KCNQ2(NMF134)-mutant mice as a model for severe neonatal epilepsy

Abstract

Aims: Mutations in the human KCNQ2 and KCNQ3 genes encoding M-channel subunits are associated with an epilepsy syndrome of newborns (BFNC). The NMF134 mouse line (Jackson lab) that carries a V182M-mutation in the S3-helix of KCNQ2 displays a reduced seizure threshold in response to electrical stimulation. We decided to use the NMF134 mouse line as novel model for the generation of treatment strategies for severe neonatal epilepsies. Methods: In vitro effects of the V182M mutation in KCNQ2 were analyzed by heterologous expression in xenopus oocytes and patch-clamp recordings from acute hippocampal brain slices. In vivo drug treatment was performed by oral application of retigabine (2×10mg/kg/d) or bumetanide (2×0.1mg/kg/d) to pregnant mice from day 11 on and subcutaneous application of the same drug concentrations to pups from days 1 to 14. Results: Homozygous NMF134 mice were weaker and smaller than their wild-type littermates and died from spontaneous seizure activity during the first postnatal weeks with a mortality rate of around 60%. No gros morphological changes were observed in hippocampal brain regions. Electrophysiological analysis of the V182M mutation in xenopus oocytes showed a 50%- reduction of M-current amplitudes due to a significant shift of voltage-dependent gating to more positive membrane potentials. CA1 pyramidal neurons of homozygous mutated mice displayed increased action potential firing upon current injection. However, sensitivity to the anticonvulsive drug retigabine was unaffected. Preliminary results of pre- and early postnatal treatment studies with retigabine showed positive effects on thriving and survival of NMF pups, but negative effects on the survival of wild-type littermates. Apart from a reduction in observed seizure frequency during the second week of life, application of the chloride-pump inhibitor bumetanide had no positive effects on both NMF pups or wild-type littermates. Conclusion: Our results emphasize the difficulty of safe and effective drug treatment of neonatal epilepsy even under conditions where a causal therapeutic option is available.