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Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder characterized by mild-to-moderate intellectual disability. [1] The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled.
Silencing of the FMR1 gene in Fragile X syndrome. FMR1 co-localizes with a rare fragile site, visible here as a gap on the long arms of the X chromosome.. A chromosomal fragile site is a specific heritable point on a chromosome that tends to form a gap or constriction and may tend to break [1] when the cell is exposed to partial replication stress. [2]
Fragile X syndrome is caused by expansion of CGG repeats in the FMR1 gene. In males without fragile X syndrome, the CGG repeat number ranges from 53 to 200 while those affected have greater than 200 repeats of this trinucleotide sequence located at the end of the X chromosome on band Xq28.3.1. [36]
Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder most frequently seen in male premutation carriers of Fragile X syndrome (FXS) over the age of 50. [4] [5] The main clinical features of FXTAS include problems of movement with cerebellar gait ataxia and action tremor.
FMR1 (Fragile X Messenger Ribonucleoprotein 1) is a human gene [5] that codes for a protein called fragile X messenger ribonucleoprotein, or FMRP. [6] This protein, most commonly found in the brain, is essential for normal cognitive development and female reproductive function.
The first trinucleotide repeat disease to be identified was fragile X syndrome, which has since been mapped to the long arm of the X chromosome. Patients carry from 230 to 4000 CGG repeats in the gene that causes fragile X syndrome, while unaffected individuals have up to 50 repeats and carriers of the disease have 60 to 230 repeats.
Fragile X syndrome is so named because of the appearance of the X chromosome in individuals with fragile X. Under an electron microscope, a region on the long arm of the chromosome resembles a thin string. Investigation showed that this region consists of a CGG repeat triplet in both normal and diseased individuals. [1]
When the repeat is present in an untranslated region, it could affect the expression of the gene in which the repeat is found (ex. fragile X) or many genes through a dominant negative effect (ex. myotonic dystrophy). [citation needed] In order to have a deleterious effect, the number of repeats must cross a certain threshold.