Gene links cannabis use and mental illness

A person's risk of developing a psychotic illness such as schizophrenia after smoking cannabis as a teenager is affected by their genetic make-up, say UK researchers. A new study to be published in the journal Biological Psychiatry shows that a variant of the COMT (catechol-O-methyl transferase) gene is linked to a five-fold increased risk of psychotic illness in people who have smoked the drug. The scientists, based at the Institute of Psychiatry at King's College London, say that neither cannabis nor the genetic variation alone is enough to trigger psychosis.

The team looked at a group of 803 people born between April 1972 and March 1973 in Dunedin, New Zealand, who have been tracked since birth as part of a long-term health study. According to a report in the Times newspaper, the participants were asked about their cannabis use at the ages of 13, 15 and 18. They were also tested to see what form of the COMT gene they had inherited, since this gene has previously been linked to schizophrenia. This illness is one of several psychotic disorders thought to involve an imbalance in levels of dopamine, a key brain chemical.

The COMT gene comes in two different varieties, known as 'val' and 'met'. In people with two met versions of the gene, the rate of psychotic illness was three per cent, regardless of whether they had smoked cannabis as a teenager or not. But in people with two val versions, the rate was 15 per cent in cannabis smokers, compared to three per cent in non-smokers. The findings suggest that the val gene variant and cannabis both affect the brain's dopamine system, delivering a 'double dose' that can be damaging. The group has previously published research showing that variations in another brain chemical gene affect how well a person copes with stressful life events, and their subsequent risk of depression.

Marjorie Wallace, chief executive of the mental health charity Sane, said that it was becoming clear that cannabis placed millions of users at risk of lasting mental illness. 'If we were able genetically to identify the vulnerable individuals in advance, we would be able to save thousands of minds, if not lives', she said. However, study leader Avshalom Caspi disagreed, saying that 'such a test would be wrong more often than it is right'. He said that since smoking cannabis has 'many other adverse effects', even people who are not genetically vulnerable should not be encouraged to use the drug.