By Simon Makin
The past few decades have seen intensive efforts to find the genetic roots of neurological disorders, from schizophrenia to autism. But the genes singled out so far have provided only sketchy clues. Even the most important genetic risk factors identified for autism, for example, may only account for a few percent of all cases.
Much frustration stems from the realization that the key mutations elevating disease risk tend to be rare, because they are less likely to be passed on to offspring. More common mutations confer only small risks (although those risks become more significant when calculated across an entire population). There are several other places to look for the missing burden of risk, and one surprising possible source has recently emerged—an idea that overturns a fundamental tenet of biology and has many researchers excited about a completely new avenue of inquiry.
Accepted dogma holds that—although every cell in the body contains its own DNA—the genetic instructions in each cell nucleus are identical. But new research has now proved this assumption wrong. There are actually several sources of spontaneous mutation in somatic (nonsex) cells, resulting in every individual containing a multitude of genomes—a situation researchers term somatic mosaicism. “The idea is something that 10 years ago would have been science fiction,” says biochemist James Eberwine of the University of Pennsylvania. “We were taught that every cell has the same DNA, but that’s not true.” There are reasons to think somatic mosaicism may be particularly important in the brain, not least because neural genes are very active.
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