Blocking a key brain chemical can reverse many of the symptoms of Fragile X Syndrome — an inherited form of mental retardation often accompanied by autism — in mice engineered to have the disease, according to a report in Tuesday's Boston Globe.

The findings, part of study published in an online journal, raise the prospect that drugs with similar effects might someday help restore brain function in human children with the syndrome. It may also help children with some forms of autism, said Susumu Tonegawa, the senior author of the paper in the online edition of the Proceedings of the National Academy of Sciences.

The research focused on blocking an enzyme called PAK. Tonegawa's research used genetic manipulation instead of drugs, but he said that he believes drug and biotech companies are already developing compounds that block the same enzyme. His lab may seek access to such compounds that target other diseases, or ask a chemist to synthesize them, said Tonegawa, a neuroscientist at Picower Institute for Learning and Memory at the Massachusetts Institute of Technology .

There are currently several drugs in development as possible treatments for people with Fragile X Syndrome, said Katie Clapp, co founder of FRAXA Research Foundation, a Newburyport based nonprofit that helped fund the research. Her 18-year-old son has the syndrome, the most common known genetic cause of autism. None of the compounds has reached the point where she would want him to try them, she said, nor are they available.

Mental retardation has long been thought to be permanent. But recent research increasingly suggests that the brain might be more fixable than previously believed. Earlier this year, scientists from Scotland reported that dramatic recoveries could be achieved in mice with Rett Syndrome, another genetic disease related to autism.

Tonegawa's paper says "that some of the abnormalities with mental retardation syndromes and autism aren't necessarily cemented in stone," said Eric Klann, a professor at Center for Neural Sciences at New York University, who was not involved with the research. "I think it gives some degree of hope."

About 100,000 Americans have Fragile X.

In people with Fragile X, the formation of neurons is abnormal, with "spines" — the little nubs where the neurons connect to each other — that are overabundant, spindly, and long. Clapp said they are "sort of like a thin, dangly . . . little wire when what you really want is a nice, thick, three-pronged, grounded thing." The thin spines tend to form weak connections.

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