The malignant transformation involves several steps, and is regulated by many cellular genes, mainly protooncogenes and tumor suppressor genes, which are also responsible for the cell cycle. These genes are translated into proteins that act promoting or inhibiting cell growth, division and differentiation, playing an important role by controlling cellular outgrowth. Protooncogenes influence positively in growth, and are extremely important as regulators of cell division and cell cycle. Once mutated or activated, these protooncogenes are turned to abnormal genes ( oncogenes ) and produce proteins that positively regulate cell growth. On the other hand, tumor suppressor genes are negative regulators of cell growth. Normally, there is a balance between these actions, yet, when a mutation occurs in those regulatory genes, an unbalance between inhibition and promotion leads to tumoral progress. For instance, loss-of-function of a tumor suppressor gene or gain-of-function of a protooncogene can result in expression of a transformed phenotype. In most cases, these functional changes lead to apoptosis that is defined as programmed cell death, a physiological mechanism that regulates cellular homeostasis by eliminating unnecessary cells2. However, when a transformed cell is able of keeping an uncontrolled growth, usually caused by gene mutations, this state of malignant changes may develop. Fortunately, all these changes occurs by genetic alterations that are acquired or inherited in the form of chromosomal genetic abnormalities, and those abnormalities can be assessed by molecular biology3.
Figure 1. Schematic presentation of the cellular compartments in relation to protooncogenes, oncogenes and tumor suppressor genes. This figure demonstrates clearly the cellular position of some cell cycle regulators by showing some integral membrane tyrosine kinases ( erbB and others ), growth factors ( sis and hst ), ras and src gene families and some membrane associated tyrosine kinases, serine-threonine kinases ( mos and raf ) and some nuclear oncoproteins. These proteins are expressed in most cells, but when a mutation occurs, this expression is abnormal and cancer can arise.
These alterations in cell cycle and genes are the basis of molecular evaluation, that can be carried out by using many methods, such as immunohistochemistry and polymerase chain reaction ( PCR ) techniques. Immunohistochemistry is an useful method, since specific monoclonal antibodies can label normal or abnormal proteins and define whether the function of certain genes is being carried out. For example, the p53 gene, a tumor suppressor gene, is present in most human cancers, and its function can be checked by verifying the presence of wild-type or mutant p53 proteins, which are present in normal and mutated cells respectively. This immunologic approach has many uses, as for example: to assess cancer origin, to define cancer cell immuno-phenotype, to detect cellular products ( hormones, cytokines, etc. ), to predict tumor behavior with specific markers and to diagnose infectious agent with in situ hybridization4.
PCR consists on amplification of DNA to amounts suitable for analysis, and was recently modified to allow identification of individual mutations by using primers, molecules capable of recognizing specific mutations from the target gene. Using this method, mutations can be assessed in urine or stool samples even when there is no clinical evidence of cancer. PCR can detect the presence of a mutant cell among 10,000 normal cells5,6. The microsatellite technique consists on a modification of PCR by the use of primers to most known mutation sites7. All these diagnostic approaches are extremely important to demonstrate alterations in genes, proteins and growth factors, which are essential to detect cancer cells before its uncontrolled growth, predict its behavior, metastatic pathways, treatment response and prognosis. The association between these markers and carcinogens are also important in relation to risk prediction and determination8.
