Gaining Awareness of Increasingly Persistent SCN1A Mutations

Abstract

SCN1A Gain-of-Function Mutation Causing an Early Onset Epileptic Encephalopathy

Clatot J, Parthasarathy S, Cohen S, McKee JL, Massey S, Somarowthu A, Goldberg EM, Helbig I. Epilepsia. 2022;1-13. doi:10.1111/epi.17444

Objective:

Loss-of-function variants in SCN1A cause Dravet syndrome, the most common genetic developmental and epileptic encephalopathy (DEE). However, emerging evidence suggests separate entities of SCN1A-related disorders due to gain-of-function variants. Here, we aim to refine the clinical, genetic, and functional electrophysiological features of a recurrent p.R1636Q gain-of-function variant, identified in four individuals at a single center.

Methods:

Individuals carrying the recurrent SCN1A p.R1636Q variant were identified through diagnostic testing. Whole cell voltage-clamp electrophysiological recording in HEK-293 T cells was performed to compare the properties of sodium channels containing wild-type Na_v1.1 or Na_v1.1-R1636Q along with both Na_vβ1 and Na_vβ2 subunits, including response to oxcarbazepine. To delineate differences from other SCN1A-related epilepsies, we analyzed electronic medical records.

Results:

All four individuals had an early onset DEE characterized by focal tonic seizures and additional seizure types starting in the first few weeks of life. Electrophysiological analysis showed a mixed gain-of-function effect with normal current density, a leftward (hyperpolarized) shift of steady-state inactivation, and slower inactivation kinetics leading to a prominent late sodium current. The observed functional changes closely paralleled effects of pathogenic variants in SCN3A and SCN8A at corresponding positions. Both wild type and variant exhibited sensitivity to block by oxcarbazepine, partially correcting electrophysiological abnormalities of the SCN1A p.R1636Q variant. Clinically, a single individual responded to treatment with oxcarbazepine. Across 51 individuals with SCN1A-related epilepsies, those with the recurrent p.R1636Q variants had the earliest ages at onset.

Significance:

The recurrent SCN1A p.R1636Q variant causes a clinical entity with a wider clinical spectrum than previously reported, characterized by neonatal onset epilepsy and absence of prominent movement disorder. Functional consequences of this variant lead to mixed loss and gain of function that is partially corrected by oxcarbazepine. The recurrent p.R1636Q variant represents one of the most common causes of early onset SCN1A-related epilepsies with separate treatment and prognosis implications.

Commentary

Loss-of-function (LOF) mutations in the voltage-gated sodium channel SCN1A gene (encoding Na_v1.1 channels) are known to cause several types of epilepsy including Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). ¹ A diagnosis of a LOF mutation in SCN1A can aid in the selection of treatment approaches. For example, sodium channel blockers such as oxcarbazepine should be avoided in patients with DS as these drugs can exacerbate disease. In contrast to the well-studied LOF SCN1A mutations, there is increasing recognition of gain-of-function (GOF) SCN1A mutations associated with familial hemiplegic migraine (FHM) and an earlier onset epileptic encephalopathy distinct from DS. For example, functional analysis of the SCN1A p.L1670W mutation found in multiple families with FHM revealed an overall GOF effect due to modification of gating properties. ² Sadleir et al, identified 9 unrelated children with early onset epileptic encephalopathy, hyperkinetic movement, and profound developmental delay; notably, 8 of the 9 children harbored the SCN1A p.Thr226Met missense mutation. ³ When tested in Chinese hamster ovary cells, Na_v1.1 channels expressing the p.T226M mutation generated hyperpolarizing shifts of the activation and inactivation curves and enhanced fast inactivation, suggesting an overall GOF effect. ⁴

Brunklaus et al., recently identified 5 patients with the recurrent SCN1A p.R1636Q mutation who presented with multiple seizure types within the first few months of life, in addition to hyperkinesia, chorea, and dystonia. ⁵ The current study by Clatot et al., ⁶ broadens the clinical spectrum of patients that harbor the SCN1A p.R1636Q mutation by describing 4 additional unrelated patients who exhibited varying severities of early onset epilepsy and motor function that ranged from normal to hyperkinesia and chorea. ⁶ Clatot et al., expressed the SCN1A p.R1636Q mutation in HEK293T cells and observed hyperpolarization of the inactivation curve, altered inactivation kinetics, and increased persistent current when compared to wild-type channels. ⁶ These observations are consistent with the functional data reported by Brunklaus and colleagues. ⁵ Interestingly, Brunklaus et al., also noted additional SCN1A variants that resulted in increased persistent sodium currents, including p.I1498 M (associated with FHM), and p.I126V and p.I1498T (associated with epilepsy). These observations provide the opportunity for treatments that target the functional consequence of the mutation (i.e, increased persistent current). For example, in the current study, the sodium channel blocker oxcarbazepine was able to significantly reduce peak and persistent sodium currents in HEK293T cells expressing the SCN1A p.R1636Q mutation. ⁶

