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Fragile X Syndrome: Mental Retardation
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Fragile X Syndrome (FXS) is the most common cause of inherited mental impairment. This impairment can range from learning disabilities to more severe cognitive or intellectual disabilities. (Sometimes referred to as mental retardation.) FXS is the most common known cause of autism or “autistic-like” behaviors. Symptoms ... can include characteristic physical and behavioral features and delays in speech and language development.
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Fragile X Syndrome is the most common inherited cause of mental retardation. The study involves researchers from both UNC and Duke University, through the Duke-UNC Brain Imaging and Analysis Center, and represents a collaboration with researchers at Stanford University, who have been awarded an identical grant to conduct this joint study.
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Fragile X syndrome (FXS) is the commonest cause of sex-linked mental retardation. It is what is known as a repeat expansion disorder. In DNA coding it is common to see repeated sequences of the nucleotides that make up the genetic strand. In Fragile X Syndrome there is an expansion of the number of repeat sequences in the Fragile X mental retardation (FMR1) gene. The nucleotides involved are cytosine (C) and guanine (G) and the repeated sequence is CGG. In the commonest form of the condition, the CGG sequence is repeated more than 200 times.
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In 1991 the long-awaited FMR-1 (fragile-X mental retardation) gene was isolated using a molecular genetic positional cloning strategy. The mutational basis of the syndrome (termed FRAXA) was recognized to be due to the expansion of a trinucleotide repeat (CGG)n present in the 5' untranslated region of the identified gene. The lack of expression of the FMR-1 gene results in non-production of the protein (fragile-X mental retardation protein, or FMRP), resulting in fragile-X syndrome. FMR-1 is the first cloned gene to be linked to human intelligence.
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Researchers have reversed almost all symptoms of fragile-X syndrome in a mouse model for the disease. In humans, fragile X is the most common form of heritable mental retardation and one of the leading known causes of autism.
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Since 1991, scientists have known which genetic alteration lies at the basis of the fragile-X-syndrome. This alteration causes the FMRP protein (Fragile X mental retardation protein, named after the syndrome) to lose its function. However, up to now, it has not been clear which bodily reactions are blocked by the loss of function of this one gene, given the fact that the FMRP controls the functioning of many other genes as well. Shedding light on this situation is one of the great challenges for researchers who want to better understand the syndrome and, consequently, the functioning and development of the brain.
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Mental Retardation