The researchers found that injecting the molecule into mice that have an equivalent of Rett syndrome helped the animals' faulty brain cells develop normally and reversed some of the disorder's symptoms.

The work, reported in the Feb. 10 online edition of the Proceedings of the National Academy of Sciences ( PNAS ), is expected to lead to new human clinical trials for a derivative of growth factor-1 (IGF-1), currently used to treat growth disorders and control blood glucose. The MIT study indicates that IGF-1 could potentially lessen the severity of symptoms of Rett syndrome.

"We demonstrate that a major underlying mechanism behind Rett syndrome in mice is that synapses in the brain remain immature and show persistent, abnormal plasticity into adulthood," said Daniela Tropea, a postdoctoral fellow at the Picower Institute and lead author of the study. "We also propose that a therapeutic based on this mechanism would be directly applicable to humans."

Injecting mice with a peptide fragment of IGF-1, used by the brain for neuronal and synaptic development, reverses a large number of symptoms of mice genetically engineered to display Rett syndrome-like symptoms.

"IGF-1 is critical for brain development. It activates molecules within neurons that make synapses mature," said study co-author Mriganka Sur, the Newton Professor of Neuroscience at the Picower Institute and head of the MIT Department of Brain and Cognitive Sciences. "This is a mechanism-based therapeutic for Rett syndrome. It is possible that this or similar therapeutics would apply to other forms of autism, which also have as their basis a persistent immaturity of synapses."

Rett syndrome, an inherited neurological disorder, causes loss of speech, reduced head size, breathing and heart abnormalities and autism-like symptoms in one out of 10,000 girls.

In 85 percent of girls with Rett syndrome, the culprit is a faulty gene coding for methyl CpG-binding protein 2, (MeCP2), critical for nerve cell maturation. A deficit in MeCP2 stops neurons from growing spines, the branch-like projections needed for cell-to-cell communication.

Recent genetic studies have shown that increasing MeCP2 expression in mice led neurons to grow new spines, indicating that the disease could be reversible. Increased IGF-1 seems to make up for the lack of MeCP2.

Daily injections of the insulin-like growth factor IGF-1 extended the life spans of infant Rett syndrome mice, improved their motor function and breathing patterns and reduced irregularities in their heart rates. In addition, their brains had more nerve-cell spines.

IGF-1 affects almost every cell in the human body, especially in muscle, cartilage, bone, liver, kidney, nerves, skin and lungs. In addition to its insulin-like effects, IGF-1 also regulates cell growth and development in nerve cells.

"This is the first realistic way for a drug-like molecule injected into the bloodstream to relieve Rett syndrome symptoms," said Whitehead member Rudolf Jaenisch, whose lab participated in the research.

web.mit/