A common SCN1A splice-site polymorphism modifies the effect of carbamazepine on cortical excitability—A pharmacogenetic transcranial magnetic stimulation study

Authors


Summary

Objective

SCN1A encodes the alpha subunit of the voltage-gated sodium channel and plays a crucial role in several epilepsy syndromes. The common SCN1A splice-site polymorphism rs3812718 (IVS5N+5 G>A) might contribute to the pathophysiology underlying genetic generalized epilepsies and is associated with electrophysiologic properties of the channel and the effect of sodium-channel blocking antiepileptic drugs. We assessed the effects of the rs3812718 genotype on cortical excitability at baseline and after administration of carbamazepine in order to investigate the mechanism of this association.

Methods

Paired-pulse transcranial magnetic stimulation (TMS) was applied in 92 healthy volunteers with the homozygous genotypes AA or GG of rs3812718 at baseline and after application of 400 mg of carbamazepine or placebo in a double-blind, randomized, crossover design. Resting motor threshold (RMT), short interval intracortical inhibition (SICI), intracortical facilitation (ICF), and cortical silent period (CSP) were determined.

Results

At baseline there was no significant difference in any TMS parameter. Genotype GG was associated with a higher carbamazepine-induced increase in CSP duration as compared to AA (multivariate analysis of covariance [MANCOVA], p = 0.013). An expected significant increase in RMT was genotype independent.

Significance

We found that the rs3812718 genotype modifies the effect of carbamazepine on CSP duration (mainly reflecting modulation of γ-aminobutyric acid (GABA)ergic inhibition), but not on RMT (mainly reflecting modulation of voltage-gated sodium channels). This provides evidence that rs3812718 affects the pharmacoresponse to carbamazepine via an effect on GABAergic cortical interneurons. Our results also confirm that TMS is useful to investigate the effect of genetic variants on cortical excitability and pharmacoresponse.

The SCN1A gene encodes the alpha subunit Nav1.1 of the voltage-gated sodium channel and plays a crucial role in the pathogenesis of several monogenic epilepsy syndromes, including genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome.[1, 2] Recent studies suggest that SCN1A may also be a susceptibility gene in seizure syndromes with complex inheritance. Schlachter et al.[3] reported that the A allele of a common SCN1A splice site polymorphism (IVS5N+5 G>A, dbSNP: rs3812718) was associated with febrile seizures. This was confirmed by some[4] but not all studies.[5, 6] A recent genome-wide association study revealed a suggestive association between genetic generalized epilepsies (GGEs) and chromosomal regions encompassing the SCN1A gene, thereby underscoring the importance of this gene for GGE.[7]

The genetic findings are supported by data showing that the A allele of rs3812718 leads to reduced expression of the neonatal exon 5N relative to the adult exon 5A.[8, 9] which can alter the electrophysiologic properties of Nav1.1 in vitro.[10]

The single nucleotide polymorphism (SNP) rs3812718 was also found to have pharmacogenetic relevance, as an influence of this polymorphism on the response to sodium channel blockers, including carbamazepine, phenytoin, and lamotrigine, in patients with epilepsy was detected by several studies,[8, 11-13] whereas the results of other studies are conflicting.[14, 15] However, the impact of the polymorphism on the human drug response in human cortical networks has not yet been assessed, leaving the mechanism linking this polymorphism to drug dosage unclear.

Multiparametric transcranial magnetic stimulation (TMS) offers the opportunity to separately examine the excitatory and inhibitory properties of the human cortex and modifying factors in vivo, including the effects of antiepileptic and other drugs.[16-21] We aimed to test two hypotheses with TMS: (1) The SNP rs3812718 is associated with baseline cortical excitability, and (2) rs3812718 is associated with altered excitability after administration of the sodium channel blocker carbamazepine (CBZ) as compared to placebo. Verification of hypothesis 1 would suggest that the common SCN1A splice-site SNP rs3812718 might play a role in the pathophysiology of complex epilepsies. Verification of hypothesis 2 would support the consideration that this SNP is of pharmacogenetic relevance.

