Tracing the Origins of Autism: A Spectrum of New Studies The etiology of a medical condition might
seem an unlikely subject to arouse intense feelings.
Yet few medical disorders have stirred up as
much passion and divisiveness among scientists
and the general public as autism has in recent
years. The heat of the controversy has even attracted
attention from periodicals such as The Wall
Street Journal, the Columbia Journalism
Review, and Wired magazine—seemingly
improbable forums for a medical debate. Why all
the furor?
At the eye of the storm is the startling climb
in the numbers of children who have been diagnosed
with one of the autism spectrum disorders (ASDs).
The most severe ASD is autistic disorder (which
often is called simply “autism”);
other forms include Asperger syndrome and the
much rarer childhood disintegrative disorder.
In the United States, the diagnosis of ASDs increased
roughly 10-fold over the course of a decade,
from 4–5 children per 10,000 in the 1980s
to 30–60 children per 10,000 in the 1990s,
according to a report in the August 2003 Journal
of Autism and Developmental Disorders. The
5 May 2006 issue of Morbidity and Mortality
Weekly Report describes the results
of two parent surveys from 2003 and 2004, which
suggested that 55–57 children per 10,000
had autism (however, an editorial note points
out that, due to the nature of the surveys, parents
of children with other ASDs may have reported
their children as having autistic disorder).
Some scientists believe that much of the upsurge
is the result of increased awareness
of ASDs or changes in diagnostic
criteria, which would
suggest that the true prevalence
of the disorders has been stable
over time. Others disagree. “It
is premature to state that there
is no increase in prevalence, ” says
W. Ian Lipkin, a professor of neurology, anatomy,
and neurobiology at Columbia University. “None
of the studies to date has been designed
to definitively address the issue. ”
The prevalence of ASDs plays into the fundamental
question of what causes these disorders. If the
number of cases is truly on the rise, then it
would seem likely that some change in the environment
is driving up the total. That’s partly
what has divided scientists into opposing camps—they
cannot agree on the relative importance of genetic
and environmental factors in the disorders’ etiology.
Alas, answering the prevalence question might
not end that debate. “Even if the prevalence
of autism were stable,” says Lipkin, “you
would not be able to rule out the possibility
of an environmental trigger.” That’s
because very little is known about the mechanisms
that cause autism, be they environmental or genetic.
“The study of autism was, until recently,
largely dominated by the field of psychology,
where characterizing the behaviors and developing
reliable instruments for diagnosis have been
major areas of research over the past few decades,” says
Irva Hertz-Picciotto, an epidemiologist at the
University of California, Davis.
Indeed, the core symptoms of ASDs—social
disinterest, repetitive and overly focused behavior,
and problems in communication, usually appearing
before 3 years of age—have been well described.
Much less research has focused on the causes
of these symptoms.
Several investigations dating back to the 1970s
indicate that identical twins have a much higher
concordance rate of ASDs than fraternal twins,
according to a report in the Spring 1998 issue
of Mental Retardation and Developmental Disabilities
Research Reviews. Those studies provide some
of the best evidence that these disorders have
a strong genetic component. But the identity
of the genes involved, much less how they produce
ASDs, has not been established. Moreover, the
concordance rate for identical twins is not 100%,
which suggests that at least some cases must
be associated with environmental or epigenetic
factors.
A few cases of ASDs have been clearly linked
to environmental insults. These include prenatal
exposure to chemical agents such as thalidomide
and valproic acid, as well as to infectious agents
such as the rubella and influenza viruses. Here
again, the concordance rate is not 100%, which
suggests that a genetic predisposition is necessary
for chemical and microbial factors to act as
triggers.
Tantalizing clues like these are prompting
scientists to reconsider the research agenda
for ASDs. Martha Herbert, a pediatric neurologist
at Harvard Medical School, and her colleagues
have been applying the methods of genomics to
identify environmentally responsive genes that
might be important in these disorders.
