Potassium channel gene mutations in epileptic encephalopathy

Abstract

The worldwide prevalence of epilepsy is between 2-3 % with many unmet clinical and health policy needs, especially in developing countries. Current estimates suggest that as many as 70 % of all epilepsy syndromes have a robust genetic etiology although the mechanisms of disease genesis are largely unknown. The improvement in genetic sequencing and testing has uncovered many genes associated with various forms of epilepsy, especially syndromes with an early onset. Many of these genes encode various ion channels, including the family of voltage-gated potassium channels, which are crucial in setting the afterhyperpolarization in neurons, preparing them to fire more action potentials. Mutations in these channels therefore bring about imbalance in the excitatory-inhibitory homeostasis in neural networks, leading to seizure development. Therefore the aim of this thesis was to characterize novel mutations of two different potassium channels encoded respectively by the KCNT1 and KCNC1 gene. The goal was to identify the biophysical changes brought about by KCNT1 and KCNC1 mutations, which were previously not reported. This thesis also looks at the effect of quinidine, which was previously reported as a possible therapeutic for KCNT1-related epilepsy, to determine the drug’s efficacy on these novel KCNT1 mutations. This thesis reports that the novel KCNT1 mutations identified from patients with autosomal dominant nocturnal frontal lobe epilepsy and epilepsy of infancy with migrating focal seizures produced potassium currents with large amplitudes and altered kinetics, similar to previously reported KCNT1 mutations in the literature; however, there was no clear genotype-phenotype pattern identified. Novel KCNT1 mutations in myoclonic atonic epilepsy and Lennox-Gastuat Syndrome, where changes in the KCNT1 gene were previously unreported, produced similar biophysics to WT, therefore indicating that KCNT1 may not be the underlying cause of epilepsy in these patients. The inclusion of single nucleotide polymorphisms in these experiments acted as controls as well as emphasized that not all genetic alterations are detrimental. The application of quinidine on some of the variants showed varied effects with some mutations having reduced and some increased currents upon its application, indicating that the drug may not be a magic-bullet treatment for all KCNT1-related epilepsies. This thesis also looked at characterizing novel KCNC1 mutations identified in patients with epileptic encephalopathy or intellectual disability without seizures. These de novo variants were compared against the recurrent p.Arg320His mutant, which was previously identified in patients suffering from progressive myoclonus epilepsy. The results obtained show varied levels of loss of function for the analysed mutants with some of them also showing changes in voltage dependence of activation and a dominant-negative effect. Therefore the results obtained in this thesis, provide a basic insight into the changes caused by newly identified mutations in two voltage-gated potassium channel genes. The data act as a framework, which can assist in the development in more complex experimental models to further understand the biophysical effects of the mutation as well as drug-protein interactions. Nonetheless, this work also emphasizes the need for in vitro experiments as a way of breaking down complex disorders such as epilepsy.