Fragile X syndrome Health Article

Definition

Fragile X syndrome is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited mental retardation in males and a significant cause of mental retardation in females.

Alternative Names

Martin-Bell syndrome; Marker X syndrome

Causes, incidence, and risk factors

Fragile X syndrome is caused by a change in the FMR1 gene. A small section of the gene code (three letters only -- CGG) is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem.

Normally, the FMR1 gene makes a protein needed for your brain to grow properly. A defect in this gene makes your body produce too little of the protein, or none at all.

Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely. You can have Fragile X syndrome even if your parents do not have it.

Symptoms

• Hyperactive behavior
• Large body size
• Large forehead or ears with a prominent jaw
• Large testicles (macro-orchidism) after the beginning of puberty
• Mental retardation
• Tendency to avoid eye contact

Family members who have fewer repeats in the FMR1 gene may not have mental retardation, but may have other problems. Women with less severe changes may have premature menopause or difficulty becoming pregnant. Both men and women may have problems with tremors and poor coordination.

Signs and tests

The person will have a family history of Fragile X syndrome (especially a male relative).

There are very few outward signs of Fragile X syndrome in babies. Some signs may include:

• Large head circumference in babies
• Mental retardation
• Oversized testes in males who have reached puberty
• Subtle differences in facial characteristics

In females, excess shyness may be the only sign of the problem.

A genetic test called polymerase chain reaction (PCR) is used to diagnose this disease. This test looks for a mutation (called a triplet repeat) in the FMR1 gene.

In the past, a specific type of chromosome analysis was done. Such testing may still be available.