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Brain defect explains resistence to touch in autism
A defect in the brain explains why many people with autism avoid touching and hugs - even from their parents, research has shown.
The problem affects individuals with Fragile X Syndrome, a well-known genetic cause of autism and inherited mental retardation.
Scientists found Fragile X results in delayed development of the sensory cortex, the brain region that responds to touch.
A domino effect triggered by the delay may cause this part of the brain to be wrongly wired, it is believed.
Fragile X is caused by a gene mutation in the female X chromosome that affects the construction of synapses, vital connection points between nerve cells.
Because boys have only one X chromosome, they are more severely affected by the syndrome than girls.
Girls have two X chromosomes, so it is less important if one is defective. To some extent it can be compensated for by the other.
Boys are generally more likely to develop autism than girls.
The US scientists, writing in the journal Neuron, studied mice with their own version of Fragile X. They discovered that in the affected animals the development of synapses was delayed in the sensory cortex.
"There is a critical period during late development when the brain is very plastic and is changing rapidly," said study leader Dr Anis Contractor, from Northwestern University Feinberg School of Medicine in Chicago.
"All the elements of this rapid development have to be co-ordinated so that the brain becomes wired correctly and therefore functions properly."
People with the syndrome suffer from "tactile defensiveness" and become anxious and socially withdrawn, said Dr Contractor.
She added: "They don't look in people's eyes, they won't hug their parents, and they are hypersensitive to touch and sound. All of this causes anxiety for family and friends as well as for the Fragile X patients themselves. Now we have the first understanding of what goes wrong in the brain."
The research raises the possibility of intervening at the right time in a child's development to prevent the problem.
The scientists carried out their study by recording electrical signals flowing through the synapses of the mice.
This allowed them to see how cell connections were developing in the sensory cortex.
The X chromosome mutation affects a gene called FMR1 (Fragile X Mental Retardation 1), blocking production of a protein that directs the formation of other synapse-building proteins.
Lacking the protein was like losing the foreman on a key construction site in the brain, said the scientists.
Fragile X is so named because it causes the X chromosome - one of the protein-wrapped packages of DNA that contain the genes - to appear broken or kinked.
"Our next step is to work out what is going wrong," said Dr
Contractor. "How does elimination of this gene FMR1 disrupt the normal developmental process?"
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