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FHF2 (FGF13) modulation of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channel inactivation gating is required for heat nociception. Transient voltage-gated sodium currents are essential for the initiation and conduction of action potentials in neurons and cardiomyocytes. The amplitude and duration of sodium currents are tuned by intracellular fibroblast growth factor homologous factors (FHFs/iFGFs) that associate with the cytoplasmic tails of voltage-gated sodium channels (Navs), and genetic ablation of Fhf genes disturbs neurological and cardiac functions. Among reported phenotypes, Fhf2 null mice undergo lethal hyperthermia-induced cardiac conduction block attributable to the combined effects of FHF2 deficiency and elevated temperature on the cardiac sodium channel (Nav1.5) inactivation rate. Fhf2 null mice also display a lack of heat nociception, while retaining other somatosensory capabilities. Here, we use electrophysiological and computational methods to show that the heat nociception deficit can be explained by the combined effects of elevated temperature and FHF2 deficiency on the fast inactivation gating of Nav1.7 and tetrodotoxin-resistant sodium channels expressed in dorsal root ganglion C fibers. Hence, neurological and cardiac heat-associated deficits in Fhf2 null mice derive from shared impacts of FHF deficiency and temperature towards Nav inactivation gating kinetics in distinct tissues. [ABSTRACT FROM AUTHOR]

Summary: Objective: Fibroblast growth factor homologous factors (FHFs) are brain and cardiac sodium channel‐binding proteins that modulate channel density and inactivation gating. A recurrent de novo gain‐of‐function missense mutation in the FHF1(FGF12) gene (p.Arg52His) is associated with early infantile epileptic encephalopathy 47 (EIEE47; Online Mendelian Inheritance in Man database 617166). To determine whether the FHF1 missense mutation is sufficient to cause EIEE and to establish an animal model for EIEE47, we sought to engineer this mutation into mice. Methods: The Arg52His mutation was introduced into fertilized eggs by CRISPR (clustered regularly interspaced short palindromic repeats) editing to generate Fhf1R52H/F+ mice. Spontaneous epileptiform events in Fhf1R52H/+ mice were assessed by cortical electroencephalography (EEG) and video monitoring. Basal heart rhythm and seizure‐induced arrhythmia were recorded by electrocardiography. Modulation of cardiac sodium channel inactivation by FHF1BR52H protein was assayed by voltage‐clamp recordings of FHF‐deficient mouse cardiomyocytes infected with adenoviruses expressing wild‐type FHF1B or FHF1BR52H protein. Results: All Fhf1R52H/+ mice experienced seizure or seizurelike episodes with lethal ending between 12 and 26 days of age. EEG recordings in 19–20‐day‐old mice confirmed sudden unexpected death in epilepsy (SUDEP) as severe tonic seizures immediately preceding loss of brain activity and death. Within 2–53 s after lethal seizure onset, heart rate abruptly declined from 572 ± 16 bpm to 108 ± 15 bpm, suggesting a parasympathetic surge accompanying seizures that may have contributed to SUDEP. Although ectopic overexpression of FHF1BR52H in cardiomyocytes induced a 15‐mV depolarizing shift in voltage of steady‐state sodium channel inactivation and slowed the rate of channel inactivation, heart rhythm was normal in Fhf1R52H/+ mice prior to seizure. Significance: The Fhf1 missense mutation p.Arg52His induces epileptic encephalopathy with full penetrance in mice. Both Fhf1 (p.Arg52His) and Scn8a (p.Asn1768Asp) missense mutations enhance sodium channel Nav1.6 currents and induce SUDEP with bradycardia in mice, suggesting an FHF1/Nav1.6 functional axis underlying altered brain sodium channel gating in epileptic encephalopathy. [ABSTRACT FROM AUTHOR]

Fibroblast growth factor homologous factors (FHFs) are intracellular proteins which regulate voltage-gated sodium (Na v) channels in the brain and other tissues. FHF dysfunction has been linked to neurological disorders including epilepsy. Here, we describe two sibling pairs and three unrelated males who presented in infancy with intractable focal seizures and severe developmental delay. Whole-exome sequencing identified hemi- and heterozygous variants in the N-terminal domain of the A isoform of FHF2 (FHF2A). The X-linked FHF2 gene (also known as FGF13) has alternative first exons which produce multiple protein isoforms that differ in their N-terminal sequence. The variants were located at highly conserved residues in the FHF2A inactivation particle that competes with the intrinsic fast inactivation mechanism of Na v channels. Functional characterization of mutant FHF2A co-expressed with wild-type Na v 1.6 (SCN8A) revealed that mutant FHF2A proteins lost the ability to induce rapid-onset, long-term blockade of the channel while retaining pro-excitatory properties. These gain-of-function effects are likely to increase neuronal excitability consistent with the epileptic potential of FHF2 variants. Our findings demonstrate that FHF2 variants are a cause of infantile-onset developmental and epileptic encephalopathy and underline the critical role of the FHF2A isoform in regulating Na v channel function. [ABSTRACT FROM AUTHOR]