ScienceDaily (Apr. 17, 2012) When a child has autism, siblings are also at risk for the disorder. New research from Washington University School of Medicine in St. Louis shows that the genetic reach of the disorder often extends to half siblings as well.

On the surface, the finding may not be surprising -- half siblings share about 25 percent of their genes. But the discovery is giving scientists new clues to how autism is inherited.

The study is published online in the journal Molecular Psychiatry.

According to principal investigator John N. Constantino, MD, the new research adds to recent evidence that even though autism is far more common in males, females still can inherit and pass along genetic risk for autism.

"We found that autism risk for half siblings is about half of what it is for full siblings," he says. "Most of the half siblings we studied had the same mothers. Given that half of the risk of transmission was lost and half was preserved among those maternal half siblings, mothers and fathers appear to be transmitting risk equally in families in which autism recurs."

Constantino, the Blanche F. Ittleson Professor of Psychiatry and Pediatrics and director of the William Greenleaf Eliot Division of Child and Adolescent Psychiatry at Washington University and psychiatrist-in-chief at St. Louis Children's Hospital, says the findings also suggest that in many families, the transmission of autism is the result of the effects of many genes -- not just one -- with each contributing a small proportion of risk.

Prior estimates of the extent to which autism is influenced by genetic factors are derived from studies of identical and fraternal twins where one, or both, are affected by the disorder. Since identical twins share 100 percent of their genes, and fraternal twins share 50 percent, inherited conditions tend to be twice as common in an identical twin pair compared to a fraternal twin pair. But twin studies of autism are too small to give precise estimates about how the disorder is inherited.

"The largest studies have included less than 300 clinically affected twin pairs," Constantino says. "And they include girls, boys and mixed twin pairs, which complicates the testing of models of inheritance in autism because the disorder is much more common in boys than girls."

Other studies have focused on siblings of children with autism, looking at how much more common autism recurrence is in siblings than the general population. But to derive more information on genetic structure from their family studies, Constantino's group looked at autism recurrence in half siblings and compared it to that in full siblings.

The researchers studied over 5,000 families in which there was a child with autism and at least one additional sibling -- the families were enrolled in a national volunteer, Internet-based family registry for autism, the Interactive Autism Network (IAN). Among those families, 619 included at least one maternal half-sibling. The researchers focused on maternal half-siblings rather than paternal half siblings because these children were more likely to live full-time with their biological mothers and to share the same environmental influences between the time they were born and the age of two, the time at which the onset of autistic syndromes occur. They compared autism recurrence among the 619 maternal half siblings to the rate among 4,832 full siblings.

Excerpt from:
Study of half siblings provides genetic clues to autism

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