Engineered flies spill secret of seizures

ScienceDaily (Oct. 11, 2012) Scientists have observed the neurological mechanism behind temperature-dependent -- febrile -- seizures by genetically engineering fruit flies to harbor a mutation analogous to one that causes epileptic seizures in people. In addition to contributing the insight on epilepsy, their new study also highlights the first use of genetic engineering to swap a human genetic disease mutation into a directly analogous gene in a fly.

In a newly reported set of experiments that show the value of a particularly precise but difficult genetic engineering technique, researchers at Brown University and the University of California-Irvine have created a Drosophila fruit fly model of epilepsy to discern the mechanism by which temperature-dependent seizures happen.

The researchers used a technique called homologous recombination -- a more precise and sophisticated technique than transgenic gene engineering -- to give flies a disease-causing mutation that is a direct analogue of the mutation that leads to febrile epileptic seizures in humans. They observed the temperature-dependent seizures in whole flies and also observed the process in their brains. What they discovered is that the mutation leads to a breakdown in the ability of certain cells that normally inhibit brain overactivity to properly regulate their electrochemical behavior.

In addition to providing insight into the neurology of febrile seizures, said Robert Reenan, professor of biology at Brown and a co-corresponding author of the paper in the Journal of Neuroscience, the study establishes

"This is the first time anyone has introduced a human disease-causing mutation overtly into the same gene that flies possess," Reenan said.

Engineering seizures

Homologous recombination (HR) starts with the transgenic technique of harnessing a transposable element (jumping gene) to insert a specially mutated gene just anywhere into the fly's DNA, but then goes beyond that to ultimately place the mutated gene into exactly the same position as the natural gene on the X chromosome. HR does this by outfitting the gene to be handled by the cell's own DNA repair mechanisms, essentially tricking the cell into putting the mutant copy into exactly the right place. Reenan's success with the technique allowed him to win a special grant from the National Institutes of Health last year.

The new paper is a result of that grant and Reenan's collaboration with neurobiologist Diane O'Dowd at UC-Irvine. Reenan and undergraduate Jeff Gilligan used HR to insert a mutated version of the para gene in fruit flies that is a direct parallel of the mutation in the human gene SCN1A that causes febrile seizures in people.

When the researchers placed flies in tubes and bathed the tubes in 104-degree F water, the mutant fruit flies had seizures after 20 seconds in which their legs would begin twitching followed by wing flapping, abdominal curling, and an inability to remain standing. After that, they remained motionless for as long as half an hour before recovering. Unaltered flies, meanwhile, exhibited no temperature-dependent seizures.

The researchers also found that seizure susceptibility was dose-dependent. Female flies with mutant strains of both copies of the para gene (females have two copies of the X chromosome) were the most susceptible to seizures. Those in whom only one copy of the gene was a mutant were less likely than those with two to seize, but more likely than the controls.

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Engineered flies spill secret of seizures