“When you realize that the widespread
changes we’re seeing in autistic brains
may occur in parallel with or even downstream
from widespread changes in the body—such
as in the immune system—and that these
changes may be environmentally triggered, you
start looking for ways to think more broadly
about genetic vulnerability. It can’t be
just about ‘brain genes,’” Herbert
says.
Some new epidemiological studies also are looking
for gene–environment interactions. According
to Diana Schendel, an epidemiologist and project
officer for autism research at the CDC, which
funds one of the projects, these initiatives
will be able to examine many possible causal
pathways to ASDs, including both genetic and
environmental causes that may lead to the development
of the disorders in different subgroups of children.
Some of these projects are already under way,
whereas others will begin soon. All of the scientists
involved, however, believe their research will
finally provide some of the answers that everyone
has been looking for.
CHARGE
The Childhood Autism Risks from Genetics and
the Environment (CHARGE) project is unique
among the large ASD epidemiological studies.
It focuses solely on autistic disorder, and it
emphasizes a search for environmental factors—including
a broad array of chemicals in food, consumer
products, and ambient air, as well as infectious
and medical exposures—that might be linked
to the disorder. The study is funded by the NIH.
CHARGE is a case–control study in which
a group of autistic children aged 2 to 5 years
is compared to a group of age-matched controls
in a population-based study. “Because of
the California Department of Developmental Services’ system
of Regional Centers [nonprofit corporations that
coordinate health care services and support for
citizens with developmental disabilities], we
have a handle on enumerating a high proportion
of the children newly diagnosed with autism in
our defined area over a specific time period,” says
Hertz-Picciotto, the principal investigator of
the CHARGE study. “Similarly, we can enumerate
the children in the same area and time period
who are not cases. We then sample from both. ”
The project was initiated in 2002 with the
goal of recruiting 1,000 to 2,000 children. Half
of the children will be autistic. The other half
will make up two control groups: one group of
children with developmental delays (but not an
ASD) and a second group of children selected
from the general population without regard to
developmental characteristics.
The advantage of the case–control design
is that scientists can acquire large numbers
of children with the disorder. By comparison,
in a cohort design researchers would need a very
large sample size, given the prevalence of autism,
to acquire the same number of cases.
Hertz-Picciotto expects to have enrolled nearly
700 children by August 2006, the end of the first
funding period. “I’ve applied for
another five-year grant,” she says, “and
I hope to be funded to enroll nine hundred in
that round, which would bring us to sixteen hundred
children. ”
The CHARGE team is looking at possible exposures
during the prenatal period and early childhood.
Some of the data will be gathered through comprehensive
interviews with parents, but Hertz-Picciotto
admits that this is not the best way to look
for exposures. “You ask people questions,
and their answers may be colored by the fact
that they know they have a child with a condition,” she
says. “They may spend a lot of time thinking
about what they might have done or what might
have gone wrong, and they may have preconceived
ideas about what caused [the disorder]. They
might not be as objective.” Such problems
with postdiagnosis interview information are
recognized as a weakness of retrospective studies.
The scientists are getting around this issue
by examining each child’s medical records
and those of the mother during pregnancy and
delivery—nonsubjective data gathered in
the course of routine obstetric care. They are
also collecting blood, urine, and hair specimens
that will be analyzed in the laboratory.
The study has already provided some intriguing
leads. “We’re finding that the immune
system seems to function at a lower level in
autism,” says Hertz-Picciotto. “That’s
an important clue. It could mean that whatever
causes autism also disrupts the immune system,
or it could be that the immune system disrupts
neural development so that something goes awry
in laying down brain circuitry prenatally or
in the early postnatal period.” [For more
information on the CHARGE study, see p.
1119, this issue.]
