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What is fragile X
syndrome?
Fragile X syndrome
is a genetic condition that causes a range of developmental problems including
learning disabilities and cognitive impairment. Usually, males are more
severely affected by this disorder than females.
Affected
individuals usually have delayed development of speech and language by age 2.
Most males with fragile X syndrome have mild to moderate intellectual
disability, while about one-third of affected females are intellectually
disabled. Children with fragile X syndrome may also have anxiety and
hyperactive behavior such as fidgeting or impulsive actions. They may have
attention deficit disorder (ADD), which includes an impaired ability to
maintain attention and difficulty focusing on specific tasks. About one-third
of individuals with fragile X syndrome have features of autism spectrum
disorders that affect communication and social interaction. Seizures occur in
about 15 percent of males and about 5 percent of females with fragile X
syndrome.
Most males and
about half of females with fragile X syndrome have characteristic physical
features that become more apparent with age. These features include a long and
narrow face, large ears, a prominent jaw and forehead, unusually flexible
fingers, flat feet, and in males, enlarged testicles (macroorchidism) after
puberty.
How common is fragile X syndrome?
Fragile X syndrome
occurs in approximately 1 in 4,000 males and 1 in 8,000 females.
What genes are related to fragile X syndrome?
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, or FMRP.
This protein helps regulate the production of other proteins and 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 FMRP. Loss or a shortage (deficiency) of
this protein disrupts nervous system functions and leads to the signs and
symptoms of fragile X syndrome.
Males and females
with 55 to 200 repeats of the CGG segment are said to have an FMR1 gene
premutation. Most people with a premutation are intellectually normal. In some
cases, however, individuals with a premutation have lower than normal amounts
of FMRP. 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. The premutation is also
associated with an increased risk of disorders called fragile X-associated
primary ovarian insufficiency (FXPOI) and fragile X-associated tremor/ataxia
syndrome (FXTAS).
Read more about the FMR1 gene.
How do people inherit fragile X syndrome?
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. X-linked dominant means that in
females (who have two X chromosomes), a mutation in one of the two copies of a
gene in each cell is sufficient to cause the disorder. In males (who have only
one X chromosome), a mutation in the only copy of a gene in each cell causes
the disorder. In most cases, males experience more severe symptoms of the
disorder than females.
In women, the FMR1 gene
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.
Where can I find information about diagnosis or management of
fragile X syndrome?
These resources
address the diagnosis or management of fragile X syndrome and may include
treatment providers.
Where can I find additional information about fragile X
syndrome?
You may find the
following resources about fragile X syndrome helpful. These materials are
written for the general public.
You may also be
interested in these resources, which are designed for healthcare professionals
and researchers.
·
OMIM
-
Genetic disorder catalog
What other names do people use for fragile X syndrome?
- FRAXA
syndrome
- fra(X)
syndrome
- FXS
- marker
X syndrome
- Martin-Bell
syndrome
- X-linked
mental retardation and macroorchidism
What if I still have specific questions about fragile X
syndrome?
Where can I find
general information about genetic conditions?
The Handbook
provides basic information about genetics in clear language.
These links provide
additional genetics resources that may be useful.
What glossary definitions help with understanding fragile X
syndrome?
You may find
definitions for these and many other terms in the Genetics Home Reference Glossary.
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