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Fragile X syndrome
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Fragile X syndrome
Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder. The average IQ in males with FXS is under 55, while affected females tend to be in the borderline to normal range, typically around 70–85. Physical features may include a long and narrow face, large ears, flexible fingers, and large testicles. About a third of those affected have features of autism such as problems with social interactions and delayed speech. Hyperactivity is common, and seizures occur in about 10%. Males are usually more affected than females.
This disorder and finding of fragile X syndrome has an X-linked dominant inheritance. It is typically caused by an expansion of the CGG triplet repeat within the FMR1 (fragile X messenger ribonucleoprotein 1) gene on the X chromosome. This results in silencing (methylation) of this part of the gene and a deficiency of the resultant protein (FMRP), which is required for the normal development of connections between neurons. Diagnosis requires genetic testing to determine the number of CGG repeats in the FMR1 gene. Normally, there are between 5 and 40 repeats; fragile X syndrome occurs with more than 200. A premutation is said to be present when the gene has between 55 and 200 repeats; females with a premutation have an increased risk of having an affected child. Testing for premutation carriers may allow for genetic counseling.
There is no cure or approved medications for treating the condition. Early intervention is recommended, as it provides the most opportunity for developing a full range of skills. These interventions may include special education, occupational therapy, speech therapy, physical therapy, or behavioral therapy. Medications may be used to treat associated anxiety, seizures, mood problems, aggressive behavior, or ADHD. Fragile X syndrome tends to show more symptoms on affected males since females have another X chromosome which can compensate for the damaged one.
Fragile X syndrome is the most common cause of inherited intellectual disability, affecting an estimated 1 per 4,000 people. It is also the most common single gene cause of autism.
Most young children do not show any physical signs of FXS. It is not until puberty that physical features of FXS begin to develop. Aside from intellectual disability, prominent characteristics of the syndrome may include an elongated face, large or protruding ears, flat feet, larger testes (macroorchidism), and low muscle tone. Recurrent otitis media (middle ear infection) and sinusitis is common during early childhood. Speech may be cluttered or nervous. Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) 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 due to variability in X-inactivation.
Individuals with FXS may present anywhere on a continuum from learning disabilities in the context of a normal intelligence quotient (IQ) 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 unaffected.
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.
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Fragile X syndrome
Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder. The average IQ in males with FXS is under 55, while affected females tend to be in the borderline to normal range, typically around 70–85. Physical features may include a long and narrow face, large ears, flexible fingers, and large testicles. About a third of those affected have features of autism such as problems with social interactions and delayed speech. Hyperactivity is common, and seizures occur in about 10%. Males are usually more affected than females.
This disorder and finding of fragile X syndrome has an X-linked dominant inheritance. It is typically caused by an expansion of the CGG triplet repeat within the FMR1 (fragile X messenger ribonucleoprotein 1) gene on the X chromosome. This results in silencing (methylation) of this part of the gene and a deficiency of the resultant protein (FMRP), which is required for the normal development of connections between neurons. Diagnosis requires genetic testing to determine the number of CGG repeats in the FMR1 gene. Normally, there are between 5 and 40 repeats; fragile X syndrome occurs with more than 200. A premutation is said to be present when the gene has between 55 and 200 repeats; females with a premutation have an increased risk of having an affected child. Testing for premutation carriers may allow for genetic counseling.
There is no cure or approved medications for treating the condition. Early intervention is recommended, as it provides the most opportunity for developing a full range of skills. These interventions may include special education, occupational therapy, speech therapy, physical therapy, or behavioral therapy. Medications may be used to treat associated anxiety, seizures, mood problems, aggressive behavior, or ADHD. Fragile X syndrome tends to show more symptoms on affected males since females have another X chromosome which can compensate for the damaged one.
Fragile X syndrome is the most common cause of inherited intellectual disability, affecting an estimated 1 per 4,000 people. It is also the most common single gene cause of autism.
Most young children do not show any physical signs of FXS. It is not until puberty that physical features of FXS begin to develop. Aside from intellectual disability, prominent characteristics of the syndrome may include an elongated face, large or protruding ears, flat feet, larger testes (macroorchidism), and low muscle tone. Recurrent otitis media (middle ear infection) and sinusitis is common during early childhood. Speech may be cluttered or nervous. Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) 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 due to variability in X-inactivation.
Individuals with FXS may present anywhere on a continuum from learning disabilities in the context of a normal intelligence quotient (IQ) 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 unaffected.
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.
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