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Thomas S. May, M.A.Medical Writer |
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Environment--Not Heredity--Responsible for Most Cancers, Researchers Find
Thomas S. May, Medical Writer
The sad truth about cancer is that, in most cases, it cannot be cured. The good news, however, is that, more often than not, it can be prevented.
Many people believe that their risk of developing cancer is mostly determined by their genetic makeup. But according to a study published in the July 13, 2000, issue of the New England Journal of Medicine, environmental factors play a much bigger role than inherited genes do in the development of most cancers.
Dr. Paul Lichtenstein, an associate professor at Sweden's Karolinska Institute, and his colleagues examined the health records of close to 90,000 Scandinavian twins, approximately 10,000 of whom had cancer. By analyzing the patterns of cancer in both "monozygotic" twins (who share all their genes) and "dizygotic" twins (who have the same degree of genetic similarity as full siblings), the scientists were able to separate the effects of genetic and environmental factors. The results showed that genetic makeup plays a relatively minor role in determining a person's risk of developing cancer.
Nature versus Nurture
"Overall, the twin of a person with cancer had an increased risk of having the same cancer," the researchers concluded. This was especially so for cancer of the stomach, colorectum, lung, breast, and prostate. But even for these cancers, genetic makeup accounted for only a minority (26 to 42%) of the risk.
Axel Skytthe of the Danish Center for Demographic Research and Epidemiology, one of the co-authors of the study, says that environmental factors are more important in the causation of cancer. "The heritability of the tendency to develop cancer is minor--only about 10 to 15 percent. This is indicated by the small difference between the rates of concordance [when both twins have the same disease] of monozygotic and dizygotic twins," he explains.
The absolute risk of the same cancer before age 75 for the monozygotic twin of a person with colorectal, breast, or prostate cancer was 11 to 18 percent, the study found. For dizygotic twins, the risk of these cancers was between 3 and 9 percent.
Cancer can be Prevented
"The estimates of absolute concordance are telling," writes Robert Hoover, MD, of the National Cancer Institute, in an editorial commenting on the above study. "For cancer at the common sites in monozygotic twins, the rate of concordance is generally less than 15 percent. Thus, the fatalism of the general public about the inevitability of genetic effects should be easily dispelled," he argues.
One of the implications of the dominant role of the environment is that we can prevent the occurrence of most cancers (or, at least, minimize the risks). Once we have identified the exact environmental factor(s) implicated in a particular type of cancer, we can reduce our risk of developing that cancer. We can, for example, minimize our risk of developing lung cancer by not smoking.
On the down side, having a low level of genetic susceptibility to cancer does not guarantee that a person will never develop the disease. For instance, even if one has a dark complexion and no family history of skin cancer, one can still develop melanoma (a form of skin cancer), as a result of repeated overexposure to the sun.
What about Genetic Testing?
Based on the results of the study by Lichtenstein and his colleagues, Dr. Hoover has questioned the usefulness of genetic testing in trying to predict cancer: "There is a low absolute probability that a cancer will develop in a person whose identical twin--a person with an identical genome and many similar exposures--has the same type of cancer. This should be instructive to some scientists and others interested in individual risk assessment who believe that, with enough information, it will be possible to predict accurately who will contract a disease and who will not," Dr. Hoover argues.
Nevertheless, some scientists say that genetic counseling is important, because it lets people find out if they carry the genes related to certain cancers. Knowing who is susceptible to certain types of cancers can motivate them to avoid the environmental variables involved, according to Anastasia Iliadou, one of the researchers involved in the study.
"Let's suppose that I eat a lot of junk food, but one
day I learn that one of my parents has been diagnosed with colorectal
cancer," Iliadou says. "I would then like to find out if I have the genes
predisposing me to colorectal cancer," she continues. "If I do, I would
try eat more broccoli--even though I've hated it since I was a kid--or
bran flakes, for instance. In other words, I would try to improve my
eating habits, in order to lower my risk of getting colorectal cancer." Iliadou also notes, however, that cancers are complex disorders and only in very rare cases is there a single gene causing cancer. "The interaction between genes and the environment is very important, and our models could not account for that," she explains. "There are many unknown genetic effects, and we still have a lot to learn about the exact nature of the interaction between genes and the environment in the development of cancer," Iliadou says.
Sidebar:Known Carcinogens*Following is a list of some of the most common environmental risk factors and the cancers they have been linked to.
Risk Factor Cancer
*Source: Harvard Report on Cancer Prevention. (Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School and the Harvard Center for Cancer Prevention)
For more information, visit the following Web sites:
http://www.yourcancerrisk.harvard.edu
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