Fragile X syndrome

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Fragile X syndrome
Classification and external resources
Location of FMR1 gene

Fragile X syndrome (FXS) is a genetic syndrome that causes a range of intellectual disabilities as well as unusual physical and behavioural characteristics. It is one of the most common causes of autism and mental retardation among boys.[1][2] Basically, these individuals lack an important gene product.

Technical account[change | change source]

Fragile X syndrome occurs as a result of a mutation of the FMR1 gene on the X chromosome. This causes an increase an increase in the number of CGG repeats in the 5' untranslated region of FMR1.[3]

In unaffected individuals, the FMR1 gene contains 5–44 repeats of the CGG codon, most commonly 29 or 30 repeats.[3][4] Between 45 and 54 repeats is considered a "grey zone". Between 55 and 200 repeats in length is described as a premutation allele. Individuals with fragile X syndrome have a full mutation of the FMR1 allele, with over 200 repeats of the CGG codon.[1][4][5]

In these individuals with a repeat expansion greater than 200, there is methylation of the CGG repeat expansion and FMR1 promoter. This silences the FMR1 gene, and its end-product is not made.

References[change | change source]

  1. 1.0 1.1 McLennan Y. et al 2011. "Fragile X Syndrome". Current Genomics 12 (3): 216–224. doi:10.2174/138920211795677886. PMC 3137006. PMID 22043169. 
  2. Budimirovic D.B. & Kaufmann W.E. 2011. "What can we learn about autism from studying fragile X syndrome?". Dev Neurosci 33 (5): 379–94. doi:10.1159/000330213. PMC 3254037. PMID 21893949. Retrieved 26 Jan 2012. 
  3. 3.0 3.1 Santoro M.R. et al 2012. Molecular mechanisms of Fragile X Syndrome: a twenty-year perspective. Annu. Rev. Pathol. Mech. Dis. 7: 219–45. [1]
  4. 4.0 4.1 Maddalena A. et al 2001.. "Technical Standards and Guidelines for Fragile X: the first of a series of disease-specific supplements to the Standards and Guidelines for Clinical Genetics Laboratories of the American College of Medical Genetics". Genetics in Medicine 3 (3): 200–205. doi:10.1097/00125817-200105000-00010. PMC 3110344. PMID 11388762. 
  5. Nolin S.L. et al 2003. "Expansion of the fragile X CGG repeat in females with premutation or intermediate alleles". American Journal of Human Genetics 72 (2): 454–64. doi:10.1086/367713. PMC 379237. PMID 12529854.