From WikiHealthCauses of cancerRelated Topics
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The cause of cancer has been heavily researched and invested in by the medical field over the last 20 years. There is no doubt that cancer is on the rise- and there has been much debate if the incidence of cancer is increasing or simply if the ability to diagnose cancer is increasing. This article assess some of the theories on the causes of cancer. As with any health ailment, understanding the 'cause' is essential in prevention of cancer, as well as treatment.
Molecular biologyCarcinogenesis (meaning literally, the creation of cancer) is the process of derangement of the rate of cell division. Cancers are caused by a series of mutations. Each mutation alters the behavior of the cell somewhat. Cancer is, ultimately, a disease of genes. Typically, a series of several mutations is required before a cell becomes a cancer cell. The process involves both oncogenes and tumor suppressor genes. Oncogenes promote cancer when "switched on" by a mutation, whereas tumor suppressor genes prevent cancer unless "switched off" by a mutation. Chromosomal translocation, such as the Philadelphia chromosome, is a special type of mutation and may involve oncogenes or tumor suppressor genes. In general, mutations in both types of genes are necessary, as a mutation limited to an oncogene would be suppressed by normal mitosis control. Mutations can have various causes. Particular causes have been linked to specific types of cancer. Tobacco smoking is associated with lung cancer. Prolonged exposure to radiation, particularly ultraviolet radiation from the sun, leads to melanoma and other skin malignancies. Breathing asbestos fibers is associated with mesothelioma. In more general terms, chemicals called mutagens and free radicals are known to cause mutations. Chronic inflammation may induce malignancy, as neutrophil granulocytes secrete free radicals. Many mutagens are also carcinogens, but some carcionogens are not mutagens. Examples of carcinogens that are not mutagens include alcohol and estrogen. These are thought to promote cancers through their stimulating effect on cellular mitosis. Mutations are more likely to occur during mitosis. Mutations can also be inherited. Inheriting certain mutations in the BRCA1 gene renders a woman much more likely to develop breast cancer and ovarian cancer, in Rb1 lead to retinablastoma, and in APC lead to colon cancer. Some types of viruses can cause mutations. They play a role in about 15% of all cancers. Tumor viruses, such as some retroviruses, herpesviruses and papillomaviruses, usually carry some oncogene or tumor suppressor inactivating gene in their genome. It is impossible to tell the initial cause for any specific cancer. However, with molecular biology, it is now possible to characterize the mutations within a tumor, and to a certain extent predict its behavior. For example, up to half the tumors are deficient in the tumor suppressor gene p53, also known as "the guardian of the genome". This mutation is associated with poor prognosis, since those tumor cells are less likely to go into apoptosis (programmed cell death) when damaged by therapy. There are more mutations that make a tumor more malignant. Telomerase mutations remove additional barriers, extending the number of times a cell can divide. Other mutations enable the tumor to grow new blood vessels to feed it, or to detach from the surrounding tissue, spreading to other parts of the body. Malignant tumors cells, such as in carcinoma, sarcoma, lymphoma or leukemia, have distinct properties:
A cell that degenerates into a tumor cell does usually not acquire all these properties at once, but its descendant cells are selected to build them. This process is called clonal evolution. A first step in the development of a tumor cell is usually a small change in the DNA, often a point mutation, which leads, among other things, to a genetic instability of the cell. The instability increases to a point where the cell loses whole chromosomes, or has double ones. Also, the DNA methylation pattern of the cell changes, activating and deactivating genes more or less at random. Cells that divide at a high rate, such as epithelials, show a higher risk of becoming tumor cells than those which divide less, for example neurons. In cellular model systems, cells are exposed to carcinogenic influences (chemicals, radiation). In these systems, the first signs of a cell developing into a tumor cell are:
Items 2-4 (above) can sometimes be traced to mutations in genes that result in a disruption of cell adhesion. Some cell adhesion proteins are tumor suppressor genes. MorphologyCancer tissue has a distinctive appearance under the microscope. Among the distinguishing traits are a large number of dividing cells, variation in nuclear size and shape, variation in cell size and shape, loss of specialized cell features, loss of normal tissue organization, and a poorly defined tumor boundary. Instead of finding a benign or malignant tumor, microscopic examination of a biopsy specimen will sometimes detect a condition called "hyperplasia". Hyperplasia refers to tissue growth based on an excessive rate of cell division, leading to a larger than usual number of cells. Nonetheless, cell structure and the orderly arrangement of cells within the tissue remain normal, and the process of hyperplasia is potentially reversible. Hyperplasia can be a normal tissue response to an irritating stimulus. For example, a callus that may form on your hand when you first learn to swing a tennis racket or a golf club is produced by hyperplasia. In addition to hyperplasia, microscopic examination of a biopsy specimen can detect another type of noncancerous condition called "dysplasia." Dysplasia is an abnormal type of excessive cell proliferation characterized by loss of normal tissue arrangement and cell structure. Often such cells revert back to normal behavior, but occasionally, they gradually become malignant. Because of their potential for becoming malignant, areas of dysplasia should be closely monitored by a health professional. Sometimes they need treatment. The most severe cases of dysplasia are sometimes referred to as "carcinoma in situ." In Latin, the term "in situ" means "in place", so carcinoma in situ refers to an uncontrolled growth of cells that remains in the original location. However, carcinoma in situ may develop into an invasive, metastatic malignancy and, therefore, is usually removed surgically, if possible. HeredityMost forms of cancer are "sporadic", and have no basis in heredity. There are, however, a number of recognised syndromes of cancer with a hereditary component. Examples are:
Environment and dietThe most consistent finding, over decades of research, is the strong association between tobacco use and cancers of many sites. Hundreds of epidemiologic studies have confirmed this association. Further support comes from the fact that lung cancer death rates in the United States have mirrored smoking patterns, with increases in smoking followed by dramatic increases in lung cancer death rates and, more recently, decreases in smoking followed by decreases in lung cancer death rates in men. Up to half of all cancer cases can be attributed to smoking, diet, and environmental pollution.
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