Fragile X syndrome overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Fragile X syndrome is the leading inherited cause of intellectual disorder and autism spectrum disorder with severe behavioral abnormalities . It is an X linked disorder, affecting both males and females. It is a genetic disease caused by CGG trinucleotide expansion (>200 CGG repeats).
Historical perspective
Fragile X syndrome was described first by Martin and Bell in 1943.
Classification
There is no established system for the classification of Fragile X syndrome.
Pathophysiology
Fragile x syndrome has an x-linked dominant inheritance. It is caused by an expansion of CGG trinucleotide repeat within FMR1 gene on X chromosome. Due to high number of CGG repeats (>200), this leads to methylation of part of gene on X chromosome that codes for Fragile X Mental retardation protein (FMRP), which is required for proper development of connections between neurons.
Causes
Fragile x Syndrome is a genetic disease which is caused by mutation in the Fragile x Mental Retardation 1(FMR1) gene in X chromosome. Generally, these mutation (>200 repeats of CGG) occurs at in the 5' untranslated region of FMR1. In around 2% of cases, Fragile X syndrome can occur as a result of point mutation in FMR1 gene.
Differential diagnosis
Fragile X syndrome must be differentiated from Down's Syndrome, Autism Spectrum Disorder, Attention deficit hyperactivity disorder (ADHD), Fragile XE syndrome (FRAXE), Klinefelter syndrome and Prader-Willi syndrome (PWS).