|Year : 2003 | Volume
| Issue : 2 | Page : 34-36
Oral cancer and basic science research: A clinician's observation
Professor of Oral and Maxillo Facial Pathology, Nair Hospital Dental College, Mumbai 400 008, India
Professor of Oral and Maxillo Facial Pathology, Nair Hospital Dental College, Mumbai 400 008
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Shetty R. Oral cancer and basic science research: A clinician's observation. J Oral Maxillofac Pathol 2003;7:34-6
| Introduction|| |
Till the protocol of treatment for oral cancer was established, bedside physicians were least interested in the experimental work of the laboratory scientist. Their attention was drawn to basic sciences when these labs attempted to resolve certain inconsistent, uncertain and confusing observations in clinical practice.
The prognosis for patients with oral malignancies continues to be poor with only a 50% five-year survival rate. Recurrences, second primaries, field cancerization and skip metastasis have hindered successful therapy of malignancies. What we expect from the basic sciences is predicting susceptibility and molecular epidemiology to design a preventive program, easier clinical and laboratory techniques to detect early non-symptomatic lesions, mathematical formulations to predict transformation of premalignant lesions, markers and molecular story tellers to replace histopathology and TNM during the decision-making phase of surgery, non-invasive or minimal invasive techniques to monitor treatment, predicting the prognosis and of course eliminate surgery which at times is more psychotraumatic than the disease itself.
Oral cancer may result from various injuries, alterations, or insults to the genes and the transformation of normal cell to malignant cell may take different genetic pathways. It is important to know these genetic events to understand the disease process. However, as the nomenclature of molecular oncology has become increasingly distant from the language clinicians use, our ability to translate and evaluate information on genetic changes is restricted.
Cancer theories arc changing continuously. The two-stage (initiation and progression) model has replaced the single sudden transformation hypothesis. It further evolved to a six hit model, ultimately settling at a multi-hit, multistage theory, which explains unexplained issues in oral carcinogenesis. As suggested by Knudson and Nowell, carcinogenesis involves the accumulation of multiple genetic events or hits in single cell. Every hit is in the form of a mutation, translocation loss, gain, insertion, inversion or amplification. As these alterations accumulate in the cell, the cell becomes functionally independent from the surrounding cells, thereby dividing more rapidly, sequestering blood vessels, deleting or amplifying signals to produce abnormal structural or functional changes and invading normal tissue at local or distant sites. The histological progression of oral carcinogenesis from hyperplasia to dysplasia followed by severe dysplasia and eventual invasion and metastasis are believed to reflect these changes. The changes affect the genes. which tightly control excitatory and inhibitory pathways regulating basic cellular functions such as cell division, differentiation and senescence. The horrific fact about these changes is that they are silent and inheritable. Thus the new cell, having acquired these changes, becomes privileged fur transformation into a cancer eel l and will require fewer hits.
| Molecular Biology Research in Oral Cancer|| |
Studies in molecular biology have increased the chances of detecting high-risk lesions and individuals and this science is emerging as molecular epidemiology. The synchronous (detected simultaneously) and metachronous (detected after a period of time) second primary tumours are being explained based on genetics. Field canceriiation and transformation of multiple cells at several sites are thought to be multiple clonal effects.
Increased susceptibility to oral cancer
Tobacco addiction is related to D2 dopamine receptor DRD2 minor alleles Al and B1. The allelic polymorphism and enzymatic activity of the enzymes Cytochrome P-450 (CYI 2) and glutathionc 5-transferase (GST) have been associated with increased risk of tobacco- associated cancer.
CYP2 converts environmental contaminants to water-soluble DNA damaging intermediates. GST is a phase II enzyme that is important in the detoxification process. These are useful follow up markers and higher levels predict recurrence. Their role in oral cancer is under investigation.
Carcinogcnes1s begins in basal and suprabasal layers as a focal overgrowth of altered stern cells, which expands upwards and laterally. It consists of six to ten genetic events, each step being a result of two sequential events, a critical mutation in the cell followed by expansion of the clone.
