Polymorphisms of the sodium voltage-gated channel, alpha subunit 1 (SCN1A -A3184G) gene among children with non-lesional epilepsy: a case-control study

Epilepsy is a multifactorial channelopathy disease with the involvement of both acquired and genetic factors [1]. Knowing the exact etiology of epilepsy is crucial for patients’ treatment and for neurobiological researchs that could direct future personalized therapies [2]. Neuronal voltage-gated sodium channels (SCN) are proteins responsible for the generation and propagation of the action potentials within the neurons. This occurs through affection of the membrane permeability to sodium ions and facilitation of the ions diffusion down an electrochemical gradient till the sodium equilibrium potential [3].

There are evidences about the role of the SCN polymorphisms in the epilepsy pathogenesis. These polymorphisms have been associated with a spectrum of epilepsy syndromes such as generalized epilepsy with febrile seizures plus, borderline severe myoclonic epilepsy of infancy, Dravet syndrome, Doose syndrome and infantile spasms [4, 5]. Most of SCN mutations are within SCN1A gene and fewer are within the SCN2A, SCN8A and SCN1B genes [6]. The SCN1A gene is located in the chromosome 2 (2q24.3) and includes 26 exons. SCN1A encodes the alpha subunit of the sodium channel NaV1.1 [7]. This type of channels conduct sodium ions through pores in the cellular membranes. Its architecture comprises four domains with six segments; four homologous domains (D1–D4), each containing six α-helical segments (S1–S6). The positively charged residues in the S4 segment or voltage-sensing helix of each domain are involved in the the graded membrane potential changes and generation of the action potentials [8].

Several studies have reported the efficacy of the sodium-channel blockers for the treatment of epilepsies due to genetic channelopathies. Carbamazepine, oxcarbazepine, phenytoin, lamotrigine, lacosamide and lidocaine prevent seizure activity through blocking the movement of sodium ions across sodium ion channels during the propagation of action potentials. This group of antiepileptic drugs (AEDs) has an additional advantage through acting on potassium channels, due to structural similarities, thus controlling genetic epilepsies due to mutations of voltage-gated potassium channel genes [9].

The SCN1A-A3184G (p.Thr1067Ala) polymorphism has been suggested to be involved in the gating of sodium channels, thus rendering them insensitive to sodium-channel blockers [10, 11]. The association of the SCN1A-A3184G polymorphism with the epilepsy risk was investigated in several non-Caucasian populations [12‐15] and in a limited number of Caucasian populations [10, 16] with inconsistent results, thus necessitating additional studies.

Some studies reported significant associations between the SCN1A polymorphisms and AEDs resistance [8]. Others tried to investigate the possible associations between these polymorphisms and AEDs choice and doses. Tate et al. reported a significant association between the SCN1A polymorphism and the dose of phenytoin and carbamazepine, and suggested a trend of reduction in the maximum dose required according to the genotype [17]. Heinzen et al. noted that individuals with the AA genotype of the rs3812718 variant need higher doses of AEDs than others with the GG genotype [18].

In the current research, we investigated the patterns and frequencies of SCN1A-A3184G (p.Thr1067Ala) polymorphism among Egyptian children and adolescents with non-lesional epilepsy, including both AEDs responders and resistant cases. We tested the association between SCN1A-A3184G genotypes and some predictors of refractory epilepsy. We also highlighted the possible pharmacological implication of studying the target polymorphism, through describing the frequently used AEDs and their possible link with the genotypes.

Epilepsy is a highly heterogeneous disease with a confirmed genetic background [23, 24]. More than one quarter of the epilepsy-related genes encode ion channel proteins, including the ligand-gated ion channels (such as gamma-aminobutyric acid receptors, N-methyl-D-aspartate receptors and nicotinic acetylcholine receptors) and the voltage-gated channels (such as Ca²⁺, K⁺ and Na⁺ channels) [25]. The first SCN1A mutation was found in epilepsy patients by 2000 [23], but now many new SCN1A mutations have been identified making it the most common epilepsy-related gene [26]. In our preliminary research, we investigated the patterns and frequencies of SCN1A-A3184G alleles and genotypes among Egyptian children and adolescents with non-lesional epilepsy. We found that the AG genotype and the A allele are the predominant models among the study participants.

In the current study, we reported insignificant differences between epilepsy cases and the control group regarding the SCN1A-A3184G genotypes and alleles. No significant differences existed between AEDs responders and resistant cases regarding these genotypes and alleles. Lack of significant differences among our Egyptian patients could be explained on the basis of the ethnic variation. This was evidenced by Baum et al. in their large multi-ethnic study, where they considered the SCN1A-A3184G (rs2298771) polymorphism as the significant ethnic-related locus among symptomatic epilepsy patients (p < 0.001) while the frequencies of the other studied SCN1A loci (rs10188577 and rs3812718) did not differ according to the ethnicity [15]. However, the relatively small sample size could be a limiting factor in interpreting our results.

