http://www.ama-assn.org/sci-pubs/amnews/pick_01/hlsc0917.htm#rbar_add
By Stephanie
Stapleton, AMNews staff. Sept. 17, 2001. Additional
information
Washington -- Science is one step closer to unlocking the genetic basis of
autism -- the puzzling neurodevelopmental disorder that effects, in some form,
at least one in 500 people.
Researchers have identified the regions of four chromosomes that appear to
be linked with the disorder, according to a study supported by the National
Institutes of Health and published in this month's American Journal of Human
Genetics.
"These findings confirm the role of genetics in autism and are a major
step in narrowing the search for the specific genes involved," said Duane
Alexander, MD, director of NIH's National Institute of Child Health and Human
Development and co-chair of NIH's Autism Coordinating Committee.
The idea that there is an inherited component to autism is not new. Earlier
studies have offered varying degrees of evidence. This study is important
because it "is another link in this chain," said Marie Bristol-Power,
PhD, the NICHD special assistant for autism. "This paper replicates
findings on two of the genetic hot spots."
Specifically, the researchers screened the DNA of more than 150 pairs of
siblings with autism -- all of whom were diagnosed using the same methods. The
study, involving subjects and researchers from seven countries, was carried out
by members of the International Molecular Genetic Study of Autism Consortium.
They found evidence that two regions on chromosomes 2 and 7 contain genes
involved in the disorder.
"The chromosomes they are on make sense in terms of autism," said
Dr. Bristol-Power. Chromosome 7 is known to be associated with many language
disorders and has been shown to be linked with autism in some earlier studies,
but not all.
Researchers were particularly excited by evidence of an autism link on
chromosome 2, since this area had recently been identified by another,
independent research group. Likely locations for autism-related genes were also
found on chromosomes 16 and 17, although this correlation was weaker, according
to the NICHD.
Overall, the study is important because it adds support to the concept that
autism is caused by more than just a single gene -- that it is multifactorial,
the result of interactions between several genes involved in brain signaling
and development, as well as environmental factors.
"It's likely we're talking about a genetic susceptibility rather than a
genetic cause," explained Dr. Bristol-Power. And it then leads to
questions about what else might contribute.
NICHD is supporting a range of research looking at the interaction between
the genes for early brain development located on chromosome 2 and environmental
influences.
In the long run, chromosome 7's role is interesting, too, because this
chromosome has been linked with other language disorders.
"What we are trying to find are biological markers and biomedical
treatments for autism," Dr. Bristol-Power said.
If this plays out, it would allow for earlier diagnosis and earlier
interventions. For instance, if a gene for language could be identified, there
would be a biological target for therapies that could at least help with the
development of language skills.
Meanwhile, findings in a separate study, published in the Aug. 30 New
England Journal of Medicine, address concerns about the possible link
between two widely used vaccines and the onset of autism and other
neurodevelopmental problems.
This study was conducted because the risk of seizures and such subsequent
disorders has not been clearly identified in children vaccinated with DTP (for
diphtheria-tetanus-pertussis) and MMR (for measles-mumps-rubella), according to
the Journal's weekly briefing materials.
These questions about risk provide context to some parents' fears that they might
be hurting their children by having them immunized. Others point to vaccination
schedules as a possible factor in their child's illness.
Centers for Disease Control and Prevention researchers concluded that the
two vaccines were associated with a short-term increased risk of fever-related
seizures, but that these seizures were not associated with an increased risk of
subsequent seizures or neurodevelopmental disabilities. This lack of
association is reassuring, according to the authors.
Using records from the Vaccine Safety Datalink project -- a study begun in
1991 to look at adverse events after childhood immunization, the researchers
examined the medical data of more than 600,000 children younger than seven
years. The children who were followed received vaccines between 1991 and 1993.
In the time since, that form of DTP vaccine has been replaced with a new form
called DTaP.
The study first sought to pinpoint children who had had seizures. From this
group, researchers gathered specific information about these seizures -- if
they were fever- or nonfever-related events and whether they were attributable
to trauma or an existing disease, such as epilepsy.
The researchers then analyzed the risk of a first seizure after DTP or MMR
vaccination. Children who had post-vaccination febrile seizures were followed
to identify the risk of subsequent seizures and other neurological problems,
including learning disorders, mental retardation and infantile autism,
according to the article.
Barbara Loe Fisher, co-founder and president of the National Vaccine
Information Center, takes exception with both the study and its findings,
saying the conclusions are "irresponsible and dangerous" and that the
methodology is flawed.
She is disheartened by what she considers to be the study's message:
"If your child has a seizure after vaccination, it doesn't matter. Keep
vaccinating them. Our message has always been that there are some children who
cannot tolerate vaccines," she said.
The NVIC calls for longitudinal studies comparing health outcomes,
morbidities and mortalities of children who have been vaccinated and those who
have not. It also supports efforts to identify biomarkers to screen children
and determine who is at risk.
But NICHD's Dr. Bristol-Power said, for now, the clues that genetics is
providing are not directly related to questions about a link between vaccines
and autism. And the size of the New England Journal study and its
findings seem to suggest that, for most children, the role of vaccines will
unlikely offer a simple answer to the "in the presence of what"
question associated with the triggers of autism, she added.
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