Genotype-phenotype correlations in SCN8A-related disorders reveal prognostic and therapeutic implications

We report detailed functional analyses and genotype-phenotype correlations in 433 individuals carrying disease-causing variants in SCN8A , encoding the voltage-gated Na+ channel NaV1.6. Five different clinical subgroups could be identified: 1) Benign familial infantile epilepsy (BFIE) (n=17, normal cognition, treatable seizures), 2) intermediate epilepsy (n=36, mild ID, partially pharmacoresponsive), 3) developmental and epileptic encephalopathy (DEE, n=191, severe ID, majority pharmacoresistant), 4) generalized epilepsy (n=21, mild to moderate ID, frequently with absence seizures), and 5) affected individuals without epilepsy (n=25, mild to moderate ID). Groups 1-3 presented with early-onset (median: four months) focal or multifocal seizures and epileptic discharges, whereas the onset of seizures in group 4 was later (median: 39 months) with generalized epileptic discharges. The epilepsy was not classifiable in 143 individuals. We performed functional studies expressing missense variants in ND7/23 neuroblastoma cells and primary neuronal cultures using recombinant tetrodotoxin insensitive human NaV1.6 channels and whole-cell patch clamping. Two variants causing DEE showed a strong gain-of-function (GOF, hyperpolarising shift of steady-state activation, strongly increased neuronal firing rate), and one variant causing BFIE or intermediate epilepsy showed a mild GOF (defective fast inactivation, less increased firing). In contrast, all three variants causing generalized epilepsy induced a loss-of-function (LOF, reduced current amplitudes, depolarising shift of steady-state activation, reduced neuronal firing). Including previous studies, functional effects were known for 165 individuals. All 133 individuals carrying GOF variants had either focal (76, groups 1-3), or unclassifiable epilepsy (37), whereas 32 with LOF variants had either generalized (14), no (11) or unclassifiable (5) epilepsy; only two had DEE. Computational modeling in the GOF group revealed a significant correlation between the severity of the electrophysiological and clinical phenotypes. GOF variant carriers responded significantly better to sodium channel blockers (SCBs) than to other anti-seizure medications, and the same applied for all individuals of groups 1-3. In conclusion, our data reveal clear genotype-phenotype correlations between age at seizure onset, type of epilepsy and gain- or loss-of-function effects of SCN8A variants. Generalized epilepsy with absence seizures is the main epilepsy phenotype of LOF variant carriers and the extent of the electrophysiological dysfunction of the GOF variants is a main determinant of the severity of the clinical phenotype in focal epilepsies. Our pharmacological data indicate that SCBs present a therapeutic treatment option in early onset SCN8A -related focal epilepsy. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was supported by the German research foundation and the Fonds Nationale de la Recherche in Luxembourg (Research Unit FOR-2715, DFG grants Le1030/15-1 and /16-1 to HL, He8155/1-1 to UBSH, He5415/7-1 to IH, FNR grant INTER/DFG/17/11583046 to PM), the German Federal Ministry for Education and Research (BMBF, Treat-ION, 01GM1907A to HL, DL and PM), the foundation no epilep (to HL to partially support LS), the Medical Faculty of the University of Tuebingen (the Fortuene program 2430-00 to SL). KS, PL, MV were funded by grant: MH CR AZV NU20-04-00279. Epi25 was supported by the National Human Genome Research Institute (NHGRI) grants UM1 HG008895 and 5U01HG009088-02, and the Stanley Center for Psychiatric Research at the Broad Institute. SI was employed by and received a salary from Ambry Genetics. SS hold the GlaxoSmithKline Endowed Chair in Genome Sciences at the Hospital for Sick Children and University of Toronto. P.S. has received speaker fees and participated at advisory boards for Biomarin, Zogenyx, GW Pharmaceuticals, and has received research funding by ENECTA BV, GW Pharmaceuticals, Kolfarma Srl., Eisai. EM was funded by grant: NIH NINDS K08 NS097633 and Foerderer Award for Excellence from The Childrens Hospital of Philadelphia Research Institute. PV received a speaker fee from Nutricia GmbH, Dr. Schar AG / SPA, Eisai. Nestle, Pediatrica. FZ and PS developed this work within the framework of the DINOGMI Department of Excellence of MIUR 2018-2022 (legge 232 del 2016). IH was also supported by The Hartwell Foundation through an Individual Biomedical Research Award. This work was also supported by the National Institute for Neurological Disorders and Stroke (K02 NS112600), including support through the Center Without Walls on ion channel function in epilepsy (Channelopathy-associated Research Center, U54 NS108874), the Eunice Kennedy Shriver National Institute of Child Health and Human Development through the Intellectual and Developmental Disabilities Research Center (IDDRC) at Childrens Hospital of Philadelphia and the University of Pennsylvania (U54 HD086984), and by intramural funds of the Childrens Hospital of Philadelphia through the Epilepsy NeuroGenetics Initiative (ENGIN). Research reported in this publication was also supported by the National Center for Advancing Translational Sciences of the National Institutes of Health under Award Number UL1TR001878. This project was also supported in part by the Institute for Translational Medicine and Therapeutics (ITMAT) Transdisciplinary Program in Translational Medicine and Therapeutics at the Perelman School of Medicine of the University of Pennsylvania. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study was approved by the local ethical committees or followed other local guidelines as following: The ethics committee of Region Sjælland, Denmark: SJ-91 Ethics Committee Antwerp University Hospital, Drie Eikenstraat 655, 2650 Edegem, Belgium Ethical Committees of the Medical Faculty of the University of Munich Ethics Committee at the Research Centre for Medical Genetics, Moscow, Russian Republic Ethics committee of the University of Leipzig (ethical approval 224/16-ek, 402/16-ek) All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. 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