Human genome 'littered' with viral DNA but problem drug users three times more likely to carry genetic fragments in regions which control reward centres and addictive behaviour

DNA implanted in the “dark parts” of the human genome by ancient virus infections may help to explain why some people are more susceptible to drug addiction, a study suggests.

Researchers from Oxford University and the University of Athens found changes in parts of the DNA which are thought to have little impact on human functions, could actually affect a suite of addiction-related behaviours.

The culprit is a fragment of viral DNA, dubbed HK2, that appears near to genes which underpin the brain’s reward system and can increase the risk of addiction by interfering with the neurotransmitter, dopamine, which reinforces pleasure seeking behaviours.

Among the general population between 5 and 10 per cent of people carry the HK2 fragment in this reward centre region, but genetic analysis of two groups of intravenous drug users form the UK and Greece found they were up to 3.6 times more likely to have this change.

The researchers say that combination of DNA fragment and location manipulates the workings of dopamine-driven addiction behaviours and is likely to be responsible for at least some of people's addictive behaviour.

“Most people think these ancient viruses are harmless,” said Dr Gkikas Magiorkinis, from the University of Athens who led the study published in the journal Proceedings of the National Academy of Science (PNAS) on Monday.

“Now we have strong proof that human endogenous retroviruses can be pathogenic [cause disease].”

The human genome is littered with fragments of DNA that were inserted by retroviruses, a group of viruses which are able to write their genetic material into a host.

Many of these infections occurred in early humans or close ancestors and are now “endogenous” and are carried by most or all humans, usually causing little to no effect as they occur in these regions long thought of as junk DNA.

“Looking into this 'dark' part of the genome will unlock more genomic secrets,” Dr Magiorkinis added.

Two other diseases are caused by retroviruses, HIV and the Human T-lymphotropic, and most people are infected directly by the virus or inherit it from a parent.

Because these viruses often lead to fatal disease, as in the case of Aids, it makes it less likely that those infected will survive to pass them on.

In HK2 the infections dates back to some of our earliest ancestors and is replicated in many of our genomes without effect, but still with the potential to cause serious harm when added to other factors that can lead to people becoming addicts.

“We know of clear biological roles for a small number of human endogenous retroviruses,” said Professor Aris Katzourakis, from the University of Oxford, who co-led the study.

“However, there has never before been strong evidence in support of a role in human biology of an endogenous retrovirus that is unfixed, in other words not shared by all individuals in the population.

“Our study shows for the first time that rare variants of HK2 can affect a complex human trait.”