* Researchers analysed 50 sets of twins, some with autism
* Identical genes allow scientists to see other influences
* Autism affects 1 in 100 in Europe, up to 1 in 50 in U.S.
By Kate Kelland
LONDON, April 23 (Reuters) - Scientists have found patterns of change in gene activity involved in autism in a study that shed light on how environmental factors can work to turn certain genes on or off and contribute to the development of the brain disorder.
In the largest study of its kind, researchers analysed data from 50 sets of twins to try to find out what might have caused some of them to develop autism while their genetically identical siblings did not.
Chloe Wong, of the Institute of Psychiatry (IoP) at King's College London, who worked on the study, explained that epigenetic changes affected levels or activity of genes without changing the underlying DNA sequence. Scientists think they are one way in which the environment interacts with the genome.
Importantly, she said in a telephone interview, epigenetic changes are also potentially reversible, so finding out more about them may point researchers towards the development of new medicines or treatments.
People with autism have varying levels of impairment across three common areas - social interaction and understanding, repetitive behaviour and interests, and language and communication.
As many as one in 50 school age children in the United States are diagnosed with autism, although some of these will be milder cases that have been diagnosed partly because of better recognition of autism symptoms by carers and doctors. In Europe experts say the rate is around one in 100 children.
Previous studies have shown there is a strong genetic component to autism. In identical twins, in around 70 percent of cases where one twin has autism, so does the other. But in 30 percent of cases, one twin has autism while the other does not.
Because identical twins share the same genetic code, this suggests non-genetic, or epigenetic, factors may be important.
Wong's team studied a particular epigenetic mechanism called DNA methylation, which acts to block the genetic sequences and can turn gene activity on or off.
They looked at DNA methylation at over 27,000 sites across the genome using samples taken from 50 identical twin pairs. Of the 50 pairs, 34 had one twin with autism and one without, five had both twins with autism, and 11 pairs were healthy controls with no autism traits or diagnoses.
The researchers found that DNA methylation at some genetic sites was consistently altered for all the children with autism, while differences at other sites were specific to certain groups of symptoms or traits of autism.
The number of DNA methylation sites across the genome was also linked to the severity of autism symptoms suggesting a quantitative relationship, said Wong, whose study was published in the journal Molecular Psychiatry on Tuesday.
"We identified distinctive patterns of DNA methylation associated with both autism diagnosis and related behaviour traits, and increasing severity of symptoms," she said.
Jonathan Mill, of the IoP and the University of Exeter, who led the study, said research into the intersection between genetic and environmental influences was crucial "because risky environmental conditions can sometimes be avoided or changed".
He said the next step was to conduct larger studies to see whether researchers can identify key epigenetic changes that are common to the majority of people with autism and use them to help develop ways of preventing or treating the disorder.
It was too early in the process to identify which environmental factors may have had an impact. (Editing by Alison Williams)