Twin studies have shown that people with schizophrenia and bipolar disorder have changes in gene activity caused by their environment, providing further evidence that such gene changes could cause the conditions.

The study was led by Jonathan Mill from the Institute of Psychiatry, King's College London, and the team scanned the genome of 22 pairs of identical twins.

In each pair chosen for the study, one twin was diagnosed with schizophrenia or bipolar disorder.

While as expected, the twins had identical DNA, their 'epigenetic' markings were different.

The changes observed do not alter the sequence of DNA but influence how active genes are by leaving chemical marks on them.

Also, the changes were on genes that have been linked with bipolar disorder and schizophrenia.

Mill's team scanned for differences in the attachment of chemical methyl groups at 27,000 sites in the genome.

While Methylation usually switches genes off, de-methylation turns them on.

Regardless of the twin's condition the most important differences, spotted in the amount of methylation, which varied by up to 20 per cent, were found in the promoter "switch" for a gene called ST6GALNAC1, which has also been linked with schizophrenia.

The function of the gene is however still not fully established but scientists think it can add sugars to proteins, which could in turn lead to an alteration of the speed or specificity of their usual function.

The research findings also revealed methylation differences in GPR24, a gene previously linked to bipolar disorder.

"We know these disorders are related, and there are clinical features shared by both," said Mill. "But our scan suggests there are some genes that might be overactive in one disease and underactive in the other."

London