Schizophrenia and bipolar disorder have same genetic causes

Schizophrenia and bipolar disorder have for decades been regarded as two distinct psychotic disorders when it comes to definitions and risk factors.

Schizophrenia is a psychotic illness that causes delusions and hallucinations - bipolar disorder also known as manic depression, causes extreme mood swings from deep depression to manic episodes.

But now a study by Swedish researchers has found the two most common psychotic disorders have the same genetic causes and questions the current separate classification of the diseases.

Though schizophrenia and bipolar disorder have always been treated as quite distinct in many ways some experts have had doubts about such strict classification and this doubt has been bolstered by modern genetic technology which has revealed that certain genes appear to affect both disorders.

To unravel the puzzle the Swedish scientists analysed the records of two million families, including 35,985 patients with schizophrenia, 40,487 patients with bipolar disorder, and the blood relatives of both to better understand whether schizophrenia and bipolar disorder have the same genetic causes.

The study covered a 30 year period from 1973 to 2004 and it was found that members of families where either schizophrenia or bipolar disorder was present had the same genetic factors which increased their risk of developing the same condition - the culprit in this risk is the genetic factors and only slightly due to shared environmental factors.

The scientists found that patients with schizophrenia are also more prone to bipolar disorder and that relatives of patients with one of the diseases are more likely to have relatives with the other - the family risk of inheriting a susceptibility to either of the disorders were assessed and found to be much the same - 64% for schizophrenia and 59% for bipolar disorder.

According to the researchers, their results provide convincing proof that schizophrenia and bipolar disorder are very much hereditary diseases, and that they share a common genetic cause and may be two versions of the same mental illness.

Authors Dr Paul Lichtenstein and Dr Christina Hultman say it is important that this common genetic background is taken into account when studying and treating schizophrenia and bipolar disorder.

They say their results show that schizophrenia and bipolar disorder have a common genetic cause and challenge the current dichotomy between schizophrenia and bipolar disorder and the knowledge of the common causes of these disorders might be beneficial for treatment options and development of psychosis medication.

The study was funded by the Swedish Council for Working Life and Social Research and the Swedish Research Council and is published in The Lancet.