Schizophrenia and bipolar disorder share a genetic cause

Schizophrenia and bipolar disorder share a genetic cause

A new study from Sweden found evidence that schizophrenia and bipolar disorder partly share a common genetic cause; if one disorder runs in the family there is a good chance that the other will too. The researchers said their finding challenges the view that these disorders are separate entities, and call for a change in the way they are currently diagnosed.

The study was the work of lead author Paul Lichtenstein, a genetic epidemiologist at the Karolinska Institutet in Stockholm, Sweden, and other colleagues from Sweden and the US, and is published in the 17 January issue of The Lancet.

For the study, Lichtenstein and colleagues examined records of all patients discharged from psychiatric hospitals in Sweden from 1973 to 2004 and found 35,985 cases of schizophrenia (0.40 percent of the population) and 40,487 cases of bipolar disorder (0.45 percent of the population). They then looked in the Swedish multi-generation register, which contains information about all children and their parents in the country, and identified over 9 million individuals living in more than 2 million nuclear families between 1973 and 2004.

By comparing the patient discharge data for schizophrenia and bipolar disorder with the register, the researchers were able to identify parents, children, brothers and sisters who shared the disorders. Using a sophisticated statistical tool they then assessed risks for the two disorders, separately and together, for biological and adoptive parents, their children, full and half siblings, of patients with either of the two disorders. The tool they used was a multivariate generalised linear mixed model, and they assessed both genetic and environmental contributions to risks.

The results showed that:

  • First degree relatives of patients whose discharge record showed they had either schizophrenia or bipolar disorder were at higher risk of having these disorders.
  • Having a mother or father with schizophrenia raised the risk of a person having the disorder by 9.9 times, compared with someone who did not.
  • Having a mother or father with schizophrenia raised the risk of a person having bipolar disorder by 5.2 times, compared with someone whose mother or father did not have schizophrenia.
  • Having a mother or father with bipolar disorder raised the risk of a person having it 6.4 times, and the risk of having schizophrenia, 2.4 times, compared with a person whose mother or father did not have bipolar disorder.
  • Having a brother or sister with one of the disorders significantly increased their risk of having them too, with half siblings having a lower risk than full siblings.
  • Overall, relatives of patients with bipolar disorder showed increased risk for schizophrenia, including adopted children whose biological parents had the disorder.
  • Heritability for schizophrenia and bipolar disorder was 64 and 59 per cent respectively.
  • For both together, the figure was 63 per cent, mostly due to additive genetic effects common to both disorders, wrote the authors.
  • Shared environmental effects were small but subtantial, they added.
Lichtenstein and colleagues concluded that:

"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," they added.

In a separate press statement reported by Scientific American, Lichtenstein said he and his colleages suggest there are hundreds if not thousands of genes involved in the development of these two disorders, and many of them overlap. However, many of them have not yet been discovered.

Lichtenstein said there were many large scale studies happening around the world searching for the genes behind these disorders, and he is also researching in this area. He said scientists should look not only for overlap between these two disorders but with other psychiatric conditions too, like depression for instance.

The study was funded by the Swedish Council for Working Life and Social Research, and the Swedish Research Council.

"Common genetic determinants of schizophrenia and bipolar disorder in Swedish families: a population-based study."

Paul Lichtenstein, Benjamin H Yip, Camilla Björk, Yudi Pawitan, Tyrone D Cannon, Patrick F Sullivan, Christina M Hultman.

The Lancet Volume 373, Issue 9659, Pages 234 - 239, 17 January 2009.


Click here for Abstract.

Sources: Journal Abstract, Scientific American.

Five mental disorders share some of the same genes (Video Medical And Professional 2020).

Section Issues On Medicine: Psychiatry