No two of us are alike -- even identical twins: Pinpointing genetic determinants of schizophrenia

The study is published in this month's PLoS ONE.

Singh looked at about one million markers of identical twins (and their two parents) where only one twin had schizophrenia. "The most informative feature of schizophrenia is that it sometimes runs in the family. So, for example, the risk of developing schizophrenia is much higher if your brother, sister, mother or father have the disease," says Singh, noting in the general population about one percent have schizophrenia. "We started with the belief that monozygotic twins are genetically identical, so if one member of identical twins has schizophrenia, then the risk for the other twin should be 100 percent, if it's all due to genes. However, studies over the years have shown that the risk of the disease in both twins is only 50 percent." That means either the twins are genetically not identical or the familial disease involves non-genetic (random) effects.

Singh and his team have now demonstrated that the monozygotic twins are not genetically identical. "So if schizophrenia is in the genes, then the difference in the genetic makeup of monozygotic twins, with only one disease twin, must have something to do with the disease." Singh found about 12 per cent of DNA can vary across individuals, "Cells are dividing as we develop and differentiate. More importantly, these cells may lose or acquire additional DNA. The genome is not static."

Dr. O'Reilly hopes this research will lead to better understanding and improved treatments for schizophrenia. "If we had a genetic test for schizophrenia, it could be applied early in the disease when it's hard to make that diagnosis," says Dr. O'Reilly.

The research was funded through the Canadian Institutes of Health Research, the Ontario Mental Health Foundation and the Schizophrenia Society of Ontario.