Homozygous Mutation in Synaptic Vesicle Glycoprotein 2A Gene Results in Intractable Epilepsy, Involuntary Movements, Microcephaly, and Developmental and Growth Retardation

BACKGROUND: Synaptic vesicle protein 2A (SV2a) is the binding site of the antiepileptic drug levetiracetam and the only known synaptic vesicle target of an epilepsy medication. To date, no pathogenic mutation in SV2A, which is the gene encoding synaptic vesicle glycoprotein 2A, has been identified in humans. We report a homozygous mutation in the SV2A gene in a patient with intractable epilepsy. METHODS: We investigated a patient with intractable epilepsy, involuntary movements, microcephaly, and developmental and growth retardation. Both parents were multiply consanguineous and an earlier-born brother of the proband had a similar course and died at 7 months of age. Detailed clinical history, imaging, electroencephalograph and metabolic testing were obtained. Full exome sequencing was performed using genomic DNA isolated from the patient and both parents. RESULTS: Exome sequencing identified a homozygous arginine to glutamine mutation in amino acid position 383 (R383Q) in exon 5 of the SV2A gene. Both parents were carriers for the R383Q variant, suggesting that R383Q is a recessive mutation. There were no other candidate alterations in the exome that could explain the phenotype in the proband. The amino acid arginine at position 383 of SV2a gene is evolutionally conserved throughout vertebrates. R383Q change is not observed in known healthy cohorts, exome databases, or the Database of Single Nucleotide Polymorphisms. The R383Q mutation is located in the second adenine binding domain in SV2a protein and may alter adenine nucleotides binding to SV2a. CONCLUSION: Our report provides the elusive evidence that an SV2A mutation can be a cause of epilepsy in humans. Levetiracetam, which binds to SV2A, was not effective as an antiepileptic medication. The location of the mutation in our patient supports an important role of adenine nucleotides binding in SV2A function.