Mechanism and therapy in DNM1 epileptic encephalopathy

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Brain cells (neurons) communicate by releasing chemical neurotransmitters. Neurotransmitters are stored in small spherical compartments within neurones called synaptic vesicles (SVs). When neurones communicate, SVs fuse with the outer surface of the neuron causing neurotransmitter release. After neurotransmitter release, these SVs are reformed by a process called endocytosis. The correct formation of SVs during endocytosis is essential for the maintenance of neurotransmitter release, since SVs are highly limited in neurons. It was originally thought that dysfunction in such an essential process would result in death of the affected individual, however over the past 10 years it is now becoming apparent that defective endocytosis underpins a series of severe forms of epilepsy called epileptic encephalopathies (EEs). As things currently stand, there are no available therapies for many individuals with EE, making understanding disease mechanisms an essential first step in generating new drugs. Mutations in the gene DNM1, are causal in a form of EE. The DNM1 gene makes a protein called dynamin-1, which is essential for SV endocytosis. As a first step in unravelling how a mutation in a gene essential for endocytosis results in EE, we generated a mouse that carries the most common human DNM1 mutation. Neurons from this mouse displayed dysfunctional SV endocytosis, and in addition had over-excitable brain activity and seizure-like behaviour. Therefore we now have an experimental tool through which to determine how epilepsy develops and in which to test new drugs. In previous work we identified a drug approved for human use that accelerates SV endocytosis. We discovered that this drug also corrected all of the defects in our mouse model of DNM1 EE (SV endocytosis, brain excitability and seizure-like events). Because it is likely that this drug affects SV endocytosis rather than dynamin-1 function itself, it has potential to be widely used in other disorders of SV endocytosis. In this programme of work we will determine how this drug works, and test its wider therapeutic potential in different models of dysfunctional SV endocytosis. People have two copies of every gene, called alleles. Individuals that have DNM1 EE only have 1 mutant allele, the other is unaffected. Interestingly, people can tolerate the complete loss of one DNM1 allele providing the other one is functional, suggesting that 1) the mutant form usually overrides the function of the unaffected version and 2) removal of the mutant allele may be a promising approach to treat the disorder. We will test this hypothesis by using gene therapy to remove the mutant DNM1 allele from the mouse model to determine whether this corrects the defects seen. A determination of how mutant DNM1 causes EE is essential to obtain, since it will provide key data on how to treat this, and other related conditions. The gene therapy approach detailed above will be critical to address this question, since we can remove the mutant DNM1 allele from specific types of neurons to determine how it is causing EE. This will be critical in determining epilepsy mechanisms. Many different mutations in the DNM1 gene have been discovered, therefore it is essential to determine whether there are similarities in how they affect dynamin-1 function and importantly how they change the function of neurons. We will also determine whether these mutations have similarities in terms of their effect, which will be highly informative in terms of future therapies.