The findings which are published in the journal Neuron, provide further evidence that these illnesses are inherited, and may in the future help doctors pinpoint which patients will respond to different types of treatments.

Experts from the University of Edinburgh, working with researchers from Mount Sinai Hospital in Toronto, Canada and RIKEN in Japan, studied two types of damage to a gene (DISC1). Previous research at the University, working with families with a high incidence of mental illness, identified this gene as being linked to schizophrenia, manic depression (bipolar affective disorder) and major depression. The gene was also found to be essential for brain signalling and plays a key role in learning, memory and mood.

To further their findings, experts looked at the behaviour of mice with two types of damage in the gene. The results suggest that one responded better to antipsychotics, used to treat schizophrenia while the other responded better to anti-depressants, used to treat mood disorders.

Prof David Porteous, Chair of Human Molecular Genetics and Medicine at the University of Edinburgh, said: ?While the causes of schizophrenia, bipolar affective disorder and major depression are unknown, all the evidence points to subtle differences in the way the brain develops and to chemical changes in the brain. Our previous work identified the DISC1 gene as an important risk factor in these types of mental illness.

"By analysing the behaviour of mice, we were able to provide further evidence of the importance of DISC1. We also found remarkable clear cut differences between the different types of damage to the gene and the treatment that was the most effective. By analysing how the brain changes and develops over time we would hope that this would lead to more effective drugs to treat such illnesses."

About one in 50 people worldwide will develop the symptoms of schizophrenia or bipolar affective disorder, with the first signs often appearing in late adolescence or early adulthood. Most cases arise in families with some sort of history of mental illness implying a strong influence of genes. Several different genes have been reported to pre-dispose to schizophrenia but DISC1 is one of the few which has been replicated by several laboratories.

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