Causes

The fragile X syndrome is a genetic disorder caused by mutation of the FMR1 gene on the X chromosome. Mutation at that site is found in 1 out of about every 1250 males and 1 out of about every 2500 females.

Normally, the FMR1 gene contains between 6 and 55 repeats of the CGG codon (tri nucleotide repeats). In people with the fragile X syndrome, the FMR1 allele has over 230 repeats of this codon.

Expansion of the CGG repeating codon to such a degree results in a methylation of that portion of the DNA, effectively silencing the expression of the FMR1 protein.

This methylation of the FMR1 locus in chromosome band Xq27.3 is believed to result in constriction and fragility of the X chromosome at that point, a phenomenon that gave the syndrome its name.

Mutation of the FMR1 gene leads to the transcriptional silencing of the fragile X-mental retardation protein, FMRP. In normal individuals, FMRP binds and facilitates the translation of a number of essential neuronal RNAs. In fragile X patients, however, these RNAs are not translated into proteins. The various sequelae of fragile X syndrome.

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