Abstracts

Rationale: Focal cortical dysplasia (FCD), a local malformation of cortical development (MCD) consists of neuronal misplacement, abnormal morphology and connectivity. It is the most common cause of medically refractory epilepsy in children and second most common in adults. Despite its prominence in intractable epilepsy, mechanisms underlying FCD associated epileptogenesis are unclear, hugely because previous animal models of MCD do not develop spontaneous seizures or fail to survive past adolescence. Therefore, we aim to develop a novel mouse model of human epilepsy associated with FCD, in which we can ask questions such as which cortical regions are more epileptogenic and is neuronal heterotopia necessary for epileptogenesis. Since hyperactivity of mammalian target of Rapamycin (mTOR) in the brain has been associated with numerous developmental disorders and brain injuries associated with epileptogenesis, we hypothesized that focal upregulation of mTOR signaling is sufficient in generating neuronal heterotopia and recurrent seizures. Methods: To constitutively upregulate mTOR activity focally in the medial prefrontal cortex (mPFC), we used an in vivo transfection technique - in utero electroporation - to manipulate protein expression in a small population of projection neurons. The delivery of a plasmid encoding a constitutively active mutant of a small GTPase, Ras homolog enriched in brain (Rheb) the canonical activator of mTOR, enables constitutive activation of endogenous mTOR. Supragranular neurons are transfected on gestation day 15 when they are born from neural precursor cells in the subventricular zone. Results: All mice (n=6) electroporated with plasmid encoding constitutively active Rheb (caRheb) developed daily seizures as recorded by transcranial electroencephalography (EEG), while all mice (n=5) electroporated with control plasmid encoding blue fluorescent protein are seizure-free. Seizures epochs last less than a minute on average and show uncanny similarities to human frontal lobe seizures, including postictal slow wave activity and interictal spikes. Immunohistochemical analysis of brain pathology reveals type 2 FCD (FCD II) in epileptic brains as defined by the latest ILAE definition, including dyslamination and heterotopia in the subgranular layer of the cortex and white matter. Widespread astrogliosis is also observed in all epileptic brains (n=5) from mPFC to cerebellum. However, mice with caRheb expression targeted to sensory-motor cortices such as somatosensory, visual, or motor cortices do not develop seizures. Surprisingly, animals with high mTOR activity but without heterotopia are also seizure-free. Conclusions: We have developed the first model of FCD-associated epilepsy that fully recapitulates the etiology, symptomology, and pathology of the human disease by selectively upregulating endogenous mTOR activity in the mPFC during cortical development. With this model we have determined that FCD in mPFC is more epileptogenic than other cortical regions, and neuronal heterotopia is necessary for epileptogenesis.