Autism: Cases on the Rise; Reason for Increase a Mystery

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The number of children diagnosed with autism or related disorders has grown at what many call an alarming rate. In the 1970s and 1980s, about one out of every 2,000 children had 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 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.

Tracking the Genetic-Environmental Interplay

More answers are coming. Pessah of UC Davis is one of the researchers in the CHARGE Study (Childhood Autism Risks from Genetics and the Environment), an ongoing study of 2,000 children. Some of the children have autism, some have developmental delay but not autism, and some are children without developmental delays.

Pessah and other researchers are focusing on how the interaction of genes and the environment play a role in autism.

Among the findings so far, he says, is that the immune system functioning of the mother may play a role in the child’s later development of autism. Pessah and his colleagues took samples from 163 mothers in the CHARGE study — 61 had children with autism, 62 had normally developing children, and 40 had children with non-autistic developmental delays. Then they isolated immune system antibodies, called IgG, from the blood of all the mothers. They took the blood samples and exposed them in the laboratory to fetal brain tissue obtained from a tissue bank.

Antibodies from the mothers of children with autism were more likely than antibodies from the other two groups to react to the fetal brain tissue, Pessah says, and there was a unique pattern to the reaction.

In an animal study, the UC Davis team then injected the antibodies into animals. The animals getting the IgG antibodies from mothers of children with autism displayed abnormal behavior, while the animals given antibodies from the mothers of normally developing children did not exhibit abnormal behaviors.

In another study, the UC Davis team found that levels of leptin, a hormone that plays a role in metabolism and weight, was much higher in children with autism than in normally developing children, especially if their autism was early in onset.

Another study, just launched by the CDC and now enrolling children, will track genetic and environmental factors that may increase risk for ASD.

Called SEED — the Study to Explore Early Development — the five-year study will follow more than 2,000 children at six sites across the U.S., says Newschaffer of Drexel, a co-principal investigator of the study. Some will have been diagnosed with ASD, some will have a developmental problem other than ASD, and a third group will be children without developmental problems.

Researchers will collect a host of genetic and environmental information, Newschaffer tells WebMD. They will find out about medical and genetic histories of the children and their parents, exposures during pregnancy to potential toxins, information about behavior, sleep problems, gastrointestinal problems, and other facts.

The hope, he says, is to find things that “stand out” — early exposure to certain substances, for instance, or certain genetic information or a specific behavior pattern — that might turn out to be markers for ASD.

Even if some environmental exposures or other findings do stand out, he says, “we are going to have to resist the temptation to say, ‘This is it,'” Newschaffer says.

Natowitz of Cleveland Clinic agrees. “There won’t be one single explanation.”