FRAGILE X

What is Fragile X?

Fragile X is the most common inherited form of mental handicap and recent genetic studies conclude that as many as one in every 1,000 children born has the problem. It occurs more in boys than girls and is associated with varying degrees of learning difficulty.

As it is a genetically inherited condition, when one child in a family is diagnosed fragile X, there are enormous implications for the brothers and sisters of that child and indeed for the cousins and second cousins too.

Until quite recently nothing at all was known about fragile X. It was first seen under the microscope in 1969, but was not associated with the particular problems of delayed development and behavioural difficulties until 1977.

How is it caused?

The condition is transmitted on the X chromosome, one of the chains of genetic material controlling inherited characteristics. Under a microscope a normal X chromosome looks like a cross, but a fragile X chromosome has an abnormal appearance at the tip. It looks as if it is broken off but not quite separated.

Why is it more common in boys than in girls?

The X chromosome is one of a pair which decides a child's sex. A boy has an X and a Y chromosome while a girl has two X chromosomes. If a girl has one "fragile" X she also has a "good" one. And it seems that the "good" chromosome usually overcomes the effects of the "fragile" one. In this way girls may unknowingly pass fragile X on to the next generation.

A few men with a "fragile" X do not show any problems but pass it on to their daughters. Such daughters will usually be of normal intelligence, but their children are at great risk of being affected.

How does Fragile X affect a child?

Boys who are affected almost always have some learning difficulty, ranging from moderate learning difficulties to severe handicap. Girls are often of normal intelligence, but up to a third have learning problems which may be mild or moderate but are occasionally severe.

The behavioural features may include delayed speech development and continuing speech difficulties, repetitive behaviour, hyperactivity and autistic-like features, such as poor eye contact and handclapping. Some sufferers may develop epilepsy.

There are physical features ascribed to fragile X which include a long narrow face with prominent bones and ears, but these are rarely obvious in affected children. It is because of this lack of obvious physical abnormalities that diagnosis is often missed or delayed.

The importance of diagnosis

The importance of diagnosing the fragile X child early is twofold. Firstly, there are many relatives who are at risk of having handicapped children and who could benefit from genetic counselling, carrier testing and the offer of prenatal diagnosis.

Secondly, the child concerned can receive specialist help and treatment. In the UK, fragile X children are sometimes not identified until their teens, and consequently precious time is lost when specialist help in the areas of education, speech and language, etc. could have been given or an appropriate programme of behaviour modification undertaken. For example, fragile X children tend to have strengths and weaknesses within their mental ability, and this information is important in the assessment of special educational needs and has implications for the planning of a remedial education programme.

The discovery, in May 1991, of the gene that causes fragile X is leading to the development of more reliable tests which will be used for identification of affected individuals and carriers as well as for pre-natal diagnosis.