WEDNESDAY, June 8 (HealthDay News) -- Girls may be more
resistant to the spontaneous genetic mishaps that explain some
cases of autism in families with no history of the disorder, two
new studies suggest.
Everyone carries a certain number of duplications or deletions
of one or more sections of DNA, something known as copy number
variants, explained Dr. Christian Schaaf, an assistant professor in
the department of molecular and human genetics at the Baylor
College of Medicine, who wrote an accompanying editorial. If you
accumulate enough of them, and the duplications and deletions occur
on certain important areas of the chromosomes, those variants may
lead to autism and other conditions.
In the studies, published in the June 9 issue of
Neuron, researchers searched copy number variants in genetic samples from more than 1,000 families with an autistic child who were recruited as part of the Simons Simplex Collection project.
Autism spectrum disorders are neurodevelopmental conditions
marked by impaired social interaction and communication, unusual
interests, repetitive behaviors and, in some cases, intellectual
In cases where only one child in a family is affected, it has
been difficult to pinpoint new copy number variants. Researchers
believed that was because the available technology did not allow
them to view the genetic material at a high enough resolution to
see what had gone wrong, Schaaf said.
The new studies made use of newer, high resolution array
analysis to seek out copy number variants that differed between the
child with autism and the parents and siblings without autism.
"Before, it was like looking at the map of the United States. There was good enough resolution to tell the states and the big cities," Schaaf explained. "Now, the resolution is so much higher, you can look at individual streets."
They were searching such "streets" for the genetic mutations
that lead to autism spectrum disorder in "simplex" cases -- those
in which only one sibling is diagnosed with the disorder -- which
researchers have long thought may be different from those that lead
to multiplex cases. "Multiplex" means that more than one child in a
family has the disorder and that the condition seems to be
In simplex cases, it's thought that new variants -- known to
scientists as "de novo" variants -- arise spontaneously when the
child is conceived and are to blame for the autism disorder, as
opposed to variants that are passed from one generation to the
The presumption was that by using a higher resolution
technology, researchers would turn up more copy number variants,
said Andy Shih, vice president of scientific affairs for Autism
But researchers didn't find a remarkable increase. In the study,
led by researchers at Cold Spring Harbor Laboratory in New York,
researchers found de novo events in 8 percent of children with an
autism spectrum disorder, compared to 2 percent of the unaffected
That's relatively consistent with other work in which the range
is anywhere from 7 to 20 percent, said Schaaf said, who added that
the studies did confirm that children with autism carry de novo
copy number variants at a higher frequency than their parents and
The researchers also made some intriguing discoveries. In this
study and a second one led by scientists at Yale University,
researchers found that children with autism were more likely to
have copy number duplications on an area of Chromosome 7 known as
Prior research has shown that
deletions on 7q11.23 is associated with Williams or
Williams-Beuren Syndrome, a rare disorder in which children are
hypersocial and have no inhibitions about talking to strangers, as
well as cardiovascular disease and developmental delays.
"That's really interesting," Schaaf said. "It appears when you're missing a little bit, you are too social, and when you have too much, you are anti-social."
In addition, it's been well-documented that boys are at least
four times more likely to have autism than girls, and the new
research hints at why this may be.
The study led by the Cold Spring Harbor Laboratory researchers
found that girls with autism are more likely to have detectable de
novo "events," that is, larger copy number variants and more genes
involved in the deletions or duplications.
This suggests that it takes more, genetically speaking, to cause
girls to have autism.
"It seems to take more hits or more damage to the genome to lead to autism in girls than in boys," Shih said. "That may start to explain some of the sex biases, that boys are more vulnerable genetically."
Interestingly, Schaaf added, most of the autistic kids in the
Simons Simplex Collection were relatively high-functioning, Schaaf
said. Among high-functioning autistic people, there's an even
greater gender difference, he noted.
Still, much is unknown, including what might make boys more
susceptible, Schaaf added.
In addition, the research does little to explain the vast
majority of autism cases. Prior research has shown that about 5
percent to 7 percent of autism cases can be explained by a single
gene disorder. Another 5 percent seems to be caused by metabolic
conditions that affect brain function.
De novo copy number variants explain anywhere from 7 percent to
20 percent of autism cases with no known cause. But for the other
70-plus percent, there is no genetic explanation.
"It doesn't mean we'll [never] know the risk factors for the other 70 percent. Some could be environmental, some could be other genetic mechanism not yet discovered or that our current technology doesn't allow us to identify," Shih said.
It seems, he added, that "we are fast approaching the limitation
of this particular technology."
Find out more about autism spectrum disorders at the
U.S. Centers for Disease Control and