Information gained from the study of GOF mutations in other sodium channels could potentially be leveraged to guide treatment strategies for patients with GOF SCN1A mutations. The SCN1A p.R1636Q mutation is located within the S4 transmembrane segment of domain IV (DIVS4), which is a highly evolutionarily conserved region of the channel. The DIVS4 region is critical for voltage sensing; thus, it is not surprising that mutations in this region can lead to epilepsy and behavioral deficits. Notably, the recurrent p.R1617Q mutation, located at the corresponding position in the paralogous SCN8A gene (encoding Na_v1.6 channels), is associated with early epileptic encephalopathy. Na_v1.6 channels expressing this mutation exhibit a slower decay of transient current and increased resurgent and persistent currents. ⁷ The SCN8A p.R1620L mutation, which affects the adjacent DIVS4 arginine residue, was identified in a patient with a number of behavioral deficits including autism, attention-deficit hyperactivity disorder, intellectual disability, and behavioral seizure-like activity but normal electrographic activity. ⁸ In vitro functional analysis of Na_v1.6 channels expressing the p.R1620L mutation revealed GOF and LOF effects. ⁸ When the SCN8A p.R1620L mutation was knocked into the mouse, heterozygous mutants (RL/+) exhibited increased seizure susceptibility, spontaneous seizures, a range of behavioral abnormalities, and similar to the in vitro assays, electrophysiological analyses from the RL/+ mutants revealed GOF and LOF effects. ⁹ Treatment of RL/+ mutants with the sodium channel blocker oxcarbazepine provided robust protection against 6 Hz- and pentylenetetrazole-induced seizures. ⁹ Consistent with the idea that sodium channel blockers may be beneficial against GOF mutations in SCN1A and SCN8A, in the current study, 1 of the 4 patients was successfully treated with oxcarbazepine. ⁶ However, while sodium channel blockers are predicted to be effective against GOF sodium channel mutations, the clinical application of these drugs is limited by tolerability, especially when high doses are needed to achieve an anti-seizure effect. Furthermore, high doses of sodium channel blockers can result in unwanted side effects. Thus, targeting specific types of sodium currents, such as persistent or resurgent currents, may yield an anti-seizure effect with less side effects.

Some anti-seizure drugs have demonstrated the ability to modulate resurgent and persistent currents. For example, while cannabidiol (CBD) is used in the treatment of DS, it has also been shown to effectively block resurgent and persistent currents caused by mutant Na_v1.6 channels. ¹⁰ Furthermore, CBD was shown to provide robust seizure protection and restore more normal social behavior in the Scn8a RL/+ mutants. ¹¹ These observations raise the possibility that CBD could be used to block persistent currents due to GOF SCN1A mutations. In the current study, 1 of the 4 patients with the SCN1A p.R1636Q mutation achieved seizure freedom with a combination of CBD, leviteracetam, topiramate, and clobazam; however, another patient was refractory to treatment with CBD, leviteracetam, topiramate, and fenfluramine. ⁶ A recent retrospective trial of patients with DS revealed that treatment with cenobamate, a novel inhibitor of persistent sodium currents, significantly reduced seizure frequency in 4 patients. ¹² In addition to blocking persistent sodium currents, cenobamate acts as a positive allosteric modulator of the GABA_A receptor. Toward the development of drugs that selectively target persistent sodium currents, Praxis Precision Medicine developed Prax330/GS967, a novel potent inhibitor of sodium persistent currents. Prax330/GS967 was found to normalize neuronal firing, protect against induced seizures and spontaneous seizures, and prolong survival in the Scn8a^N1768D/+ mutants. ¹³ Given the availability of drugs that target persistent sodium currents, blocking persistent sodium currents may be a treatment option for patients with GOF SCN1A mutations.

Recent studies, including the current study by Clatot and colleagues, expand our understanding of GOF SCN1A mutations and underscore the benefit of using functional data to help inform treatment strategies. Since routine functional analysis of identified variants is not feasible, the use of in silico tools to predict the functional consequence of a mutation should also be considered. Furthermore, we can potentially leverage our understanding of GOF mutations in other sodium channels (e.g, SCN8A) to help identify drugs that have demonstrated efficacy in preclinical models or patients with GOF sodium channel mutations, and ultimately, improve care for patients with GOF SCN1A mutations.

Jennifer C. Wong, PhD
Department of Human Genetics Emory University