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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...

    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.

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