A mutant gene can trigger learning difficulties by preventing brain cells communicating with each other effectively, research shows.
The defect is carried on the X chromosome
A team from Emory University examined the key gene mutation behind fragile X Syndrome - the most common cause of inherited learning difficulties.
They were able to use drugs to reverse the effects of the mutation in brain cells taken from rats.
The study appears in Proceedings of the National Academy of Sciences.
FRAGILE X SYNDROME
Boys usually more severely affected
Main problem is mental impairment
Other symptoms include hyperactivity, attention deficit disorder, emotional and behavioural problems, anxiety and mood swings
There may also be characteristic facial features, such as a long face and large ears
Other physical features include flat feet and hyperextensible joints
The researchers hope their work could aid development of the first ever treatment for the condition.
The Emory team first identified a mutant form of the FMR1 gene as the cause of fragile X Syndrome in 1991.
People who carry this form of the gene - which is found on the X chromosome - fail to make a key protein called FMRP.
The latest work shows that this chemical imbalance weakens the connections - or synapses - that cells make with their neighbours by effectively putting key receptors which facilitate communication out of commission.
This breakdown in cell-to-cell communication can result in the mental deficits typical of fragile X.
Precisely targeted drug therapy was able to restore the number of receptors in the synapses to their correct level - and ensure that cells were able to talk to each other properly.
Lead researcher Professor Stephen Warren said: "We have now explained the fundamental defect in the brain in fragile X syndrome and, most importantly, found that we can correct this problem in the laboratory.
"This is quite exciting, progressing from the identification of the gene in 1991 to now believing we will be able to treat a previously untreatable condition.
"Our next steps will be to continue screening and identifying the best drugs to try and correct the deficiencies that result from fragile X syndrome."
Further work needed
Dr Michael Tranfaglia, medical director of the US FRAXA Research Foundation, said the study was "very exciting".
But Dr Mark Hirst, scientific adviser to UK Fragile X Society, said while the study was interesting, the work was still at an early stage.
"The challenge now is to take these observations on isolated cells and study these processes in the living brain and ask whether malfunctioning brain cells can be manipulated using drugs to restore their normal function."
Lynne Zwink, of the Fragile X Society, said that despite being the most common known cause of inherited intellectual disability, there was little public awareness of the condition.
It causes difficulties in learning from mild to severe intellectual disability as well as difficulties with social, language, emotional and behavioural problems. In severe cases, children display behaviours similar to autism.
Ms Zwink said: "The demands of children place an enormous strain on parents; who are often caring for more than one child affected by fragile X in their family."