Scientists have discovered two defective genes that cause dangerous thickening of the heart, the leading medical cause of sudden death among young athletes.

Glitches in these genes result in an inherited disorder known as familial hypertrophic cardiomyopathy. Victims are prone to suffer heart rhythm problems or to sudden death from cardiac arrest.

Among the most notable victims was basketball star Hank Gathers of Loyola Marymount, who collapsed during a game last March. Experts are uncertain whether he had the inherited form of the disease.

The latest discovery by researchers at Harvard Medical School is the first time scientists have pinpointed the precise genes responsible for an inherited abnormality of the heart.

Cardiomyopathy causes thickening of the walls of the heart. While familial hypertrophic cardiomyopathy is rare -- affecting about three in 100,000 people -- other forms are common side effects of a variety of diseases, including high blood pressure.

Thickening of the heart muscle also can result from high blood pressure, rheumatic heart disease, diabetes, hardening of the arteries and other diseases.

By untangling the precise cause of the inherited form of the disease, doctors hope they will shed light on the mysterious origins of the far more common varieties of cardiomyopathy. "It has significance far beyond this relatively rare disease," said Christine Seidman, one of the Harvard researchers. "We'll gain significant insights in secondary causes of hypertrophy (thickening), which is a very common phenomenon."

Inherited cardiomyopathy runs in families. Parents with the disease have a 50 percent chance of passing it on to their children. Researchers studied two families with the disorder and found that it was caused by two adjacent genes that make proteins that are essential for making the heart muscle contract.

These genes contain the code for proteins called myosin heavy chain polypeptides. Each gene contains about 30,000 bits of information. In one family, the disease resulted from a mistake in just one of those bits of code. In the other, people with the disease inherited an extra gene that was a hybrid of the two heart protein genes.

Finding the genes that cause inherited cardiomyopathy has been a goal of researchers at labs in the U.S. and Canada for several years. "The work is extremely important," said Neal Epstein, who had been working on the disease at the National Heart, Lung and Blood Institute.

He noted that some people who carry one of the faulty genes show no outward signs of their condition, even though they may suffer its consequences. Knowing the precise genetic defect should now enable physicians to determine whether they are at risk.

Seidman said this can be especially important when one child has been stricken with the disease but parents are unsure whether their other children also have it. --