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Schizophrenia: An Unpleasant Side Effect of Natural Selection?

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bluebrain Recent studies indicate that schizophrenic conditions may stem from a genetically-triggered maladaptation involving the gene DISC1, which, according to research, has been selected for in evolution even though it contributes to schizophrenia. Compare this with sickle-cell anemia: it is caused by having two mutated copies of a certain gene, while those with just one copy of the mutation are naturally protected against malaria.

Discover Online reports:

One of the key tenets of Darwinism is that adaptations that work against the survival of a species are destined to disappear. So why does schizophrenia continue to linger on? Could it be that it confers some advantage?

For years, scientists struggled to identify an adaptive advantage that might explain schizophrenia’s persistence. Researchers from various disciplines volleyed ideas back and forth. Some argued that the genes implicated in the disease promoted creativity; others believed that schizophrenics were frustrated cult leaders—unorthodox thinkers constitutionally “engineered” to lead segments of humanity to break off from the herd, but who lacked the charisma to effect much change. None of the theories gained much traction.

New research is pointing to a different possibility: There may be no adaptive advantage provided by schizophrenia in and of itself, but rather from some genes that contribute to the disease. According to a study published in the Proceedings of the Royal Society, there is evidence that some of the gene variants associated with schizophrenia—especially a mutation in a gene called disrupted-in-schizophrenia 1 (DISC1)—have been selected for by evolution. This supports the idea that the disease may be a maladaptive combination of mutations that individually have the potential to enhance fitness. It could be a more complicated version of the familiar case of sickle cell anemia: having two mutant copies of a certain gene causes the disease, whereas having only one mutant copy provides protection against malaria.

A recent study headed up by Johns Hopkins University neuroscientists may have found what kind of process goes awry in schizophrenic brains. Researchers found that DISC1 regulates the migration of new neurons in the adult brain. When the levels of DISC1 were reduced in mice during adult neurogenesis, the newborn neurons sped up and overshot their intended targets within the hippocampus, says Xin Duan, a study collaborator. When the neurons finally reached their destinations, they forged an unusual number of connections with neighboring cells, a series of events that might give rise to the abnormal—and quite crippling—brain functions associated with schizophrenia, according to Hongjun Song, a Johns Hopkins neurologist who also worked on the study. It is possible, Song says, that further research will lead to a drug that treats schizophrenia by restoring normal neurogenesis.

So what evolutionary advantage could schizophrenia-related genes bring to people who have some of the genes but not the disease? For now, this remains one of the many open questions about this puzzling condition.

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