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Fragile X Syndrome: Genes
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The gene for the fragile X protein, FMR1, was discovered in 1991. The defect, a trinucleotide repeat, was a new molecular mechanism of disease that has since been found in other inherited disorders, such as Huntington's disease. The number of repeats in the CGG (cytosine-guanine-guanine) sequence correlates roughly with clinical severity. Persons with 50-200 repeats are carriers who may show no or mild signs. With higher number of repeats, DNA methylation inactivates the gene, according to Owen Rennert, M.D., FAAP, professor and chairman of pediatrics, and head, division of genetics, Georgetown University.
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Fragile X syndrome is detected through a DNA analysis that almost always requires drawing blood. The technique for identifying fragile X syndrome is a specialized process and not all genetic labs have this capability. For those that do have this capability, the procedure is virtually 100 percent reliable. Fragile X can be detected prenatally or in newborns through DNA testing. Also, the carrier status of parents can be accurately determined. However, these tests are not routinely done and must be specifically requested.
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Although there is currently no cure for fragile X syndrome, scientists are making great progress in understanding the biology of the disorder. In the mid-to late 1990s, Stephen Warren and colleagues determined that the FMR1 gene product, named FMRP, is an RNA-binding protein that shuttles in and out of the nucleus and is involved in binding various messenger RNAs. Moreover, scientists successfully developed mice that lack the FMR1 gene, which will greatly aid research. Symptoms of fragile X mice include learning disabilities, hyperactivity, and, in males, enlarged testicles. Prevailing hypotheses about FMRP suggest that this protein is involved in forming neural connections in the developing brain.
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To understand the details of the neuronal pathology of Fragile X syndrome, the researchers, led by Huibert Mansvelder, studied mice in which the same gene that causes the disease in humans had been knocked out. The scientists performed a detailed analysis of the electrophysiological properties of neurons in the prefrontal cortex, a region responsible for higher cognitive functions, including learning and memory, that are affected in humans with the disorder.
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Since the discovery of the Fragile X gene in 1991, there has been tremendous progress in the understanding of this disorder. Preimplantation genetic screening, using molecular genetic screening of in vitro fertilized embryos followed by implantation of embryos that are free of the disorder, may be available to would-be parents in the near future.
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The DNA test is able to analyze the structure of the fragile X gene in great detail. DNA is like a twisted ladder and the rungs of the ladder carry the mutation or the change in the DNA. There is an excessive number of chemical repeats, called CGG repeats, down the rungs of the DNA ladder where the fragile X gene is located. All people have the fragile X gene and it normally produces the fragile X protein, which is very important for normal brain development. Most people have between 5 to 44 CGG repeats in their fragile X gene. Carriers, on the other hand, have between 55 to 200 repeats.
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