For the first time, scientists in the US have reversed symptoms of mental retardation and autism in laboratory mice bred specially to display the symptoms of Fragile X Syndrome - the leading inherited cause of mental retardation and the most common genetic cause of autism.

In this syndrome, the dendritic spines - small protrusions that are responsible for communication between brain cells - appear in greater numbers but are longer and thinner, so transmit weaker electrical signals.

Researchers created a strain of mice that showed typical Fragile X Syndrome symptoms - hyperactivity, purposeless and repetitive movements - then reversed these symptoms by blocking the action of an enzyme (p21-activated kinase, or 'PAK') which affects the number, size and shape of connections between brain cells.

They found that not only were structural abnormalities in connections between brain cells corrected by inhibiting this enzyme, but proper electrical communication between the cells was also restored.

The fact that the mice were not treated until a few weeks after symptoms of disease first appeared suggests that PAK inhibitors could have a therapeutic role in reversing already established mental impairments in Fragile X children.

There are existing chemical compounds that block the activity of PAK and these could form the basis of drugs to treat Fragile X Syndrome, and possibly autism as well.

First published in September 2007

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