Antiepileptic activity of preferential inhibitors of persistent sodium current

Use-Dependent Block of Human Cardiac Sodium Channels by GS967

GS-458967, 6-(4-(Trifluoromethoxy)phenyl)-3-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine (GS967) is a recently described, novel, sodium channel inhibitor exhibiting potent antiarrhythmic effects in various in vitro and in vivo models. The antiarrhythmic mechanism has been attributed to preferential suppression of late sodium current. However, there has been no reported systematic investigation of the effects of this compound on isolated sodium channels. Here, we examined the effects of GS967 on peak (INaP) and late (INaL) sodium current recorded from cells that heterologously expressed human cardiac voltage-gated sodium channel, the principle cardiac sodium channel. As previously described, we observed that GS967 exerted tonic block of INaL (63%) to a significantly greater extent than INaP (19%). However, GS967 also caused a reduction of INaP in a frequency-dependent manner, consistent with use-dependent block (UDB). GS967 evoked more potent UDB of INaP (IC50 = 0.07 µM) than ranolazine (16 µM) and lidocaine (17 µM). Use-dependent block was best explained by a significant slowing of recovery from fast and slow inactivation with a significant enhancement of slow inactivation in the presence of GS967. Furthermore, GS967 was found to exert these same effects on a prototypical long QT syndrome mutation (delKPQ). An engineered mutation at an interaction site for local anesthetic agents (F1760A) partially attenuated the effect of GS967 on UDB, but had no effect on tonic INaL block. We conclude that GS967 is a preferential inhibitor of INaL, but it also exerts previously unreported strong effects on slow inactivation and recovery from inactivation, resulting in substantial UDB that is not entirely dependent on a known interaction site for local anesthetic agents.

Cacna1g is a genetic modifier of epilepsy caused by mutation of voltage-gated sodium channel Scn2a

More than 1,200 mutations in neuronal voltage-gated sodium channel (VGSC) genes have been identified in patients with several epilepsy syndromes. A common feature of genetic epilepsies is variable expressivity among individuals with the same mutation. The Scn2a(Q54) transgenic mouse model has a mutation in Scn2a that results in spontaneous epilepsy. Scn2a(Q54) phenotype severity varies depending on the genetic strain background, making it a useful model for identifying and characterizing epilepsy modifier genes. Scn2a(Q54) mice on the [C57BL/6JxSJL/J]F1 background exhibit earlier seizure onset, elevated spontaneous seizure frequency, and decreased survival compared to Scn2a(Q54) mice congenic on the C57BL/6J strain. Genetic mapping and RNA-Seq analysis identified Cacna1g as a candidate modifier gene at the Moe1 locus, which influences Scn2a(Q54) phenotype severity. In this study, we evaluated the modifier potential of Cacna1g, encoding the Cav3.1 voltage-gated calcium channel, by testing whether transgenic alteration of Cacna1g expression modifies severity of the Scn2a(Q54) seizure phenotype. Scn2a(Q54) mice exhibited increased spontaneous seizure frequency with elevated Cacna1g expression and decreased seizure frequency with decreased Cacna1g expression. These results provide support for Cacna1g as an epilepsy modifier gene and suggest that modulation of Cav3.1 may be an effective therapeutic strategy.

Molecular subtyping and improved treatment of neurodevelopmental disease

The next-generation sequencing revolution has substantially increased our understanding of the mutated genes that underlie complex neurodevelopmental disease. Exome sequencing has enabled us to estimate the number of genes involved in the etiology of neurodevelopmental disease, whereas targeted sequencing approaches have provided the means for quick and cost-effective sequencing of thousands of patient samples to assess the significance of individual genes. By leveraging such technologies and clinical exome sequencing, a genotype-first approach has emerged in which patients with a common genotype are first identified and then clinically reassessed as a group. This approach has proven a powerful methodology for refining disease subtypes. We propose that the molecular characterization of these genetic subtypes has important implications for diagnostics and also for future drug development. Classifying patients into subgroups with a common genetic etiology and applying treatments tailored to the specific molecular defect they carry is likely to improve management of neurodevelopmental disease in the future.

