How cannabis can trigger schizophrenia

Smoking cannabis affects brain chemistry so seriously it can trigger schizophrenia, scientists warn today.

Researchers have found the first evidence that marijuana can cause genetic abnormalities associated with the mental illness, which affects about one in every 100 people.

Many scientists have been warning for years that cannabis can trigger hallucinations and delusions similar to symptoms found in schizophrenia.

Previous studies have suggested that using the drug before the age of 18 raises the risk of the condition in later life by six-fold.

Its ability to cause psychotic conditions is probably related to long-term changes in the brain caused by a substance found in marijuana called tetrahydrocannabinol (THC).

THC affects chemicals in the brain that transmit information from one nerve cell to another.

Disrupting the delicate chemical balance can result in memory loss, anxiety and other conditions.

Rare genetic changes associated with a heightened risk of schizophrenia have been revealed by two independent studies.

The surveys have identified sections of the human genome that, when deleted, can elevate the risk of developing schizophrenia by up to 15 times compared with the general population.

Schizophrenia is a serious mental health problem and affects around 1 in every 100 people at some point during their lives. Genetic factors are thought to account for more than 70% of cases. But unlike many diseases with a genetic basis - and in common with many other psychiatric disorders - researchers have struggled to locate genes linked to the occurrence of the disease.

The new findings come from two large-scale studies done by multi-national collaborations, both of which are published in Nature this week. The first study comes from the International Schizophrenia Consortium (ISC), a collaboration of researchers from 12 institutions in Europe, the United States and Australia; the second comes from the SGENE consortium, made up of 18 institutions across Europe, the United States and China.

The new Japanese research was led by clinical psychiatrist Dr Hiroshi Ujike of Okayama University and reported in the journal Molecular Psychiatry.

The team examined the gene-controlling 'receptors' in the brain that respond to marijuana in 121 Japanese schizophrenics and a similar number of healthy men and women. 'These sites are where marijuana acts on the brain,' said Dr Ujike.

The scientists found distinct abnormalities in DNA among the schizophrenics.

Although these deletions do not offer a complete explanation of the genetic basis of schizophrenia, the replication of the findings between the two studies is important. “The fact that there are two independent studies that come at the problem from different angles and using different methodologies is fantastic for psychiatric genetics,” says Pamela Sklar of Harvard Medical School in Cambridge, Massachusetts, part of the ISC study.