INTERNATIONAL. Research on large Middle Eastern families has helped scientists pinpoint six new genes implicated in autism, a new study published on Thursday said.

The research "strongly supports the emerging idea that autism stems from disruptions in the brain's ability to form new connections in response to experience, consistent with autism's onset during the first year of life when many of these connections are normally made," a team led by researchers at Children's Hospital Boston and members of the Autism Consortium said.

In traditional Arab societies, cousins commonly wed, making it more likely that rare mutations will be expressed, the team said in study published in Science. And Middle Eastern families typically have significantly more children than those in the west, which makes them relatively more useful in genetic studies.

The research has already identified large sections of the genetic code that are de-activated in autistic individuals. At least six genes linked to learning may be involved.

Autism runs in families, and is known to be affected by certain inherited genes. However, scientists have so far only managed to uncover an estimated 15% to 20% of its genetic causes.

When a mother and father share a recent ancestor, the chances of offspring being affected by autism roughly doubles. This gave scientists the inspiration to investigate families from Middle Eastern countries where it is normal for cousins to marry.

By comparing DNA from family members with and without autism, the scientists were able to close in on recessive mutations shared by people with the condition.

The Homozygosity Mapping Collaborative for Autism recruited 104 families from across the Arabic Middle East, Turkey and Pakistan, in which there was a high incidence of autism; 88 of the families included cousin marriages.

A team led by Howard Hughes Medical Institute investigator Christopher Walsh visited Turkey, Dubai, Kuwait and Saudi Arabia to confirm the diagnoses, researchers said.

In their mapping, Walsh of Children's Hospital Boston, along with geneticist Eric Morrow of Massachusetts General, and Seung-Yun Yoo looked to compare the DNA of relatives with and without autism. They also looked for recessive mutations, where the disease is seen only when a child has two copies of a particular gene.

"We check each set of chromosomes from beginning to end looking for one place where the child has two identical pieces of DNA on both chromosomes," said Walsh.

"Eventually we find a spot where all affected children have two identical chunks of DNA, and where unaffected children have something different."

Slightly more than 6% of the 88 families had rare inherited deletions in DNA regions linked to autism, the research found. The 88 families were from eight countries: Jordan, Saudi Arabia, Kuwait, Oman, Pakistan, Qatar, Turkey and the United Arab Emirates.

"Interestingly, not all the affected genes were actually deleted, but only prevented from turning on, offering hope that therapies could be developed to reactivate the genes," the researchers added in their statement.

Many genes can contribute to autism, researchers say, which makes it impossible to search for just one critical gene that causes the disease, as with cystic fibrosis or Huntington's disease.

In total five deletions were identified affecting at least six recognised genes. The genes have different functions, but all play a role in "reinforcing" connections between neurons, a process widely thought to be the basis of learning and memory.

This network can be disrupted in a myriad of ways, and may be one mechanisms that people with a variety of autism-linked mutations share," said Dr Michael Greenberg, also from the Children's Hospital Boston.

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