Gain-of-function FHF1 mutation causes early-onset epileptic encephalopathy with cerebellar atrophy

Abstract

Objective: Voltage-gated sodium channel (Na v )-encoding genes are among early-onset epileptic encephalopathies (EOEE) targets, suggesting that other genes encoding Na v -binding proteins, such as fibroblast growth factor homologous factors (FHFs), may also play roles in these disorders. Methods:To identify additional genes for EOEE, we performed whole-exome sequencing in a family quintet with 2 siblings with a lethal disease characterized by EOEE and cerebellar atrophy. The pathogenic nature and functional consequences of the identified sequence alteration were determined by electrophysiologic studies in vitro and in vivo.Results: A de novo heterozygous missense mutation was identified in the FHF1 gene (FHF1A R114H , FHF1B R52H ) in the 2 affected siblings. The mutant FHF1 proteins had a strong gain-of-function phenotype in transfected Neuro2A cells, enhancing the depolarizing shifts in Na v 1.6 voltage-dependent fast inactivation, predicting increased neuronal excitability. Surprisingly, the gain-of-function effect is predicted to result from weaker interaction of mutant FHF1 with the Na v cytoplasmic tail. Transgenic overexpression of mutant FHF1B in zebrafish larvae enhanced epileptiform discharges, demonstrating the epileptic potential of this FHF1 mutation in the affected children.Conclusions: Our data demonstrate that gain-of-function FHF mutations can cause neurologic disorder, and expand the repertoire of genetic causes (FHF1) and mechanisms (altered Na v gating) underlying EOEE and cerebellar atrophy. Neurology ® 2016;86:2162-2170 GLOSSARY EOEE 5 early-onset epileptic encephalopathies; FHF 5 fibroblast growth factor homologous factor; Na v 5 voltage-gated sodium channel; OE 5 overexpression; WT 5 wild-type.Early-onset epileptic encephalopathies (EOEE) are characterized by progressive diffuse brain dysfunction with recurrent seizures starting during the neonatal or early infantile periods. These epilepsies are among the most severe, with the child typically experiencing multiple seizure types, which are often refractory to antiepileptic drugs, in the setting of developmental delay or regression. Epileptic encephalopathies can worsen over time, and the epileptic activity itself may contribute to severe cognitive, neurologic, and behavioral impairments above and beyond what might be expected from the underlying pathology alone.1 EOEE represent a broad spectrum of phenotypes that are highly heterogeneous at the clinical and molecular levels. Whereas a number of genetically determined (monogenic) forms have been recognized, in clinical practice many cases remain of unknown etiology.2 Of the genetic mutations known to be linked to EOEE, several are missense mutations in SCN8A encoding the a pore-forming subunits of the voltage-gated sodium channel (Na v ) 1.6, 3-5 with some of these mutations acting in a gain-offunction manner to enhance sodium channel current.3,4 So far there has been no report of EOEE caused by mutation of sodium channel-binding proteins, such as fibroblast growth factor *These aut...

FHF1 (FGF12) epileptic encephalopathy

“…3 Recently, a de novo FHF1 mutation (p.R52H) was reported in early-onset EE in 2 siblings. 4 We report 3 patients from unrelated families with the same FHF1 p.R52H mutation. The 5 cases together frame the FHF1 R52H EE from infancy to adulthood.…”

“…The table summarizes their clinical features and those of the recently published 4 original sib-pair. Salient features of the latter pair include neonatal-onset intractable epilepsy, profound intellectual disability, severe feeding difficulties, MRI initially unremarkable and subsequently exhibiting cerebellar atrophy, ataxia, and death in SE.…”

“…Siekierska et al 4 demonstrated in vitro and in vivo that R52H is a toxic gain-of-function mutation. To date, thousands of patients with EE have undergone exome sequencing.…”

De novo FGF12 mutation in 2 patients with neonatal-onset epilepsy

“…2 The mutation is a de novo heterozygous G>A transition at position 155 of the complementary DNA (NM_004113) which results in a p.R52H missense substitution. Sanger sequencing was performed in both trios and confirmed that the mutation was only present in the affected probands.…”

“…Recently, WES in a family quintet with 2 affected siblings with early-onset epileptic encephalopathy (EOEE) and cerebellar atrophy identified fibroblast growth factor 12 ( FGF12 , also known as FHF1 ) as a putative gene responsible. 2 A de novo gain-of-function variant (p.R52H) was found in 2 affected siblings with lethal disease. FGF12 encodes a cytosolic protein that interacts with neuronal sodium channels and increases their voltage-dependent fast inactivation.…”