Funded by Autism Speaks – an advocacy group based out of the University of Toronto, and the Hospital for Sick Children, a new study suggests that most siblings diagnosed of autism do not share the same genetic factors that predispose them to developing the disorder. Parents of autistic children may not really find this revelation surprising, but medical researchers and doctors do.

Using whole-gene sequencing procedure, researchers analyzed genetic materials from 85 families with two autism children each. Whole-gene sequencing maps an individual’s entire genome, and not strands of samples like some other techniques.

Focusing only on 100 genetic glitches that lead to autism in children, the researchers found that 30% of the 85 sibling pairs shared the same mutation while 70% of them did not. They further found that sibling pairs that had the same genetic glitches were identical, and similar in social skills and habits; while those with different generic risks were different in everything.

“We anticipated that, more often than not, there would be shared inheritance” in siblings, said the project’s research director, Dr. Stephen Scherer, a professor of medicine at the University of Toronto, and also the director of the Center for Applied Genomics at the Hospital for Sick Children. “That wasn’t the case.”

Findings in this research will change the way clinical practice in autism is done. Some hospitals analyze the genetic profile of the eldest affected sibling to try to understand an infant, or to advise parents of the odds of having another child with the same disorder. That approach is not informative in most cases, the study authors said.

“This is very important work for people like me,” said Valerie South, an emergency nurse in Oakville, Ontario, outside Toronto. Her sons Cameron, 20, and Thomas, 14, have severe autism. In a family of four or more, the odds of having two children with autism is about one in 10,000.

Her husband had in 1998 contacted doctors to learn about the risks of having another child with autism – even though they had Cameron and an older son, Mitchell, who has no developmental problems.

“We wanted Mitchell, who is a great brother, not to have to carry the burden all by himself,” Ms. South said. “They told us that the odds of having another Cameron were tiny. And even if it happened, they said, it wouldn’t be a severe case.”

They did not get another Mitchell — or another Cameron. They got Thomas, who shares Cameron’s diagnosis, but who is very different. Thomas will walk right up to strangers, Ms. South said, while Cameron draws back. Thomas loves his iPad, whereas Cameron has no interest in computers. Thomas is an “escape artist,” continually on the move, while Cameron prefers to park himself in one place.

According to Helen Tager-Flusberg, a developmental neuroscientist at Boston University, who was not involved in the research, “This study makes us step back and realize we’re not necessarily going to get as much predictive value out of genetic mapping as we thought.”

“The study is very well designed, the end result is somewhat surprising, and it reiterates the complexity of the underlying genetics of autism,” said Dr. Yong-hui Jiang, an associate professor in the department of pediatrics and neurobiology at the Duke University School of Medicine – who among others agree that the finding was important and convincing enough.

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