Researchers have identified new genetic variants that increase the risk of schizophrenia, opening up the way to new methods for classifying and diagnosing people with the illness, media reports said Thursday.

This finding, published in two independent studies in the journal Nature, raises the possibility that the genetic component of the disease is due to a very large number of variants, each of which is very rare, rather than to a handful of common variants.

The variants are discovered by two groups, one led by Dr. Kari Stefansson of Decode Genetics in Iceland and the other by Dr. Pamela Sklar of the Massachusetts General Hospital.

In the studies, the researchers analysed the genes of 6,000 to 10,000 people from around the world, half of whom had schizophrenia.

They found one mutation on chromosome 1, two on chromosome 15 and confirmed a variant associated with the condition on chromosome 22. These changes can increase the risk of developing schizophrenia by up to 15 times.

In each case certain genes are knocked out, or deleted, on the chromosome. They are also found in some people with autism and other psychological disorders, suggesting the conditions may be related, the researchers said.

"What is beginning to emerge is that a lot of the risk of brain diseases is conferred by rare deletions," Dr. Stefansson said. The three variants discovered by his and Dr. Sklar's group all involve the deletion of large sections of DNA from specific sites in patients’ genomes.

Their report follows a finding in March from researchers at the University of Washington in Seattle that rare deletions and duplications of DNA figure prominently in schizophrenia.

The human DNA can be thought of as a very long string of letters — about 3 billion of them — that sometimes form words (genes). Each newly identified deletion removes a section of about half a million to 2 million letters.

"This is tremendous" for basic research into the disease, said Dr. Linda Brzustowicz of Rutgers University. But since the deletions found so far are related to such a small fraction of schizophrenia cases, she said it's too early for companies to offer to test people for them.

"This work opens up an entirely new way to think about schizophrenia and eventually will suggest avenues for researching therapies for the sake of patients and families suffering from this terrible disorder," said Pamela Sklar of the Broad Institute.

Schizophrenia, characterised by hallucinations, delusions and disordered thinking, is far more common in men than in women and is usually diagnosed in late adolescence or early adulthood.

(Xinhua News Agency July 31, 2008)