Preclinical validation of endocannabinoid system drug targets for the treatment of Dravet syndrome

Abstract

Dravet syndrome (DS) is a paediatric encephalopathy associated with intractable seizures and detrimental behavioural impairments; with a dire need for improved therapeutic options. DS results largely from loss-of-function mutations in the voltage-gated sodium channel gene SCN1A. Heterozygous deletion of Scn1a in mice mimics epilepsy features seen in patients but severity of the DS phenotype is background strain-dependent in mice, indicating the presence of phenotype-modifying genes that could serve as drug targets. Comparing expression between seizure-resistant and seizure-susceptible strains of mice can identify candidate targets, which can then be evaluated pharmacologically or genetically for their ability to modify the DS phenotype. The endocannabinoid system holds therapeutic potential as it regulates neuronal activity, so we used a DS mouse model to explore this system for potential druggable targets. We measured less cannabinoid receptor 1 (CB1R) expression but more G protein-coupled receptor 55 (GPR55) expression, along with lower levels of their respective endogenous ligands, 2-AG and LPI, in the hippocampus of mice on the seizure-susceptible strain. We then showed that potentiation of endocannabinoid signalling with a MAGL inhibitor or a CB1R positive allosteric modulator, and heterozygous deletion of the Gpr55 geneexhibited anticonvulsant properties in the mouse model, supporting the role of CB1R and Gpr55 for the therapeutic management of DS. During our investigations we also discovered additional targets with phenotype-modifying potential that require further investigation. Finally, we also improved upon the DS mouse model by characterising adolescent behavioural phenotype to better model the behavioural impairments observed in paediatric DS patients. We identified impairments in measures of social, cognitive and attentional behaviours, and thus provide an early-intervention platform to evaluate potential behaviour-modifying molecular targets or drugs.