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The findings could point the way to deciding who will benefit from gene testing when assessing risks for colon cancer, the study's authors said.
Two "germline mutations" -- DNA changes passed down from parent to offspring -- are often implicated when multiple colorectal adenomas (polyps) are found in people with a condition called familial adenomatous polyposis, noted a team led by Dr. Shilpa Grover, of Brigham and Women's Hospital in Boston.
However, there is no consensus on when genetic testing for the two genes, known as APC and MUTYH, should be done.
In the new study, published in the Aug. 1 issue of the Journal of the American Medical Association, Grover and colleagues looked at data on almost 8,700 people who had undergone full genetic mapping between 2004 and 2011.
The researchers found that 80 percent of people with 1,000 or more polyps also carried the APC mutation, whereas just 2 percent carried the MUTYH mutation. Rates of the APC mutation were also much more prevalent than MUTYH among people with between 100 and 999 polyps, the study found.
However, the rate of either of the mutations was similarly low (estimated at 10 percent of patients or less) for people with fewer adenomas (between 10 and 99 polyps).
The bottom line: The number of colorectal polyps a patient had "was strongly associated" with the presence or absence of these genetic mutations, the researchers said.
Based on the finding, the investigators reasoned that testing for APC or MUTYH might be considered for anyone with 10 or more polyps detected on colonoscopy. "However, our results are derived from a selected cohort [group] of high-risk individuals and need to be validated in larger populations of unselected patients," the researchers added in a journal news release.
Two experts said the information in the study was valuable.
"This study is important because it helps to correlate the number of colon polyps with genetic testing for certain genetic abnormalities," said Dr. David Feldman, an attending physician at Beth Israel Medical Center in New York City.
Familial adenomatous polyposis "causes many polyps and usually [when] untreated leads to colon cancer before age 40," he added. "The study's findings may be helpful in counseling people with familial adenomatous polyposis and family members, and could lead to changes in the future in screening for polyps in people with [the condition] and families of these patients."
Another expert cautioned, however, that it's still too early for concrete recommendations on genetic testing for patients at risk of colon cancer.
"This study suggests that patients with 10 or more adenomas be considered for genetic testing, [but] this recommendation needs to be carefully considered in the context of the patient population evaluated in this study and the potential positive or negative effects of genetic testing," said Dr. David Bernstein, chief of hepatology and acting chief of gastroenterology at North Shore University Hospital in Manhasset, N.Y.
And Feldman noted that while many people will have a positive finding of polyps on colonoscopy, familial adenomatous polyposis remains relatively rare.
"Colon polyps are found commonly in people undergoing screening colonoscopy. Up to 85 percent of people undergoing a screening colonoscopy will be found to have colon polyps," he said. "Less than 10 percent of these patients found on screening colonoscopy will have familial adenomatous polyposis. Most patients with colon polyps will have sporadic polyps for which no genetic testing is currently available."
-- E.J. Mundell
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