There are many complex social and environmental factors that influence whether a person becomes addicted to drugs or not. However, there are also suspected genetic components, and new research reveals a big factor in whether or not someone injects themselves with drugs like heroin is the presence of DNA inherited from an ancient retrovirus.
The virus HERV-K HML-2 has been with humanity at least as long as we have been Homo Sapiens, a minimum of 250,000 years. It's also been found in Neanderthal and Denisovan remnants. Back then it made its way into the germline of our ancestors, ensuring its pieces of DNA would be passed on from parent to child.
Like other retroviruses, HERV-K HML-2 can turn up in different parts of the genome, and its effects vary by location. One place it manifests is within the RASGRF2 gene. Knowing that RASGRF2 influences dopamine production, Professor Aris Katzourakis of Oxford University suspected HERV-K HML-2 might influence susceptibility to addiction, which is strongly related to dopamine response.
Katzourakis co-led a team that compared the frequency with which HERV-K HML-2 has integrated itself into RASGRF2 among people who acquired other viruses through intravenous drug use – a strong indication of drug abuse – with control populations.
Among 202 HIV-positive people in Greece, 14 percent of those who had been infected by sharing needles were RASGRF2-integrated, as Katourakis notes in the Proceedings of the National Academy of Sciences. Just 6 percent of those who had acquired HIV through other means had HERV-K HML-2 in the relevant location.
Katourakis and co-authors confirmed the findings by comparing a UK population who had hepatitis C, and a long-term history of intravenous drug use, with those who had been infected through bleeding disorders. Here the difference in RASGRF2-integration was even sharper; 34 percent to 9.5 percent.
Confirmation came when the authors introduced HERV-K HML-2 into an equivalent position in the genomes of mice, and saw that it affected the transcription of the RASGRF2 gene.
Although the human genome contains bits of DNA from many retroviruses that infected us during our evolutionary past, HERV-K HML-2 is the only one thought to still be proliferating, turning up in new locations within our chromosomes. There is something of an irony that HIV is also a retrovirus, although one that has not made its way into the human germline.
As the figures make clear, most people with RASGRF2-integrated HERV-K HML-2 won't become addicted to opiates, just as the majority of people with addiction problems don't have this particular genetic twist. However, the work does indicate that people with RASGRF2-integration are much more vulnerable, and may benefit from interventions that prevent addiction from occurring, if they are willing to be tested for its presence.