian, nasopharyngeal and head / neck cancer cells. [3] Gene therapy can also be used as an adjuvant to surgery (at the resected tumour margins). This review highlights various gene therapy methods that are available for combating OSCC.


   History of Gene Therapy Top


Joshua Lederberg and Edward Tatum laid out the fundamental tenets for gene therapy. [4] The science leading to gene therapy took a large step forward when Michael et al. succeeded in transferring a gene (TK gene, which codes for thymidine kinase) into mammalian cells in 1977. [5],[6] On September 14, 1990, the first approved gene therapy clinical trial took place when Ashanthi De Silva, a 4 year-old girl with Adenosine Deaminase (ADA)-deficiency / Severe Combined Immunodeficiency (SCID) syndrome, was given her own T cells engineered with a retroviral vector carrying a normal ADA gene by the NIH (National Institutes of Health) team of Anderson, Blaese and Rosenberg. [7]

As of March 2004, 619 gene therapy / transfer clinical protocols have been submitted to the NIH / FDA (Food and Drug Administration) for approval: cancer-405 (65%), monogenic diseases (17 different genetic diseases including SCID, haemophilia and cystic fibrosis) -58 (9%), infectious diseases (primarily human immunodeficiency virus, HIV) -40 (6%), other diseases primarily peripheral artery disease and coronary artery disease 69 (11%) and marker or nontherapeutic trials-47 (9%). [8]


   Molecular Basis of Oral Cancer Top


The crucial event in the transformation of a premalignant cell to a malignant cell is the inactivation of cellular negative regulators-tumour suppressor genes. [9],[10] Mutations of the p53 tumour suppressor gene result either in the production of structurally altered protein or complete loss of the protein. p53 has been shown to be functionally inactivated in oral tumours and restoration of p53 in oral cancer lines and tumours induced in animal models has been shown to reverse the malignant phenotype. [11] Apart from p53 mutations, there are several other factors, which contribute to the development of cancer; these include mutations in proto-oncogenes and mutations in genes coding for cell adhesion molecules. Certain viruses have also been found to be associated with oral cancer. [12]


   Concept of Gene Therapy Top


Gene therapy is a therapeutic intervention based on the modification of the genetic material of the living cells. The term "gene therapy" refers to replacing or repairing a defective gene in the diseased cell's genome in order to restore normal cell function and tissue integrity.

A successful gene therapy requires that the:

  • Genetic malfunction/nature of a disease is clearly understood.
  • Therapeutic material can be delivered to the target cells in the affected tissue or organ.
  • Therapeutic material is active for the intended duration and delivers the intended benefit to the target cells.
  • Harmful side effects, if any, are manageable.


Therapeutic material can be delivered to the target cells in two main ways. First, it can be inserted into cells from the affected tissue outside the body and these cells then returned to the body (ex vivo). Second, it can be delivered directly into the body at the required site (in vivo). The ex vivo approach has not been utilized in oral cancer because superficial lesions usually lend themselves to the direct injection of genetic material. Either way, a 'delivery vehicle' called a vector is used to introduce the therapeutic material into the patient's target cells. [13]


   Viral and Nonviral Vectors Top


Vectors are most commonly based on modified viruses because these can target and enter cells efficiently. To date, some 70% of gene therapy trials approved in the UK have involved viral vectors (retro, adeno and herpes viruses). The majority of viral-mediated gene therapy trials in patients with oral cancer have used adenoviruses. The transduction efficiency and ability of recombinant adenovirus to inhibit the tumour were demonstrated using human squamous cell carcinoma cell lines. The efficacy of this viral vector was found to be 100%. [14] A dose of up to 2 x 10 12 viral particles is well-tolerated.

These particles are either administered through intraperitoneal, intrahepatic arterial, intratumoural or intravenous routes. [15] Potential safety problems of viral vectors led to the development of efficient nonviral vectors. Among the nonviral vectors are electroporation, [16] microinjection and use of ballistic particles which are physical methods, whereas liposomes, [17] proteins and calcium phosphate are biochemical methods employed to deliver DNA into the host cells. Clinical trials have demonstrated that electroporation was safe and efficient in SCC. Particle bombardment (biolistics / gene gun) has also been studied to deliver genes to the oral mucosa in preclinical animal models. Two novel gene transfection reagents, Metafectane and GeneJammer were examined for their ability to deliver a reporter gene to SCC VII murine squamous carcinoma cell lines. Beneficial results were obtained, thus proving the efficiency of nonviral vectors. [18]


   Gene Therapy Strategies for Oral Squamous Cell Carcinoma Top


Oral squamous cell carcinoma is an attractive tumour target due to its frequent genetic mutations and accessibility for intratumoural administration.

Gene addition therapy

Gene addition or augmentation therapy aims at introducing a normal, functional copy of a gene into the genome so as to restore the normal function of the cell. Introduction of drug-resistant genes into normal cells (to protect them against chemotherapy) and genes that inhibit tumour angiogenesis are the other strategies used in gene augmentation therapy. Several genetic alterations have been described in oral cancer including mutations of p53, p16 and p21. Since the protein p53 plays a role in cell-cycle regulation and apoptosis, p53 gene transfer was initially tested in squamous cell carcinoma patients by injecting the primary or regional tumour with an adenoviral vector expressing wild-type p53.

