Mislocalization of KCNQ2 Channels as a Pathogenic Mechanism in KCNQ2 Developmental and Epileptic Encephalopathy

Abstract

KCNQ2 potassium channel variants are linked to developmental and epileptic encephalopathy (DEE). However, the mechanisms by which pathogenic variants, especially those outside known hotspots, such as the S4–S5 linker, lead to disease remain unknown. Here, we examined the H228R variant, a pathogenic mutation in the S4–S5 linker associated with DEE. We tested whether H228R induces KCNQ2 channel mistargeting in addition to its biophysical effects, given recent evidence of impaired trafficking in KCNQ2 DEE variants. We confirmed the H228R variant as a loss-of-function (LOF) when expressed as a homomer and as a dominant-negative when coexpressed with wild-type (WT) KCNQ3. Surprisingly, it exhibited some gain-of-function effects when coexpressed with WT KCNQ2. To determine its cellular localization in vivo, we used male and female heterozygous Kcnq2 H228R knock-in mice, some of which die prematurely despite lack of increases in hippocampal excitatory neuron intrinsic excitability. We validated two different KCNQ2 antibodies in hippocampus via immunohistochemistry. These antibodies detected KCNQ2 in axons, with signal loss observed in Kcnq2 knock-out mice. Using these antibodies, we found that the H228R variant caused KCNQ2 channels to concentrate in the soma, strongly reducing their presence in axons. Further, analysis of heterozygous mice expressing both a FLAG-tagged WT KCNQ2 and H228R revealed that the FLAG-WT KCNQ2 could still traffic to axons, indicating that some KCNQ2 channels are correctly targeted within neurons. In summary, our results demonstrate that the LOF H228R variant disrupts the localization of variant KCNQ2 channels, suggesting mislocalization as a general endophenotype of KCNQ2 encephalopathy.