Progressive epileptic encephalopathy associated with a novel HCN2 mutation

So far mutations in HCN2 gene, encoding for the hyperpolarization-activated cyclic nucleotide-gated channel 2, have been related to mild epileptic clinical phenotypes. In this study, a novel HCN2 mutation was found in a proband, now aged 7 years old, with a congenital encephalopathy characterized by drug resistant epilepsy, severe developmental delay, ataxia, dystonia and cerebral visual impairment. A convulsive status epilepticus at 5 months of age marked the onset of epilepsy. The HCN2 mutation affects an aminoacid located in the S6 transmembrane helix (p.Gly460Asp) and is carried in heterozygosis. To describe the functional consequences of the mutant channel, whole-cell patch-clamp experiments were performed in HEK293 cells expressing HCN2 wild type (WT) or p.Gly460Asp channel. A coexpression of the same amount of plasmid encoding for the wt or the mutant form of the channel mimed the heterozygous condition. A complete abolishment of the current was observed considering the mutant compared to the WT channel (-9.9±1.2 pA/pF, n=20 vs -31.2±8.4 pA/pF, n=44 respectively; p<0.05) and the heterozygous condition led to a significant reduction of the current density (-20.1±5.1 pA/pF n=35; p<0.05). WT and heterozygotic channels shared overlapping activation curves (V1/2 and k -92.9±0.3 mV and 5.6±0.3, n=29 vs -91.6±0.2 mV and 5.6±0.2, n=16 respectively) and no significant differences were present in their kinetics of both activation and deactivation. In conclusion, this is the first study linking HCN2 to progressive epileptic encephalopathy. The Gly460Asp mutation seems to act as a loss-of-function that could potentially affect the control of neuronal excitability and therefore explain the proband pathological condition.