Calcium Imaging of Neuronal Activity in Drosophila Can Identify Anticonvulsive Compounds

Although there are now a number of antiepileptic drugs (AEDs) available, approximately one-third of epilepsy patients respond poorly to drug intervention. The reasons for this are complex, but are probably reflective of the increasing number of identified mutations that predispose individuals to this disease. Thus, there is a clear requirement for the development of novel treatments to address this unmet clinical need. The existence of gene mutations that mimic a seizure-like behaviour in the fruit fly, Drosophila melanogaster, offers the possibility to exploit the powerful genetics of this insect to identify novel cellular targets to facilitate design of more effective AEDs. In this study we use neuronal expression of GCaMP, a potent calcium reporter, to image neuronal activity using a non-invasive and rapid method. Expression in motoneurons in the isolated CNS of third instar larvae shows waves of calcium-activity that pass between segments of the ventral nerve cord. Time between calcium peaks, in the same neurons, between adjacent segments usually show a temporal separation of greater than 200 ms. Exposure to proconvulsants (picrotoxin or 4-aminopyridine) reduces separation to below 200 ms showing increased synchrony of activity across adjacent segments. Increased synchrony, characteristic of epilepsy, is similarly observed in genetic seizure mutants: bangsenseless¹ (bss¹) and paralytic^K1270T (para^K1270T). Exposure of bss¹ to clinically-used antiepileptic drugs (phenytoin or gabapentin) significantly reduces synchrony. In this study we use the measure of synchronicity to evaluate the effectiveness of known and novel anticonvulsive compounds (antipain, isethionate, etopiside rapamycin and dipyramidole) to reduce seizure-like CNS activity. We further show that such compounds also reduce the Drosophila voltage-gated persistent Na⁺ current (I_NaP) in an identified motoneuron (aCC). Our combined assays provide a rapid and reliable method to screen unknown compounds for potential to function as anticonvulsants.

Pathogenic mechanism of recurrent mutations of SCN8A in epileptic encephalopathy

Objective

The early infantile epileptic encephalopathy type 13 (EIEE13, OMIM #614558) results from de novo missense mutations of SCN8A encoding the voltage‐gated sodium channel Na_v1.6. More than 20% of patients have recurrent mutations in residues Arg1617 or Arg1872. Our goal was to determine the functional effects of these mutations on channel properties.

Methods

Clinical exome sequencing was carried out on patients with early‐onset seizures, developmental delay, and cognitive impairment. Two mutations identified here, p.Arg1872Leu and p.Arg1872Gln, and two previously identified mutations, p.Arg1872Trp and p.Arg1617Gln, were introduced into Na_v1.6 cDNA, and effects on electrophysiological properties were characterized in transfected ND7/23 cells. Interactions with FGF14, G‐protein subunit Gβγ, and sodium channel subunit β1 were assessed by coimmunoprecipitation.

Results

We identified two patients with the novel mutation p.Arg1872Leu and one patient with the recurrent mutation p.Arg1872Gln. The three mutations of Arg1872 and the mutation of Arg1617 all impaired the sodium channel transition from open state to inactivated state, resulting in channel hyperactivity. Other observed abnormalities contributing to elevated channel activity were increased persistent current, increased peak current density, hyperpolarizing shift in voltage dependence of activation, and depolarizing shift in steady‐state inactivation. Protein interactions were not affected.

Interpretation

Recurrent mutations at Arg1617 and Arg1872 lead to elevated Na_v1.6 channel activity by impairing channel inactivation. Channel hyperactivity is the major pathogenic mechanism for gain‐of‐function mutations of SCN8A. EIEE13 differs mechanistically from Dravet syndrome, which is caused by loss‐of‐function mutations of SCN1A. This distinction has important consequences for selection of antiepileptic drugs and the development of gene‐ and mutation‐specific treatments.

