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Autism Cases on the Rise; Reason for Increase a Mystery

Scientists are scouring genetic and evironmental data to find a cause for the rise in autism.

Zeroing In on the Genetics of Autism continued...

Parents who give birth to a child with ASD have up to an 8% chance of having another child who is also affected, the CDC estimates.

Many U.S. couples have delayed childbearing, and the older ages of both the mother and the father have been linked with a higher risk of having children with ASD, according to a report in the journal Pediatrics. With age could come increased risk for genetic mutations or other genetic problems.

Specific genetic problems help explain only a small percentage of autism cases so far. "We  know that major chromosomal abnormalities are identified in about 5% of ASD," says Milunsky of Boston University. "We know that Fragile X syndrome is responsible for about 3%." Fragile X syndrome, a family of genetic conditions, is the most common cause of inherited mental impairment, and also the most common known cause of autism or autism-like behaviors.

"Hot spots" of genetic instability may play a role, researchers say. For instance, a team of researchers reported in The New England Journal of Medicine that duplications and deletions on a specific chromosome seem to be associated with some cases of autism.

Specific genes or problems on chromosomes are implicated in a small number of ASD cases, Milunskey writes in a report on autism research published in the Archives of Disease in Childhood. For instance, maternal duplication on a specific chromosome region has been linked to about 1% of those with ASD.  

"We are homing in on those 'hotspot' regions and identifying some of the single genes involved in either the direct causation or the susceptibility to ASD," Milunsky says.

But genetics is not the whole story, he and other experts say.

Zeroing in on Environmental Triggers

A variety of environmental triggers is under investigation as a cause or contributing factor to the development of ASD, especially in a genetically vulnerable child.

Exposure to pesticides during pregnancy may boost risk. In a study published in Environmental Health Perspectives, researchers compared 465 children diagnosed with ASD with nearly 7,000 children without the diagnosis, noting whether the mothers lived near agricultural areas using pesticides.

The risk of having ASD increased with the poundage of pesticides applied and with the proximity of the women's homes to the fields.

Besides pesticide exposure, exposure to organic pollutants that have built up in the environment are another area of concern, says Pessah of UC Davis. For instance, polychlorinated biphenyls or PCBs, substances previously found in electrical equipment, fluorescent lighting and other products, are no longer produced in the U.S. but linger in the environment, he says. "Particular types of PCBs are developmental neurotoxins," he says.

Another toxin to the brain is mercury in its organic form. But according to a report published in Pediatrics, there is no evidence that children with autism in the U.S. have increased mercury concentrations or environmental exposures. Though many parents of children with ASD believe their child's condition was caused by vaccines that used to contain thimerosal (a mercury-containing preservative), the Institute of Medicine concludes there is no causal association.

Even so, many autism organizations remain convinced there is a link. The vaccine-autism debate reignited in early March 2008, after federal officials conceded to award compensation to the family of a 9-year-old Georgia girl who developed autism-like symptoms as a toddler after getting routine childhood vaccinations. Officials said the childhood vaccines given to the girl in 2000, before thimerosal was phased out, aggravated a pre-existing condition that then manifested as autism-like symptoms. The pre-existing condition was a disorder of the mitochondria, the "power sources" of the cell, according to the family.

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