Pathological mechanisms underlying Progressive Myoclonus Epilepsy

Lead Research Organisation: University College London
Department Name: Institute of Neurology

Abstract

Progressive Myoclonus Epilepsy (PME) is a devastating neurological syndrome linked to mutations in several genes, including the Golgi t-SNARE GOSR2. Patients with GOSR2-PME exhibit three main symptoms: myoclonus (involuntary muscle spasms), ataxia (inability to control body movements) and epileptic seizures. These symptoms begin very early (around 2-6 years old), increase in severity with age and eventually lead to premature mortality, often between ages of 20-40, due to respiratory complications. While certain drugs can limit aspects of the disease (such as seizure frequency), GOSR2-PME is generally drug-resistant and is therefore incurable. Thus, there is an urgent need to acquire a fuller understanding of the cellular alterations that lead to nervous system dysfunction in GOSR2-PME. To help achieve this goal, we have generated a range of new in vivo models of this disorder, utilising the fruit fly, Drosophila melanogaster. Despite several hundred million years of evolutionary divergence, the Drosophila genome contains a single homologue of GOSR2 (membrin) that we have found to be functionally interchangeable with human GOSR2. We have generated mutations in Drosophila membrin that correspond to human PME mutations, and have found that these have profound impacts on Drosophila development, particularly the development of synapses at the neuromuscular junction. We will now take advantage of the extensive genetic toolkit and rapid generation time of Drosophila to uncover the molecular and cellular basis of these effects. Given the evolutionary conservation of GOSR2, we expect our findings to be readily transferable to human patients, and to significantly broaden our understanding of the pathological mechanisms underlying PME.

Technical Summary

We are investigating how mutations in GOSR2 - a Golgi t-SNARE fundamentally required for ER-Golgi trafficking in the secretory pathway - cause a devastating neurological disorder, Progressive Myoclonus Epilepsy (PME). GOSR2-PME is characterised by myoclonus, ataxia and tonic-clonic seizures. Symptoms begin early in life and increase in severity with time, such that patients often die between the ages of 20-40 years old. The secretory pathway is universally required for transport and secretion of proteins, lipids and complex carbohydrates. Paradoxically, mutations in GOSR2 cause a relatively selective neuronal phenotype, yet the mechanisms by which GOSR2 mutations impact neuronal development and/or circuit function in PME are unclear. To address this, we have developed novel Drosophila models of GOSR2-PME. We have found that PME-linked mutations in the Drosophila orthologue of GOSR2 (membrin) result in profound changes in organismal development and synaptic morphology. PME-mutant Drosophila larvae exhibit reduced locomotion and pharate adults are unable to successfully eclose from the pupal case, likely due to motor defects. At the Drosophila neuromuscular junction, we observe significant synaptic retraction and abnormal morphology of presynaptic boutons, coupled with alterations in the presynaptic cytoskeleton. These neurodevelopmental alterations are intriguing, since GOSR2-PME patients exhibit signs of sensory neuronopathy and motorneuron denervation. Our Drosophila model will now provide a platform to dissect the molecular basis of these alterations in synaptic development, and significantly enhance our understanding of PME pathophysiology.

Planned Impact

The underlying goal of this proposal is to further our understanding of the molecular and cellular mechanisms underlying Progressive Myoclonus Epilepsy (PME), particularly PME linked to mutations in the Golgi t-SNARE GOSR2. In addition to direct benefits to the scientific community, this project holds promise of long-term benefits for PME patients.

While rare, PME is a truly devastating syndrome. Severe myoclonus and ataxia are often observed early in patient life (as young as 2 years of age), and early mortality is common. Several genes have been linked to PME, yet for the majority it is still unclear how mutations in these loci result in nervous system dysfunction. The acquisition of such knowledge is a fundamental prerequisite for the development of a therapeutic strategy for PME, since this disorder is highly drug resistant and currently untreatable.

Given the severity of patient symptoms and the frequent early mortality observed in patient cohorts, expanding the number of in vivo models of PME is an essential step in this process. The fruit fly, Drosophila, represents an ideal organism to model PME, given the presence of strong homologues of the majority of PME-linked loci in the Drosophila genome and the wealth of genetic tools available for gene manipulation. Currently, only one PME model has been developed in Drosophila, focused on the planar cell polarity gene prickle. We note that synaptic development has not been investigated in this model. Thus, our data represents the first investigation of how a PME gene impacts the development of motorneurons. This is particularly relevant given that motorneuron denervation has been documented in patients with GOSR2-PME.

Our study will provide several new PME-linked tools to the community of Drosophila researchers, particularly transgenes encoding wild-type and PME-mutant isoforms of Drosophila Membrin and human GOSR2. These fly lines will help to expand the range of possible future studies in Drosophila involving PME. The neurodevelopmental alterations we have identified in our PME model will help to guide future research, with the aim of uncovering the molecular mechanisms underlying GOSR2-PME, and potentially other PME subtypes.

Finally, the post-doctoral researcher will gain advanced knowledge of electrophysiological recordings at the Drosophila larval NMJ, immuno-fluorescent microscopy, and Drosophila genetics. Since there is a high demand for skilled electrophysiologists, we anticipate that the techniques acquired during this project will significantly help their future job prospects.

Publications

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Title Drosophila and human Membrin transgenes 
Description We have generated a series of six transgenes encoding Drosophila or human GOSR2/Membrin - either wild-type in sequence or containing one of two mutations linked to progressive myoclonus epilepsy (PME). By expressing these transgenes globally in a Drosophila membrin null background, we generated four models of GOSR2/Membrin-linked PME and two corresponding controls. 
Type Of Material Model of mechanisms or symptoms - non-mammalian in vivo 
Year Produced 2017 
Provided To Others? Yes  
Impact These tools were essential to our recent article in Cell Reports that suggested, for the first time, a mechanistic basis by which mutations in the Golgi SNARE protein GOSR2/Membrin cause PME. Generation of Drosophila models of this disease allowed us to test how Membrin mutations impacted several aspects of neurodevelopment and function, including dendritic growth, dendritic and axonal cargo trafficking, presynaptic stability, spontaneous and evoked neurotransmission. We were also able to show that our Drosophila models recapitulate aspects of PME pathology, including ataxia and seizures. 
URL https://www.ncbi.nlm.nih.gov/pubmed/28978487