Alteration of regulatory pathway
Refer following figure [Figure 1]
| Cytogenetic Changes Responsible for Deregulation|| |
Alteration of bands on the short (p) and Iong (q) arms of chromosome, like loss at 3p (97%), loss at Ip, 5q, 6q, 8p, 9p, 11q, 13q, 18q, 21q (50%). gain at 3q26 (87%) leads to genetic deregulation (Study of tumour cell chromosome by Bovori 1941). Genetic changes occur as point mutation, amplification, rearrangement! Translocation/inversion, deletion, insertion and loss of heterozygosity (LOH). Chromosome break point occurs at the centeromeric region of chromosome 1, 3, 8, 14, 15.
Genetic changes might be dominant (gain of function - proto-oncogenes) or recessive (loss of function-tumour suppressor gene). Genetic instability is due to microsatellite instability (MI) or loss of heterozygosity (LOH). These changes are seen early in carcinogenesis and not seen in normal tissue, Thus they are highly specific for cancer. Such changes at 9p and 3q are highly implicated.
These are altered proto-oncogenes, which are the growth-promoting regulator genes (housekeeping genes). Proto-oncogenes are conserved through evolution in nature. They are components of metabolic processes i.e. cell proliferation and differentiation. These genes code for the functional and the regulatory proteins, such as growth factor, growth factor receptor, protein kinase, nuclear transcription factor and, cell signalling transducing factor.
Oncogenes of myc and ras families are commonly associated with solid tumours. Epithelial growth factor is associated with erbB-1 in SCC. In 7-52 % of oral cancer cases, amplification of mvc, ras, erb B-1 occurs. The bcl-2 oncogene, an inhibitor of apoptosis, responsible for the longevity of cell is deregulated in oral cancer and is over expressed in 21 % of oral cancers.
Point mutation of oncogenes
In oral cancer, point mutation of H ras , which has role in guanosine triphosphate dehydrolysing enzyme system (GTPase) activity, occurs. Function of ras is like an on/off switch, active when bound to GTP and inactive when hound to GDP.
Point mutations are studied by PCR Analysis, DOT BLOT of the PCR products, hybridization, specific oligonucleotide probe and, direct sequencing of PCR products. H ras LOH is seen in Indian cancers. Oncogene changes are observed in cancer in Indian subcontinent, as it is associated with chewing habits while in western countries activation of oncogenes are not seen. Amplification of oncogenes is often associated with advanced cancers.
Growth factors act by autocrine (cell stimulation by its own growth factor) or by paracrine (cell stimulation by external factors) stimulation.
Transforming growth factor α (TCF α)
TGFα is over expressed in oral cancer, early in hyperplastic epithelium and later in inflammatory infiltrate surrounding epithelium. It stimulates the cell by binding to the epidermal growth factor receptor (EGFR). It promotes neovascularization and mitogenesis. It is over expressed in normal mucosa of oral cancer patients, who subsequently develop second primaries. TGFa and EGFR over expression have a shorter survival than over expression of EGFR alone.
Cell surface receptor
Cell surface receptor binding translates extracellular signals through the cell membrane by activating a cascade of' biochemical reactions. EGFR is an important oncoprotein in oral cancer because the gene is amplified, thus producing more receptors. Oral mucosa has 50% more receptors compared to other mucosal surfaces. An EGFR positive tumour responds well to chemotherapy.
Intra cellular messengers
ras gene encodes closely related proteins that are located on the cytoplasmic side of the cell membrane and transducts messages from the cell surface receptors to the intra cellular regulatory enzyme, ras transmits mitogenic signals by binding GTP and GDP. Hydrolysis of GTP and GDP ends the signal.
In oral cancer, K-ras and H -ras are activated by point mutation. It cannot convert GTP to GDP and stimulate cell proliferation.
