“Multiple lines of evidence suggest a central role for the endocannabinoid system (ECS) in the neuronal development and cognitive function and in the pathogenesis of fragile X syndrome (FXS). This review describes the ECS, its role in the central nervous system, how it is dysregulated in FXS, and the potential role of cannabidiol as a treatment for FXS. FXS is caused by deficiency or absence of the fragile X messenger ribonucleoprotein 1 (FMR1) protein, FMRP, typically due to the presence of >200 cytosine, guanine, guanine sequence repeats leading to methylation of the FMR1 gene promoter. The absence of FMRP, following FMR1 gene-silencing, disrupts ECS signaling, which has been implicated in FXS pathogenesis. The ECS facilitates synaptic homeostasis and plasticity through the cannabinoid receptor 1, CB₁, on presynaptic terminals, resulting in feedback inhibition of neuronal signaling. ECS-mediated feedback inhibition and synaptic plasticity are thought to be disrupted in FXS, leading to overstimulation, desensitization, and internalization of presynaptic CB₁ receptors. Cannabidiol may help restore synaptic homeostasis by acting as a negative allosteric modulator of CB₁, thereby attenuating the receptor overstimulation, desensitization, and internalization. Moreover, cannabidiol affects DNA methylation, serotonin 5HT_1A signal transduction, gamma-aminobutyric acid receptor signaling, and dopamine D₂ and D₃ receptor signaling, which may contribute to beneficial effects in patients with FXS. Consistent with these proposed mechanisms of action of cannabidiol in FXS, in the CONNECT-FX trial the transdermal cannabidiol gel, ZYN002, was associated with improvements in measures of social avoidance, irritability, and social interaction, particularly in patients who are most affected, showing ≥90% methylation of the FMR1 gene.”

https://pubmed.ncbi.nlm.nih.gov/36624400/

“FXS is caused by deficiency or absence of FMRP, typically due to the presence of >200 CGG repeats and methylation in the promoter region of the FMR1 gene. The absence of FMRP downregulates the ECS signaling, which has been implicated in FXS pathogenesis. Synaptic homeostasis and plasticity may be regulated by the ECS through the postsynaptic “on demand” production of endocannabinoids, which then bind to CB₁ receptors on presynaptic terminals, resulting in regulation of glutamate signaling and GABAergic signaling. The ECS-mediated feedback inhibition and synaptic plasticity are thought to be disrupted in FXS due to dysregulation of enzymes that are integral to the ECS (e.g., DAGL), leading to overstimulation, desensitization, and internalization of presynaptic CB₁ receptors.

Cannabidiol may help restore synaptic homeostasis by acting as a negative allosteric modulator of CB₁, thereby attenuating CB₁ receptor overstimulation, internalization, and desensitization. Moreover, cannabidiol has effects on DNA methylation, 5HT_1A signal transduction, GABA_A receptor signaling, and dopamine D₂ and D₃ receptor signaling, which may contribute to beneficial effects in patients with FXS. Consistent with these proposed mechanisms of action of cannabidiol in FXS, the transdermal cannabidiol gel, ZYN002, was associated with improvements in measures of social avoidance, irritability, and social interaction in the CONNECT-FX trial, particularly among patients with ≥90% methylation of the FMR1 gene.”

https://jneurodevdisorders.biomedcentral.com/articles/10.1186/s11689-023-09475-z