What is Fragile-X syndrome?
Fragile X is a common inherited syndrome that causes intellectual disability. It affects one in every 2,000 males and one in every 6,000-8,000 females.
- Fragile-X syndrome is caused by a mutation in a single gene, the Fragile-X mental retardation (FMR1) gene, which is located on the X chromosome.
- Normally, this gene contains a stretch of DNA in which a three base sequence (CGG) is repeated up to about 30 times in a row.
- In people with Fragile-X syndrome this three base sequence of DNA is repeated over 200 times. This abnormal repetition of bases creates the microscopic appearance of a gap on the long arm of the X chromosome (see illustration below), hence the name, ‘fragile’-X syndrome.
- The FMR1 gene is responsible for providing instructions for making ‘Fragile-X mental retardation 1 protein’ (FMRP).
- This protein is required for the brain to develop normally. It plays a role in the development of synapses, specialised connections between our nerve cells which are critical for relaying nerve impulses.
- The Fragile-X mutation results in an FMR1 gene that is switched off, which means that FMRP is no longer produced, preventing normal neuronal development.
- Symptoms of Fragile-X syndrome are more severe in males compared with females. Because females have two X chromosomes, an inherited damaged FMR1 gene can be compensated for by an inherited healthy FMR1 gene. Unlike females, males only have one X chromosome, so an inherited damaged copy is their only copy.
- Fragile-X syndrome is the leading known genetic cause of autism (around 2-6 per cent of children with autism are diagnosed with Fragile-X syndrome).
- It is also the leading known cause of inherited intellectual disability.
Symptoms in boys
- Boys with Fragile-X syndrome have developmental delay, meaning they reach developmental milestones, such as sitting and talking, later than children without the condition.
- They show moderate to severe learning disabilities, combined with behavioural problems such as hyperactivity and autism.
- The physical signs of the disease become more pronounced as they get older. These include:
- a characteristic long face with jutting jaw
- large ears
- high forehead
- enlarged testicles
- flat feet
- unusually flexible finger joints.
Symptoms in girls
- Girls with Fragile-X syndrome have mild behavioural problems.
- 50 per cent of girls with the syndrome will show evidence of mental retardation.
- Like boys with the syndrome, girls tend to have large ears.
- Testing for Fragile-X syndrome is recommended if:
- one or more of the disease symptoms are evident in a child
- the mother is known to carry a pre- or full mutation in the FMR1 gene. A pre-mutation is a FMR1 gene with 55 to 200 repeats of the CGG triplet. Women with the pre-mutation have an increased risk of having a child with Fragile-X syndrome.
- if there is a history of unexplained mental retardation in the family.
- Testing is carried out by looking directly at the FMR1 gene in cells obtained from a blood sample. The number of repeated CGG triplets is counted within the FMR1 gene to identify if there are more than 200 repeats.
- Prenatal diagnosis testing is also possible to determine whether or not the baby has inherited the abnormal gene.
- There is currently no cure for Fragile-X syndrome.
- Drugs can be used to prevent seizures and treat some of the common behavioural problems.
- Children with Fragile-X syndrome often benefit from specialist education including occupational and speech therapy.
This page was last updated on 2016-01-25