The disease known as drug addiction shares many features with other chronic illnesses—one of which is heritability, meaning a tendency to run in families. Scientists are now studying how genes can play a role in making a person vulnerable to drug addiction, or in protecting a person against drug addiction.
While the environment a person grows up in, along with a person's behavior, influences whether he or she becomes addicted to drugs, genetics plays a key role as well. Scientists estimate that genetic factors account for 40 to 60 percent of a person's vulnerability to addiction.
The National Institute on Drug Abuse (NIDA) is currently supporting a major research effort to identify gene variations that make a person vulnerable to drug addiction. This effort involves studying DNA (deoxyribonucleic acid), which directs the development of every human cell (Figure 1). By mapping DNA sequences in drug addicts, scientists have been able to isolate gene sequences that indicate a greater risk of becoming addicted to drugs. These gene sequences contain the instructions for producing specific proteins, which perform most of a body's life functions. The way these proteins function, or don't function, can indicate how vulnerable a person is to drug addiction (Figure 2).
A major finding about the genetics of drug addiction was reported in 2004 by investigators at the Howard Hughes Medical Institute at Duke University Medical Center. The researchers were able to identify a specific protein—PSD-95—that had a relationship both to drug addiction and to learning and memory. Mice that had low levels of PSD-95 took longer to learn their way around a maze, and also were much more sensitive to cocaine.
The researchers concluded that mice with normal amounts of PSD-95 were more likely to learn their way around the maze and less likely to become addicted to cocaine. Because cocaine leads to sharp increases in the neurotransmitter dopamine, which is responsible for feelings of pleasure, or the high that drug users crave, PSD-95 likely is involved in other types of addiction. According to Marc G. Caron, Ph.D., an investigator who was part of the research team, PSD-95 "likely plays a role in addiction to other drugs—including nicotine, alcohol, morphine, and heroin—because they all exert effects through dopamine."
Another important research breakthrough in 2004 was reported by a team led by Dr. Paul Greengard, a Nobel Prize-winning neurobiologist and NIDA-funded researcher at Rockefeller University in New York City. Dr. Greengard's team found that almost every known drug of abuse—including cocaine, opiates, and amphetamines—works through a brain protein known as DARPP-32. DARPP-32 is involved as a go-between in the actions of virtually all neurotransmitters (chemical brain messengers) in all parts of the brain. When DARPP-32 was removed from the brains of mice, the mice no longer responded to drugs of abuse.
Research into the role of genes in drug addiction has shown that natural variations in proteins—which are encoded by a person's genes—can lead to differences in how vulnerable that person is to drugs of abuse. Continued study of genetic factors in drug addiction can provide new ways for understanding the disease of drug addiction, and can lead to new therapies for preventing and treating it.
"Drug Addiction, Learning Share Common Brain Protein," Duke University press release. Accessed at www.dukehealth.org/health_library/news/7415.
Greengard, Paul. "Signal Integration in the Brain," NIDA Science Meeting Summaries and Special Reports. Accessed at http://archives.drugabuse.gov/meetings/apa/signalintegration.html.
"Research Report Series on Cocaine Abuse and Addiction," NIDA Research Reports. Accessed at www.drugabuse.gov/ResearchReports/Cocaine/Cocaine.html.
(Image Credits: Art adapted from: U.S. Department of Energy Genome Program)