Adenoviral p53 (Ad-p53) was demonstrated to be safe and well-tolerated. [19] Studies on Ad-p53 as a surgical adjuvant showed good results. [20] The reconstitution of wild-type p53 function with p53-expressing adenovirus and combinational therapy using ionizing radiation and recombinant Ad-p53 has been reported to have a significant tumour-suppressant effect on various cells. The survival of SCC cell lines was inhibited after transfection with recombinant p53-expressing adenovirus. [21]

Antisense RNA therapy

The treatment of genetic disorders by introducing a remedial gene that prevents the expression of a specific defective gene is called as "Antisense therapy." Gene expression can usually be inhibited by RNA that is complementary to the strand of DNA expressing the gene. Such therapy can be directed towards carcinoma cells whose malignant phenotype is dependent upon the expression of particular oncogenes such as myc, fos and ras. Inhibition of expression of these oncogenes may alter the phenotype, thus arresting the tumour growth [22],[23] [Figure - 1]. Specific types of HPV are associated with oral cancers. Certain genes of the human papillomavirus (HPV) such as E6 and E7 show continuous expression in growing tumours. Although there have been no clinical trials of gene therapy for HPV-associated oral cancers, in vitro studies show that expression of these genes could be inhibited by antisense therapy. [24]

Suicide gene therapy
"Suicide" gene therapy involves the introduction of a gene into a cell that enables a prodrug to be activated into an active cytotoxic drug. The most extensively studied approach utilizes the Herpes Simplex Virus-Thymidine Kinase (HSV-TK)/ ganciclovir system. The HSV-TK gene encodes a viral enzyme that phosphorylates ganciclovir into its monophosphate, which is then further phosphorylated by the host's intracellular enzymes into the active triphosphate. The triphosphate inhibits DNA polymerase activity thus terminating DNA synthesis, resulting in cell death. Thus, this system selectively targets cancer cells into which the HSV-TK gene has been introduced.

Ganciclovir (GCV) is an excellent substrate for HSV-TK and a poor substrate for mammalian thymidine kinase [Figure - 2]. This selectivity of ganciclovir facilitates the achievement of cytotoxic levels in transfected cells leaving the normal cells unharmed. [25] Reports suggest that this method led to apoptosis of an OSCC cell line. Morphological changes like chromatin condensation, cell shrinkage, blebbing of cell membrane and ballooning of cells were observed. Flow cytometric analysis showed a maximum of about 65% of the cells in the early phase of apoptosis. In addition, DNA fragmentation was investigated in vivo using nude mice which were implanted with SAS (squamous cell carcinoma) cell line and treated with AdHSV-TK (Adenoviruses vector containing HSV-TK gene) and GCV examination of tumor sections confirmed the presence of DNA-fragmentation-positive cells in the treatment group.

  1. Introduction of Ganciclovir-the prodrug.
  2. HSV/TK gene is introduced into the tumour cell.
  3. Thymidine kinase protein is produced by the TK gene.
  4. Virally encoded thymidine Kinase phosphorylates ganciclovir to its monophosphate.
  5. Intracellular enzymes convert Gancilovir monophosphate into a triphosphate.
  6. The active Gancilovir triphosphate induces apoptosis of the tumour cell.


Immunologic gene therapy

The immunologic gene therapy approach to oral cancer involves either increasing the immunogenic potential of tumour cells or augmenting the patient's immune response to a tumour. Biological molecules produced by tumour cells are found to elicit strong immune response. T-cells are the major immune cells involved in antitumour immunity. [27] Studies in animal models have shown that administration of interleukin-2 (IL-2), tumour necrosis factor alpha (TNF-α), IL-4, interferon-gamma (IFN-γ), IFN-α, granulocyte macrophage colony-stimulating factor (GM-CSF) and IL-6 had enhanced killer cell-mediated cytotoxic effects. The feasibility and efficacy of a combination of nonviral, lipid-formulated murine interleukin-2 (mIL-2) and polymer-formulated, murine interleukin-12 (mIL-12) gene therapy for squamous cell carcinoma have been investigated in preclinical models. The use of combined mIL-2 and mIL-12 gene therapy resulted in significant anti-tumour effects, most likely due to increased activation of cytolytic T lymphocyte and natural killer cells. [28]

Summary

Advances in understanding and manipulating genes have set the stage for scientists to alter patient's genetic material to fight or prevent disease. Gene therapy has now been applied successfully to a wide range of conditions such as malignancies of the breast, prostate, colon, lungs etc. and other degenerative disorders like Parkinson's and Alzheimer's disease. Gene therapy for oral cancer is currently under investigation in clinical trials. The safety of gene therapy in OSCC was established in Phase I trials and the efficacy in combination with chemotherapy or radiation therapy was demonstrated in Phase II trials. Presently, clinical trials of gene therapy for OSCC as adjuvant treatment are under way (Phase III). [29] The ongoing research in the field of molecular biology combined with the unfolding of the mystery of the human genome has made cancer-based gene therapy more viable and promising.

 
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Correspondence Address:
T R Saraswathi
Department of Oral and Maxillo-Facial Pathology, Meenakshi Ammal Dental College and Hospitals, Chennai - 600 095, Tamilnadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.33787

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