Epilepsy Caused by an Abnormal Alternative Splicing with Dosage Effect of the SV2A Gene in a Chicken Model

Marine Douaud,Katia Feve,David Gourichon,Michèle Tixier-Boichard,Frédérique Pitel,Simon Boitard,Fabienne Pituello,Cesira Batini,Gérard Coquerelle,Agathe Vieaud,Eric Leguern,Alain Vignal,Robert Naquet

doi:10.1371/journal.pone.0026932

Abstract

Photosensitive reflex epilepsy is caused by the combination of an individual's enhanced sensitivity with relevant light stimuli, such as stroboscopic lights or video games. This is the most common reflex epilepsy in humans; it is characterized by the photoparoxysmal response, which is an abnormal electroencephalographic reaction, and seizures triggered by intermittent light stimulation. Here, by using genetic mapping, sequencing and functional analyses, we report that a mutation in the acceptor site of the second intron of SV2A (the gene encoding synaptic vesicle glycoprotein 2A) is causing photosensitive reflex epilepsy in a unique vertebrate model, the Fepi chicken strain, a spontaneous model where the neurological disorder is inherited as an autosomal recessive mutation. This mutation causes an aberrant splicing event and significantly reduces the level of SV2A mRNA in homozygous carriers. Levetiracetam, a second generation antiepileptic drug, is known to bind SV2A, and SV2A knock-out mice develop seizures soon after birth and usually die within three weeks. The Fepi chicken survives to adulthood and responds to levetiracetam, suggesting that the low-level expression of SV2A in these animals is sufficient to allow survival, but does not protect against seizures. Thus, the Fepi chicken model shows that the role of the SV2A pathway in the brain is conserved between birds and mammals, in spite of a large phylogenetic distance. The Fepi model appears particularly useful for further studies of physiopathology of reflex epilepsy, in comparison with induced models of epilepsy in rodents. Consequently, SV2A is a very attractive candidate gene for analysis in the context of both mono- and polygenic generalized epilepsies in humans.

Highlights

Genetic reflex epilepsy (GRE), which was first described by Morgan and Morgan (1939) [1], is a type of idiopathic epilepsy in which a stimulus of any sensory modality evokes paroxysmal manifestations only in genetically predisposed subjects
Several studies have strongly supported the notion that there is a genetic etiology for photosensitive epilepsy, but no causative gene or mutation has yet been identified [4,5,6,7]
Because most of the SV2A gene sequence was not found in the chicken sequence assembly nor among the published chicken genes or EST, we aligned chicken chrUn_random sequences to those from other model organisms and identified new genomic chicken SV2A fragments homologous to the mouse sequence (Fig. S4). Combining this with partial cloning and sequencing of chicken SV2A provided new SNP markers that were used to narrow the genetic mapping interval to within a 13-kb portion of SV2A in the fourth generation of our pedigree, and determined that the causative mutation laid between markers GCT1888 and GCT2123 (Fig. 1b and 1c)

Summary

Introduction

Genetic reflex epilepsy (GRE), which was first described by Morgan and Morgan (1939) [1], is a type of idiopathic epilepsy in which a stimulus of any sensory modality evokes paroxysmal manifestations only in genetically predisposed subjects. This substitution, suggested to be the causative mutation, leads to aberrant splicing of the SV2A gene, and is responsible for a dosage effect explaining the phenotype observed in the Fepi strain. Combining this with partial cloning and sequencing of chicken SV2A provided new SNP markers that were used to narrow the genetic mapping interval to within a 13-kb portion of SV2A in the fourth generation of our pedigree, and determined that the causative mutation laid between markers GCT1888 and GCT2123 (Fig. 1b and 1c).

Results

Conclusion