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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 Introduction The sad truth about cancer is that, in most cases, it cannot be cured. But results of a new study find 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 New England Journal of Medicine (July 13, 2000), environment plays 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 nearly 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 only share some, like non-twin
siblings do), the scientists were able to separate the cancers that were
caused by genetics from the ones caused by environmental factors. The
results showed that genetic makeup plays a relatively minor role in
determining a person's risk of developing cancer.
"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 (26-42%) of the risk.
Environmental factors are more important in the causation of cancer, according to Axel Skytthe of the Danish Center for Demographic Research and Epidemiology, and one of the study's co-authors. "The heritability of the tendency to develop cancer is minor--only about 10-15%. 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 11-18%,
the study found. For dizygotic twins, the risk of these cancers was
between 3% and 9%. 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 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. Basically, the study results say we can prevent the occurrence of most cancers, or at least minimize the risks. Once we have identified the exact environmental factor(s) that cause a particular type of cancer, we can reduce our risk of developing that cancer. For example, by not smoking, we reduce the risk of getting lung cancer. On
the down side, just because you aren't genetically susceptible to cancer
does not guarantee that you will never develop the disease. For instance,
even if you have a dark complexion and no family history of skin cancer,
you can still develop melanoma (a form of skin cancer) as a result of
repeated overexposure to the sun. Genetic Testing?
Based on the results of the study, Dr. Hoover questions 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...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 certain environmental variables, according to Anastasia Iliadou, one of the study's researchers.
"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. If I
do, I would try to 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." (It has been said that green vegetables and dietary fiber can
prevent cancer.)
Iliadou also notes, however, that cancers are complex 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.
Thomas S. May is a freelance
medical writer. Source: Medscape Health
Reprinted from Medscape Health for Consumers.
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