Cancer is caused in most instances by mutation or by some other abnormal activation of cellular genes that control cell growth and cell mitosis. Proto-oncogenes are normal genes that code for various proteins that control cell adhesion, growth, and vision. If mutated or excessively activated, proto-oncogenes can become abnormally functioning oncogenes capable of causing cancer. As many as 100 different oncogenes have been discovered in human cancers.

Also present in all cells are antioncogenes, also called tumor suppressor genes, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer.

For several reasons, only a minute fraction of the cells that mutate in the body ever lead to cancer. First, most mutated cells have less survival capability than do normal cells, and they simply die. Second, only a few of the mutated cells that survive become cancerous, because even most mutated cells still have normal feedback controls that prevent excessive growth. Third, cells that are potentially cancerous are often destroyed by the body’s immune system before they grow into a cancer. Most mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body’s immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In people whose immune systems have been suppressed, such as in persons taking immunosuppressant drugs after kidney or heart transplantation, the probability that a cancer will develop is multiplied as much as fivefold. Fourth, the simultaneous presence of several different activated oncogenes is usually required to cause a cancer. For instance, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because another mutant gene is not present simultaneously to form the needed blood vessels.

What is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be to ask why all of us do not develop millions or billions of mutant cancerous cells. The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, along with the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet despite these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics.

Thus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence. However, the probability of mutations can be greatly increased when a person is exposed to certain chemical, physical, or biological factors, including the following:

 1. It is well known that ionizing radiation, such as x-rays, gamma rays, particle radiation from radioactive substances, and even ultraviolet light, can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, causing many mutations.

 2. Chemical substances of certain types also have a high propensity for causing mutations. It was dis covered long ago that various aniline dye derivatives are likely to cause cancer, and thus workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called carcinogens. The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. These carcinogens cause about one quarter of all cancer deaths.

 3. Physical irritants can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells. The more rapid the mitosis, the greater the chance for mutation.

4. In many families, there is a strong hereditary tendency to cancer. This hereditary tendency results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.

 5. In laboratory animals, certain types of viruses can cause some kinds of cancer, including leukemia. This phenomenon usually occurs in one of two ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes, thereby causing a mutation that leads to cancer. In the case of RNA viruses, some of these viruses carry with them an enzyme called reverse transcriptase that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer.

 Invasive Characteristic of the Cancer Cell. The major differences between a cancer cell and a normal cell are as follows:

 1. The cancer cell does not respect usual cellular growth limits, because these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells.

 2. Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths.

3. Some cancers also produce angiogenic factors that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth.

Why Do Cancer Cells Kill? The answer to the question of why cancer cells kill is usually simple. Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, with their number multiplying day by day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually sustain nutritive death.

مواضيع ذات صلة

العوامل البيئية المسببة للسرطان

الأورام الصماوية للبنكرياس

متلازمة الأورام الغدية الصماوية المتعددة Multiple Endocrine Neoplasia (MEN) Syndrome

خصائص الخلايا السرطانية

Testicular germ cell tumours

Prostate carcinoma

Renal cell carcinoma

المسببات الكيميائية للسرطان chemical causes of cancer

الوراثة والسرطان Genes associated with cancer

السرطان اللبي ( النخاعي ) Medullary carcinoma

Hepatocellular carcinoma (liver cell cancer)

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