Fragile X is caused by a repeating trinucleotide triplet CGG. In the normal population, the number of CGG repeats is less than 55. The presence of clinical symptoms can be detected when the repeat is amplified beyond 55 repeats.
The three nucleotide sequence CGG is present in abnormally high amounts, resulting in signs and symptoms of fragile X. Clinical presentation varies depending on the amount of trinucleotide repeats present.
In patients with fragile X syndrome, the expanded CGG triplet repeats are hypermethylated and the expression of the FMR1 gene (fragile X mental retardation 1 gene) is repressed, which leads to the absence of the fragile X mental retardation protein (FMRP) and subsequent mental retardation.
Fragile X is inherited in an X-linked dominant fashion.
Patients display developmental delay with an IQ in the range of 20-60.
Fragile X patients may display features which meet criteria for concomitant autism spectrum disorder. Autism is a pervasive developmental delay disorder characterized by language impairment as well as deficits in social interaction. The disorder is recognized by repetitive behavior and significant focus on objects rather than people.
Patients characteristically display large everted ears.
Patients characteristically display large mandible.
Patients characteristically display a long face.
Large testes are a distinctive feature in fragile X syndrome, which is observed in a high proportion of patients.
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