U.S. scientists have discovered bipolar disorder -- also known as manic-depression -- might actually be a polygenic disorder.

Francis McMahon and colleagues at the National Institute of Mental Health conducted the first genome-wide association study of bipolar disorder using pooled DNA from two independent case-control samples of European ancestry. The case sample consisted of unrelated individuals selected from families with at least one affected sibling pair, while the control sample consisted of individuals who did not meet criteria for major depression and denied a history of bipolar disorder or psychosis.

More than 550,000 single-nucleotide polymorphism or SNPs, were used to measure the genetic variation between the samples. SNPs that occurred more often in the people with bipolar disorder were examined individually.

The researchers found no specific gene appears to be necessary or sufficient for disease. Instead, several genes, each of modest effect, were shown to contribute to the risk of bipolar disorder -- with each person's disease risk being influenced by the total burden of risk alleles they carry. Several of the implicated genes lie within regions previously linked to schizophrenia.

The study is reported online this week in the journal Molecular Psychiatry.

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