Though researchers have long thought that environmental stress factors - things like family dysfunction, low socio-economic status and discrimination - play an important role in cardiovascular disease and type II diabetes, not much is known about physiological factors that link stress to the diseases' development. The study followed 523 pairs of twins over the past ten years.
"Cardiovascular disease and type II diabetes and their co-morbidity pose an important health challenge to the United States," says Dr. Frank Treiber, vice president for research and principal investigator on the study. "What we're looking at are environmental issues and the role they play. in causing these diseases. It's often the combination of genetic and environmental risk factors that is driving the development of these diseases," Dr. Frank Treiber added.
"How those factors are related to physiological changes that then cause the development of sub clinical disease is unknown at this point. A twin study allows you to tease out the genetic contributions by comparing identical and fraternal twins," Dr. Treiber said.
MCG researchers have been studying the twin sets since 1997. When they started the study, their average age was 10; they will be 19, on average, as they start the new study. By comparing identical twins, who share the same genetic material, to fraternal twins, who are, on average, like other siblings in terms of the genetic material they share, researchers can determine whether risk factors such as high blood pressure and insulin levels are due to genetics or environmental factors. They believe the cumulative impact of stressful environments will predict cardiovascular disease and type II diabetes.
Researchers will evaluate them two more times over the next four years, asking about stress factors, including their living environments and how they cope with stress. "The twin design allows you to assess whether they are experiencing the same environmental factors now as when they were younger and in the same household," Dr. Treiber said.
"Early in life, children tend to model their parents' behavior in how they cope with stress and perceive the world. As they've gotten older, they've been exposed to different things, different environments. By comparing them and determining whether they've developed early signs of these diseases, we can tell how much is attributable to genetics and how much is environmental stress," Dr. Treiber added.
Past phases of the study have revealed that among both the black and white twins, genetics account for about half of the differences in blood pressure and reactions to stress. With the current phase, researchers also hope to find out whether the black twins, whose race tends to develop hypertension earlier and more often than whites, are more impacted by stress. One theory is that blacks have a higher risk of stress due to things such as discrimination, unfair treatment and unsafe neighbourhoods.
"The value of the longitudinal study is that we'll be able to look at the changes over time. Not only the physical changes as they mature, but also the changes in their social and emotional development," ," Dr. Treiber said. "We have a greater chance to intervene and alter environmental factors, for example by teaching people how to better deal with stress," said Dr.Harold Snieder, MCG adjunct professor of paediatrics, chair of the Genetic Epidemiology and Biostatistics Unit at the University of Groningen, The Netherlands, and a co-investigator on the study.