A multinational group of scientists has concluded that about a third of the genetic underpinnings of schizophrenia are caused by the interactions of many thousand common genetic variants. While each variant may be harmless on its own, the combined impact vastly increases the risk of both schizophrenia and bipolar disorder.

The results imply that the genetic basis of the 2 common psychoses  are more complex than had been thought, and that the conditions can develop not just from a rare, devastating genetic variant, but from dozens or hundreds of common ones.

The write-up is in Nature.
 
“This is an enormous first for our field,” co-author Patrick Sullivan told BurrillReport. “We now have the outline of the puzzle, we just need to… see how (the pieces) fit together,” added the professor of psychiatry at the UNC School of Medicine.
 
Although scientists have long recognized that schizophrenia has a genetic basis, it’s only recently that they’ve started pinpointing offending genetic loci.
 
To reach their conclusions, Sullivan and colleagues isolated 30,000 genetic variants that occurred more frequently in 3,000 people having schizophrenia than in a control group without the condition. The team confirmed the differences in 2 other populations of schizophrenic patients and 2 groups with bipolar disorder.

The genetic link between schizophrenia and bipolar disorder had not previously been recognized.
 
“While our study finds a surprising number of genetic effects, we expect that future work will assemble them into meaningful pathways that will teach us about the biology of schizophrenia and bipolar disorder,” senior author Pamela Sklar told Burrill.
 
The scientists also found that a particular HLA haplotype increased the risk for both psychoses. Since HLA proteins are involved in the immune response to infections, they speculated that an infectious agent might be an etiologic factor.