Scientists eye an enzyme as target in fighting autism

Scientists eye an enzyme as target in fighting autism
Wed Jun 27, 9:11 AM ET
Associated Press

US researchers have reversed the symptoms of mental retardation and autism in mice by inhibiting an enzyme that affects the connections between brain cells, researchers said Wednesday.

In a series of experiments on mice, the MIT investigators showed that they could undo the brain damage seen in a condition called Fragile X syndrome by inhibiting a key brain chemical called PAK.

In humans, Fragile X syndrome (FXS) is the leading cause of mental retardation and the most common genetic cause of autism -- the complex and devastating developmental disorder that is now being diagnosed in increasing numbers of children.

The study raises the intriguing possibility that the brain damage seen in children with the condition can be rolled back and identifies a specific target for potential drug therapies.

"It opens up a new avenue for drug research to treat this condition," said Susumu Tonegawa, a neuroscientist at the Massachusetts Institute of Technology in Cambridge, Massachusetts, and lead author of the paper.

MIT researchers began by creating a batch of mice that had been genetically modified to have Fragile X, a condition in which the neurons of the brain are structurally abnormal and functionally impaired compared to regular nerve cells.

These transgenic mice had many of the behavioral problems seen in kids with the condition: hyperactivity, attention deficits, repetitive behaviors and poor social skills.

The investigators then cross-bred these mice with another batch of mice that had been genetically modified to inhibit the activity of the PAK (p21-activated kinase) enzyme which is instrumental in shaping the formation of neuronal connections in the brain.

The researchers knew that when PAK was inactivated, the mice developed neurons that had short, fat dendritic spines, with a higher-than-usual capacity for relaying the electrical impulses that pass between brain cells.

In other words, the shape and function of the dendritic spines in the PAK mice was just the reverse of those seen in the brain cells of the mice with Fragile X syndrome.

The researchers gambled that the two abnormalities would cancel each other out, and that's exactly what the experiment showed.

The cross-bred mice had been genetically engineered so that the inactivation of the PAK enzyme began two weeks into the mouse's life cycle, which in human terms would be several years after birth.

Tests and autopsies showed that the PAK-blocking action restored electrical communication between neurons in the brains of the double mutant mice, correcting their behavioral abnormalities in the process.

"This is very exciting because it suggests that PAK inhibitors could be used for therapeutic purposes to reverse already established mental impairments in fragile X children," said Eric Klann, a professor at New York University's Center for Neural Science.

The study was conducted by Tonegawa and a postdoctoral student at MIT's Picower Institute for Learning and Memory and appears in this week's edition of the Proceedings of the National Academy of Sciences.