A Study among the Genotype, Functional Alternations, and Phenotype of 9 SCN1A Mutations in Epilepsy Patients

Abstract

Mutations in the voltage-gated sodium channel na v 1.1 (SCN1A) are linked to various epileptic phenotypes with different severities, however, the consequences of newly identified SCN1A variants on patient phenotype is uncertain so far. The functional impact of nine SCN1A variants, including five novel variants identified in this study, was studied using whole-cell patch-clamp recordings measurement of mutant na v 1.1 channels expressed in HEK293T mammalian cells. E78X, W384X, E1587K, and R1596C channels failed to produce measurable sodium currents, indicating complete loss of channel function. E788K and M909K variants resulted in partial loss of function by exhibiting reduced current density, depolarizing shifts of the activation and hyperpolarizing shifts of the inactivation curves, and slower recovery from inactivation. Hyperpolarizing shifts of the activation and inactivation curves were observed in D249E channels along with slower recovery from inactivation. Slower recovery from inactivation was observed in E78D and T1934I with reduced current density in T1934I channels. Various functional effects were observed with the lack of sodium current being mainly associated with severe phenotypes and milder symptoms with less damaging channel alteration. In vitro functional analysis is thus fundamental for elucidation of the molecular mechanisms of epilepsy, to guide patients' treatment, and finally indicate misdiagnosis of SCN1A related epilepsies.

Dravet variantSCN1AA1783Vimpairs interneuron firing predominantly by altered channel activation

“…Consistent with our data pronounced shifts of voltage-dependence of activation have also been described for other LOF mutations associated with epilepsy, such as SCN1A I1656M (Lossin et al, 2003), SCN1A D249E (Kluckova et al, 2020) and SCN1A R859C (Barela et al, 2006).…”

“…NaV1.1 variants are associated with a variety of neurological diseases including familial febrile seizures, generalized epilepsy with febrile seizures plus (GEFS+), sporadic/familial hemiplegic migraine and Dravet syndrome resembling a clinical phenotype at the severe end of the spectrum (Gambardella & Marini, 2009). The high diversity of SCN1A associated disorders can be explained by a variety of gain and loss of function variants (Catterall, Kalume, & Oakley, 2010) which impair the channel on different levels including changed RNA transcription (Lange et al, 2019), reduced protein expression, altered membrane trafficking (Thompson, Porter, Kahlig, Daniels, & George, 2012), impaired ßsubunit interaction (Spampanato et al, 2004) and channel dysfunction (Kluckova et al, 2020).…”

Dravet Variant SCN1AA1783V Impairs Interneuron Firing Predominantly by Altered Channel Activation

“…This predicted change on afterhyperpolarization was reflected as a nonsignificant tendency in our brain slice culture recordings (Supplementary Table 2). Consistent with our data, pronounced shifts of voltagedependence of activation have also been described for other LOF mutations associated with epilepsy, such as SCN1A I1656M (Lossin et al, 2003), SCN1A D249E (Kluckova et al, 2020), and SCN1A R859C (Barela et al, 2006). Similar to SCN1A A1783V , these variants also feature unaltered current density in comparison to the WT.…”

“…SCN1A variants are associated with multiple neurological diseases including generalized epilepsy with febrile seizures plus, sporadic/familial hemiplegic migraine, and DS resembling a clinical phenotype at the severe end of the spectrum (Gambardella and Marini, 2009). The high diversity of SCN1A associated disorders can be explained by a variety of gain and loss of function variants (Catterall et al, 2010), which impair the channel on different levels including transcriptional changes (Lange et al, 2019), reduced protein expression, altered membrane trafficking (Thompson et al, 2012), impaired ß-subunit interaction (Spampanato et al, 2004), and channel gating dysfunction (Kluckova et al, 2020).…”

Data_Sheet_1.pdf

“…SCN1A variants are associated with multiple neurological diseases including generalized epilepsy with febrile seizures plus, sporadic/familial hemiplegic migraine, and DS resembling a clinical phenotype at the severe end of the spectrum (Gambardella and Marini, 2009). The high diversity of SCN1A associated disorders can be explained by a variety of gain and loss of function variants (Catterall et al, 2010), which impair the channel on different levels including transcriptional changes (Lange et al, 2019), reduced protein expression, altered membrane trafficking (Thompson et al, 2012), impaired ß-subunit interaction (Spampanato et al, 2004), and channel gating dysfunction (Kluckova et al, 2020).…”