The p53 Tumor Suppressor Gene Triggers Cell Suicide
One particular tumor suppressor gene codes for a protein called "p53" that can trigger cell suicide (apoptosis). In cells that have undergone DNA damage, the p53 protein acts like a "brake pedal" and halts cell growth and division. If the damage can not be repaired, the p53 protein eventually initiates cell suicide, thereby preventing the genetically damaged cell from growing out of control.
DNA Repair Genes
A third class of genes implicated in cancer are called "DNA repair genes." DNA repair genes code for proteins whose normal function is to correct errors that arise when cells duplicate their DNA prior to cell division. Mutations in DNA repair genes can lead to a failure in DNA repair, which in turn allows subsequent mutations in tumor suppressor genes and proto-oncogenes to accumulate. People with a condition called xeroderma pigmentosum have an inherited defect in a DNA repair gene. As a result, they cannot effectively repair the DNA damage that normally occurs when skin cells are exposed to sunlight, and so they exhibit an abnormally high incidence of skin cancer. Certain forms of hereditary colon cancer also involve defects in DNA repair.
Cancer Tends to Involve Multiple Mutations
Cancer often arises because of the accumulation of mutations involving oncogenes, tumor suppressor genes, and DNA repair genes. For example, colon cancer can begin with a defect in a tumor suppressor gene that allows excessive cell proliferation. The proliferating cells then tend to acquire subsequent mutations involving a DNA repair gene, an oncogene, and several other tumor suppressor genes. The accumulated damage yields a highly malignant, metastatic tumor. In other words, creating a cancer cell requires that the brakes on cell growth (tumor suppressor genes) be released at the same time that the accelerators for cell growth (oncogenes) are being activated.
Since exposure to carcinogens (cancer-causing agents) is responsible for triggering most human cancers, people can reduce their cancer risk by taking steps to avoid such agents. Hence the first step in cancer prevention is to identify the behaviors or exposures to particular kinds of carcinogens and viruses that represent the greatest cancer hazard.
The use of tobacco products has been implicated in roughly one out of every three cancer deaths, making it the largest single cause of death from cancer. Cigarette smoking is responsible for nearly all cases of lung cancer, and has also been implicated in cancer of the mouth, larynx, esophagus, stomach, pancreas, kidney, and bladder. Pipe smoke, cigars, and smokeless tobacco are risky as well. Avoiding tobacco is, therefore, the single most effective lifestyle decision any person can make in attempting to prevent cancer.
Protect Yourself Against Sunlight
Skin cancer caused by exposure to sunlight is the most frequently observed type of human cancer. Because skin cancer is often easy to cure, the danger posed by sunlight is perhaps not taken seriously enough. Therefore it is important to know that a more serious form of skin cancer, called melanoma, is also associated with sun exposure. Melanomas are potentially lethal tumors. Risk of melanoma and other forms of skin cancer can be significantly reduced by avoiding excessive exposure to the sun and wearing clothing to shield the skin from ultraviolet radiation. Sunscreen lotions may also protect against some forms of skin cancer, if sun exposure cannot be avoided.
Limiting Alcohol and Tobacco Consumption
Drinking excessive amounts of alcohol is linked to an increased risk for several kinds of cancer, especially those of the mouth, throat, and esophagus. The combination of alcohol and tobacco appears to be especially dangerous and is even riskier than would be expected by just adding the effects of the two together.
Diet: Limit Fats and Calories
Studies suggest that differences in diet may also play a role in determining cancer risk. But in contrast to the clear-cut identification of tobacco, sunlight, and alcohol, the exact identity of the dietary components that influence cancer risk has been difficult to determine. Limiting fat consumption and calorie intake appears to be one possible strategy to decrease risk of some cancers, because people who consume large amounts of meat (which is rich in fat) and large numbers of calories exhibit an increased cancer risk, especially for colon cancer.
Diet: Consume Fruits and Vegetables
In contrast to factors such as fat and calories, which appear to increase cancer risk, other components of the diet may decrease cancer risk. The most compelling evidence has been obtained for fruits and vegetables, whose consumption has been strongly correlated with a reduction in cancer risk. Although the chemical components in these foods responsible for this protective effect are yet to be identified, eating at least five servings of fruits and vegetables each day is recommended by many groups.
Avoid Cancer Viruses
Actions can also be taken to avoid exposure to the small number of viruses that have been implicated in human cancers. The most common cancer-causing virus in the United States is the human papillomavirus (HPV), which is involved in the transmission of cervical cancer. Since this virus is sexually transmitted, its spread can be combatted using the same "safe sex" approaches that are recommended to prevent the spread of HIV; e.g., limiting exposure to multiple sexual partners. There is also an HPV vaccine available to young women which can decrease their chances of getting cervical cancer.
Avoid Carcinogens at Work and at Home
Because people spend so much time at work and at home, assessments should be made of possible carcinogens in these environments. Some occupational carcinogens have been identified because people who work together and have been exposed to the same substances have developed a particular kind of cancer at increased frequency. For example, cancer rates in construction workers who handle asbestos have been found to be much higher than normal. A potential hazard in the home is radon, a radioactive gas that can seep into houses from underground rock formations found in certain areas of the country. Simple test kits for radon are available.
The fact that many environmental chemicals can cause cancer has fostered the idea that industrial pollution is a frequent cause of cancer. However, the frequency of most human cancers (adjusted for age) has remained relatively constant over the past half century, in spite of increasing industrial pollution. Hence, in spite of evidence that industrial chemicals can cause cancer in people who work with them or in people who live nearby, industrial pollution does not appear to be a major cause of most cancers in the population at large.