What is Fragile X Syndrome?

By Dr Ananya Mandal, MD

Fragile X syndrome is a genetic condition that shows typical physical attributes along with behavioural and developmental anomalies in a child. It is also called Martin Bell syndrome or a Marker X syndrome.

How is FXS inherited?

FXS is the most common cause of genetic learning disability. In this condition one of the genes on the X chromosome is faulty.

Each of the males possesses an X and a Y sex chromosome while females contain 2 X chromosomes in all their cells. The specific gene on the X chromosome makes a protein necessary for brain development. If it is faulty there is a deficiency of this protein.

The gene is called the FMR1 gene. The Faulty X chromosome is seen to have a band like constriction near the tip that appears as if the chromosome is broken off at the end.

Who is most affected by FXS?

Since boys have a single X chromosome, they are more severely affected than girls who may have one normal FMR1 gene on the second healthy X chromosome. Affected girls may have only mild disabilities. These girls however are carriers of the condition and they may pass it on to their male offspring.

Some affected persons may also have a small change or mutation and thus may be carriers. They pass on the full mutation to the next generation.

Diamgrammatic Representation of the Transmission Pattern of Fragile X

How common is FXS?

Fragile X affects about one in 3,600 men and one in 4,000 to 6,000 women. It affects all races and ethnicities. It may be carried by those with mild affliction of the FMR1 gene and may be passed on to the offspring.

In the UK, the overall prevalence of FXS in the general population is 2.3 per 10,000 or 1 in 4,425 population. Two to four times as many females as males are carriers of the defect. Only a third of females carry the abnormal gene responsible for learning disabilities.

Symptoms of FXS

The most common symptom of the FXS is intellectual impairment. There is a wide range of difference in IQ among sufferers. Some patients may have a normal IQ and shows no sign of fragile X and some may have severe learning difficulties. The degree of impairment is dependent on the extent of damage to the gene.

In addition to intellectual deficits there are also emotional and behavioural problems. This may include features of:

  • Attention deficit disorder
  • Hyperactivity
  • Impulsivity
  • Anxiety
  • Mood swings

There are typical facial features and physical attributes of FXS. These include long face and large ears, flat feet, high forehead, large testicles, large jaws, and extremely flexible joints.

Diagnosis of FXS

Fragile X can be diagnosed using a DNA analysis. DNA can be checked in an unborn fetus as well. This is done by taking a sample of the amniotic fluid or tissues from within the womb. This is called chorionic villus sampling or amniocentesis and can be performed at about 11 weeks of pregnancy.

Treatment of FXS

As of now there is no cure or treatment for FXS. Children with FXS need behavioural therapy, counselling and emotional and educational support. Children with delayed speech and language development need the help of speech and language therapists.

Behavioural therapy is useful for children with attention deficit, impulsivity and other behavioural problems. Some children cope in mainstream schools. However, those with impaired intelligence and learning difficulties may benefit from special schools to meet their special educational needs.

Reviewed by April Cashin-Garbutt, BA Hons (Cantab)

Sources

Last Updated: Nov 4, 2012

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