Expression of mRNA encoding two gain-of-function cyfip2 variants associated with DEE65 results in spontaneous seizures in Xenopus laevis tadpoles

Developmental and epileptic encephalopathies (DEE) are a genetically diverse group of disorders with similar early clinical presentations. DEE65 is caused by de novo, non-synonymous, gain-of-function mutations in CYFIP2. It presents in early infancy as hypotonia, epileptic spasms and global developmental delay. While modelling loss-of-function mutations can be done using knockdown or knockout techniques to reduce the amount of functional protein, modelling gain-of-function mutations requires different approaches. Here, we show that transient ectopic expression of the Arg87Cys pathogenic variant of cyfip2 mRNA in Xenopus laevis tadpoles resulted in increased seizure-related behaviours such as rapid darting and swimming in circles. In contrast, expression of a second pathological variant, Tyr108Cys, did not alter tadpole behaviour. Expression of either pathogenic variant resulted in spontaneous epileptic activity in the brain. For both variants, neuronal hyperactivity was reduced by treating the tadpole with 5 mM of the anti-seizure drug valproate (VPA). mRNA overexpression of gain-of-function variants in X. laevis tadpoles may be useful both for understanding the aetiology of DEE and for pre-clinical drug testing.