Impaired Color Recognition in HCN1 Epilepsy: A Single Case Report

Retinal Dysfunction in a Mouse Model of HCN1 Genetic Epilepsy

Pathogenic variants in HCN1 are associated with a range of epilepsy syndromes including a developmental and epileptic encephalopathy. The recurrent de novo HCN1 pathogenic variant (M305L) results in a cation leak, allowing the flux of excitatory ions at potentials where the wild-type channels are closed. The Hcn1M294L mouse recapitulates patient seizure and behavioral phenotypes. As HCN1 channels are highly expressed in rod and cone photoreceptor inner segments, where they shape the light response, mutated channels are likely to impact visual function. Electroretinogram (ERG) recordings from male and female mice Hcn1M294L mice revealed a significant decrease in the photoreceptor sensitivity to light, as well as attenuated bipolar cell (P2) and retinal ganglion cell responses. Hcn1M294L mice also showed attenuated ERG responses to flickering lights. ERG abnormalities are consistent with the response recorded from a single female human subject. There was no impact of the variant on the structure or expression of the Hcn1 protein in the retina. In silico modeling of photoreceptors revealed that the mutated HCN1 channel dramatically reduced light-induced hyperpolarization, resulting in more Ca2+ flux during the response when compared with the wild-type situation. We propose that the light-induced change in glutamate release from photoreceptors during a stimulus will be diminished, significantly blunting the dynamic range of this response. Our data highlight the importance of HCN1 channels to retinal function and suggest that patients with HCN1 pathogenic variants are likely to have a dramatically reduced sensitivity to light and a limited ability to process temporal information.SIGNIFICANCE STATEMENT Pathogenic variants in HCN1 are emerging as an important cause of catastrophic epilepsy. HCN1 channels are ubiquitously expressed throughout the body, including the retina. Electroretinogram recordings from a mouse model of HCN1 genetic epilepsy showed a marked decrease in the photoreceptor sensitivity to light and a reduced ability to respond to high rates of light flicker. No morphologic deficits were noted. Simulation data suggest that the mutated HCN1 channel blunts light-induced hyperpolarization and consequently limits the dynamic range of this response. Our results provide insights into the role HCN1 channels play in retinal function as well as highlighting the need to consider retinal dysfunction in disease caused by HCN1 variants. The characteristic changes in the electroretinogram open the possibility of using this tool as a biomarker for this HCN1 epilepsy variant and to facilitate development of treatments.

Efficacy of antiseizure medication in a mouse model of HCN1 developmental and epileptic encephalopathy

Acquisition of drug-sensitivity profiles is challenging in rare epilepsies. Anecdotal evidence suggests that antiseizure medications that block sodium channels as their primary mechanism of action exacerbate seizures in HCN1 developmental and epileptic encephalopathies (DEEs), whereas sodium valproate is effective for some patients. The Hcn1 M294L heterozygous knock-in (Hcn1M294L ) mouse carries the homologue of the recurrent gain-of-function HCN1 M305L pathogenic variant and recapitulates the seizure and some behavioral phenotypes observed in patients. We used this mouse model to study drug efficacy in HCN1 DEE. Hcn1M294L mice display epileptiform spiking on electrocorticography (ECoG), which we used as a quantifiable measure of drug effect. Phenytoin, lamotrigine, and retigabine significantly increased ECoG spike frequency, with lamotrigine and retigabine triggering seizures in a subset of the mice tested. In addition, there was a strong trend for carbamazepine to increase spiking. In contrast, levetiracetam, diazepam, sodium valproate, and ethosuximide all significantly reduced ECoG spike frequency. Drugs that reduced spiking did not cause any consistent ECoG spectral changes, whereas drugs that increased spiking all increased power in the slower delta and/or theta bands. These data provide a framework on which to build our understanding of gain-of-function HCN1 DEE pharmacosensitivity in the clinical setting.