ABC
The Autism Birth Cohort (ABC) Study, now under
way in Norway, is a large prospective design
that is expected to gather information on 100,000
babies. The work is being led by scientists at
the Mailman School of Public Health at Columbia
University, who are collaborating with colleagues
at the Norwegian Institute of Public Health,
with funding from the U.S. National Institute
of Neurological Disorders and Stroke.
“When you want to know why some people
are more at risk than others in a population,
then that’s best answered using a cohort
design,” says Ezra Susser, an epidemiologist
at Columbia University and a co-investigator
on the ABC project. “When we think about
environmental causes of [ASDs], we’re probably
interested in phenomena that occur prior to birth
or perhaps shortly after birth. So you want to
collect prospective data from people as early
as possible in pregnancy.” Because ASDs
are not common, the study will need large numbers
of children to have enough statistical power,
according to Susser.
So far the ABC team has recruited 75,000 pregnant
Norwegian mothers, but Susser is hoping for more. “We’ve
got enough to look for an environmental risk
factor, but you need larger numbers for studying
gene–environment interactions, which could
turn out to be important,” he says. It’s
possible the team could acquire greater numbers
by collaborating with other studies. One candidate
for collaboration is the Avon Longitudinal Study
of Parents and Children in the United Kingdom,
which is looking at the complex ways in which
environmental features may relate to optimal
development and health in children. But there’s
been no agreement yet, Susser says.
![chart2](graph2-sm.jpg) |
image:
Corbis
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Even so, the ABC scientists are optimistic
about their study. “Little is known about
the natural history of [ASDs],” says Lipkin,
who is the principal investigator of the project. “By
starting prenatally, we’re collecting detailed,
critical information about environmental exposures
in an unbiased fashion. ”
The scientists are also collecting plasma,
serum, RNA, and DNA. “We have extraordinary
biological materials,” says Lipkin. “We
can pursue biomarkers as well as exposure to
toxicants and infection. We also have maternal
DNA, paternal DNA, and the child’s DNA
[so-called trio data]; thus we can look for the
appearance of novel mutations,” he adds.
The ABC researchers will follow the children
through time, with parents answering questionnaires
about the health and social interactions of their
children as they reach 6, 18, and 36 months of
age. “It may be that the developmental
trajectory tells us much more than a single time
point can ever tell us about the pathogenesis
of [ASDs],” says Mady Hornig, a physician-scientist
at Columbia University who participates in the
project.
Despite their enthusiasm for the project’s
potential, the ABC scientists feel they could
accomplish much more if they only had the funding. “The
pity of it is we have no money to do the biological
work,” says Lipkin. “We can collect
the samples and do the questionnaires, but we’ve
been unable to get funding to look for any of
the environmental factors. We’re collecting
blood, but we won’t know whether there’s
a biomarker until we do a biomarker analysis.
We have funds to collect RNA, but in order to
do the transcript profiling we need approximately
four hundred dollars per sample,” he says.
Lipkin adds that there’s only so much
that one can do with questionnaire data. “We
do ask about infection and diet, but that’s
not the same as having a lab value that can validate
what was reported, and then look at a direct
correlation with the outcome,” he says.
Lipkin believes that part of the problem is
that searching for environmental factors goes
against the current research paradigm in ASDs. “The
focus is on genetic factors,” he says. “Infectious
diseases, toxicology, and immunology receive
short shrift. The ABC is clearly the right opportunity
to pursue these other leads because we have the
ideal samples to survey prenatally and postnatally,” he
says.
The scientists are just now receiving the responses
to the 36-month questionnaire. “It’ll
probably be another two years before we have
our first report,” Hornig says. Funds are
now in place to study the children at 36 months;
however, the team hopes to follow them for a
lifetime, according to Hornig.
CADDRE
In response to the Children’s Health
Act of 2000, the CDC established and funds six
Centers for Autism and Developmental Disabilities
Research and Epidemiology (CADDRE) to investigate
potential risk factors for ASDs. The multisite
approach offers a study group that is geographically
and demographically more representative of the
general U.S. population than a smaller regional
study could provide, according to Craig Newschaffer,
an epidemiologist and principal investigator
at the Johns Hopkins Bloomberg School of Public
Health CADDRE site.
