Seizures, behavioral deficits, and adverse drug responses in two new genetic mouse models of HCN1 epileptic encephalopathy

Abstract

De novo mutations in voltage- and ligand-gated channels have been associated with an increasing number of cases of developmental and epileptic encephalopathies, which often fail to respond to classic antiseizure medications. Here, we examine two knock-in mouse models replicating de novo sequence variations in the HCN1 voltage-gated channel gene, p.G391D and p.M153I (Hcn1G380D/+ and Hcn1M142I/+ in mouse), associated with severe drug-resistant neonatal- and childhood-onset epilepsy, respectively. Heterozygous mice from both lines displayed spontaneous generalized tonic-clonic seizures. Animals replicating the p.G391D variant had an overall more severe phenotype, with pronounced alterations in the levels and distribution of HCN1 protein, including disrupted targeting to the axon terminals of basket cell interneurons. In line with clinical reports from patients with pathogenic HCN1 sequence variations, administration of the antiepileptic Na+ channel antagonists lamotrigine and phenytoin resulted in the paradoxical induction of seizures in both mouse lines, consistent with an effect to further impair inhibitory neuron function. We also show that these variants can render HCN1 channels unresponsive to classic antagonists, indicating the need to screen mutated channels to identify novel compounds with diverse mechanism of action. Our results underscore the necessity of tailoring effective therapies for specific channel gene variants, and how strongly validated animal models may provide an invaluable tool towards reaching this objective.

The HCN1 hyperpolarization-activated cyclic nucleotide-gated channel enhances evoked GABA release from parvalbumin positive interneurons

“…Patients carrying the more severe HCN1 mutations also show cognitive impairments and autistic traits. While recent studies have shown the impact of function-impairing HCN channel mutations in pyramidal cells in the cortex and hippocampus (Bleakley et al, 2021; Merseburg et al, 2022) on epileptiform activity, the reduction of inhibition, reported in this study can be expected to contribute to excessive excitability in the hippocampal network and investigating inhibitory inputs from PV+ INs in these pathological mouse lines may be indicated.…”

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

“…Carbamazepine also induced an increase in spiking and caused a seizure in two mice. Lamotrigine and phenytoin have been reported to cause increased seizure frequency in three HCN1 DEE patients to date 2,13,14 . Furthermore, two additional heterozygous knock‐in mouse models of GOF HCN1 DEE, Hcn1 G380D and Hcn1 M143L , also experienced lamotrigine‐ and phenytoin‐mediated seizures 14 .…”

“…DS is thought to be caused by reduced γ‐aminobutyric acid (GABA)ergic inhibitory neuron activity, with sodium channel–blocking drugs proposed to further exacerbate this disinhibition causing increased seizure susceptibility 23 . In contrast, HCN1 channels are expressed predominantly in pyramidal (excitatory) neurons, with GOF pathogenic variants resulting in a depolarized resting potential 13,14 . However, HCN1 channels are found in some inhibitory interneurons, especially those expressing parvalbumin 24 .…”

“…Sodium valproate is the only antiseizure medication (ASM) that has been reported to be an effective treatment for more than two patients with HCN1 epilepsy 2,4,13 . There is also anecdotal evidence that drugs that work primarily through sodium channel block, including lamotrigine and phenytoin, can exacerbate seizures 2,13,14 . A systematic study of the effect of common ASMs on HCN1 epilepsy is challenging given the rarity of the condition.…”

“…HCN1 epilepsy has been modeled in mice 13,14 . The Hcn1 M294L heterozygous knock‐in mouse is engineered to carry the mouse homologue of the human HCN1 M305L pathogenic variant 2,13 .…”