Pathophysiology of KCNQ2-related Disorders

SUMMARY | Pathogenic variants in KCNQ2 are among the most commonly discovered genetic etiologies of neonatal and infantile onset epilepsy. KCNQ2 encodes the voltage-gated potassium (KV) channel KV7.2, which forms hetero-tetrameric complexes with the related channel KV7.3 to generate M-current, a voltage-gated, slowly activating and deactivating current widely distributed in central and peripheral neurons. Activation of M-current at subthreshold membrane potentials opposes cell depolarization by incoming stimuli and dampens neuronal excitability. The clinical spectrum of KCNQ2-related disorders ranges from self-limited familial neonatal epilepsy (SLFNE) to sporadic cases of severe developmental and epileptic encephalopathy (DEE). It is not known why DEE patients suffer long-lasting intellectual and developmental disabilities, while SLFNE patients have normal neurodevelopment. This gap in knowledge hinders deployment and timing of optimal therapies. Current thinking posits that SLFNE patients harbor mutations that cause loss-of-function and haploinsufficiency, whereas DEE patients harbor mutations that exert dominant-negative effects or rarely gain-of-function. However, this inference does not fully explain the heterogenity in clinical outcomes. This is likely because this it is based on work in heterologous systems solely examining the effects of Kv7.2 mutations on channel function without considering the impact on trafficking, localization, and differential assembly of KV7.2 channels with other KV7 subunits in neurons. The objective of this proposal is to determine how KCNQ2 mutations impair the assembly and physiology of neuronal KV7 ion channels. We will utilize our multidisciplinary expertise and established toolbox of high throughput technologies and orthogonal cell models, including unique patient-specific induced pluripotent stem cell (iPSC) derived neurons and unique mouse models, to advance understanding of pathophysiological mechanisms that drive divergent phenotype severity in KCNQ2-related disorders. In Aim 1, we will employ our unique, large collection of patient-specific iPSC lines with heterozygous pathogenic KCNQ2 variants to determine drivers of phenotype divergence in this disorder, to assess trafficking and localization of mutant KV7.2, and to determine the subunit composition of KV7 channels in human neurons. In Aim 2, we will extend our investigations of channel composition in vivo using an existing portfolio of genetically engineered mice that express epitope-tagged endogenous KV7 channels subunits in brain. We will determine if pathogenic KV7.2 variants disrupt the normal time, region and cell type dependent formation of KV7 channel complexes. Finally in Aim 3, we will investigate the functional and pharmacological properties of pathogenic KCNQ2 variants when co-expressed with KV7.5, and determine if KCNQ2-DEE variants differ from KCNQ2-SLFNE variants in their responses to intracellular PIP2, a major regulator of neuronal M-current. Collectively, our work will fill fundamental gaps in our understanding of Kv7 channel diversity and the impact of pathogenic variants, and inform therapeutic strategies that seek to modify KCNQ2 patient outcomes.