Text A. How Cancer Develops.

 

Cancer cells develop from normal cells in a complex process called transformation. The first step in the process is initiation, in which a change in the cell's genetic material primes the cell to become cancerous. The change in the cell's genetic material is brought about by an agent called carcinogen—such as a chemical, virus, radiation, or sunlight. However, not all cells are equally susceptible to carcinogens. A genetic flaw in the cell or another agent, called a promoter, may it more susceptible. Even chronic physical irritation may make cells more susceptible to be­coming cancerous. In the next step, promotion, a cell that has been initiated becomes cancerous.

Promotion has no effect on noninitiated cells. Thus, several factors, often the combination of a susceptible cell and a carcinogen, are needed to cause cancer.

In the process by which a normal cell becomes a cancerous cell, ultimately its DNA undergoes change. Changes in a cell's genetic material are often hard to detect, but sometimes a change in the size or shape of one specific chromosome indicates a certain type of cancer. For example, an abnormal chromosome called the Philadelphia chromosome is found in about 80 percent of the people with chronic myelocytic leukemia. A ge­netic changes have also been identified in brain tumors and cancers of the colon, breast, lung, and bone.

A series of chromosomal changes may be needed for some cancers to develop. Studies of familial polyposis of the colon (a hereditary in­testinal disorder in which polyps develop and become cancerous) have suggested how this might work in colon cancer: The normal lining of the colon begins to grow more actively (hyperproliferate) because the cells no longer have a sup­pressor gene on chromosome 5 that normally controls the growth of the lining. A slight change in DNA then promotes changes to form an ade­noma (a benign tumor). Another gene (the RAS oncogene) makes the adenoma grow more actively. The subsequent loss of a suppressor gene on chromosome 18 further stimulates the adenoma, and finally the loss of a gene on chromosome 17 converts the benign adenoma to cancer. Additional changes may make the cancer metastasize.

Even when a cell becomes cancerous, the immune system can often destroy it before it replicates and becomes established as a cancer. Cancer is more likely to develop when the immune system isn't functioning normally, as in people with AIDS, those receiving immunosuppressive drugs, and those with certain autoimmune eases. However, the immune system is not foolproof; cancer can escape the immune system protective surveillance even when it's functioning normally.

 

 

Carcinogens: Chemicals That Can Cause Cancer

Chemical Type of Cancer

Environmental and industrial

Arsenic Lung

Asbestos Lung pleura

Aromatic amines Bladder

Benzene Leukemia

Chromates Lung

Nickel Lung, nasal sinuses

Vinyl chloride Liver

Associated with lifestyle

Alcohol Esophagus, mouth, throat

Betel nuts mouth, throat

Tobacco Head, neck, lungs,

esophagus, bladder

Used in medicine

Alkylating agents Leukemia, bladder

Diethylstilbestrol Liver, vagina (if exposed before birth)

Oxymetholone Liver

Thorotrast Blood Vessels

 

Notes:

susceptible восприимчивый, впечатлительный

benign tumor доброкачественная опухоль

surveillance надзор

 

Text B. Risk Factors.

A host of genetic and environmental factors increases the risk of developing cancer.

Family history is one important factor. Some families have a significantly higher risk of developing certain cancers than other families. For example, a woman's risk of developing breast cancer increases 1.5 to 3 times if her mother or sister had breast cancer. Some breast cancers are linked to a specific gene mutation, more common some ethnic groups and families. Women with this gene mutation have an 80 to 90 percent chance (developing breast cancer and a 40 to 50 percent chance of developing ovarian cancer. Researchers have found that 1 percent of Ashkenazi Jewish women have this gene mutation. Many other cancers, including some skin cancers and colon cancers, tend to run in families as well.

People with chromosomal abnormalities have an increased risk of cancer. For example, people with Down syndrome, who have three instead of the usual two chromosomes numbered 21, have a 12 to 20 times higher risk of developing acute leukemia.

A number of environmental factors increase the risk of cancer. One of the most important is cigarette smoking. Smoking cigarettes substantially increases the risk of developing cancers of lung, mouth, larynx, and bladder.

Extended exposure to ultraviolet radiation, primarily from sunlight, causes skin cancer. Ionizing radiation, which is particularly carcinogenic, is used in x-rays, produced in nuclear power plants; and atomic bomb explosions, and reaches the earth from space. For example, survivors of the atomic bombs dropped on Hiroshima and Naga­saki during World War II have an increased risk of developing leukemia. Exposure to uranium in mine workers has been linked to the development of lung cancer 15 to 20 years later; the risk is greatly increased if the mine workers also smoked cigarettes. Long-term exposure to ionizing radia­tion predisposes people to develop cancer of the blood cells, including acute leukemia.

Diet is another important risk factor for cancer, particularly cancers of the gastrointestinal sys­tem. A high-fiber diet reduces the likelihood of developing colon cancer. A diet high in smoked and pickled foods increases the chance of devel­oping stomach cancer. Current evidence suggests that following a diet in which less than 30 percent of the total calories comes from fat reduces the risk of colon, breast, and possibly prostate can­cer. People who drink large amounts of alcohol are at much higher risk of esophageal cancer.

Many chemicals are known to cause cancer and many others are suspected of causing cancer. Ex­posure to certain common chemicals can greatly increase a person's chance of developing cancer, often years later. For example, asbestos exposure may cause lung cancer and mesothelioma (cancer of the pleura). A cancer is even more prevalent in cigarette smokers who were exposed to asbestos.

The risk of cancer also varies according to where people live. The risk of colon and breast cancers is low in Japan, yet in Japanese immi­grants to the United States, the risk increases and eventually equals that of the rest of the American population. The Japanese have extremely high rates of stomach cancer. However, for Japanese born in the United States, the incidence is lower. This geographic variation in cancer risk is prob­ably multifactorial: a combination of genetics, diet, and environment.

Several viruses are known to cause cancer in humans and several others are suspected of caus­ing cancer. The papillomavirus that causes genital warts is probably one cause of cervical cancer in women. Cytomegalovirus causes Kaposi's sar­coma. Hepatitis B virus can cause liver cancer, although whether it's a carcinogen or a promoter isn't known. In Africa, the Epstein-Barr virus causes Burkitt's lymphoma; in China, it causes cancers of the nose and pharynx. Obviously, some additional factor, either environmental or genetic, is needed for the Epstein-Barr virus to cause cancer. Some human retroviruses, such as the human immunodeficiency virus, cause lymphomas and other cancers of the blood system.

Infection with the parasite Schistosoma (Bilharzia) may cause bladder cancer by chronically ir­ritating the bladder. Yet, other causes of chronic bladder irritation don't cause cancer. Infection with Clonorchis, found mainly in the Far East, can lead to cancer of the pancreas and bile ducts.

Notes:

genetic and environmental factors генетические факторы и факторы внешней среды

likelihood of developing вероятность развития

current evidence текущее подтверждение,

свидетельство