A Loss-of-Function HCN4 Mutation Associated With Familial Benign Myoclonic Epilepsy in Infancy Causes Increased Neuronal Excitability

Abstract

HCN channels are highly expressed and functionally relevant in neurons and increasing evidence demonstrates their involvement in the etiology of human epilepsies. Among HCN isoforms, HCN4 is important in cardiac tissue, where it underlies pacemaker activity. Despite being expressed also in deep structures of the brain, mutations of this channel functionally shown to be associated with epilepsy have not been reported yet. Using Next Generation Sequencing for the screening of patients with idiopathic epilepsy, we identified the p.Arg550Cys (c.1648C>T) heterozygous mutation on HCN4 in two brothers affected by benign myoclonic epilepsy of infancy. Functional characterization in heterologous expression system and in neurons showed that the mutation determines a loss of function of HCN4 contribution to activity and an increase of neuronal discharge, potentially predisposing to epilepsy. Expressed in cardiomyocytes, mutant channels activate at slightly more negative voltages than wild-type (WT), in accordance with borderline bradycardia. While HCN4 variants have been frequently associated with cardiac arrhythmias, these data represent the first experimental evidence that functional alteration of HCN4 can also be involved in human epilepsy through a loss-of-function effect and associated increased neuronal excitability. Since HCN4 appears to be highly expressed in deep brain structures only early during development, our data provide a potential explanation for a link between dysfunctional HCN4 and infantile epilepsy. These findings suggest that it may be useful to include HCN4 screening to extend the knowledge of the genetic causes of infantile epilepsies, potentially paving the way for the identification of innovative therapeutic strategies.

The hyperpolarization‐activated cyclic nucleotide‐gated 4 channel as a potential anti‐seizure drug target

“…Powell et al, 2008; Strauss et al, 2004). There is also an association between neuronal hyperexcitability and genetic change in HCN1 , HCN2 and HCN4 (Becker et al, 2017; Bonzanni et al, 2018; Campostrini et al, 2018; Dibbens et al, 2010; Marini et al, 2018; Nava et al, 2014). This relationship is complex, with evidence for both increased and decreased I h associating with heightened neuronal excitability.…”

Refining critical regions in 15q24 microdeletion syndrome pertaining to autism

“…Recent studies showed it is highly expressed in habenula and thalamus, but is low expression level in amygdala, cortex, and hippocampus, which are known ASD associated regions. (Oyrer et al, 2019;Seo et al, 2015) and a loss of function mutation in HCN4 is associated with development of infantile epilepsy (Campostrini et al, 2018).…”

“…Recent studies showed it is highly expressed in habenula and thalamus, but is low expression level in amygdala, cortex, and hippocampus, which are known ASD associated regions. (Oyrer et al, ; Seo et al, 2015) and a loss of function mutation in HCN4 is associated with development of infantile epilepsy (Campostrini et al, ). ADPGK is ADP‐dependent glucokinase and an alternative, glycolytic enzyme mediating generation of the oxidative signal whose downregulation inhibits oxidative signal and induces NF‐kB‐dependent gene expression, which makes T cell activation (Kaminski et al, ).…”

Ectopic HCN4 expression drives mTOR-dependent epilepsy

“…Another study reported no change in HCN4 expression in the hippocampus following febrile seizures despite changes in HCN1 (56). Loss-of-function mutations in HCN4 has also recently been associated with benign myoclonic epilepsy (57). More recently, increased HCN4 expression in the hippocampal dentate gyrus has been reported in individuals with SUDEP (58).…”