Down's syndrome's patients have an extra chromosome

US and Swiss-based researchers have found a gene they think is responsible for mental retardation in Down's.

The team hope that a drug could now be discovered to "reactivate" a part of brain that does not work properly in patients with the genetic disorder.

However, they stress that their work is at an early stage, and any treatment would be some years away.

The human cell nucleus contains 23 pairs of chromosomes, which carry genetic information necessary for developing and running the body correctly.

In Down's syndrome, the baby carries an extra copy of chromosome 21, and its presence interferes with several important body systems.

Too many genes

Unlike many gene defects, which happen because a key gene is missing, the Down's defects arise because they have an extra copy of certain genes on the third chromosome 21.

We definitely think we are on the right track to restoring memory and learning in individuals with Down's syndrome Professor William Mobley, Stanford University

This means that they are producing too much of certain body chemicals, throwing finely tuned mechanisms which govern the development and function of vital organs into disarray.

Children and adults with Down's have a distinctive facial appearance with protruding eyes and tongues.

More importantly, they often have heart defects, are generally more prone to heart disease, cannot regulate temperature well in infancy, and are vulnerable to infections.

On top of this, most children with Down's have varying degrees of mental retardation.

The research team, based both at Stanford University in California and the University of Geneva, claims to have found a gene on chromosome 21 which plays a role in retardation.

In experiments in mice which have a similar genetic fault - and which share many genes with humans - they have found a gene which appears to hinder the transport of an important brain chemical into a certain part of the forebrain.

This has already been linked with retardation.

Reversal of fortune

Chemistry and Industry reports that this gene could now be the target for drugs to "downregulate" the action of the extra gene - although these drugs are some time away.

Remarkably, there is some hope that a drug to reverse this gene defect could actually reverse retardation to some degree even in adults.

This area of the forebrain does not require new brain cells - it already has cells, but they are simply not working properly.

The theory is that a drug to restore their proper activity could have a marked effect.

Professor William Mobley, from Stanford University, said: "We definitely think we are on the right track to restoring memory and learning in individuals with Down's syndrome.

"With the perfect drug at the right dose, we can reduce protein production by the extra copy of the gene.

"We think that this could reverse mental retardation."

No certain improvement

However, this is far from certain, as no-one knows how newly-reactivated neurons would behave, or how they would fit into the wider workings of the brain.

Professor Mobley said that it was possible there would be absolutely no effect whatsoever.

Dr Jonathan Cooper, from the Institute of Psychiatry, told BBC News Online: "If it's a single gene involved in the degeneration of these cells, then it should be possible to target it, although this may be technically complicated to do".

"However, if you can restore these cell populations, it would be important to know that they are fully functional, which presently remains unclear"

"This is a very good research team and this idea shows great potential - although it is at a very early stage.

"There is likely to be some way to go before this can translate into treatments in humans."

A spokesman for the Down's Syndrome Association told BBC News Online: "The findings of William Mobley and his team are interesting and we would welcome anything that improves the lives of people with Down's syndrome.