According to Newschaffer, the CADDRE sites
will use a case cohort design in which the exposure
patterns of the ASD cases are compared to a random
sample of children living in the same geographic
area. A third study group, consisting of neurodevelopmentally
impaired children who do not have an ASD, will
round out the sample populations. The investigators
hope to enroll a total of 650 to 900 children,
aged 3 to 5 years, in each study group across
all the sites, making CADDRE the largest study
of its kind in the United States, says Newschaffer.
A uniform protocol across the sites will allow
the scientists to pool their data.
CADDRE will collect and archive blood, cheek
cell, and hair samples from the children in order
to investigate a broad range of potential risk
factors. “We’re not focused on the
environment as much as CHARGE is,” says
Newschaffer, “but we are collecting data
on questionnaires and reviewing medical records
on exposure, in addition to the biosampling for
exposures. ”
The scientists should have sufficient numbers
to look at gene–environment interactions. “We
are collecting DNA from the parents and the kids
from each of the groups. We’ll have trio
data in each of the three groups, a potentially
powerful design,” says Newschaffer.
CADDRE scientists will also characterize the
behavior of the children, as well as describe
any comorbid medical conditions and atypical
physical features. The goal is to sort out different
etiologic subgroups within the autism spectrum.
As Newschaffer explains, “There are a lot
of possible reasons why we’ve had a hard
time coming up with genetic and nongenetic risk
factors. One of them is that autism is likely
a heterogeneous condition, with different etiologies
producing kids with what appear to be similar
phenotypic profiles. If you don’t separate
out the different etiologic groups, it’s
going to be very hard to find an association
with a gene or an exposure. If we limit our analyses
to kids that have a certain profile, we’re
going to be able to make some informed guesses
about what profiles might allow risk factors
to emerge,” he says. The CADDRE sites will
begin recruiting children into the study in the
fall of 2006.
More Studies, More Acronyms
There are several other smaller epidemiological
studies in the works. In California, scientists
are tapping into specimen banks that have stored
blood samples taken from mothers during pregnancy
and from their children at birth. The Early Markers
for Autism (EMA) study employs a case–
control design, with about 100 children with an ASD (primarily autism), 100
who are developmentally delayed, and 200 from the general population. “We
can correlate what’s happening in the mom and the baby, which is really
exciting,” says Lisa Croen, a perinatal epidemiologist at the Kaiser
Permanente Division of Research in California and the project’s principal
investigator.
EMA is a multidisciplinary collaboration with
epidemiologists, geneticists, immunologists,
neurovirologists, and endocrinologists, according
to Croen. “Because autism is so complex,
it’s important for all these researchers
to communicate with each other. I think EMA is
a model for how to do research in autism,” she
says. EMA is unique, according to Croen, because
the study will be looking for biological markers
of ASDs very early in development, during gestation,
and at birth. “This allows us to focus
on mechanisms that may be leading to autism rather
than mechanisms that are consequences of having
autism,” she says.
The EMA scientists are investigating genetic
and nongenetic factors, with a focus on the immune
dysregulation hypothesis of ASDs. “We’re
measuring different kinds of immune markers,
including immunoglobulin levels and antibodies
to specific infectious agents, cytokines, and
autoantibodies,” says Croen. “We’re
looking for things that distinguish kids who
are subsequently diagnosed with autism from those
who aren’t. This will help us understand
the pathobiology of autism—the mechanisms
that are leading to the dysregulation in development. ”
The three-year EMA is currently in its last
year. “We still have lots of analyses to
do,” says Croen, “but we’re
beginning to write some papers. We’re finding
differences between the children in levels of
certain proteins measured in the circulating
blood collected from mothers during pregnancy.
