Posts tagged fragile x syndrome
Articles and news from the latest research reports.
The Unstable Repeats—Three Evolving Faces of Neurological Disease
Disorders characterized by expansion of an unstable nucleotide repeat account for a number of inherited neurological diseases. Here, we review examples of unstable repeat disorders that nicely illustrate three of the major pathogenic mechanisms associated with these diseases: loss of function typically by disrupting transcription of the mutated gene, RNA toxic gain of function, and protein toxic gain of function. In addition to providing insight into the mechanisms underlying these devastating neurological disorders, the study of these unstable microsatellite repeat disorders has provided insight into very basic aspects of neuroscience.
Fragile X makes brain cells talk too much
The most common inherited form of mental retardation and autism, fragile X syndrome, turns some brain cells into chatterboxes, scientists at Washington University School of Medicine in St. Louis report.
The extra talk may make it harder for brain cells to identify and attend to important signals, potentially establishing an intriguing parallel at the cellular level to the attention problems seen in autism.
According to the researchers, understanding the effects of this altered signaling will be important to developing successful treatments for fragile X and autism.
“We don’t know precisely how information is encoded in the brain, but we presume that some signals are important and some are noise,” says senior author Vitaly Klyachko, PhD, assistant professor of cell biology and physiology. “Our theoretical model suggests that the changes we detected may make it much more difficult for brain cells to distinguish the important signals from the noise.”
The findings appear Feb. 20 in Neuron.
Fragile X is caused by mutations in a gene called Fmr1. This gene is found on the X chromosome, one of the two sex chromosomes. Females have two copies of that chromosome, while males only have one. As a result, males have fragile X syndrome more often than females, and the effects in males tend to be more severe.
Symptoms of fragile X include mental retardation, hyperactivity, epilepsy, impulsive behavior, and delays in the development of speech and walking. Fragile X also affects anatomy, leading to unusually large heads, flat feet, large body size and distinctive facial features. Thirty percent of fragile X patients are autistic.
Scientists deleted the Fmr1 gene many years ago in mice to create a model of fragile X. Without Fmr1, the mice have abnormalities in brain cells and social and behavioral deficits similar to those seen in human fragile X.
According to Klyachko, nearly all fragile X mouse studies in the past two decades have focused on how Fmr1 loss affects dendrites, the branches of nerve cells that receive signals. In contrast, his new study finds significant changes in axons, the branches of nerve cells that send signals.
Normally, signals travel down the axon as surges of electrical energy. These surges only last for tiny fractions of a second, briefly causing the axon to release compounds known as neurotransmitters into the short gap between nerve cells. The neurotransmitters cross the gap and bind to their receptors on the dendrite to convey the signal.
When Klyachko monitored electrical surges along axons in the fragile X mice, though, he discovered that they lasted significantly longer. This caused release of more of neurotransmitters from the axon. When it should have stopped talking, the axon continued to chatter.
“The axons are putting out much more neurotransmitter than they should, and we think this confuses the system and overloads the circuitry,” Klyachko explains. “It may also create problems in terms of brain cells using up their resources much more quickly than they normally would.”
Infusing synthetic copies of the gene’s protein, called FMRP, into brain cells from the mouse model rapidly restored the electrical surges to their normal length.
Additional experiments revealed that FMRP works by interacting with one of the biggest channels on the surfaces of axons. These channels let electrically charged potassium ions into the axons, helping to shape and control the duration of the electrical surge.
In healthy brain cells, the main function of these channels is to prevent the electrical surge from getting too long. With FMRP gone, the channel is active for a shorter time, prolonging the surge and overwhelming the dendrite with too much chatter.
Klyachko and his colleagues are now studying the connections between FMRP and the channel it interacts with in axons. They hope to learn more about how information is encoded and processed at the level of individual brain cells. These insights one day may help clinicians better diagnose and treat many kinds of mental disorders.
Neuroscientists find excessive protein synthesis linked to autistic-like behaviors
Autistic-like behaviors can be partially remedied by normalizing excessive levels of protein synthesis in the brain, a team of researchers has found in a study of laboratory mice. The findings, which appear in the latest issue of Nature, provide a pathway to the creation of pharmaceuticals aimed at treating autism spectrum disorders (ASD) that are associated with diminished social interaction skills, impaired communication ability, and repetitive behaviors.
"The creation of a drug to address ASD will be difficult, but these findings offer a potential route to get there," said Eric Klann, a professor at NYU’s Center for Neural Science and the study’s senior author. "We have not only confirmed a common link for several such disorders, but also have raised the exciting possibility that the behavioral afflictions of those individuals with ASD can be addressed."
The study’s other co-authors included researchers from the University of California, San Francisco (UCSF) and three French institutions: Aix-Marseille Universite’; Institut National de la Santé et de la Recherche Médicale (INSERM); and Le Centre National de la Recherche Scientifique (CNRS).
The researchers focused on the EIF4E gene, whose mutation is associated with autism. The mutation causing autism was proposed to increase levels of the eIF4E, the protein product of EIF4E, and lead to exaggerated protein synthesis. Excessive eIF4E signaling and exaggerated protein synthesis also may play a role in a range of neurological disorders, including fragile X syndrome (FXS).
In their experiments, the researchers examined mice with increased levels of eIF4E. They found that these mice had exaggerated levels of protein synthesis in the brain and exhibited behaviors similar to those found in autistic individuals—repetitive behaviors, such as repeatedly burying marbles, diminished social interaction (the study monitored interactions with other mice), and behavioral inflexibility (the afflicted mice were unable to navigate mazes that had been slightly altered from ones they had previously solved). The researchers also found altered communication between neurons in brain regions linked to the abnormal behaviors.
