X-linked recessive inheritance
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FRAXA syndrome

Fragile X Syndrome is the most common inherited cause of mental retardation, and is associated with autism. more...

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Causes

The fragile X syndrome is a genetic disorder caused by mutation of the FMR1 gene on the X chromosome. Mutation at that site is found in 1 out of about every 2000 males and 1 out of about every 4000 females.

Normally, the FMR1 gene contains between 6 and 53 repeats of the CGG codon (trinucleotide repeats). In people with the fragile X syndrome, the FMR1 allele has over 230 repeats of this codon.

Expansion of the CGG repeating codon to such a degree results in a methylation of that portion of the DNA, effectively silencing the expression of the FMR1 protein.

This methylation of the FMR1 locus in chromosome band Xq27.3 is believed to result in constriction and fragility of the X chromosome at that point, a phenomenon that gave the syndrome its name.

The mutation and methylation of the FMR1 gene lead to the transcriptional silencing of the fragile X-mental retardation protein, FMRP. In normal individuals, FMRP binds and facilitates the translation of a number of essential neuronal RNAs. In fragile X patients, however, these RNAs are not translated into proteins. The various sequelae of fragile X syndrome result.

Transmission of the Fragile X

The diagram (above) of X-linked recessive inheritance is not entirely inappropriate but it markedly oversimplifies the situation and does not provide a sufficient foundation for genetic counseling with the fragile X syndrome.

Because males normally have only one copy of the X chromosome, those males with significant trinucleotide expansion at the FMR1 locus are symptomatic. They are mentally retarded and may show various physical features of the fragile X syndrome.

Females have two X chromosomes and thus have double the chance of having a working FMR1 allele. Females carrying one X chromosome with an expanded FMR1 gene can have some signs and symptoms of the disorder or be normal.

Males with the fragile X cannot transmit it to any of their sons (since males contribute a Y chromosome, not an X, to their male offspring.)

Females carrying one copy of the fragile X can transmit it to their sons or daughters. Sons who receive the fragile X are at high risk for mental retardation. Daughters who receive the fragile X may appear normal or they may be mentally retarded, usually to a lesser degree than boys with the syndrome.

Symptoms

Aside from mental retardation, prominent characteristics of the syndrome include an elongated face, large or protruding ears, large testicles (macroorchidism), and low muscle tone. Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) and atypical social development, particularly shyness and limited eye contact. Some individuals with the fragile X syndrome also meet the diagnostic criteria for autism.

Read more at Wikipedia.org


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Something's not quite right - FRAXA Research Foundation
From Pediatrics for Parents, 9/1/03 by Debra Borchert

Developmental delays and learning disabilities come in as many different forms as there are children. As we learn to understand each, it's important to have conditions correctly diagnosed so children can benefit from early intervention and therapies that will help them grow to their fullest potentials.

Fragile X Syndrome

You've probably never heard of the most common form of inherited mental impairment, fragile X syndrome. Because of its recent discovery in 1991, many doctors and pediatricians haven't heard of it either. Fragile X was the first gene uncovered in the Human Genome Project. Researchers estimate that fragile X affects approximately one in every 2,000 boys and one in every 4,000 girls. But even today, many people with fragile X are still not correctly diagnosed.

Completely Typical At Birth

Most children with fragile X appear completely normal at birth, but gradually they experience developmental delays. As the delays become evident, parents grow confused.

"My daughter still isn't sitting up by herself."

"He's still not talking at 18 months."

"He cries all the time and I feel like a bad parent."

Frustrated, parents visit specialist after specialist in hope of discovering the cause for their children's delayed development.

What's wrong?

Fragile X syndrome is a genetic condition caused by a single gene whose code is changed. The defect causes a disruption between the sending and receiving of messages required for proper brain development and functioning. When this gene is altered, it can cause developmental delays and mild to severe learning disabilities, including mental retardation.

Characteristics

Children may have some or none of the common characteristics, which contributes to the difficulty in diagnosing fragile X.

Some of the most common physical characteristics include:

* Eye & vision impairments

* Elongated face

* Flat feet

* High arched palate

* Hyper-extensible joints (double jointed)

* Large testicles (evident after puberty)

* Low muscle tone

* Prominent ears

Some of the most common behavioral characteristics include:

* Anxiety & shyness

* Autism & autistic-like behavior

* Hand biting & hand-flapping

* Hyperactivity & short attention spans

* Language delays

* Perseveration--repetition of the same actions or words

* Poor eye contact

Testing

Because children with fragile X have normal births, and parents are unaware of this condition, affected families may have more than one fragile X child. Since the gene can pass silently through generations, undetected, and then affect a child, anyone with undiagnosed mental retardation in his or her family should be tested.

A simple DNA blood test is available to determine if a child is affected or if a person carries the fragile X gene. A carrier is a man or woman who may show no signs of impairment, but whose gene changes are passed on to their children. One in every 260 women carries this gene and with each pregnancy she has a 50 percent chance of passing it on. The same test is used for prenatal diagnosis.

Treatment

Currently, only the symptoms of fragile X can be treated. Early intervention programs involving occupational, physical, speech, and sensory integration therapies are most often helpful.

Research

Research on treating the "root cause" of fragile X is rapidly moving forward. Only one protein is missing with the syndrome, so compared to other conditions that involve many other proteins and genes, solving the problem of fragile X syndrome is relatively simple. Research on this gene contributes to finding cures and treatments to other conditions such as Downs syndrome, autism, and Alzheimer's.

FRAXA Research Foundation

The FRAXA Research Foundation was founded to support scientific research aimed at finding a treatment and a cure for fragile X syndrome. With Nobel Prize winning scientists, Dr. James Watson and Dr. Eric Kandel, serving on the advisory board of the FRAXA Research Foundation, there is great hope for future generations.

For more information, contact: The National Fragile X Association 800-688-8765 www.fragilex.org

FRAXA Research Foundation

978-462-1866

Web site: www.fraxa.org

Debra Borchert is author of the soon-to-be-published, Fragile Secret, about her life with her three siblings who are affected by fragile X syndrome. She can be reached by e-mail at debra@fragilesecret.com

COPYRIGHT 2003 Pediatrics for Parents, Inc.
COPYRIGHT 2004 Gale Group

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