Name: 脆弱X綜合症 (Fragile X syndrome)
Uploaded: 2021-01-14T05:33:14.000Z
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Fragile X syndrome, also known as Martin–Bell syndrome, or Escalante's syndrome, is a genetic

關係子句

syndrome that is the most widespread single-gene cause of autism and inherited cause of intellectual

disability especially among boys. It results in a spectrum of intellectual disabilities

ranging from mild to severe as well as physical characteristics such as an elongated face,

large or protruding ears, and large testes, and behavioral characteristics such as stereotypic

movements, and social anxiety. Fragile X syndrome is associated with the

expansion of the CGG trinucleotide repeat affecting the Fragile X mental retardation

1 gene on the X chromosome, resulting in a failure to express the fragile X mental retardation

protein, which is required for normal neural development. Depending on the length of the

CGG repeat, an allele may be classified as normal, a premutation, or full mutation. A

definitive diagnosis of fragile X syndrome is made through genetic testing to determine

the number of CGG repeats. Testing for premutation carriers can also be carried out to allow

for genetic counseling. The first complete DNA sequence of the repeat expansion in someone

with the full mutation was generated by scientists in 2012 using SMRT sequencing.

There is currently no drug treatment that has shown benefit specifically for fragile

X syndrome. However, medications are commonly used to treat symptoms of attention deficit

and hyperactivity, anxiety, and aggression. Supportive management is important in optimizing

functioning in individuals with fragile X syndrome, and may involve speech therapy,

occupational therapy, and individualized educational and behavioral programs.

Aside from intellectual disability, prominent characteristics of the syndrome may include

an elongated face, large or protruding ears, flat feet, larger testes, and low muscle tone.

Recurrent otitis media and sinusitis is common during early childhood. Speech may be cluttered

or nervous. Behavioral characteristics may include stereotypic movements and atypical

social development, particularly shyness, limited eye contact, memory problems, and

difficulty with face encoding. Some individuals with fragile X syndrome also meet the diagnostic

criteria for autism. Males with a full mutation display virtually

complete penetrance and will therefore almost always display symptoms of FXS, while females

with a full mutation generally display a penetrance of about 50% as a result of having a second,

normal X chromosome. Females with FXS may have symptoms ranging from mild to severe,

although they are generally less affected than males.

Physical phenotype Large, protruding ears

Hyperextensible finger joints Hyperextensible thumbs

Intellectual development Individuals with FXS may present anywhere

on a continuum from learning disabilities in the context of a normal intelligence quotient

to severe intellectual disability, with an average IQ of 40 in males who have complete

silencing of the FMR1 gene. Females, who tend to be less affected, generally have an IQ

which is normal or borderline with learning difficulties. The main difficulties in individuals

with FXS are with working and short-term memory, executive function, visual memory, visual-spatial

relationships, and mathematics, with verbal abilities being relatively spared.

Data on intellectual development in FXS are limited. However, there is some evidence that

standardized IQ decreases over time in the majority of cases, apparently as a result

of slowed intellectual development. A longitudinal study looking at pairs of siblings where one

child was affected and the other was not found that affected children had an intellectual

learning rate which was 55% slower than unaffected children.

When both autism and FXS are present, a greater language deficit and lower IQ is observed

as compared to children with only FXS. Autism

Fragile X syndrome co-occurs with autism in about 5% of cases and is a suspected genetic

cause of the autism in these cases. This finding has resulted in screening for FMR1 mutation

to be considered mandatory in children diagnosed with autism. Of those with fragile X syndrome,

prevalence of concurrent autism spectrum disorder has been estimated to be between 15 and 60%,

with the variation due to differences in diagnostic methods and the high frequency of autistic

features in individuals with fragile X syndrome not meeting the DSM criteria for an ASD.

Although individuals with FXS have difficulties in forming friendships, those with FXS and

ASD characteristically also have difficulties with reciprocal conversation with their peers.

Social withdrawal behaviors, including avoidance and indifference, appear to be the best predictors

of ASD in FXS, with avoidance appearing to be correlated more with social anxiety while

indifference was more strongly correlated to severe ASD. When both autism and FXS are

present, a greater language deficit and lower IQ is observed as compared to children with

only FXS. Genetic mouse models of FXS have also been

shown to have autistic-like behaviors. Social interaction

FXS is characterized by social anxiety, including poor eye contact, gaze aversion, prolonged

time to commence social interaction, and challenges forming peer relationships. Social anxiety

is one of the most common features associated with FXS, with up to 75% of males in one series

characterized as having excessive shyness and 50% having panic attacks. Social anxiety

in individuals with FXS is related to challenges with face encoding, the ability to recognize

a face that one has seen before. It appears that individuals with FXS are interested

in social interaction and display greater empathy than groups with other causes of intellectual

disability, but display anxiety and withdrawal when placed in unfamiliar situations with

unfamiliar people. This may range from mild social withdrawal, which is predominantly

associated with shyness, to severe social withdrawal, which may be associated with co-existing

autism spectrum disorder. Females with FXS frequently display shyness,

social anxiety and social avoidance or withdrawal. In addition, premutation in females has been

found to be associated with social anxiety. The size of DNA insertion is related to severity

of attention problems and withdrawal symptoms. Individuals with FXS show decreased activation

in the prefrontal regions of the brain. These regions are associated with social cognition.

Psychiatric Attention deficit hyperactivity disorder is

found in the majority of males with FXS and 30% of females, making it the most common

psychiatric diagnosis in those with FXS. Hyperactivity and disruptive behavior peak in the preschool

years and then gradually decline with age, although inattentive symptoms are generally

lifelong. Aside from the characteristic social phobia

features, a range of other anxiety symptoms are very commonly associated with FXS, with

symptoms typically spanning a number of psychiatric diagnoses but not fulfilling any of the criteria

in full. Behaviors such as hand flapping and biting, as well as aggression, can be an expression

of anxiety. Although only a minority will meet the criteria for obsessive-compulsive

disorder, a significant majority will feature obsessive-type symptoms. However, as individuals

with FXS generally find these behaviors pleasurable, unlike individuals with OCD, they are more

frequently referred to as stereotypic behaviors. Mood symptoms in individuals with FXS rarely

meet diagnostic criteria for a major mood disorder as they are typically not of sustained

duration. Instead, these are usually transient and related to stressors, and may involve

labile mood, irritability, self-injury and aggression.

Individuals with fragile X-associated tremor/ataxia syndrome are likely to experience combinations

of dementia, mood, and anxiety disorders. Males with the FMR1 premutation and clinical

evidence of FXTAS were found to have increased occurrence of somatization, obsessive–compulsive

disorder, interpersonal sensitivity, depression, phobic anxiety, and psychoticism.

Hypersensitivity and repetitive behavior Children with fragile X have very short attention

spans, are hyperactive, and show hypersensitivity to visual, auditory, tactile, and olfactory