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
"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
"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
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