A gene that raises someone's risk of becoming addicted to cannabis has been discovered.
Around 9 percent of people who use cannabis develop a dependence on the drug, which leads to withdrawal symptoms when someone stops using it. While the causes of cannabis addiction are partly environmental, studies in twins have suggested that genetics accounts for 51 to 70 percent of a person's likelihood of developing the condition.
'We discovered that the disorder was associated with a genetic variant. This variant affects how much of a certain nicotine receptor is formed in the brain,' said study author Dr Ditte Demontis of Aarhus University, Denmark.
The researchers compared the genomes of 2000 people in Denmark diagnosed with cannabis-use disorder, and 50,000 people who did not have the condition. Comparing the genetic information with patient health records, they found a mutation in the nicotine receptor encoding gene CHRNA2 was linked to cannabis addiction.
The researchers then tested the findings against a second, larger cohort from Iceland – comparing the genomes of 5500 people diagnosed with the condition with 300,000 people without, which bore out the connection with CHRNA2.
People who had the mutation linked to cannabis addiction in this gene had lower levels of the receptor in the brain. However, a mechanism linking this mutation and cannabis addiction two has yet to be found, as the study only found a correlation between the two. In addition, although the study was relatively large for its kind, the authors acknowledge limitations, including a focus on northern Europeans.
Cannabis addiction is linked to other health problems, including psychosis, bipolar disorder, anxiety and cognitive impairment. Dr Demontis said she hoped that pursuing this line of enquiry further would lead to new treatments for cannabis-use disorder.
'People who abuse cannabis often do worse in the education system, and our results show that this can be partly explained by genetics,' said Dr Demontis. 'Our hope is to be able to improve treatment and perhaps in the long-term even prevent this abuse.'
Professor Joel Gelernter, a psychiatrist at Yale School of Medicine, told Scientific American: 'I think it's quite likely that [the research] adds something that's really of interest for our biological understanding of the nature of cannabis dependence and why some people are more likely to become cannabis dependent than others.'