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

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_v1.6 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.