Pubdate: Tue, 12 Apr 2005
Source: Times, The (UK)
Copyright: 2005 Times Newspapers Ltd
Author: Mark Henderson
Bookmark: (Cannabis - United Kingdom)


ONE in four people carries genes that increases vulnerability to psychotic 
illnesses if he or she smokes cannabis as a teenager, scientists have found.

A common genetic profile that makes cannabis five times more likely to 
trigger schizophrenia and similar disorders has been identified, increasing 
pressure on the Government to reverse the drug's reclassification from 
Class B to Class C.

The increased risk applies to people who inherit variants of a gene named 
COMT who also smoked cannabis as teenagers. About a quarter of the 
population have this genetic make-up, and up to 15 per cent of the group 
are likely to develop psychotic conditions if exposed to the drug early in 

Neither the drug nor the gene raises the risk of psychosis by itself.

The study, led by Avshalom Caspi and Terrie Moffitt, of the Institute of 
Psychiatry at King's College London, offers the best explanation yet for 
the way that cannabis has a devastating psychiatric impact on some users 
but leaves most unharmed. Scientists had suspected that genetic factors 
were responsible for this divide, but a gene had not been pinpointed.

The findings, to be published in Biological Psychiatry, also reinforce a 
growing consensus that nature and nurture are not mutually exclusive forces 
but combine to affect behaviour and health. The King's team has previously 
identified genes that raise the risk of depression or aggression, but only 
in conjunction with environmental influences.

Mental health campaigners said that the results vindicated their concerns 
about the decision last year to downgrade cannabis to a Class C drug, which 
means that possession is no longer an arrestable offence.

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. About fifteen million Britons have tried 
cannabis, and between two million and five million are regular users, 
according to the Home Office British Crime Survey. The research suggests 
that a quarter could be at risk.

The evidence will be considered by a review of the drug's classification 
announced last month by the Home Secretary. It may be possible to develop a 
test for genetic susceptibility to cannabis. "If we were able genetically 
to identify the vulnerable individuals in advance, we would be able to save 
thousands of minds, if not lives," Ms Wallace said.

Dr Caspi, however, rejected the idea of screening based on the COMT gene. 
"Such a test would be wrong more often than it is right. Cannabis has many 
other adverse effects, especially on developing teenagers, on respiratory 
health and possibly on cognitive function. Effects may be pronounced among 
a genetically vulnerable group but that doesn't mean we should encourage 
others not genetically vulnerable to use cannabis."

The King's team tracked 803 men and women born in Dunedin, New Zealand, in 
1972 and 1973, who were enrolled at birth in a research project. Each was 
interviewed at 13, 15 and 18 about cannabis use, tested to determine which 
type of COMT genes they had inherited, and followed up at 26 for signs of 
mental illness.

COMT was chosen as it is known to play a part in the production of 
dopamine, a brain-signalling chemical that is abnormal in schizophrenia. It 
comes in two variants, known as valine or methionine, and every person has 
two copies, one from each parent.

Among people with two methionine variants, the rate of psychotic illness 
was 3 per cent, the background rate for the general population, regardless 
of whether they had used cannabis as teenagers.

Among those with two valine variants the rate was 3 per cent for non-users 
but 15 per cent for those who had smoked cannabis in their teens.

Dr Caspi said research had shown that the valine gene variant and cannabis 
affect the brain's dopamine system in similar fashion, suggesting that they 
deliver a "double dose" that can be damaging. The work needs to be 
replicated by others to confirm the findings, Dr Caspi said. It also is 
possible that the gene involved is not COMT but a neighbour.


*Cannabis was reclassified from a Class B to a Class C drug in January 
2004. Possession remains illegal, but is not an arrestable offence. The 
Home Secretary has asked for a review by November

*The Home Office estimates that fifteen million people have tried cannabis, 
two million to five million are regular users and reclassification has 
saved 199,000 hours' police work

*Liberalisation campaigners argue that millions smoke the drug with fewer 
ill-effects than others suffer from alcohol or tobacco

*A recent study at Maastricht University found that cannabis doubles the 
risk of schizophrenia, hallucinations and paranoia among a genetically 
susceptible group
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MAP posted-by: Jay Bergstrom