Fragile X syndrome


What happens because of the CGG trinucleotide expansion in fragile X syndrome?

In fragile X syndrome, the body silences or turns off the Fragile X mental retardation 1 (FMR1) gene due to the CGG trinucleotide expansion of greater than 200 repeats by a process called hypermethylation. The way that genes work in the body is that the gene serves as a blueprint, and each gene is translated into a protein that ultimately carries out the job of that gene in the body. This process happens multiple times so that the body has the proper amount of that specific protein (which varies depending on which gene and how much is needed).

Because the FMR1 gene is silenced, little to no fragile X mental retardation protein (FMRP) is made. The lack of FMRP leads to abnormal brain development, which causes the cognitive and developmental delay seen in people with Fragile X syndrome. FMRP plays a role in the production of other proteins too; not having the proper amount of these proteins can lead to the other characteristics we see in persons with fragile X syndrome.

Sherman S, Pletcher BA, Driscoll DA. Fragile X syndrome: Diagnostic and carrier testing. Genetics in Medicine. 2005;7(8):584-587. doi:10.1097/01.GIM.0000182468.22666.dd.

Hagerman R, Lauterborn J, Au J, Berry-Kravis E. Fragile X Syndrome and Targeted Treatment Trials. Results and problems in cell differentiation. 2012;54:297-335. doi:10.1007/978-3-642-21649-7_17.

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