DALLAS (AP) - Blacks who have a rogue version of a blood-clotting gene have six times the risk of heart disease compared with other blacks, according to researchers.

``We have identified a genetic marker for predicting increased heart attack risk in African-Americans,'' said Dr. Kenneth K. Wu, lead author of the study in the March 13 issue of Circulation, an American Heart Association journal.

He said the work shows the importance of investigating genetic risk factors for heart disease by ethnic origin. The rogue gene was not associated with higher heart disease risk in whites.

The gene that was studied is the blueprint for a protein called thrombomodulin, which converts the enzyme thrombin from a clotting agent into an anticlotting agent. The protein plays a key role in protecting blood vessels.

A common variance in the gene occurs when an amino acid called alanine is replaced by valine. This change previously has been associated with heart attack risk.

The researchers examined nearly 16,000 middle-aged Americans between 1987 and 1989, including their blood pressures and samples of blood and DNA. They compared 376 people (87 black, 289 white) with heart disease to a group of 461 (105 black, 356 white) who did not develop heart disease.

Sixteen percent of blacks who suffered heart disease had at least one copy of the rogue gene, which is inherited, whereas only 7 percent of healthy blacks had the variant.

After accounting for other risk factors such as hypertension and diabetes, the researchers calculated that having the rogue gene increased the risk of coronary heart disease in blacks by a factor of six.

``I think it's interesting, but the numbers are very small so the results have to be interpreted with extreme caution,'' said James de Lemos, assistant professor of medicine at the University of Texas Southwestern Medical Center at Dallas, who was not involved in the study.

Wu said the reasons for the increased risk in blacks with the rogue gene aren't known.

One possibility is that the thrombomodulin gene isn't the cause of the increase, but is a ``marker'' for another variant that is the cause.

Another theory is that the rogue gene actually is the cause, somehow exerting more influence over the thrombomodulin protein in blacks as compared to whites, Wu said.

If the findings are confirmed as expected by larger studies, doctors may begin screening blacks with heart disease for the variant gene to help them minimize their overall risk, Wu said.

``This probably would be useful to discern the risk among blacks who already have coronary heart disease or have suffered a heart attack and prevent a recurrence: modify lifestyle, increase treatment of hypertension and diabetes, decrease body weight,'' Wu said.

Copyright 2001 The Associated Press. All rights reserved.