Twin Studies Reveal Genetic Components Leading To Cardiac Disease

Main Category: Cardiovascular / Cardiology
Also Included In: Genetics
Article Date: 12 Jan 2008 - 0:00 PDT

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Twins provide a unique living laboratory for the study of heredity. Daniel O'Connor, M.D., Professor of Medicine and Pharmacology at the University of California, San Diego (UCSD) School of Medicine, has studied about 265 twin pairs over the past few years, which has led him to some surprising discoveries. In a series of studies published in 2007 in the American Heart Association journals Circulation and Hypertension, O'Connor identified particular genes that influence human heart health in unexpected ways.

"By studying many traits and genes, we have started to put together unexpected stories," said O'Connor. "Factors linked to cardiac disease such as hypertension or elevated blood glucose and lipids were just amorphous risk factors before. Now we know that they run in families, not only because of a shared environment, but because they are determined in part by particular genes."

Studying genetic variations in twins provides scientists with data on traits that are heritable, or passed down through human DNA, versus those determined by the environment, age or how a person cares for his or her health, factors such as medical care or diet and exercise.

For example, by studying twins, O'Connor's team identified a strong effect of heredity in the occurrence of a blood protein associated with increased blood pressure, body mass index, and insulin resistance, among other factors. Its presence indicates a heightened state of inflammation in the body pointing to cardiovascular risk.

The UCSD team has also described the effect of genetics on the blood vessel lining, or endothelium. Endothelial function, or dysfunction, is the basis of every common vascular disease, such as heart attack or stroke.

"When blood vessels stop contracting and relaxing properly, it is the first indication of vascular dysfunction," said O'Connor.

Studying about 300 twins, the researchers found a previously unsuspected link between catecholamines chemical compounds produced in the adrenal gland that are released in reaction to stress and the endothelial system, also showing that this link is due to a genetic variation and therefore runs in families.

"We're just scratching the surface," he said. O'Connor and his research team have looked at about 900 of the 30,000 genes on the human genome. They hope to more densely and systematically evaluate genotypes by studying blood samples from more twin pairs, in order to more extensively map genetic components that lead to increased risk of heart and other diseases.

"Behavioral studies with twins have been conducted for decades, but in the post-genomic era, we can now factor in genotypes and determine the effect of specific genes on traits," O'Connor said. By studying both genetically identical (monozygotic) and fraternal (dizygotic, twins who share half of their genetic information) twin pairs, researchers can more readily pinpoint specific genes in people who are the same age. A formula measuring the difference in the frequency of traits between the siblings in each twin pair establishes what is called the index of heritability.

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