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Smoking Linked to Some Colon Cancers

WebMD Health News

Nov. 14, 2000 -- This Thursday, the American Cancer Society sponsors the 24th annual Great American Smokeout. While there are countless reasons to stop smoking, here's one more: It appears that a genetic alteration that occurs in certain colon cancers is linked to cigarette smoking, with smokers almost twice as likely to have this mutation as nonsmokers.

Researchers from the University of Utah found that the genetic alteration, known as microsatellite instability, was strongly linked to smoking in those picking up the habit at a young age and in those who had smoked for many years. They estimated that 21% of all colon cancers associated with this genetic alteration can be linked to smoking. Their findings were reported in the Nov. 15 issue of The Journal of the National Cancer Institute.

"We have known for a long time that there are differences in the genetic characteristics of tumors," study author Martha L. Slattery, PhD, tells WebMD. "The association between cigarette smoking and colon cancer has been believed to be small, because most studies have been inconclusive. This study suggests there may be a link between smoking and specific subsets of colon cancer." Slattery is a professor of epidemiology at the University of Utah.

Microsatellite instability is a genetic error found in the "letter" pairs that make up DNA, the basic set of instructions found in every cell of the body. This instability has been shown to interfere with a cell's ability to identify and correct the DNA mutations that lead to cancer. While microsatellite instability is common in people with an inherited form of colon cancer called hereditary nonpolyposis colorectal cancer, these family-linked tumors account for just 2% to 5% of all colon cancers. The microsatellite instability error, however, is believed to be present in roughly 15% of all colon cancers.

"This study is unique because it linked smoking to this mismatched repair," Michael Thun, MD, who heads epidemiological research for the American Cancer Society, tells WebMD. "This is some of the best evidence that smoking plays a role in at least some colorectal cancers."

The University of Utah researchers found that colon cancer patients with microsatellite instability were more likely to smoke 20 or more cigarettes a day, were more likely to begin smoking at a young age, and were more likely to have smoked for 35 years or more, compared to colon cancer patients without the genetic error. They found a twofold increased risk associated with cigarette smoking and microsatellite instability in tumors.

The authors conclude that smoking represents the largest risk factor for microsatellite instability in tumors identified to date. Their findings also could help researchers better understand the pathways involved in the progression to colon cancer. Numerous studies have definitely linked cigarette smoking with the formation of colon polyps known as adenomas, which are precursors to colorectal cancer. But studies evaluating smoking in colon and rectal cancers have been far less conclusive.

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