ST. LOUIS, MISSOURI--Scientists hoping to find a simple set of genes that dictates a predisposition to addiction are likely to be frustrated, according to new research, which reinforces the notion that not only are addictions genetically complex, but they also overlap with each other to some extent.
Studies with twins indicate that addiction to nicotine, alcohol, and other drugs is partly inherited. Health professionals would love to understand the genes responsible, but the handful identified so far likely represent only a small portion of those at work.
To see if a general set of addiction genes can explain most bad habits, geneticist Kirk Wilhelmsen of the University of North Carolina, Chapel Hill, examined how smoking and other addictive behaviors were passed down in two different populations of people. The first included almost 400 California families in which at least three people smoked. People who scored highest on a standard nicotine addiction test or who had high numbers of smoking withdrawal symptoms sported a particular genetic sequence on chromosome 6, the researchers reported here today at the annual meeting for the American Association for the Advancement of Science (which publishes ScienceNOW). In contrast, those for whom cigarettes were particularly relaxing shared sequences on chromosome 15. This group also had a unique region on chromosome 4 that appears to be linked to the amount of time people take in the morning to light up. This same region is known to harbor many genes involved in alcohol metabolism.
The story was not the same in Native American families in southern California, a group with high rates of drug dependence. Wilhelmsen's team found that the same region of chromosome 6 predicted several forms of substance abuse, but not smoking. But they could not find a region on chromosome 4 that linked with alcohol dependency in the Native Americans, suggesting that genes that predispose people to addiction differ by population. Another difference was that chromosome 6 in Native Americans also linked to higher fat as measured by body mass index (BMI). ("What's more addicting than that bag of potato chips in the night?" Wilhelmsen asks.)
The link between BMI and drug addiction in the Native American population was "totally surprising," says science historian Michael Fortun of Rensselaer Polytechnic Institute in Troy, New York, who studies the genetics of addiction. The work also highlights how complicated dependency is, he adds, but "understanding the genetics is essential for teasing out the components of addiction."