An analysis of 9 million Swedish people from a 30-year period has shown that bipolar disorder and schizophrenia share common genetic causes. The findings are presented in an Article in this week's edition of The Lancet, written by Dr Paul Lichtenstein and Dr Christina Hultman, Karolinska Institutet, Stockholm, Sweden, and colleagues.
Whether these two conditions are the clinical outcomes of distinct or shared processes is much debated in psychiatry. The authors aimed to solve this dilemma using data from Sweden's multi-generation register, featuring some 9 million individuals from 2 million families in the period 1973-2004. Risks for schizophrenia, bipolar disorder, and their occurrence with each other were assessed for biological and adoptive parents, offspring, full siblings and half-siblings of people with one of the diseases.
First-degree relatives of people with either schizophrenia (35,985 people) or bipolar disorder (40,487) were at increased risk of these conditions. Full siblings were nine times more likely than the general population to have schizophrenia and eight times more likely to have bipolar disorder. Maternal half-siblings were 3.6 times more likely to have schizophrenia and 4.5 more times more likely to have bipolar disorder than the general population; the risk was lower for paternal half-siblings, who were 2.7 times more likely to have schizophrenia and 2.4 times more likely to have bipolar disorder. When relatives of people with bipolar disorder were analysed, increased risks for schizophrenia also existed for all relationships — including adopted children to their biological parents with bipolar disorder. Heritability — ie, the 'proportion' of a trait that is inherited genetically rather than learnt from/affected by the environment — was found to be 64% for schizophrenia and 59% for bipolar disorder. The causes for the comorbidity between the two conditions were to a large extent (63%) due to genetic factors.
The authors conclude: "Similar to molecular genetic studies, we showed evidence that schizophrenia and bipolar disorder partly share a common genetic cause. These results challenge the current nosological* dichotomy between schizophrenia and bipolar disorder, and are consistent with a reappraisal of these disorders as distinct diagnostic entities... Within clinical practice, the underlying structure of psychosis and the knowledge of the common causes of these disorders might be beneficial for treatment options and development of psychosis medication."
In an accompanying Comment, Professor Michael Owen and Professor Nick Craddick, University of Cardiff, UK, ask if the two conditions are no longer to be regarded as distinct, with what should this scenario be replaced? They discuss efforts to reformulate the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM) as well as WHO's International Classification of Disease, and conclude: "In our view the new diagnostic criteria must encourage the careful measurement and reappraisal of psychopathology...The resulting detailed clinical diagnosis will allow the efficacy of current and future treatments to be monitored in individuals and better serve research into cause, classification, and treatment."
Dr Paul Lichtenstein, Karolinska Institutet, Stockholm, Sweden T) +46 (0) 733 093 324 E) paul.lichtenstein@ki.se
Dr Christina Hultman, Karolinska Institutet, Stockholm, Sweden T) +46 (0) 8 524 838 93 E) Christina.Hultman@ki.se
Katarina Sternudd, Press Office, Karonlinska Institutet, Stockholm, Sweden T) +46 (0)8-524 838 95 / +46 (0)70-2243895 E) katarina.sternudd@ki.se
Professor Michael Owen, University of Cardiff, UK T) +44 (0)29 2074 2020 E) owenmj@cardiff.ac.uk
Notes to editors: *Nosology: Classification of diseases
For full Article and Comment see: http://press.thelancet.com/bipolar.pdf
Journal
The Lancet