To remedy to these autistic-like behaviors, the researchers then tested a drug, 4EGI-1, which diminishes protein synthesis induced by the increased levels of eIF4E. Through this drug, they hypothesized that they could return the afflicted mice’s protein production to normal levels, and, with it, reverse autistic-like behaviors.
The subsequent experiments confirmed their hypotheses. The mice were less likely to engage in repetitive behaviors, more likely to interact with other mice, and were successful in navigating mazes that differed from those they previously solved, thereby showing enhanced behavioral flexibility. Additional investigation revealed that these changes were likely due to a reduction in protein production—the levels of newly synthesized proteins in the brains of these mice were similar to those of normal mice.
"These findings highlight an invaluable mouse model for autism in which many drugs that target eIF4E can be tested," added co-author Davide Ruggero, an associate professor at UCSF’s School of Medicine and Department of Urology. "These include novel compounds that we are developing to target eIF4E hyperactivation in cancer that may also be potentially therapeutic for autistic patients."
(Source: eurekalert.org)
Fragile X Protein Linked to Nearly 100 Genes Involved in Autism
Doctors have known for many years that patients with fragile X syndrome, the most common form of inherited intellectual disability, are often also diagnosed with autism. But little has been known about how the two diagnoses are related.
Now a collaborative research effort at Duke University Medical Center and Rockefeller University has pinpointed the precise genetic footprint that links the two. The findings, published online in the journal Nature on Dec. 12, 2012, point the way toward new genetic testing that could more precisely diagnose and categorize the spectrum of autism-related disorders.
Fragile X syndrome is the most well understood single-gene cause of autism. It results from defects on a small part of the genetic code for a protein that researchers have dubbed the fragile X mental retardation protein, or FMRP.
Normally, FMRP plays an important role controlling production of other proteins in the brain and other organs. It does this by looking for specific genetic patterns located on the messages encoding proteins. When it locates these genetic flags, it attaches to them and, along with other signals, controls where and when protein is made.
In fragile X syndrome, this process breaks down because a defect in the gene causes the body to produce too little, or in some cases, none of the FMRP protein. As a result, additional proteins it would normally regulate are made in the wrong place and at the wrong time. Until now, little was known about how this process worked in people with the autism.
Using a combination of laboratory experiments and advanced bioinformatics, the research team, led by Thomas Tuschl, PhD, a Howard Hughes Medical Institute investigator at Rockefeller University, and Uwe Ohler, PhD, an associate professor in Biostatistics and Bioinformatics at the Duke Institute for Genome Sciences & Policy, identified both the genetic flags that FMRP is looking for and the genes it targets.
(Image courtesy of www.sueblimely.com)
Boosting natural marijuana-like brain chemicals treats fragile X syndrome symptoms
UCI study points to role endocannabinoids play in common genetic cause of autism
American and European scientists have found that increasing natural marijuana-like chemicals in the brain can help correct behavioral issues related to fragile X syndrome, the most common known genetic cause of autism.
The work indicates potential treatments for anxiety and cognitive defects in people with this condition. Results appear online in Nature Communications.
Daniele Piomelli of UC Irvine and Olivier Manzoni of INSERM, the French national research agency, led the study, which identified compounds that inhibit enzymes blocking endocannabinoid transmitters called 2-AG in the striatum and cortex regions of the brain.
These transmitters allow for the efficient transport of electrical signals at synapses, structures through which information passes between neurons. In fragile X syndrome, regional synapse communication is severely limited, giving rise to certain cognitive and behavioral problems.
Fragile X syndrome is caused by a mutation of the FMR1 gene on the X chromosome. People born with it are mentally disabled; generally experience crawling, walking and language delays; tend to avoid eye contact; may be hyperactive or impulsive; and have such notable physical characteristics as an elongated face, flat feet and large ears.
The researchers stress that their findings, while promising, do not point to a cure for the condition.
“What we hope is to one day increase the ability of people with fragile X syndrome to socialize and engage in normal cognitive functions,” said Piomelli, a UCI professor of anatomy & neurobiology and the Louise Turner Arnold Chair in the Neurosciences.
The study involved mice genetically altered with FMR1 mutations that exhibited symptoms of fragile X syndrome. Treated with novel compounds that correct 2-AG protein signaling in brain cells, these mice showed dramatic behavioral improvements in maze tests measuring anxiety and open-space acceptance.
While other work has focused on pharmacological treatments for behavioral issues associated with fragile X syndrome, Piomelli noted that this is the first to identify the role endocannabinoids play in the neurobiology of the condition.
About endocannabinoids
Endocannabinoid compounds are created naturally in the body and share a similar chemical structure with THC, the primary psychoactive component of the marijuana plant, Cannabis. Endocannabinoids are distinctive because they link with protein molecule receptors — called cannabinoid receptors — on the surface of cells. For instance, when a person smokes marijuana, the cannabinoid THC activates these receptors. Because the body’s natural cannabinoids control a variety of factors — such as pain, mood and appetite — they’re attractive targets for drug discovery and development. Piomelli is one of the world’s leading endocannabinoid researchers. His groundbreaking work is showing that this system can be exploited by new treatments to combat anxiety, pain, depression and obesity.
(Source: today.uci.edu)