Drug Addicts More Likely to Have Ancient Virus in Their Genome

An illustration of the hepatitis C virus. Scientists studied those who caught the condition via intravenous drug use. Getty Images

Drug addicts are more likely to carry an ancient virus which could affect the production of dopamine than the rest of the population, a study has found.

Scientists studied a type of ancient retrovirus called HERV-K HML-2 (HK2). A retrovirus is a form of bug which transcribes its RNA into the DNA of a host cell to multiply. Retroviruses can either spread exogenously between individuals, like HIV, or endogenously from parents to offspring, but they were not previously believed to be harmful in humans.

We already know that our set of genetic instructions called the human genome contains fragments of ancient retroviruses from the germline, or cellular lineage, of our primate ancestors. This happened because retroviruses kept reintegrating into our genomes.

HK2 is believed to be the remaining ancient retrovirus that proliferated the human germline 250,000 years ago (considered to be relatively recent by the standards of evolutionary biology). The authors of the study published in the journal Proceedings of the National Academy of Sciences believe an uncommon type of HK2 called RASGRF2 could be linked to addiction.

"By showing strong genetic links for addiction, we advocate towards destigmatizing addiction and intensification of pharmacological support of addicts," Dr. Aris Katzourakis, professor of evolution and genomics at the University of Oxford and author of the study, told Newsweek.

Researchers at the University of Oxford, U.K., and the National-Kapodistrian University of Athens, showed this type of HK2 can manipulate neighboring genes, including one which plays a role in how dopamine is released in the brain. Dopamine is a neurotransmitter connected to rewarding experiences, and is therefore implicated in addiction.

"Some endogenous retroviruses have been tentatively linked to pathologies in humans, but despite intense research efforts for over 20 years, it has been hard to distinguish cause from effect," Katzourakis said.

"We show the strongest evidence provided to date that an endogenous retrovirus is linked to harmful effects in humans, by showing a link between an integrated retrovirus and addictive behavior."

Read more: How to know if you are addicted to video games and how to get help

To conduct their study, the researchers in Greece recruited 202 HIV positive participants, and found RASGRF2 was 2.5 times more common in patients who were infected via intravenous drugs than those who were infected by other means. And in a separate part of the study conducted in the U.K., 184 hepatitis C patients were 3.6 times more likely to have RASGRF2 in their genes if they had suffered chronic drug abuse than those who had not.

At first, when the researchers found a link between those who injected drugs and RASGRF2, they thought it might be a fluke.

"However, we followed this up in a second completely independent cohort and also found the association there, and also performed laboratory experiments in a cell line that also confirmed our result," said Katzourakis.

"We have identified a plausible pathway for how the virus influences the behavior of the host," explained Katzourakis. "This particular integration is inside a gene known as RASGRF2, which is involved in dopamine response. This influences the reward behavior, and we believe that by changing the expression of RASGRF2, which the virus does do in a cell line, it is influencing the reward seeking behavior of the individual.

But the results stopped short of proving that viruses can change our behavior or make us more prone to addiction, he said.

"We do not know if HERV-K HML-2 is 'alive' and spreads among humans nowadays, but we have strong evidence that uncommon proviruses can be pathogenic," he said.

"The next important question is whether uncommon proviruses are the 'echo' of the 'old-dead' epidemic, or more uncommon viruses are generated in present human populations."