A Swedish registry study involving more than 2 million families confirms that first-degree relatives of patients with schizophrenia or bipolar disease are at increased risk for both disorders.
"These findings highlight the need for identification of genes for schizophrenia, bipolar disorder, as well as for genes common to both disorders," lead investigator Dr Paul Lichtenstein said.
Epidemiological studies and genetics research have suggested a possible overlap in genetic susceptibility between the two disorders, but the data have been inconsistent, Lichtenstein at Karolinska Institutet in Stockholm and his associates note in The Lancet.
By linking the data on the Swedish multi-generation register and the national hospital discharge register for the period between 1973 and 2004, the team identified close to 36 000 individuals with schizophrenia and more than 40 000 with bipolar disorder.
What the study found
Compared with the general population, full siblings of the patients with schizophrenia were 9.0 times more likely to have schizophrenia and 3.7 times more likely to have bipolar disorder. For siblings of the patients with bipolar disorder, the relative risks were 7.9 for bipolar disorders 3.9 for schizophrenia. Half-siblings were also at elevated risk, though of lower magnitude, the results show.
The evidence was reinforced by the finding that adopted children whose biological parents or siblings had one of the disorders remained at higher risk, whereas adoptees not genetically related to patients with a family history of these diseases were no more likely than the general population to be diagnosed.
"Thus," the investigators conclude, "schizophrenia and bipolar disorder share common genetic causes." They speculate that "some genes are probably associated with the risk for both disorders and some with risk for only one disorder."
While the findings do challenge the idea that schizophrenia and bipolar disorder have separate origins, Lichtenstein said that in many cases, the two disorders are still distinct entities.
Findings seen as 'timely'
"There are clearly some typical schizophrenia cases and clearly some bipolar ones," he said. "But then there is everything in between. I think about it more as a continuum of psychotic symptoms."
He predicts that "maybe we will in the future have specific medications for different types of symptoms (rather than diseases)."
In a related commentary, Dr's Michael J. Owen and Nick Craddock at Cardiff University, UK, note that these findings are "especially timely...because efforts to re-formulate DSM V and ICD 11 are underway."
"In our view," they say, "the new diagnostic criteria must encourage the careful measurement and reappraisal of psychopathology," to develop a detailed clinical diagnosis that allows "the efficacy of current and future treatments to be monitored in individuals and better serve research into cause, classification, and treatment." – (Reuters Health, January 2009)
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