Our results also agree with Chou et al. who reported insignificant differences between 83 control subjects and 104 Taiwanese epileptic children regarding SCN1A-A3184G genotypes and alleles [14]. We also agree with Kang et al. who investigated 311 children for SCN1A-A3184G (rs2298771) and SCN2A-G56A (rs17183814) polymorphisms and tested their association with refractory seizures. They reported insignificant differences between controls and cases as regards the SCN1A-A3184G polymorphism, but the A allele of SCN2A-G56A polymorphism was associated with the refractory seizures [27]. Moreover, similar findings were described in a German study where SCN1A-A3184G polymorphism was not considered as a major contributor to the idiopathic generalized epilepsy [28]. We also agree with Hosseini et al. who evaluated the frequency of the SCN1A (rs2298771 & 7,601,520) and the ABCB1 (rs1045642) polymorphisms within the Iranian population with idiopathic refractory epilepsy. They enrolled 81 healthy subjects and 34 patients with idiopathic refractory epilepsy, and reported insignificant differences between the studied groups [29]. There is also agreement with another Chinese study including 471 patients with epilepsy (272 AEDs responders and 199 AEDs resistant cases). Authors studied the association of the responsiveness to AEDs with SCN1A, SCN2A, and SCN3A polymorphisms, and correlated any association with the mRNA expression of the neuronal sodium channels. They suggested an association between SCN2A IVS7-32A > G and the AEDs response, without any evidence of an effect on the mRNA expression or splicing. However, they reported a negative association between the SCN1A-A3184G polymorphism and the resistance to AEDs [30].

On the other hand, Lakhan et al. suggested that the AG genotype of the SCN1A-A3184G polymorphism was more frequent among North Indian epilepsy patients [p = 0.005; 95% confidence interval = 1.19–2.61, odds ratio = 1.76] [12]. In a meta-analysis study done by Li et al., the SCN1A-A3184G (rs2298771) polymorphism was significantly associated with the response to AEDs [31].

In the current study, we did not find the GG genotype of the SCN1A-A3184G polymorphism among the studied population. This is in contrast to a Slovenian study where the G allele was associated with a lower epilepsy risk and a high remission rate among patients with epilepsy [7]. These discrepancies could be mainly explained by the interethnic differences in the SCN1A alleles/genotypes distribution [20].

Interestingly, the current study suggested significant associations between the heterozygous AG genotype in the epilepsy cases with the higher seizure frequency and generalized epileptiform EEG activity. Possible associations were also suggested between the AG-genotyped patients with refractory epilepsy and the earlier age of onset of seizures and duration since the last seizure attack. On reviewing literature, no previous reports tested the associations between the SCN1A-A3184G (rs2298771) polymorphism and the predictors of refractory epilepsy. We would like to emphasize that the higher seizure frequency, generalized epileptiform EEG activity and the earlier age of onset of seizures were proved to be significant predictors of the AEDs response in previous studies [32, 33]. This observation could open the gate for considering the AG genotype of the SCN1A-A3184G polymorphism as a predictor for treatment response, and aid in designing the personalized medicine for epileptic children according to their genotype [34]. However, large scale multi-center studies are still needed to prove such associations.

Several studies were carried out to select the most appropriate AEDs in different types of epilepsy [9, 35]. The current study suggested that levetiracetam and carbamazepine were more frequently used in the AG-genotyped responders than those with the AA genotype. Regarding the AEDs resistant group, topiramate and valproic acid were more frequently used in children with the AG genotype while carbamazepine was more frequently used in the AA genotype. This observation highlights the possible role of the SCN1A-A3184G genotypes in the selection of the appropriate AEDs.

On reviewing literature, several studies were carried out to detect the possible pharmacological implications of the SCN1A-A3184G polymorphism in epilepsy management. A recent meta-analysis including eight studies reported that Asian patients with epilepsy and the GG genotype of the SCN1A-A3184G polymorphism are at a higher risk of carbamazepine resistance [36]. On the other hand, another meta-analysis including 18 studies (2546 patients) reported no association between the same polymorphism and the carbamazepine resistance [37]. Regarding the valproic acid, a Chinese study suggested that SCN1A-A3184G polymorphism has no effect on the drug response [38]. Actually, gathering data from different ethnic groups expand the knowledge about the genetic background of epilepsy and help in personalized medicine strategy.

No significant differences were found between Egyptian children and adolescents with non-lesional epilepsy and healthy controls regarding the frequency of SCN1A-A3184G polymorphism. We suggested significant associations between the AG genotype of the SCN1A-A3184G polymorphism and some predictors of refractory epilepsy. The SCN1A-A3184G genotypes might affect AEDs selection. Multicenter large-scale studies are still needed to validate our findings.