Fragile X syndrome is marked by autism-like symptoms
Mar 18, 2004 - 1:43:38 AM
People with fragile X syndrome, the most common inherited developmental disability, have reduced blood levels of a protein vital for brain development and function, researchers at the School of Medicine have found. These lowered levels are linked to abnormal activity patterns in the brain.
It is exciting to think that a biological marker we can measure in the blood is correlated with vital brain function, said Allan Reiss, MD, professor and director of the Stanford Psychiatry Neuroimaging Laboratory and Behavioral Neurogenetics Research Center in the Department of Psychiatry and Behavioral Sciences and the study's senior author.
These sets of images show brain regions where there was significant activation during the response inhibition tasks used to compare activity in fragile X brains (blue) with typically developing brains (red). Photo: Courtesy of Vinod Menon
Additionally, in people with fragile X syndrome, researchers found that background brain activity outside the realm of problem solving does not decrease as expected when the individual is confronted with a complex task. In unaffected people, the brain smoothly redirects resources to other tasks as needed. This may explain why people with fragile X can't produce cognitive resources when needed.
The findings, published March 1 in the Proceedings of the National Academy of Sciences, will enable a more targeted approach to the development of treatments for the disorder.
Fragile X syndrome is so named because it results from a mutation of a gene at a fragile site on the X chromosome where structural gaps may occur. It affects roughly 1 in 3,600 males and 1 in 4,000 to 6,000 females, according to the National Fragile X Foundation.
The mutation arises when a repetitive DNA segment of a gene known as FMR1 expands up to hundreds or thousands of times. The FMR1 gene normally produces fragile X mental retardation protein, which regulates the production of other proteins controlling how nerve synapses grow and change in response to learning.
Males with fragile X syndrome produce little or no fragile X protein. They also have severe manifestations of the disease, including autistic-like behaviors, hyperactivity and mental retardation. Affected females often have less extreme symptoms such as attention deficit, shyness, anxiety and learning problems, although some may show autistic behavior and mental retardation.
Such a broad spectrum of severity in females corresponds to a wide range of brain activation patterns and blood levels of fragile X protein. This range makes females particularly fitting subjects for studying the association between the protein levels and brain activity in individuals with fragile X syndrome.
The effect of genetic factors on brain function is a topic of increasing interest within the field of cognitive neuroscience, and fragile X syndrome provides an excellent model to investigate the effect of a single gene on human brain function said first author Vinod Menon, PhD, associate professor of psychiatry and behavioral sciences and a member of Stanford's neurosciences program.
Menon, Reiss and colleagues examined whether reductions in brain activation are correlated with levels of fragile X mental retardation protein in the blood. In previous studies the researchers had shown that individuals with performance deficits had reduced brain activity in regions known to be associated with the tasks being performed. They conducted the current study to shed light on whether the reduced brain activity observed was simply a function of poor performance or the result of faulty neural processing.
The study, funded by the National Institutes of Health with support from the Canel Family Fund, observed 18 females ages 10-22 who had the gene mutation that causes fragile X syndrome, and for comparison, 16 typically developing age-matched females. Study subjects performed a series of tasks while undergoing an MRI that allows researchers to monitor brain activity. The method tracks changes in blood oxygen levels as a marker for changes in blood flow that, in turn, are closely correlated to nerve cell activity in the brain.
The so-called response inhibition task researchers used was simple, addressing the ability to control impulsive behavior. Subjects were shown different letters of the alphabet that flashed one at a time on a computer screen. They were asked to respond by pressing a key in every case except when they saw the letter X. The first task was a Go task, in which the letter X never appeared and in this way subjects were allowed to build up a tendency to respond. Immediately afterward, subjects performed a Go/No Go task in which the letter X did appear in the lineup, at which point the subject had to control the previously built impulse to respond. Statistical correlations were made between observed reduction in brain activity compared with typically developing individuals and the levels of fragile X mental retardation protein found in blood samples taken from each subject.
Individuals with fragile X syndrome performed the response inhibition task as well as normally developing people, so the observed differences in brain activity could not be attributed simply to performance deficits. Among the participants with fragile X, brain activity decreased in key areas involved in response inhibition in proportion to fragile X protein levels. We are particularly excited to have a marker for this condition that gives us a tool to begin to query associations across multiple scientific levels including genetic, brain function and behavior, said Reiss. The study brings neuroscience and psychiatry together in a unique way.
The work is part of a comprehensive research program at Stanford directed by Reiss and devoted to studying fragile X syndrome and other genetic and neurodevelopmental disorders that affect learning, behavior and development in children. The research team plans to expand brain imaging research to test other cognitive and behavioral functions with the disorder, integrating knowledge gained from genetic, physiological and behavioral studies. They are recruiting preschoolers, children and adolescents for ongoing studies to determine, among other things, the timing, amount and type of effective interventions.
Individuals with fragile X syndrome or other causes of developmental disability are encouraged to participate in this study. Call (888) 411-2672 or e-mail [email protected] for more information.
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