Schizophrenia and bipolar disorder can be traced to the same genetic variants, according to a trio of new studies by an international group of researchers.
The findings represent a step toward understanding what causes the mental illnesses.
"The implications are that we can say and feel confident that we are making steady progress in detecting and characterizing the genetic factors that are involved in schizophrenia," said Pamela Sklar, a doctor at the Massachusetts General Hospital department of psychiatry and a co-author of one of the studies.
The International Schizophrenia Consortium, led by researchers from 11 institutes in Europe and the United States, tested hundreds of thousands of DNA sequence variations in more than 8,000 individuals with schizophrenia, and 19,000 without.
Researchers found that across all DNA samples, the same large group of genetic variants was more common in the schizophrenia patients, even though the data had been collected by different investigators and tested in different laboratories.
The discovery that this same group of variants was also common in individuals with bipolar disorder was striking, since the two conditions are considered to be distinct, although related, illnesses.
Schizophrenia is a psychotic disorder that affects about 1 per cent of the world's population. It usually strikes late in adolescence or early adulthood, and is characterized by persistent delusions and hallucinations.
Although treatment for the disease is widely available, it is often limited in its completeness. Patients have a tendency to relapse, Dr. Sklar said.
Bipolar disorder, which used to be known as manic depression, is characterized by episodes of extremely elevated moods combined with depressive episodes.
Unlike disorders such as Huntington's disease, in which a defect at a single genetic location is responsible, there are multiple genes that contribute to a person's susceptibility to schizophrenia, with a variant of any single gene contributing only a little bit to a person's risk.
Dr. Sklar hopes that by identifying the group of variants that is linked to schizophrenia, researchers will be able to identify meaningful aspects of the biology of both illnesses and work toward new treatments. However, she cautions that although the two disorders share genes, it is important to remember they are separate.
"It may mean that some of the aspects of biology are shared, and that's what we need to sort out," said Dr. Sklar.
Allan Young, the director of the Institute of Mental Health at the University of British Columbia in Vancouver, sees the study as just another piece in a larger puzzle.
"The real challenge is looking at the next step, which is looking at the change in brain function, which is the underlying illness," he said.
The danger of looking at schizophrenia from a purely genetic standpoint, said Dr. Young, is that most people only have a very simple view of that model.
"They think that if my mum and dad had it, then I'm definitely going to get it," he said, when in fact it is much more complicated. "Having it in the family increases the risk, but nothing is inevitable really," he said, pointing out that there are numerous genes that contribute to developing schizophrenia, and they are not necessarily inherited directly.