McTague, Amy; (2018) The Molecular Genetic Investigation of Epilepsy of Infancy with Migrating Focal Seizures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Epilepsy of infancy with migrating focal seizures (EIMFS) is characterised by the onset of frequent focal seizures in the first 6 months of life, a typical migratory EEG pattern and severe developmental delay. In this thesis, I report a cohort of patients with EIMFS, delineate clinical features and investigate the molecular genetic basis of this syndrome. In 2012, heterozygous mutations in the sodium-gated potassium channel KCNT1, were described in patients with EIMFS. Using a variety of genetic techniques, I have identified 12 patients with mutations in this gene. Four are novel, previously unreported mutations. Functional investigations, including protein homology modelling and electrophysiology in a xenopus oocyte model showed that all novel KCNT1 variants were gain-of-function mutations. In addition, I describe a new genetic cause of EIMFS. Within my cohort, I identified a consanguineous family with two affected children. Autozygosity mapping and whole exome sequencing revealed a novel, homozygous mutation in SLC12A5. SLC12A5 encodes KCC2, the neuronal potassium chloride co-transporter that determines the direction and polarity of GABA-mediated signalling. Through international collaboration, I found a second family with two affected children harbouring compound heterozygous SLC12A5 mutations. All three SLC12A5 variants were investigated using an overexpression HEK293 cell model. Immunoblotting and immunohistochemistry revealed decreased cell surface expression of mutant KCC2. Electrophysiology experiments showed a depolarization of the chloride reversal potential and a delayed response to chloride loading. Taken together, these results indicate that loss of KCC2 function is likely to result in abnormal neuronal inhibition in this form of EIMFS. The genetic heterogeneity in EIMFS is strong evidence that a wide variety of different pathogenic mechanisms can result in the severe epilepsy and abnormal neurodevelopment observed in this condition. Further elucidation of causative genes in both animal and cell models is needed to identify novel therapeutic targets for this devastating disorder.

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