Dept. of Genetic Engineering and Biotechnology
Roll-10, 3rd year
Fragile X Syndrome
Fragile X syndrome (FXS) is the most common known cause of inherited intellectual disability.
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. It is also called Martin–Bell syndrome, Marker X syndrome
& in South American countries it is most commonly known as Escalante's syndrome.
FXS is an X-linked disorder. It is a single gene disorder carried by either a mother or a father.
Impairmentin this disease canrange 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 also can include characteristic
physical and behavioral features and delays in speech and language development.
It is the first known disease of trinucleotide repeat disorder. Fragile X syndrome is associated
with the expansion of the CGG trinucleotide repeat affecting the Fragile X mental retardation 1
(FMR1) gene on the X chromosome, resulting in a failure to express the fragile X mental
retardation protein (FMRP), which is required for normal neural development.
WHY IT IS CALLED FRAGILE X SYNDROME
A fragile site is a heritable point in a chromosome that tends to
constrict and thus may tends to break under replication stress.
In FXS, there is a rare, folic acid-sensitive site at Xq27.3. Unstable CGG repeat in the tip of the long arm of X chromosome may
cause constriction which gives the impression that the tip is
ready to detach from the rest of the chromosome. Thus it the
name Fragile X Syndrome comes.
TIMELINE OF FXS
1943- J. Purdon Martin and Julia Bell discovered that a kind of mental retardation was linked to
X chromosome from a pedigree analysis. Later this disease was named as FXS.
1969- Herbert Lubs discovered an unusual "marker X chromosome" in affected male. He also
developed a chromosomal test for Fragile X in order to accurately identify affected patients.
1970- Herbert Lubs’s test was used worldwide.Frederick Hecht coined the term "fragile site".
1991- Scientists discovered the FMR1 (fragile X mental retardation 1) gene that causes Fragile X
Based on the best available evidence:
Approximately one-million Americans carry the Fragile X mutation, including approximately 100,000 with fragile X syndrome, and are at risk for developing a Fragile Xassociated Disorder.
Approximately 1 in 3600 to 4000 male in the world is born with the full mutation for
Approximately 1 in 4000 to 6000 females in the world is born with the full mutation for
Fragile X. Approximately 1 in 800 men in the world is carriers of the Fragile X premutation.
Approximately 1 in 260 women in the world are carriers of the Fragile X premutation.
The vast majority of males with the full mutation will have fragile X syndrome. Approximately
50% of females with the full mutation will have some features of fragile X syndrome. The reason it is lower in females is that, while all males with an FMR1 full mutation will have fragile X
syndrome, some females with an FMR1 full mutation will not have behavioral, cognitive or
physical features of FXS. FXS is also known as the most common single gene cause of autism.
Between 2-6 percent of all children diagnosed with autism are reportedly linked with the Fragile X gene mutation. Concurrently, approximately one-third of all children diagnosed with fragile X syndrome also have autism.
Fragile X Syndrome is an X-linked dominant condition with reduced penetration (referring to its
partially recessive nature). FXS is an "X-linked" condition, meaning that the gene is on the X
chromosome. It is inherited in dominant pattern meaning that only one copy of mutated gene
is sufficient to cause the condition. Since a woman has two X chromosomes a female with a
premutation or full mutation has a 50% chance of passing on the X with the mutation in each
pregnancy. If she has a premutation, and it is passed on to offspring, it can remain a premutation or it can expand to a full mutation. If she has a full mutation and it is passed on, it will remain a full mutation. Additionally, in other X-linked conditions all males who carry the gene are
affected, however in Fragile X Syndrome, unaffected males can carry the gene in the premutation form and have no symptoms of Fragile X Syndrome. Males with the premutation will pass it
on to all of their daughters and none of their sons. 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. Chances of expanding are related to the size of the mother’s premutation. By
contrast, the premutation in men does not expand to more than 200 repeats during meiosis as
it is passed to the next generation. Men pass the premutation only to their daughters as full
mutation is lost during spermatogenesis. Their sons receive a Y chromosome, which does not
include the FMR1 gene.
This is the display of a punnet square involving the gene that causes Fragile X Syndrome. The
simulation illustrates what could potentially happen if either a mother or a father were carrying
the FMR1 gene and then passed it on to their offspring. In Diagram A, the father has the FMR1
gene which he then passes on to 50% of his offspring, two daughters. In Diagram B, the mother
has the FMR1 gene which she passes on to 50% of her offspring, one girl and one boy.
CHANGES IN THE FMR1 GENE
Fragile X syndrome is a genetic disorder which occurs as a result of a mutation of the fragile X
mental retardation 1 (FMR1) gene on the X chromosome, most commonly an increase in the
number of CGG trinucleotide repeats in the 5' untranslated region of FMR1. 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.FMR1 gene has four allelic forms which aredistinguished
by the number of CGG repeat present in the gene. In unaffected individuals, the FMR1 gene
contains 5-44 repeats of the CGG codon, most commonly 29 or 30 repeats. 45 to 54 repeats are
considered as "grey zone", with a premutation allele generally considered to be between 50
and 200 repeats in length. Fragile X syndrome patient have a full mutation of the FMR1 allele,
with over 200 repeats of the CGG. Premutation individual carry an unstable mutation which
can expand in future generations. The full mutation causes the gene to shut down a methylated
region of the FMR-1 gene. Premuations display few or no symptoms of FXS while the full mutation significantly magnifies symptoms. Normally, the FMR-1 gene produces an important protein called FMRP. When the gene is turned off, the individual does not make fragile X mental
retardation protein (FMRP). The lack of this specific protein is believed to cause Fragile X Syndrome. 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.