I think the study has much to contribute to our
understanding of the biology of what might be
going wrong. ”
Croen is also an investigator on the California
Autism Twin Study (CATS), which expects to recruit
300 identical and fraternal twin pairs born between
1987 and 1999 in which at least one of the twins
has an ASD. Comparing the twin pairs will allow
the scientists to estimate the heritability of
ASDs—the relative genetic and environmental
contributions to the disorder. “Knowing
the behavioral and developmental differences
between the twins might help us understand the
effects of gene expression, the in utero environment,
and environmental triggers,” Croen says.
![chart3](graph3-sm.jpg) |
Hertz-Picciotto is also excited about a five-year
study that she and her colleagues hope to begin
soon. Unlike CHARGE, the new effort, called MARBLES
(Markers for Autism Risk in Babies—Learning
Early Signs), will be a prospective study in
which data will be gathered before the children
are diagnosed. Pregnant women who already have
at least one child with autism will be enrolled
right at the beginning of pregnancy. The mothers
will keep diaries about their symptoms and health-related
events, and the researchers will collect cord
blood samples and placentas.
Based on previous research, Hertz-Picciotto
expects that about 1 in 10 siblings of the autistic
children will also have the disorder, and perhaps
1 in 4 or 5 will be “on spectrum” with
a related but less severe condition such as Asperger
syndrome, or with some symptoms of the broad
behavioral phenotype, such as language delays
and atypical social skills. “This work
is complementary to the case–control approach,
and should provide us with a lot of information
that will build on what we find in CHARGE. It
should be a phenomenal resource,” she says.
You Say You Want a Revolution
In April 2004, the U.S. DHHS issued a publication, Congressional
Appropriations Committee Report on the State
of Autism Research, describing recommendations
made by a panel of expert scientists convened
by the Interagency Autism Coordinating Committee
(IACC). The IACC panel suggested an ambitious
agenda, which included the goal of identifying
environmental risk factors and their associated
developmental windows within a four- to six-year
period, as well as identifying genetic and
nongenetic causes of ASDs and their interactions
within seven to ten years.
Hertz-Picciotto, a member of the IACC panel,
thinks these goals should be taken with a grain
of salt. “I’m optimistic that we
will have identified some environmental risk
factors, and may have excluded a few others,
between 2008 and 2010—but by no means will
we have the final word. The genetics and the
gene–environment interactions may be even
tougher. Unfortunately, I don’t see enough
groups working on the environmental contribution
to autism, so it may be slower than projected,” she
says.
Mark Blaxill, vice president of SafeMinds,
a parent-led advocacy group, also believes that
environmental risk factors don’t receive
enough consideration. “The CDC has not
addressed the crisis in autism responsibly,” he
says. “They should be raising the alarm,
and they have failed to do so. They should be
asking why so many children are sick. Instead,
they’ve tried to suggest a degree of doubt
about the increases, and that diverts attention
and funding from environmental causes. ”
Schendel responds, “It is clear that
more children than ever before are being classified
as having an ASD. It is important that we treat
common developmental disorders, and especially
the ASDs, as conditions of urgent public health
concern. The CDC’s efforts in addressing
this public health concern include funding for
ASD monitoring programs to understand ASD trends,
funding for research into the genetic and environmental
causes of ASDs, and education and outreach programs
to promote early identification and timely intervention
for all children with developmental problems. ”
Despite the promise of the new epidemiological
studies, some researchers are still dismayed,
as one scientist put it, that “geneticists
are running the show, and ignoring the environmental
aspects.” What would it take for things
to change? Blaxill invokes the ideas of philosopher
Thomas Kuhn, who suggested that scientific revolutions
occur when an old paradigm is replaced by a new
one. “I believe we’re in the middle
of a paradigm shift,” Blaxill says. “The
dramatic explosion of autism rates does not fit
the genetic model. It’s an anomaly that
will kill the old paradigm.”
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