Fragile X syndrome Dr Trisha Macnair Fragile X syndrome (or Martin-Bell syndrome) is the most common inherited cause of learning disability.

In this article

What causes it? What are the symptoms?

Who's affected? How is it diagnosed?

What's the treatment? Advice and support

In fragile X, one of the genes on the X chromosome is faulty. When normal, this gene makes a protein necessary for brain development, but when it's faulty little or none of the protein is made.

The gene that causes fragile X is called FMR1 (fragile X mental retardation 1) gene. A constriction can be detected near the tip of the X chromosome, which makes it fragile and susceptible to breaking.

Boys are usually more severely affected as they have only one X chromosome. Girls have a second X chromosome, which can be used instead of the faulty one, and may have only mild disabilities. They may also be carriers of the condition.

The main problem in fragile X is mental impairment. This can range from very minor, so that the person has a normal IQ and shows no sign of fragile X, to learning difficulties. How badly someone is affected depends on the degree of change in the gene.

Other symptoms include hyperactivity, attention deficit disorder, emotional and behavioural problems, anxiety and mood swings.

There may also be characteristic facial features, such as a long face and large ears. Other physical features include flat feet and hyperextensible joints.

Fragile X affects about one in 3,600 men and one in 4,000 to 6,000 women of all races and ethnic groups. It shows an X-linked recessive pattern of inheritance and changes in the gene can become more serious as it's passed from parent to child.

Some people have only a very minor change in the FMR1 gene (called premutation) and may have no, or minimal, fragile X symptoms. But those with full mutation, where larger changes in the gene exist, demonstrate more severe signs of the condition.

Despite being carriers, some men aren't affected (even though they have no normal copy of their X chromosome). But men who have the full mutation are almost always affected.

Premutation carriers (those who don't have the full mutation) don't have learning disabilities. In women with the full mutation, a third have a below-normal IQ, a third have a borderline-normal IQ and the remaining third have a normal IQ.

Fragile X can be diagnosed using a DNA test. Antenatal testing is possible using chorionic villus sampling or amniocentesis.

As yet there are no specific treatments or a cure for fragile X, but emotional and educational support for children and their families are vital to help them reach their best potential.

The Fragile X Society

Tel: 01371 875100
Website: www.fragilex.org.uk

This article was last medically reviewed by Dr Rob Hicks in April 2008