Don't Blame Your Parents for Cancer

Environmental factors like smoking, diet, at fault, study says

By Adam Marcus HealthSCOUT Reporter WEDNESDAY, July 12 (HealthSCOUT) -- All cancers are at bottom genetic, the product of DNA errors that allow cells to divide unchecked. But that doesn't necessarily mean these mutations are passed from parent to child. Indeed, researchers have long known that most cancers aren't inherited. The question, though, is how much -- or, how little -- a role legacy plays.

New research from Scandinavian scientists suggests that the vast bulk of human cancers results not from what you inherit but from cell mutations caused by environmental factors -- everything from your diet and exposure to carcinogens in tobacco to viruses that trigger tumors.

And for most of the tumors that do have a strong genetic component, the researchers say, the exact genes that cause them still haven't been found.

Jeff Boyd, a cancer geneticist at the Memorial Sloan-Kettering Cancer Center in New York City, says the latest work should reassure people who have relatives with cancer. "Cancer is not inevitable, even in a context of a strong genetic predisposition or family history" of the disease.

What's more, cautions Boyd, who was not one of the researchers on the study, the term "environmental" is used too loosely in the report, which appears in this week's issue of The New England Journal of Medicine.

"What most Americans will read into this is that the majority of cancers result from exposure" to toxins in the air or water. That, he says, "is entirely inaccurate. In fact, there are very, very small numbers of human cancers that can reliably be attributed" to such chemicals," he says.

In the latest work, led by Paul Lichtenstein, of the Karolinska Institute in Stockholm, Sweden, researchers from around Scandinavia studied the contribution of heredity to cancers in nearly 45,000 pairs of twins, roughly 90,000 people, in the region.

Twins are an excellent living laboratory for studies of heredity. Since they have at least 50 percent of their genes in common, variations in cancer risks and rates can reasonably be attributed to non-genetic factors.

Nearly 11,000 of the subjects contracted a least one form of cancer. But despite their shared genes, twins of cancer patients weren't very likely to also develop the disease, Lichtenstein's group found.

To be sure, some tumors were more strongly genetic. Genes were implicated in 42 percent of a person's risk of prostate cancer, 35 percent of the risk of colorectal cancer, and 27 percent of their odds of developing breast cancer. And cancer risks were higher in identical twins than in fraternal twins, who share only half their genes.

Yet on average, the researchers claim, heredity is at most a mild force. "This finding indicates that the environment has the principle role" in cancer, they write. "The relatively large effect of heritability in cancer at a few sites (such as prostate and colorectal cancer) suggests major gaps in our knowledge of the genetics of cancer."

Although the study has several shortcomings, including its inability to look at the interaction between genes and the environment, it "provides new and valuable information for the nature-vs.-nurture debate," writes Dr. Robert Hoover, of the National Cancer Institute, in an editorial accompanying the journal article.

Given the overwhelming impact of environmental risk factors on the disease, Hoover writes, most attempts to use genetics to determine who will contract cancer and who won't "seemed doomed."

Kay Huebner, a cancer geneticist at Thomas Jefferson University's Kimmel Cancer Center in Philadelphia, says the Scandinavian research illustrates how much work is needed to uncover the genes that do up the risk of tumors. "It certainly indicates that we haven't found the familial genes for prostate cancer" and perhaps colorectal cancer, Huebner says.

What To Do

Figuring out what's nature's fault and what's the environment's fault when it comes to cancer needs much more study.

While all the genes for inherited cancers may not be identified, there are ways to maximize your chances of beating the disease even if you get it. Screening tests, such as routine mammography for breast tumors and the fecal occult blood test for colorectal polyps, can catch cancers in early, treatable stages

-- cin (cin@cinn.cin), August 05, 2000