Rahul Balakrishnan

Background: Fragile X Syndrome (FXS) in Autism Spectrum Disorder (ASD) presents with significant anxiety that lacks treatment in the pediatric population. FXS is characterized by CGG repeats in the FMR1 gene resulting in symptoms of anxiety, hyperactivity, and social avoidance.¹ Recent literature suggests that these symptoms may arise due to dysregulation of the endocannabinoid system, resulting in dysregulation of excitation-inhibition in the cortex.¹ Preclinical trials have shown that increasing the endogenous ligands of AEA and 2-AG in FMR1 knockout (KO) mice reverses the anxious phenotype in various behavioral assays.¹ Because of this, a promising therapeutic response to anxiety for FXS in children is transdermal cannabidiol gel, which works through restoration of the endocannabinoid system.^1-3

Objective: In this narrative review, we explored the neural mechanisms by which cannabinoid treatments can resolve anxiety in Fragile X Syndrome and Autism Spectrum Disorder.

Search Methods: An online search in the PubMed database was conducted from 2018 to 2024 using the following keywords: “Fragile X Syndrome”, “Autism Spectrum Disorder”, “Neural mechanism”, “Cannabinoid”.

Results: CB1KO mice were subjected to the three-chamber test, direct social interaction, elevated plus maze, and acoustic startle tests.⁴ The CB1KO mice in both genders demonstrated deficits in social interaction and social motivation.⁴ Additionally, the CB1KO mice demonstrated a reduced increase in ultrasonic vocalization (USV) frequency that persisted in adulthood, indicating a developmental delay of communication.⁴ In line with this, Rimonabant, a CB1 inverse agonist, and DO34, a diacylglycerol lipase inhibitor that decreased 2-arachidonoylglycerol (2-AG), were administered in mice to isolate which aspect of the endocannabinoid system mediates anxiety.⁵ The mice treated with DO34 showed decreased USVs, reduced time in the open arms of EPM, and increased locomotor activity,⁵ indicating that 2-AG mediates the hyperactive phenotype of anxiety characteristic of FXS.⁵ Following this, JZL-184 was administered in FMR1KO mice to observe the effects of 2-AG amplification on anxiety.³ JZL-184 increased the amount of time the mice spent in the center and in the open arms in the OFT and EPM.³ However, the JZL-184 mice also demonstrated increased control of resting gamma-band power in EEG following administration for 1 day.³ Following this, VSN16R, a cannabinoid-like compound that acts on BK channels, was administered in FMR1KO mice at chronic doses for 4 weeks.² VSN16R significantly corrected the total distance traveled, normalized locomotor activity, corrected deficits in fear conditioning response, marble burying, stereotypy, and latency to attack in the mice, suggesting that cannabinoid treatment may resolve the anxious phenotype in FXS.² In response to this, a clinical trial of 20 children (6–17 years old) with FXS were initially screened for anxious symptoms and then received ZYN002, a transdermal CBD gel, twice daily for 12 weeks alongside their standard treatment for FXS.¹ Following this, the children were screened again for anxiety.¹ Overall, the medication was well tolerated, and there were significant improvements in anxiety, social avoidance, and stereotyped behaviors.¹

Conclusions: Studies have found that cannabinoid treatment in children with FXS leads to improvements of anxiety symptoms. VSN16R demonstrated specific behavioral improvements in FMR1KO mice across various aspects of anxiety in FXS. Currently, two clinical trials recruiting children to evaluate cannabinoid treatments, which may show promise for treatment of behavioral symptoms in FXS.

Works Cited:

1. Heussler H, Cohen J, Silove N, et al. A phase 1/2, open-label assessment of the safety, tolerability, and efficacy of transdermal cannabidiol (ZYN002) for the treatment of pediatric fragile X syndrome. J Neurodev Disord. 2019;11(1):16.  doi:10.1186/s11689-019-9277-x
2. Hurley MJ, Deacon RMJ, Chan AWE, Baker D, Selwood DL, Cogram P. Reversal of behavioural phenotype by the cannabinoid-like compound VSN16R in fragile X syndrome mice. Brain. 2022;145(1):76-82. doi:10.1093/brain/awab246
3. Pirbhoy PS, Jonak CR, Syed R, et al. Increased 2-arachidonoyl-sn-glycerol levels normalize cortical responses to sound and improve behaviors in Fmr1 KO mice. J Neurodev Disord. 2021;13(1):47. doi:10.1186/s11689-021-09394-x
4. Fyke W, Premoli M, Echeverry Alzate V, et al. Communication and social interaction in the cannabinoid-type 1 receptor null mouse: Implications for autism spectrum disorder. Autism Res. 2021;14(9):1854-1872. doi:10.1002/aur.2562
5. Fyke W, Alarcon JM, Velinov M, Chadman KK. Pharmacological inhibition of the primary endocannabinoid producing enzyme, DGL-α, induces autism spectrum disorder-like and co-morbid phenotypes in adult C57BL/J mice. Autism Res. 2021;14(7):1375-1389. doi:10.1002/aur.2520