Down's syndrome is caused by an extra chromosome
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Researchers believe they have identified a molecule that could be targeted to treat mental impairment in people with Down's syndrome.
A team at the Institute of Psychiatry at King's College London found people with Down's syndrome have higher levels of myo-inositol in their brains.
They also found increased levels of this molecule are associated with reduced intellectual ability.
The study is published in Archives of General Psychiatry.
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We are now carrying out more studies to see if we can reduce the concentration of myo-inositol in the brains of people with Down's
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The researchers also suspect that high levels of myo-inositol could play a role in predisposing people with Down syndrome to early-onset Alzheimer's disease.
The molecule is known to promote the formation of amyloid plaques - a hallmark of Alzheimer's.
Once they reach the age of 40, almost all people with Down's syndrome show the brain characteristics of Alzheimer's disease - though they do not all go on to develop dementia.
The combination of pre-existing mental retardation with an increasing overlying dementia is difficult to treat, and expensive to manage.
Reducing concentration
Lead researcher Professor Declan Murphy said: "We have shown in this study that adults with Down's syndrome have a significantly higher concentration of myo-inositol in the hippocampal region of their brains, and this increase is associated with a reduced cognitive ability.
"We are now carrying out more studies to see if we can reduce the concentration of myo-inositol in the brains of people with Down's.
"We hope that if we can do this, it will be a new way of treating this devastating disorder."
Down's syndrome is the most common genetic cause of mental impairment.
It is caused when a child has three copies of chromosome 21, rather than the usual two.
The latest research has shown that one of the genes on chromosome 21 controls production of a protein that pumps the molecule myo-inositol into the brain.
The increased levels of myo-inositol in the brains of people with Down's syndrome could be explained by the fact that these people have an extra copy of the gene that makes this pump.
Not a cure
The Down's Syndrome Association said it welcomed any research that may have a beneficial effect on the lives of people with the condition.
In a statement, the charity said: "We are very pleased that scientists are producing results that help us to understand the reasons behind Down's syndrome's associated learning disability.
"However, the Institute of Psychiatry's research does not herald a 'cure' for the condition, and any treatment available is still a long way in the future."
The researchers used sophisticated scan technology to measure the concentrations of myo-inositol in the brains of 38 adults with Down's syndrome and 42 healthy controls.