It's In Their Genes: Study Of Twins Connects Smoking Addiction With Major Depression

"Some people with a history of depression may become smokers as a way of self-medicating," said Qiang John Fu, M.D., Ph.D., assistant professor of community health in biostatistics at Saint Louis University School of Public Health. "Some people who are smokers might become depressed when they try to give up cigarettes and can't.

"When I tried to find something to explain this correlation, I discovered the answer lay partly in a person's genes that are associated with conduct disorder, which is extreme rebellious behavior of teens and children," Dr. Fu continued. "My findings are an alternate explanation about why nicotine dependence and major depression go together."

Dr. Fu also found that the genes that increased a person's risk of developing major depression and nicotine addiction are found in those who have conduct disorder, such as stealing, vandalizing, running away from home and fighting. These people are likely to become addicted to other drugs and behave impulsively, he said.

Dr. Fu and his team analyzed 3,360 pairs of middle-aged, predominantly Caucasian twins who served in the military during the Vietnam War.

Slightly more than half were identical twins who had a 100-percent genetic match and about 45 percent were fraternal twins who shared half their genes. Researchers compared the answers from the twins, and used a mathematical model to estimate the genetic and environmental influences on nicotine addiction and major depression.

"Our data showed that both major depression and nicotine dependence were highly genetically correlated with conduct disorder," Dr. Fu said.

The research also helps to explain why smoking seems to run in some families, Dr. Fu said.

"Maybe Dad and Mom have a certain personality, which is why they may be more likely to smoke or to be depressed. That personality trait may be based in their genes," he said.

The research points geneticists in a new direction to determine the influences of a personality trait, Dr. Fu said. In addition, clinicians could use his findings to identify those who are at risk of developing major depression or nicotine addiction.

"When they see people with a history of conduct disorder, they may be able to predict those people who could develop major depression or nicotine dependence," Dr. Fu said.

The research was funded by grants from the National Institutes of Health. It appeared in the June issue of Twin Research and Human Genetics.