May 18, 2009
Fragile X syndrome is a genetic condition involving mutations in a gene on the X chromosome. Fragile X syndrome is the most common form of inherited mental retardation in males. and a significant cause of mental retardation in females.
Fragile X syndrome is caused by mutations in the FMR1 gene on the X chromosome at the location q27.3. In the FMR1 gene, there is a particular region that contains a DNA segment of three nucleotides (the building blocks of DNA) that are repeated multiple times. These repeats are called CGG trinucleotide repeats.
The normal number of CGG repeats in the FMR1 gene ranges from 5 to 40.
In people with fragile X syndrome, the CGG segment is repeated more than 200 times.
Males and females with 41 to 55 repeats are said to be in the "grey zone" (also called intermediate range).
- No consensus exists regarding the precise size of repeats that should be considered intermediate. Some researches define the "grey zone" as 41 to 58 repeats.
- Some studies have suggested that females with over 35 repeats are at risk to develop premature ovarian failure, in which menstrual periods stop by age 40
Males and females with 55 to 200 repeats of the CGG segment are said to have an FMR1 premutation.
- Most people with a FMR1 premutation are intellectually normal.
- In some cases, however, people with a FMR1 premutation have lower than normal amounts of the fragile X mental retardation 1 protein. As a result, they may have mild versions of the physical features seen in fragile X syndrome (such as prominent ears) and may experience emotional problems such as anxiety or depression. Some children with a premutation may have learning disabilities or autistic-like behavior.
- About 20% of females with a FMR1 premutation have premature ovarian failure.
- Males, and some females, with a FMR1 premutation have an increased risk of developing a disorder known as fragile X-associated tremor/ataxia syndrome (FXTAS). This disorder is characterized by progressive problems with movement (ataxia), tremor, memory loss, loss of sensation in the lower extremities (peripheral neuropathy), and mental and behavioral changes.
For more information about Fragile X syndrome visit: http://AccessDNA.com/condition/Fragile_X_Syndrome/153