De Novo Loss-of-Function Mutations in CHD2 Cause a Fever-Sensitive Myoclonic Epileptic Encephalopathy Sharing Features with Dravet Syndrome

Abstract

Dravet syndrome is a severe epilepsy syndrome characterized by infantile onset of therapy-resistant, fever-sensitive seizures followed by cognitive decline. Mutations in SCN1A explain about 75% of cases with Dravet syndrome; 90% of these mutations arise de novo. We studied a cohort of nine Dravet-syndrome-affected individuals without an SCN1A mutation (these included some atypical cases with onset at up to 2 years of age) by using whole-exome sequencing in proband-parent trios. In two individuals, we identified a de novo loss-of-function mutation in CHD2 (encoding chromodomain helicase DNA binding protein 2). A third CHD2 mutation was identified in an epileptic proband of a second (stage 2) cohort. All three individuals with a CHD2 mutation had intellectual disability and fever-sensitive generalized seizures, as well as prominent myoclonic seizures starting in the second year of life or later. To explore the functional relevance of CHD2 haploinsufficiency in an in vivo model system, we knocked down chd2 in zebrafish by using targeted morpholino antisense oligomers. chd2-knockdown larvae exhibited altered locomotor activity, and the epileptic nature of this seizure-like behavior was confirmed by field-potential recordings that revealed epileptiform discharges similar to seizures in affected persons. Both altered locomotor activity and epileptiform discharges were absent in appropriate control larvae. Our study provides evidence that de novo loss-of-function mutations in CHD2 are a cause of epileptic encephalopathy with generalized seizures.

Depdc5 knockdown causes mTOR‐dependent motor hyperactivity in zebrafish

“…Reports of genetically and chemically induced zebrafish models of epilepsy (reviewed in 19, 24, 37, 38), have established a number of behavioral paradigms that are used as metrics in this field. These measures are generally obtained at larval stages 3–7 dpf upon evoked or spontaneous swimming episodes 20, 21, 28, 39.…”

“…This phenotype could be measured automatically by movement quantification and therefore could represent an early screening parameter for hyperactivity in zebrafish. At later stages of larval development, extensive research on both drug‐induced and genetic models of epilepsy have revealed a stereotyped swimming characteristic of epileptogenic‐like activity, where individual larvae swim in a circular, corkscrew‐like trajectory 20, 22, 24. To describe this particular phenotype, we have quantified the rotational angle of the swimming trajectory and obtained a reliable metric for comparing the epilepsy‐like phenotypic features in zebrafish.…”

“…The first report of chemically induced seizures using the convulsant drug pentylenetetrazole (PTZ) documented stereotypical swimming abnormalities in 1‐week‐old larvae, consisting of circular trajectories, which correlated with increased synchronized neuronal activity in the zebrafish optic tectum as evidenced by field recordings 20. Subsequent studies have successfully reproduced these effects in genetic models of epilepsy in zebrafish 21, 22, 23, 24. Recently, a zebrafish homozygous null model for the mTOR regulator protein, TSC2, was reported to display epileptiform activity and defective locomotion responses,25 validating the use of this vertebrate model organism for epilepsies related to the mTOR pathway.…”

Cross-species pharmacological characterization of the allylglycine seizure model in mice and larval zebrafish

“…It combines easy handling and relatively high-throughput screening with the complexity of a whole vertebrate organism. Within the past decade, the usefulness of zebrafish in epilepsy research has been validated through studies involving pharmacologically induced acute seizure models [13][14][15] as well as zebrafish models of genetic epileptic syndromes (Angelman's [16], Lowe's [17], BNFC [18], EAST [19], and Dravet [20,21]). More recently, some of these zebrafish models have been applied in high-throughput epilepsy drug discovery (PTZ [22] and Dravet [20]).…”

Lessons learned from gene identification studies in Mendelian epilepsy disorders

“…Selecting phenotypically matched cases can thus enlarge the chance of finding independent variants in the same gene (eg, ref. 15).…”