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Tiny genetic variations may hold the key to schizophrenia, scientists claim.
A third of all inherited cases of the condition may be caused by a combination of mutations which would have little effect but which cumulatively lead to the mental illness.
The study by scientists from universities including Harvard and published in Nature journal is the biggest yet into the genetics of the illness.
It involved three teams who analysed genetic data from 8,014 schizophrenia sufferers, comparing them with samples from 19,090 people who did not have the condition. The findings suggest schizophrenia is more complex than thought, and can arise from common genetic variants as well as rare ones.
The researchers said many of the genetic variations they identified played individually a small role in raising the risk of passing schizophrenia down the generations.
Dr Shaun Purcell, from Harvard University, who led one of the teams, said: "Cumulatively, they play a major role, accounting for at least one third - and probably more - of disease risk."
Dr Pamela Sklar, of Massachusetts General Hospital, said: "We fully expect that future work will assemble them into meaningful pathways that will teach us about the biology of schizophrenia."
The teams highlighted genes found on Chromosome Six in area known as the Major Histocompatibility Complex, which plays a role in the immune system, and in controlling when other genes are switched on and off.
Scientists also uncovered similarities to bipolar disorder, challenging medical orthodoxy which had seen the two as different. They found 30,000 genetic variations between those with schizophrenia and those who were healthy.
Dr Thomas Insel, of the US National Institute of Mental Health, said: "These new results recommend a fresh look at our diagnostic categories.
"If some of the same genetic risks underlie schizophrenia and bipolar disorder, perhaps these disorders originate from some common vulnerability in brain development."
The researchers believe the study may help explain why environmental factors also seem to affect disease risk - such as evidence that children whose mothers contract flu while pregnant have a higher risk.
Schizophrenia affects up to one per cent of adults. Symptoms can include delusions, hallucinations, paranoia, and depression. Its is thought up to 90 per cent of cases may be inherited.
