KCC2 and epilepsy in a mouse model of tuberous sclerosis complex

PROJECT SUMMARY/ABSTRACT Tuberous sclerosis complex (TSC) is a relatively common genetic disorder, which features hamartoma or tumor growth in multiple organs, including the brain, and causes a variety of neurological and neuropsychiatric symptoms, including epilepsy, intellectual disability, and autism. Mutation of the TSC1 or TSC2 genes leads to hyperactivation of the mechanistic target of rapamycin (mTOR) pathway, which drives tumor growth and epileptogenesis in TSC. Epilepsy occurs in up to 90% of TSC patients and is intractable to treatment in the majority of cases, often leading to life-long disabling seizures. While significant advances in treatment of epilepsy in TSC have been made, including recent FDA approval of an mTOR inhibitor for epilepsy, most TSC patients continue to have intractable seizures and significant side effects, which strongly affect quality of life and may exacerbate co-morbidities of cognitive and behavioral disorders. So, novel therapeutic approaches to epilepsy in TSC are greatly needed. An intriguing, unexplored avenue to investigate epilepsy in TSC relates to the regulation of intracellular chloride homeostasis, which is critical for facilitating synaptic inhibition in the brain, particularly as driven by gamma-aminobutyric acid A (GABAA) receptors. The K+-Cl- cotransporter 2 (KCC2) extrudes chloride from neurons, leading to a relatively low intracellular concentration and negative reversal potential for chloride. Thus, activation of GABAA receptors results in a hyperpolarizing chloride influx and synaptic inhibition of neurons in the normal mature brain. However, during early brain development or under certain pathological conditions, KCC2 expression may be relatively low, leading to elevated intracellular chloride concentrations and a depolarizing response to GABA, increasing neuronal excitability and the propensity for seizures. Reduced KCC2 expression in pathological brain specimens from TSC patients with epilepsy have been reported, but the functional consequences of this abnormality and its potential role in causing epilepsy in TSC have not been previously investigated. In this grant, we test the novel hypothesis that decreased KCC2 expression may cause or contribute to epilepsy in mouse models of TSC and that pharmacological modulators that increase KCC2 expression may be effective treatments for seizures in TSC. This work is innovative in investigating a novel mechanism of epileptogenesis in the genetic epilepsy of TSC. The findings from these studies may have strong clinical significance and impact for developing novel therapies for epilepsy in TSC. In particular, some KCC2 modulators are already FDA-approved for other indications and could be repurposed and tested in clinical trials for epilepsy in TSC patients, potentially making the path to clinical translation and applications of this grant relatively immediate. In addition, as TSC is often viewed as a model disease, findings from this proposal may have relevance for the potential role of KCC2 in other epilepsies due to other causes.