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Fragile X syndrome is a genetic condition that causes a range of developmental problems including learning disabilities and mental retardation. Usually, males are more severely affected by this disorder than females.
Males and females with fragile X syndrome may have anxiety and hyperactive behavior such as fidgeting, excessive physical movements, or impulsive actions. They may also have attention deficit disorder, which includes an impaired ability to maintain attention and difficulty focusing on specific tasks. About one-third of males with fragile X syndrome also have autism or autistic-like behavior that affects communication and social interaction. Seizures occur in about 15 percent of males and about 5 percent of females with fragile X syndrome.
Many males with fragile X syndrome have characteristic physical features that become more apparent with age. These features include a long and narrow face, large ears, prominent jaw and forehead, unusually flexible fingers, and enlarged testicles (macroorchidism) after puberty.
Fragile X syndrome occurs in approximately 1 in 4,000 males and 1 in 8,000 females.
Mutations in the FMR1 gene cause fragile X syndrome.
The FMR1 gene provides instructions for making a protein called fragile X mental retardation 1 protein, whose function is not fully understood. This protein likely plays a role in the development of synapses, which are specialized connections between nerve cells. Synapses are critical for relaying nerve impulses.
Nearly all cases of fragile X syndrome are caused by a mutation in which a DNA segment, known as the CGG triplet repeat, is expanded within the FMR1 gene. Normally, this DNA segment is repeated from 5 to about 40 times. In people with fragile X syndrome, however, the CGG segment is repeated more than 200 times. The abnormally expanded CGG segment turns off (silences) the FMR1 gene, which prevents the gene from producing fragile X mental retardation 1 protein. Loss or a shortage (deficiency) of this protein disrupts nervous system functions and leads to the signs and symptoms of fragile X syndrome.
In a small percentage of cases, other types of mutations cause fragile X syndrome. These mutations delete part or all of the FMR1 gene, or change one of the building blocks (amino acids) used to make the fragile X mental retardation 1 protein. As a result, no protein is produced, or the protein's function is impaired because its size or shape is altered.
Males and females with 55 to 200 repeats of the CGG segment are said to have an FMR1 premutation. Most people with a premutation are intellectually normal. In some cases, however, individuals with a premutation have lower than normal amounts of the fragile X mental retardation 1 protein. As a result, they may have mild versions of the physical features seen in fragile X syndrome (such as prominent ears) and may experience emotional problems such as anxiety or depression. Some children with a premutation may have learning disabilities or autistic-like behavior. About 20 percent of women with a premutation have premature ovarian failure, in which menstrual periods stop by age 40. Men, and some women, with a premutation have an increased risk of developing a disorder known as fragile X-associated tremor/ataxia syndrome (FXTAS). This disorder is characterized by progressive problems with movement (ataxia), tremor, memory loss, loss of sensation in the lower extremities (peripheral neuropathy), and mental and behavioral changes.
Fragile X syndrome is inherited in an X-linked dominant pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. (The Y chromosome is the other sex chromosome.) The inheritance is dominant if one copy of the altered gene in each cell is sufficient to cause the condition.
In women, the FMR1 premutation on the X chromosome can expand to more than 200 CGG repeats in cells that develop into eggs. This means that women with the premutation have an increased risk of having a child with fragile X syndrome. By contrast, the premutation in men does not expand to more than 200 repeats as it is passed to the next generation. (Men pass the premutation only to their daughters. Their sons receive a Y chromosome, which does not include the FMR1 gene.)
'''Fragile X Syndrome''' is the most common inherited cause of mental impairment, and the most common known cause of autism. Fragile X syndrome is a genetic disorder caused by a mutation of the FMR1 gene on the X chromosome, a mutation found in 1 out of every 2000 males and 1 out of every 4000 females. Typically the FMR1 gene contains between 6 and 53 repeats of the CGG codon. In people with the disorder, the FMR1 allele has over 230 repeats. Expansion to such a degree results in a methylation of that portion of the DNA, effectively silencing the expression of the FRM1 protein. The characteristic constriction of the X chromosome at the chromosomal locus Xq27.3 is caused by this methylation. Because men have only one copy of the X chromosome, males with sufficient trinucleotide expansion are symptomatic, while females have two X chromosomes and thus double the chance of a working allele (unless parents are related). Aside from mental retardation, the most obvious indicators include physical differences and behaviors commonly associated with autism. Of the former, the most readily visible are an elongated face and large or protruding ears, but others are also frequently present. Of the latter, behavioral stereotypy and atypical social development are the most frequently observed. While there is no current cure for the syndrome, there is hope that further understanding of the FMR1 gene would allow to counteract the underlying genetic cause. Currently, the syndrome can be treated through behavioral therapy, special education, and when necessary, treatment of physical abnormalities. Persons with Fragile X in their family histories are advised to seek genetic counseling to assess the likelihood of having children who are affected, and how severe any impairments may be in affected descendants. [ Read More ]
Fragile X Association of Southern California - Fragile X Syndrome is the leading inherited cause of developmental disabilities and mental impairment worldwide. It affects 1 in 2,000 males and 1 in 4,000 females. It is estimated that 1 in 259 females are carries of the premutation.
FRAXA Research Foundation Home Page - Non-profit organization run by parents. Fighting to find a cure for Fragile X Syndrome and helping Fragile X Family's.
Fragile X Syndrome - Diagnostic and Carrier Testing - A Policy Statement from the American College of Medical Genetics.
Fragile X Syndrome - Single page on recognition of the condition in young children.
Fragile X Fact's Page - Describing Fragile X Syndrome.
Carolina Fragile X Project - A series of studies examining the impact of fragile X syndrome (FXS) on individuals, families and the agencies that serve them.
Queensland Fragile X Association - About QFXA, news, events, links and contacts. Also information of fragile X syndrome.
Online Support Group - Support group for Family's who are dealing with Fragile X Syndrome.
Fact Sheets for Health Professionals: Fragile X Syndrome - From the Victorian Government, Australia.
Maryland Fragile X Resource Group - A community of families in and around the Washington-Baltimore metro area that have children affected by Fragile X Syndrome. Includes news, upcoming events, and resources.