These are proteins which stimulate other genes to be activated. It is important in intracellular pathway. C- myc helps in regulating cell proliferation and differentiation. C- myc is over expressed in poorly differentiated tumours and poor prognosis cases. They include PRADI and Cyclin D-l. They are cell cycle promoters and are over expressed in head and neck cancer.
Tumour suppressor genes
Accumulation of activated genes appears to be of primary importance, but these alone are not sufficient to result in oral cancer. Inactivation of negative regulatory tumour suppressor genes is required. Oncogenes show mutations at only one of the gene copies, while tumour suppressor genes are inactivated by point mutation, deletion and rearrangement in both gene copies. Thus they are difficult to identify because they are negative phenotype or no longer present within the cell. Only two genes, p53 and doe-1 are known for tumour suppressor activity in oral cancer. Deregulation of these has effect on cell cycle, chromosome stability, senescence, apoptosis and control of cell proliferation.
p53 gene and protein is the most commonly investigated molecule in human cancer. The gene is located at 17p 13.1. It functions as a `guardian of cell' by providing the molecular brake and maintaining the genomic stability. When DNA damage occurs, cell produces p53, which stops cell division by arresting cells at G,-S boundary, induces DNA repair and triggers programmed cell death or apoptosis.
Tumour suppression of p53 is negated by point mutation, deletion of alleles, binding to viral protein (E6 or E7 of HPV 16/18), p53 is studied by PCR SSCP (presence of point mutation), PCR (LOH), immunohistochemical detection of p53 protein, and presence of IIPV 16/18 infection. Point mutations result in structurally altered protein that sequesters the wild type protein (wild p53). Most mutations of p53 genes extend the half life of p53 protein resulting in their accumulation in the nuclei of the malignant cells.
Restoration of p53 function in oral cancer cell lines and in oral tumours induced in animal models resulted in the reversion of the malignant phenotype.
doc-1 is mutated in oral cancer and re-expressed causing reversing back to normal. doc-1 is similar to gene product induced in mouse fibroblast by tumour necrosis factora (TNFα). In normal epithelial cells, TNFα decreases proliferation and increases differentiation. It may be helped by interferonα. Therefore doc-1 is important in regulation of TNFα induced keratinization.
Other tumour suppressor genes
Include Rh, p16-CDK41, APC (Adenomatous Polyposis Coli) and FHIT (Fragile Histidine Triad).
Oral growth suppressor and signal pathways
Extracellular proteins such as TNFγ and TGFβ may provide important growth inhibitory signals in oral epithelial cell biology. TGFβ inhibits cell proliferation whose activity is related to Rb gene.
Cell surface molecules
These are important in cell proliferation and angiogenesis. E-cadherin is a cell to cell adhesion molecule associated with both invasion and metastasis. DCC (Deleted in Colon Cancer) is a cell to cell contact inhibitor and is mutated in oral cancer. Thrombospondin-1 (TSP-1) is an extracellular glycoprotein deregulated in oral cancer allowing malignant epithelial cells to induce angiogenesis. RAR-B retinoic acid receptor is a nuclear transcription factor down regulated in oral cancer. µPA (urokinase type Plasminogen Activating factor, MMP (Matrix Metalloproteinase) and cathepsins help in consecutive destruction of extracellular matrix (ECM), thereby assisting in tumour cell invasion and metastasis.
Cell adhesion molecules
They maintain cell to cell interaction, cell to matrix interactions, tissue integrity and regulate cell movement and migration. They include integrins (α and β subunits). Loss of α2β1, α3β1, and α6β4 is seen in poorly differentiated oral cancers and shows focal interruption of basement membrane. α6 increases in metastasis.
Tumour metastasis suppressor gene
Presence of NM23 protein shows less lymph node involvement.
| Conclusion|| |
In conclusion it can be stated that changes such as LOH, mutations of H ras and p53, and binding to F6 and E.7 of HPV 16/18 cause immortalization of the cell and that is cancer.
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[Figure - 1]