Fragile X Syndrome

Abstract

Fragile X syndrome is an X-linked dominant hereditary condition, which results in mental deficits and developmental delays. This disorder is sometimes referred to as Martin-Bell syndrome. It was first described by Martin and Bell, who in 1943 investigated a large family, in which multiple male relatives had mental deficits. After Down's syndrome, fragile X syndrome is the next most common cause of developmental delays and mental deficiencies. The name fragile X was given to this syndrome because the X chromosome has a broken appearance under the microscope.

In fragile X syndrome, there is a defect in FMR1 (fragile X mental retardation -1) gene, which is located on X chromosome. This defective gene carries an abnormally high number of CGG trinucleotides. The increased number of the trinucleotides in the coding gene renders the protein product of the gene inactive. Normally a FMR1 gene contains less than 54 of CGG repeats. If the number of repeats is increased from 55 to 200, it is considered a premutation; and if the number of repeats becomes greater than 200, the full mutation occurs. Fragile X chromosome syndrome affects all racial/ethnic groups equally. The incidence of this disorder in the USA is estimated to be 1 in 4000-6000 males and 1 in 8000 females. The incidence rates in other countries is not known, however some data from Europe indicate that the rates may me similar to those in the US.

Read full article for:

• Clinical features
• Characteristic features of premutation and full mutation
• Diagnosis and genetic testing for fragile X syndrome
• Fragile X tremor-ataxia syndrome in adults
• Treatment options

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