Abstracts

Rationale: Juvenile Myoclonic Epilepsy (JME), a common and lifelong epilepsy, is the most common cause of primary grand mal seizures and accounts for 10 to 12% of all epilepsies. Mutations in myoclonin/EFHC1 cause JME in 20% of JME families and in 9% of consecutive clinic cases of JME.The myoclonin/EFHC1 protein enhances dendritically located R type voltage dependent calcium currents in HEK cells and induces apoptosis in hippocampal cells in culture. Because the functions of EFHC1 and the mechanisms underlying seizure susceptibility and pathogenesis remain unclear, we are currently using Drosophila melanogaster to unravel the cellular role of EFHC1 and generate a model for JME. Methods: We identified two Drosophila homologues of myoclonin/EFHC1 which display a similar degree of homology with their human counterpart. We are currently focusing on the study of one of these two fly genes named CG8959. We have generated CG8959 knockout mutants and transgenic Drosophila lines for targeted overexpression of the CG8959 encoded protein. We are using genetic, molecular and cell biological approaches to establish the phenotypic consequences of modulating CG8959 expression. Results: CG8959 knockout mutant flies appear phenotypically normal. However, quantitative analysis reveals decreased overall number of branches and boutons in the neuromuscular junction synapse. CG8959 overexpression reduces structure of arbor, branches and stems of neuromuscular junction synapses and dendrite arborization in peripheral sensory neurons. Moreover, CG8959 overexpression in the posterior compartment of the fly wing causes loss of posterior margin and wing veins. Modification of this phenotype by mutations in the Notch gene suggests a functional interaction between myoclonin/EFHC1 proteins and the Notch pathway. Conclusions: Modulation of myoclonin/EFHC1 expression in Drosophila affects neuromuscular junction and dendrite arborization. Maintenance of a population of dynamic microtubules is important for axonal extension, synaptic bouton formation and dendritic arborization. Thus changes in microtubule organization could be the common theme underlying the NMJ synapse and dendritic arbor phenotypes produced by loss and overexpression of CG8959. We will discuss how myoclonin/EFHC1 proteins may impact microtubule stability resulting in arborization defects of neuronal processes and how these defects could be at the basis of seizure susceptibility and epileptogenesis.