Because schizophrenia tends to run in families, researchers have long sought a "schizophrenia gene." But scientists now realize that most inherited disease factors aren't single genes, but combinations of different genes.
These combinations can be quite complex. Now modern technology -- and DNA samples from huge numbers of people -- allow researchers to scan the entire human genome for differences between people with and without specific diseases.
Three such genome-wide studies -- involving large, international teams of scientists -- appear in the July 2 issue of Nature.
The studies point to single genetic variants involved in immune responses and in brain development. None of these genes increases schizophrenia risk by itself, but the changes come together to exert a powerful effect.
"Cumulatively they play a major role, accounting for at least one-third -- and probably much more -- of disease risk," Harvard's Shaun Purcell, PhD, co-leader of one of the research teams, says in a news release.
Interestingly, people with bipolar disorder have many of the same genetic changes seen in people with schizophrenia. This suggests these two distinct mental disorders may be linked.
There's still a lot more work to do. It's not at all clear why this constellation of changes predisposes a person to mental illness. And while knowledge of these changes offers insights into the biology of schizophrenia and bipolar disorder, the findings cannot be used as a genetic test to diagnose these diseases or predict risk.