NETWORK OVERCOMPENSATION MAY DRIVE EPILEPTOGENESIS IN A <EM>KCNA2</EM> LOSS-OF-FUNCTION MOUSE MODEL

Fast voltage activated potassium current plays a critical role in regulating neural excitability. K_V1.2 encoded by KCNA2 is a key player in this current and its de novo variants have been linked to developmental and epileptic encephalopathies. Patients with the recurrent loss-of-function variant p.(Pro405Leu) (P405L) exhibit focal seizures and mild developmental delay.
In order to describe the developmental pathophysiology we generated a Kcna2^+/P405L knock-in mouse model and performed in-vivo phenotyping, intracranial video EEG monitoring, immunohistochemistry, Golgi-stainings, whole-cell patch-clamp recordings of excitatory and inhibitory neurons as well single-nuclei RNA sequencing of cortical and hippocampal-formation tissue at two developmental time points.
Heterozygous Kcna2^+/P405L mice exhibited focal and bilateral tonic-clonic seizures with premature seizure induced death (SUDEP) occurring within the first few months. Using c-Fos stainings, we observed more active neurons in somatosensory cortex and the hippocampus of mutant animals with seizures. Surprisingly, at developmental stages prior to seizure onset we found reduced pyramidal neuron excitability and decreased excitatory network activity, which normalized around P30 at the time of seizure onset. Notably, this was accompanied by an initialy increased afterhyperpolarization. RNA-Seq analysis suggests that axonal and synaptic pathways are upregulated in Kcna2^+/P405L mice, especially at seizure onset. Consistent with this, Golgi stainings revealed alterations in spine-type composition, with a shift toward an over-mature spine profile in mutant mice.
In conclusion, we established a new mouse model for a loss-of-function in KCNA2. Counterintuitively, the mechanism of seizure susceptibility here does not appear to arise from increased pyramidal cell excitability, but rather from synaptic overcompensation.