Molecular Fingerprint Identified For Cocaine Addiction

Applying DNA microarray technology, the most powerful method for gene expression profiling, Dr. Hemby analyzed brain tissue from 10 human subjects who had overdosed on cocaine and an equal number of controls. Among the 9,000 genes scrutinized per subject, more than 400 turned out to have become disregulated-either turned on or off due to long-term cocaine use.

"For the first time, we have looked at a portion of the human genome and determined the effects of a drug like cocaine," says Dr. Hemby, director of the Emory DNA Microarray Facility and an assistant professor of pharmacology in the Emory University School of Medicine. "It's going to take a long time to work this out, but we're setting up a framework that we can take into studies of opiate and alcohol addiction and other human diseases that will ultimately lead to the development of new treatments."

The identification of genetic markers for addiction could be the most significant advance in drug addiction research in decades. Much of the work was made possible by the recent development of DNA microarray technology, which enable the assessment of thousands of genes simultaneously. Hemby noted the tool has been a boon for researchers who study complex mammalian behavior like cocaine addiction that are dictated by the coordinate changes in gene expression of many genes.

Identifying genes associated with cocaine addiction has been constrained by the limitations of conventional methods of gene expression profiling that can only focus on single genes. One of the greatest successes in identifying genes associated with cocaine use was the characterization by Yerkes' Dr. Mike Kuhar of Cocaine and Methamphetamine Transcript (CART).

In addition to analyzing many genes simultaneously, DNA microarray technology can identify genes whose functions are either known or unknown. The complex process of gene expression profiling using this method begins with the fabrication of a gene chip made up of DNA copies derived from more than 40,000 human genes. RNA isolated from post-mortem brain tissue is reverse-transcribed into complimentary DNA and used as a "probe" with the gene chip. If the RNA has the same sequence as one of the spots on the chip, the "probe" binds or hybridizes with the DNA. In the final step, a microarray reader scans the hybridized slide to determine those genes that are up-regulated, down-regulated, or unchanged.

With the human genome now completely mapped, Dr. Hemby hopes to soon have access to a complete library from which to make gene chips. Given the number of genes already identified, he expects to find several more disregulated genes involved in the cocaine addiction process. He noted that while finding the disregulated genes has marked a significant advance, equally important is identifying those that remain unchanged by long-term cocaine use.

Dr. Hemby's research opens several new frontiers in drug addiction research. Conventional drug addiction studies typically have attempted to use animals to model this uniquely human condition, with its myriad of molecular processes that cannot be accurately reproduced in the laboratory. While many researchers continue to focus on identifying brain receptors for drugs, Dr. Hemby says that mounting evidence clearly shows that genes form the biological underpinnings for addiction.

"Addiction research generally has taken a global approach to understanding how a drug like cocaine affects the brain," says Dr. Hemby. "Conventional tests, such as functional MRIs, provide a broad measure of what is taking place during drug self-administration. But it doesn't give us a panel of things that are changing."

To model the molecular changes that take place with long-term cocaine use, Dr. Hemby is developing a gene expression model in rats. "We're asking about what effects are produced at the initial exposure to cocaine, 30 days later, and after years of exposure. We also want to know what happens when an addict relapses," Dr. Hemby says.

As scientists achieve greater understanding of the biology of cocaine addiction, they hope to develop medications that effectively treat addiction without serious side effects. These medications would target specific aspects of the biochemical pathways that promote craving for cocaine. The National Institute on Drug Abuse has made it a national priority to develop a therapeutic substitute for cocaine, corresponding to the heroin substitute methadone.

Dr. Hemby cautions that it may not be possible to wholly conquer addiction. "It isn't reasonable to believe that we can cure cocaine addicts," he says. "Any therapeutic approach should instead be designed to prevent relapse."

Dr. Hemby recently extended his drug addiction studies to heroin, a highly addictive derivative of opium that produces an unparalleled physical dependency. In studies of rats, many of whom have self-administered heroin for months, Dr. Hemby is using gene expression profiling to map the effects of heroin on the neural systems in a part of the brain involved in the rewarding effects of drugs. As with his cocaine study, he hopes to observe the molecular changes over the long term and determine whether they are reversible. He also wants to gauge the effect of heroin exposure after prolonged abstinence.