FMRP-MOLECULAR BASIS OF FXS
Full mutation in FMR1 gene prevents production of FMRP. FMRP is found in highest concentrations in the brain and testes. It is a translational regulator and regulates local protein synthesis
in dendrites and synapses. It appears to bind selectively to around 4% of mRNA that function at
many synapses in mammals. These mRNA targets have been found to be located in the neuronal dendrites and brain tissue from FXS patients shows abnormal dendritic spines. Also FMRP has
been found to regulate many signaling pathway such as mGluR pathway, dopamine pathway,
GABA pathway. Recent research on mGluR pathway shows that lack of FMRP is partially responsible for the symptoms caused in FXS. FMRP negatively regulate protein synthesis stimulated by mGluR activation. When FMRP is not produced, mGluR signaling causes increased syn-
thesis of those proteins which are responsible for causing long term depression (LTD). The absence of FMRP in dopamine and GABA pathways causes attention deficit, hyperactivity problems and anxiety symptoms found in FXS.
SIGNS AND SYMPTOMS
Behavior problems associated with fragile X syndrome includes:
o Delay in crawling, walking, or twisting
o Hand clapping or hand biting
o Hyperactive or impulsive behavior
o Mental retardation
o Speech and language delay
o Tendency to avoid eye contact
o Learning disability (Low IQ in male)
o Large, protruding ears
o Long face
o High-arched palate
o Hyper extensible finger joints
o Double-jointed thumbs
o Flat feet
o Soft skin
o Post-pubescent macroorchidism (Larger testes in men after puberty)
o Hypotonia (low muscle tone)
o Higher risk of developing partial seizures(14% male and 6% female have seizure)
o A key neurological feature found in FXS patients is, in certain areas of their brain
neurons have immature and dense dendritic spines. The dendrites are sites at
which the majority of excitatory synapses occur. Similar abnormalities are found
in other mental retardation.
o Fragile X associated tremor/ataxia syndrome (FXTAS)- in premutation individual
o Fragile X related primary ovarian insufficiency (FXPOI)- 20% of premutation female individual
o Attention deficit hyperactivity disorder (ADHD)- majority of males with FXS and
Determine the number of CGG repeat but can’t detect methylation pattern. Both
premutation and full mutation can be detected.
Southern blot of DNA
Determine both number of CGG repeat and methylation pattern using a methylase enzyme. Both premutation and full mutation can be detected.
Willemsen et al developed a diagnostic test based on the use of Antibody to FMRP to
detect presence or absence of FMR protein in lymphocyte or hair root. This method can
develop full mutation in male but is not useful in mosaic males or female as some FMRP
Methylation specific melting curve analysis is a recently developed method.it is based
on the fact transcription error seen on the FMR1 gene results in hypermethylation (Dahl
et al-2007). It can be used in males only but allows rapid identification
FXS can be identified in babies before birth byo amniocentesis
o chorionic villus sampling
FXS due to missense mutations or deletions involving FMR1 requires sequencing of the FMR1
gene for identification
At this point of time there is currently no cure available to prevent FXS. However, there are alternatives towards helping patients improve their lives by assisting them come to terms with
their disability and overcome various obstacles. Management includes such approaches as
speech therapy, occupational therapy, and physical therapy. The expertise of psychologists,
special education teachers, and genetic counselors may also be beneficial. Drugs may be used
to treat hyperactivity, seizures, and other problems. Folic acid has been found to improve
hyperactivity and attention deficits in some pre-adolescent males with fragile X syndrome. Ritalin for ADD, Prozac for aggression, Melatonin for sleep disturbances, and Lithium carbonate for
mood instability can be used. Though these medicines help to prevent some of the symptoms,
however there are side effects. Establishing a regular routine, avoiding over stimulation, and
using calming techniques may also help in the management of behavioral problems. Mainstreaming of children with fragile X syndrome into regular classrooms is encouraged because
they do well imitating behavior. Peer tutoring and positive reinforcement are also encouraged.
Doctors must first evaluate the different characteristics of the patients before administrating a
long term treatment plan.
Recent studies have focused on a number of critical areas. The role of FMRP's RNA partners,
many of which have now been validated through in vitro assays, is of primary importance. Also
being examined is the function the various domains of FMRP and its translation of polyribosome on the mRNA as a messenger ribonucleoprotein, basically an RNA-binding protein, which
is still relatively unknown. Evidence from mouse models shows that mGluR5 antagonists can
rescue dendritic spine abnormalities and seizures, as well as cognitive and behavioral problems,
and may show promise in the treatment of FXS. Two new drugs, AFQ-056 (mavoglurant) and
dipraglurant, as well as the repurposed drug fenobam are currently undergoing human trials for
the treatment of FXS. There is also early evidence for the efficacy of arbaclofen, a GABAB agonist, in improving social withdrawal in individuals with FXS and ASD.Fragile X is also known as the
most prominent cause of autism; hopefully when FXS has been uncovered there could be more
scientific advances that could help to lessen or even possibly get rid of the effects of autism.
Although Fragile X Syndrome might appear to be a very rare genetic it is not too distant as one
might think. Nevertheless, it is more common than people think and misunderstood Syndrome.
Even if there is no cure it does not mean that people suffering from its affects can’t get better.
With better research, medication, and individual therapy being provided, scientists have been
able to assist patients in improving their standard of living by developing their motor skills and
mental capacity. Maybe in future gene therapy and protein supplement will finally be able to
cure this disease
Pg. 1……………………. A. Introduction
B. Name and Brief History
Pg. 2-3…………………. A. Incidence
B. Inheritance pattern
C. Changes in FMR1 gene
Pg. 3-4………………….. A. Changes in FMR1 gene
B. FMR protein
Pg. 5…………………….A. Signs and symptoms
Pg. 6…………………….. A. Diagnosis
Pg. 7…………………….. A. Treatment