Commonalities in the neurobiology between autism and fragile X

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There is a close association between autism and fragile X syndrome (FXS) with 30% of males with FXS having autism and 2 to 7% of children with autism having the fragile X mutation. The protein that is missing or deficient in FXS, FMRP, is an RNA binding and transport protein which regulates the translation of many messages important for synaptic plasticity. Typically FMRP inhibits the translation of these messages, such that protein production increases when FMRP is absent. Some of these proteins are known to also cause autism when they are mutated including neuroligin 3 and 4 and the SHANK protein. Therefore, when FMRP is missing there is dysregulation of other proteins that are known to cause autism. FMRP is an important inhibitor of protein production in the metabotropic glutamate receptor 5 pathway (mGluR5) which leads to long term depression (LTD) or the weakening of synaptic connections. Therefore, when FMRP is missing there is enhanced mGluR5 activity leading to enhanced LTD and weak or immature synaptic connections. The use of mGluR5 antagonists to reverse the LTD in the animal models of FXS has led to reversal of the learning, behaviour and dendritic spine abnormalities in these animals. There are now initial studies taking place in humans regarding the use of mGluR5 antagonists to improve behaviour and cognition in FXS. It is likely that these mGluR5 antagonists will also be helpful in a subgroup of patients with non fragile X autism who have similar problems with hyperactivity, hyperarousal and anxiety to those seen in FXS. A second cause of autism is the fragile X premutation but this mechanism of involvement is related to RNA toxicity which perhaps stimulates neuroimmune problems and may mimic other causes of autism. Neurons with the premutation are more vulnerable to environmental toxicity and oxidative stress leading to early cell death.