Fragile X syndrome (FXS) is a monogenetic disorder caused by a mutation in the FMR1 gene and the most common form of inherited intellectual disability and autism spectrum disorder (ASD). Patients with FXS show a range of typical physical features such as macro-orchidism in males, a long and narrow face, large and protruding ears, and hyperextensible joints. Common comorbidities of FXS are neuropsychiatric disorders such as hyperactivity, depression and anxiety.
The mutation of FMR1 in FXS disrupts production of the FMR1 gene product, the fragile mental retardation protein (FMRP). The main function of FMRP is to locally act as a translational repressor for target mRNAs and thereby regulate de novo protein synthesis and ultimately synaptic plasticity. FMRP, together with the mTOR pathway and the ERK pathway regulates expression of target mRNAsn mediated by stimulation of Group I metabotropic glutamate receptors (mGluR) and thereby regulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) internalisation and thus long term depression (LTD). LTD is a form of synaptic plasticity which is involved in learning and memory. Lack of FMRP leads to exaggerated mGluR dependant LTD, which accounts for most of FXS pathogenesis.