Fragile X syndrome
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Fragile X Syndrome is the most common inherited cause of mental impairment, and the most common known cause of autism. Fragile X syndrome is a genetic disorder caused by a mutation of the FMR1 gene on the X chromosome, a mutation found in 1 out of every 2000 males and 1 out of every 4000 females. Typically the FMR1 gene contains between 6 and 53 repeats of the CGG codon. In people with the disorder, the FMR1 allele has over 230 repeats. Expansion to such a degree results in a methylation of that portion of the DNA, effectively silencing the expression of the FRM1 protein. The characteristic constriction of the X chromosome at the chromosomal locus Xq27.3 is caused by this methylation.
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X-linked recessive inheritance
Because men have only one copy of the X chromosome, males with sufficient trinucleotide expansion are symptomatic, while females have two X chromosomes and thus double the chance of a working allele (unless parents are related). Aside from mental retardation, the most obvious indicators include physical differences and behaviors commonly associated with autism. Of the former, the most readily visible are an elongated face and large or protruding ears, but others are also frequently present. Of the latter, behavioral stereotypy and atypical social development are the most frequently observed.
While there is no current cure for the syndrome, there is hope that further understanding of the FMR1 gene would allow to counteract the underlying genetic cause. Currently, the syndrome can be treated through behavioral therapy, special education, and when necessary, treatment of physical abnormalities. Persons with Fragile X in their family histories are advised to seek genetic counseling to assess the likelihood of having children who are affected, and how severe any impairments may be in affected descendants.
External links
- FraXA.org (http://www.fraxa.org) - The Fragile X Research Foundation
- FragileX.org (http://www.fragilex.org) - The National Fragile X Foundation
- Stanford.edu (http://www.stanford.edu/group/hopes/rltdsci/trinuc/f9.html) - 'Trinucleotide Repeat Disorders Part 9: Non-Polyglutamine Diseases: Descriptions of other diseases that involve codon repeat expansions' (September 18, 2002)
- NIH.gov (http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=309550) - 'Fragile Site Mental Retardation 1 Gene; FMR1' (Fragile X syndrome OMIM entry)
- SpectrumHaven.com (http://www.spectrumhaven.com) - 'Fun Chat Ideas: Asperger, ADHD, PDD-NOS, ADD, Fragile-X Autism, Dyspraxia, Hyperlexia' (Support group forum for Fragile X syndrome and other autism spectrum disorders)
References
- GeneClinics.org (http://www.geneclinics.org/servlet/access?db=geneclinics&site=gt&id=8888891&key=0bTjwXe1x5G0x&gry=&fcn=y&fw=VKeC&filename=/profiles/fragilex/index.html) - 'Fragile X Syndrome: FRAXA, FXS, Fragile X Mental Retardation, Marker X Syndrome, Martin-Bell Syndrome. Includes: Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS), Robert A. Saul, MD, FACMG, Jack C. Tarleton, PhD, FACMG, GeneReview (September 13, 2004)
- Nature.com (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nrn/journal/v6/n5/full/nrn1667_fs.html) - ' From MRNP Trafficking to Spine Dysmorphogenesis: The Roots of Fragile X Syndrome', Claudia Bagni1, & William T. Greenough Nature Reviews Neuroscience, 6, pp 376-387 (2005)de:Fragiles X-Syndrom