The vast majority of cancers are caused not by inherited defects in people's genes, as many believe in this age of genetics, but by environmental and behavioral factors, according to the largest cancer study ever to enter the "nature vs. nurture" debate.
"Environmental factors are more important than gene factors, and that's important to remember, especially since everyone thinks that everything is solved now that we have the human genome in our computers," said Paul Lichtenstein of Stockholm's Karolinska Institute, who led the study of 89,576 twins reported in today's issue of the New England Journal of Medicine.
The report doesn't negate the influence of genes, finding that they, too, play significant roles, especially in the cases of prostate and colorectal cancer.
But it underscores the conclusion that comes from a host of other studies: Many cancers can be avoided by healthy lifestyle choices such as eating a good diet, exercising and eschewing tobacco.
All told, however, the environmental contribution to cancer has been presumed by many experts to be as low as 50 percent. And, given the recent revolution in molecular biology, much of the modern search for the causes of cancer has focused on genes.
"I think there is this kind of fatalistic approach to genes that the general public seems to have now -- that if your mom, dad, sister or brother had something that you're doomed to have it, too," said Dr. Robert Hoover, director of the epidemiology and biostatistics program at the National Cancer Institute in Bethesda, Md., who wrote a commentary accompanying the report.
In fact, he said, though there are indeed genes that cause high rates of cancer in some families, such fatalism, for the most part, is unwarranted.
Overall, genetic factors seemed to account for between 21 percent and 42 percent of risk, depending on the type of cancer, with an average of about 30 percent.
The researchers and other experts noted, however, that it is still important for people to pay attention to their family histories, because even a 30 percent genetic influence on cancer is still sizable.
The study was conducted on 44,788 pairs of twins born between 1870 and 1958 in Sweden, Denmark and Finland by researchers and scientists in the three nations. Cancer registries revealed that 10,803 of these people developed cancer.
To tease apart the relative contribution of genes vs. environment, the scientists compared cancers in identical twins to non-identical, or fraternal, twins.
Identical twins share the same set of genes, while fraternal twins on average share only half their genes. Thus, if a particular cancer is largely caused by genes, identical twins would be much more likely than fraternal twins to have the same illness. By contrast, if environment is the main cause, then identical and fraternal twins would have about the same cancer risk.
Even when an identical twin had a cancer, the scientists pointed out, the chance that the other twin got that cancer was very low.
The authors cautioned that they were only able to reach firm or reasonably firm conclusions about the most common cancers -- breast, colorectal, prostate, stomach and lung -- because the cancer registry data weren't complete, and even with such a huge passel of twins, the numbers of rarer cancers were too small to yield significant results.
But Lichtenstein said he believes it's likely that many rarer cancers will follow similar patterns.
On average, environmental factors caused about twice as many cancers as inborn genetic factors. The study did not identify what exactly in the environment put people at risk for specific types of cancer, but researchers said cigarettes, poor diet, lack of exercise, radiation and pollution were among the prime culprits.
Prostate cancer had the strongest genetic component, accounting for 42 percent of the risk, followed by colorectal (35 percent) and breast cancer (27 percent).
The large influence of environment and lifestyle observed in this and other studies flies in the face of a gene-centric mood in the general public fostered partly by the frenzy surrounding the recently announced mapping of most of the human genetic code, Lichtenstein said.
"I think it is important to say that even though the genome project has a lot of promises for us and for the treatment of cancer and other diseases, it won't explain all cancers," he said. "It will not be a miracle and solve all problems."
Dr. Thomas Mack, professor of preventive medicine at the Keck School of Medicine at the University of Southern California, called the study "a good paper -- it was done very well."
While there are definitely genetic and lifestyle differences between Scandinavia and the United States, his own, U.S.-based twin research has reached similar conclusions, Mack said.
The high heritability rate of prostate cancer does not come as a surprise, experts say. An earlier study suggested that this cancer runs more strongly in families than colon or breast cancer, said Dr. Patrick Walsh, chief of urology at the Johns Hopkins Medical Institutions in Baltimore. Thus, he said, it's particularly important that men ascertain their family history for this disease and be screened if necessary.
So far, while several prostate cancer genes have been located, attempts to pin down environmental causes have yielded little.
This is in stark contrast to other cancers, where -- though much is still unknown, and results are often contradictory -- a host of studies have pointed fingers at environmental influences.
Smoking, for instance, is linked not only to lung cancer, but to cancers of the stomach and mouth. Infection with human papillomavirus increases risk for cervical cancer. A woman's lifetime exposure to estrogen, as well as exercise, seems to influence her risk for breast cancer. Physical activity, amount of red meat, fruits and vegetables and other aspects of the diet have been implicated in colorectal cancer, and others.
"You can't choose your parents," said Devra Lee Davis, a cancer epidemiologist at Carnegie Mellon University in Pittsburgh.
"What you can do is control your exposures in your environment."
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