The importance of the knowledge about molecular epidemiology goes beyond the cited advances, so that this approach might play a pivotal role in our understanding about cancer. Nowadays, some important advances had been established, and many others are under study. For example, the p53 gene is a tumor supressor gene, and is present in most cancers. Its use in esophageal cancer screening in not defined, but in association with other markers, the prevalence of positive cells to molecular abnormalities can determine its use in screening among high-risk populations. This is due to the p53 marker variable prevalence in this cancer, it can vary from a prevalence of 80% in high-risk populations to 35% in low-risk. Complementary, bcl-2 is normally present only in the basal layer of normal mucosa, and have been found out of this layer in 60% of severe squamous dysplasia and 50% of in situ carcinomas. In addition, there are other markers such as RB that is present in 41%, the 11q13 area and cyclin D which are present in 20-25% and the c-myc gene that is mutated in 14-25% of this cancers. Another importance of this markers is diagnosis, so that p53’s presence in esophageal dysplasia can indicate a late stage in this carcinogenesis, and makes the probability of a cancer extremelly increased, due to the low prevalence of this marker in normal mucosal cells9. Also, in ovarian cancer, this gene had been defined as a marker of poor prognosis and, in contrast, bcl-2 (another gene) had been found to be a marker of good prognosis10. p53 is responsible for a worse prognosis in head and neck cancer, and in sarcoma as well11,12.Also, p53 is indicated as responsible for tamoxifen resistence in breast cancer, what suggests it can interfere in treatment response13. These are only a small piece of all the associations that had been found in the last years, so that its is impossible to describe all of them in this paper.
All these associations of molecular markers of cancer ( oncogenic markers ) are extremely important to the development of a better approach to cancer, in terms of diagnose, treatment and increase survival. Although, the methods are getting each time more sensitive and specific, we still have limits on our molecular view, and even the limit between research and reality. As a matter of fact, there is a long way until we have concrete data and conclusions on what is the best for our patients.
1. Perkins AS, Van Woude GF. Principles of molecular cell biology of cancer: Oncogenes. In: DeVita VT, Hellman S, Rosemberg AS. Cancer: Principles and practice of oncology. Fourth edition, J B Lippincott Co., Philadelphia, 1993.
2. Schulte-Hermann R, Grasl-Kraupp B, Bursch W. Tumor development and apoptosis. Int. Arch. Allergy Immunol. 1994; 105: 363-367.
3. Sidransky D, Boyle J, Koch W. Molecular screening: Prospects for a new approach. Arch. Otolaringol. Head Neck Surg. 1993; 119: 1187-1190.
4. Wakamatsu A, Simões AB, Kanamura CT, et al. Manual de imuno-histoquímica. Sociedade Brasileira de Patologia, São Paulo, 1995.
5. Sidransky D, Von Eschenbach A, Tsai YC, et al. Identification of p53 gene mutations in bladder cancers and urine samples. Science 1991; 252: 706-709.
6. Sidransky D, Tokino T, Hamilton SR, et al. Identification of ras oncogene mutations in the stool of patinets with curable colorectal tumors. Science 1992; 256: 102-105.
7. Paulson TG, Wright FA, Parker BA, et al. Microsatellite instability correlates with reduced survival and poor disease prognosis in breast cancer. Cancer Res. 1996; 56: 4021-4026.
8. Soussi T. The p53 tumour suppressor gene: a model for molecular epidemiology of human cancer. Molecular Medicine Today, Jan 1996: 32-37.
9. Montesano R, Hollstein M, Hainaut P. Genetic alterations in esophageal cancer and their relevance to etiology and pathogenesis: a review. Int. J. Cancer ( Pred. Oncol.) 1996, 69: 225-235.
10. Herod JJO, Eliopoulus AG, Warwick J, et al. The prognostic significance of bcl-2 and p53 expression in ovarian cancer.
11. Gallo O, Chiarelli I, Bianchi S, et al. Loss of p53 gene mutation after irradation is associated with increased aggressiveness in recurring head and neck cacer. Clin. Cancer Res.1996; 2: 1577-1582.
12. Hieken TJ, Gupta TKD. Mutant p53 expression: A marker of diminished survival in well-differentiated soft tissue sarcoma. Clin. Cancer Res. 1996; 2: 1391-1395.
13. Guillot C, Felette N, Courtois S, et al. Alteration of p53 damage response by tamoxifen treatment. Clin. Cancer Res.1996; 2: 1439-1444.
14. D’as-Eipper C, Subramanian T, Chinnadurai G. bfl-1, a bcl-2 homologue, suppresses p53-induced apoptosis and exhibits potent cooperative transforming activity. Cancer Res.1996; 56: 3879-3882.
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