WASHINGTON (AP) -- People of eastern European Jewish ancestry are two to three times more likely to develop colon cancer if they have a mutation of a single gene, according to a new study.

The study of DNA from more than 3,000 Ashkenazi Jews in New York and northern Israel appears Friday in the journal Science.

Earlier studies had shown that mutation of both copies of the BLM gene causes a disorder called Bloom syndrome. The study shows that it takes only a single flawed BLM gene to increase the risk of colon cancer.

Kenneth Offit, a Memorial Sloan-Kettering Cancer Center researcher and senior author of the study, said the conclusion is based on a finding that Ashkenazi Jews diagnosed with colon cancer were two to three times more likely to have a single mutation of the BLM gene.

"This means there is about a three times greater risk for colon cancer in an individual carrying a single copy of the mutated gene," said Offit.

The study evaluated the genes of 1,244 Ashkenazi Jews with colon cancer and 1,839 who did not have the disease.

Bloom syndrome, caused by a mutation of both copies of the BLM gene, is linked to a number of physical conditions, including short stature, and to a predisposition for many types of cancer. Bloom syndrome occurs in all ethnic groups, but is most common among Ashkenazi Jews, or Jews of Eastern European descent.

Offit said the researchers looked for evidence that the single mutation also caused other cancers associated with Bloom syndrome, but no linkage was found.

Only about 1 percent of the Ashkenazi Jews have the single BLM gene mutation, said Offit. A much smaller percentage, he said, have the double mutations.

Dr. Neal J. Meropol, a colon cancer researcher at the Fox Chase Cancer Center in Philadelphia, said the study by Offit and his colleagues is part of a growing effort to identify specific and subtle genetic risk factors for various types of cancer.

"Over the next several years, there are likely to be many gene alternations identified that modify one's risk of developing a particular kind of cancer," said Meropol. "This doesn't mean that it is a certainty that one will get cancer, but it may prompt you to get (medical) screening or to alter behaviors that also confer risk."

Smoking has been linked to colon cancer. A diet overly rich in fats and sparse in fiber has also been linked to colon cancer, although this connection is not universally accepted among researchers.

There are an estimated 10 million Ashkenazi Jews worldwide, with about 2.5 million in Israel and about 1 million in the New York City metropolitan area. The Ashkenazi trace their origins to Eastern Europe, but they live in many countries.

Colon cancer is the third leading cause of cancer death in the United States. There are about 148,000 new cases of colon cancer diagnosed and about 56,000 people die of the disease each year.

Offit said studying the biological mechanism linking the BLM mutation to colon cancer may lead to a basic understanding of what causes all colon cancers.

"Although this finding relates only to Ashkenazi Jews, we think the mechanism will be the same for all colon cancer," said Offit.

The BLM gene is known to play a role in correcting DNA errors inside cells. When a cell is formed, there sometimes are DNA mistakes. Genes such as BLM detect and, in some cases, correct those flaws. When these mistakes are not corrected because genes such as BLM are not working, it is possible that a cell will retain DNA errors that could cause cells to proliferate without control, forming tumors.

Offit said eventually it may be possible to conduct genetic screening to identify patients with BLM mutations, but for now "tests such as this one are not ready for clinical use."

Instead, he said, people should have regular colonoscopy screenings on a schedule that is suggested by their family history of colon cancer. In families where cancer or colon polyps have been found, he said, the screening may start at an earlier age and be scheduled more often than the generally recommended five-year interval.

Offit said colon cancer is considered a completely preventable disease. The cancer is generally preceded by formation of colon polyps. If these polyps are removed during regular colon screening tests, then the cancer does not form, he said.

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