Hlf is a genetic modifier of epilepsy caused by voltage-gated sodium channel mutations

Mutations in voltage-gated sodium channel genes cause several types of human epilepsies. Often, individuals with the same sodium channel mutation exhibit diverse phenotypes. This suggests that factors beyond the primary mutation influence disease severity, including genetic modifiers. Mouse epilepsy models with voltage-gated sodium channel mutations exhibit strain-dependent phenotype variability, supporting a contribution of genetic modifiers in epilepsy. The Scn2a(Q54) (Q54) mouse model has a strain-dependent epilepsy phenotype. Q54 mice on the C57BL/6J (B6) strain exhibit delayed seizure onset and improved survival compared to [B6xSJL/J]F1.Q54 mice. We previously mapped two dominant modifier loci that influence Q54 seizure susceptibility and identified Hlf (hepatic leukemia factor) as a candidate modifier gene at one locus. Hlf and other PAR bZIP transcription factors had previously been associated with spontaneous seizures in mice thought to be caused by down-regulation of the pyridoxine pathway. An Hlf targeted knockout mouse model was used to evaluate the effect of Hlf deletion on Q54 phenotype severity. Hlf(KO/KO);Q54 double mutant mice exhibited elevated frequency and reduced survival compared to Q54 controls. To determine if direct modulation of the pyridoxine pathway could alter the Q54 phenotype, mice were maintained on a pyridoxine-deficient diet for 6 weeks. Dietary pyridoxine deficiency resulted in elevated seizure frequency and decreased survival in Q54 mice compared to control diet. To determine if Hlf could modify other epilepsies, Hlf(KO/+) mice were crossed with the Scn1a(KO/+) Dravet syndrome mouse model to examine the effect on premature lethality. Hlf(KO/+);Scn1a(KO/+) offspring exhibited decreased survival compared to Scn1a(KO/+) controls. Together these results demonstrate that Hlf is a genetic modifier of epilepsy caused by voltage-gated sodium channel mutations and that modulation of the pyridoxine pathway can also influence phenotype severity.

Seizure suppression through manipulating splicing of a voltage-gated sodium channel

Voltage-gated persistent sodium current (INaP) is a tractable target for antiepileptic drugs. Using a strategy focused on INaP reduction, Lin et al. identify 95 regulators of voltage-gated sodium channel splicing for which RNAi knockdown reduces seizure duration in Drosophila. Manipulation of splicing regulators could improve control of epilepsy.

Seizure can result from increased voltage-gated persistent sodium current expression. Although many clinically-approved antiepileptic drugs target voltage-gated persistent sodium current, none exclusively repress this current without also adversely affecting the transient voltage-gated sodium current. Achieving a more selective block has significant potential for the treatment of epilepsy. Recent studies show that voltage-gated persistent sodium current amplitude is regulated by alternative splicing offering the possibility of a novel route for seizure control. In this study we identify 291 splicing regulators that, on knockdown, alter splicing of the Drosophila voltage-gated sodium channel to favour inclusion of exon K, rather than the mutually exclusive exon L. This change is associated with both a significant reduction in voltage-gated persistent sodium current, without change to transient voltage-gated sodium current, and to rescue of seizure in this model insect. RNA interference mediated knock-down, in two different seizure mutants, shows that 95 of these regulators are sufficient to significantly reduce seizure duration. Moreover, most suppress seizure activity in both mutants, indicative that they are part of well conserved pathways and likely, therefore, to be optimal candidates to take forward to mammalian studies. We provide proof-of-principle for such studies by showing that inhibition of a selection of regulators, using small molecule inhibitors, is similarly effective to reduce seizure. Splicing of the Drosophila sodium channel shows many similarities to its mammalian counterparts, including altering the amplitude of voltage-gated persistent sodium current. Our study provides the impetus to investigate whether manipulation of splicing of mammalian voltage-gated sodium channels may be exploitable to provide effective seizure control.