Subjects and Methods

Subjects

The number of subjects was determined by a sample size calculation. With σ = 2.5 and a speculated difference of 1.5 between the genotypes on RMT, that were determined as detectable difference, and power of 1−β = 80%, 46 participants were needed for each genotype group using t-tests. Considering the distribution of the genotypes in the general population and the dropout rate, we genotyped 271 healthy volunteers in order to identify volunteers with rs3812718 genotype AA (approximately 25% of the general population) and GG (approximately 22% of the general population). Healthy subjects were recruited from the geographical area served by the Marburg University Hospital. All subjects were of European ancestry.

Main inclusion criteria were the following: right-handedness as determined by the Edinburgh Handedness Inventory (EHI, EHI-score ≥ 80), age 18–60 years, reliable contraception for women (Pearl index < 0.1), the cognitive and physical ability to understand the experimental procedure, willingness to ingest the study medication (CBZ, placebo) and to be investigated by TMS. Main exclusion criteria included neurologic/psychiatric diseases, serious medical conditions such as cancer, cardiac problems and endocrinologic disorders, epileptic seizures, nonepileptic seizures, use of central nervous system (CNS)–active drugs, metal implants in neck or head, as well as pregnancy or lactation.

Subjects were instructed to refrain from smoking or taking CNS-active substances like caffeine for at least 24 h before each TMS investigation.

The study was approved by the local institutional review board (IRB) and the German Federal Institute of Drugs and Medical Devices (BfArM; eudraCT number 2008-003392-40).

Methods

After a detailed explanation of the experimental procedure and providing written informed consent, the SCN1A SNP rs3812718 was genotyped on an ABI Prism 7900HT Sequence Detection System (Life Technologies Corporation, Carlsbad, CA, U.S.A.) using a TaqMan SNP Genotyping Assay (Assay ID: C__25982233_10; Life Technologies Corporation). Subjects with the genotypes AA and GG were further investigated using TMS. TMS was performed on two different days at least 2 weeks apart before (baseline) and 5 h after intake of either placebo or 400 mg of CBZ in a randomized (block randomization, permutated block size), crossover, double-blind study design.

Female subjects had a pregnancy test on both days. In addition, all subjects underwent electrocardiography (ECG) before the first TMS session and were excluded from the study if the ECG showed any abnormalities.

Transcranial magnetic stimulation measurements

Transcranial magnetic stimulation (TMS) was delivered through a figure-eight–shaped magnetic coil (external diameter of each loop 9 cm; current in the junction of the coil directed from anterior to posterior). The coil was connected to two magnetic stimulators with a monophasic current waveform (Magstim 200) via a Bistim Module (Magstim; Whitland, Dyfeld, United Kingdom). Maximum stimulus intensity was 2T.

Subjects were seated in an armchair with the head fixed in a plastic foam headrest. The coil was placed flat on the skull at an angle of 45 degrees to the sagittal plane, inducing a current in the brain approximately perpendicular to the central sulcus, flowing from posterior to anterior. The optimal coil position was determined by recording motor evoked potentials (MEPs) while varying the coil position. The coil position leading to the highest peak-to-peak amplitude of the MEP was marked directly on the scalp to ensure accurate coil repositioning.

In each subject, the left, presumably dominant hemisphere was evaluated. Motor evoked potentials were recorded using surface electromyography (EMG) Ag/AgCl electrodes placed over the right abductor digiti minimi muscle (ADM) in a belly-tendon montage. The raw signal was amplified, filtered (20 Hz–10 kHz) and recorded with a PC using a commercially available data collection and averaging program (Magnetix; Center of Sensorimotor Research, Munich, Germany) for offline analysis.

The TMS parameters resting motor threshold (RMT), short-interval intracortical inhibition (SICI), facilitation (ICF), and cortical stimulation-induced silent period (CSP) were used to investigate motor cortex excitability.

The RMT was defined as the minimal stimulus intensity required to induce a MEP of more than 50 μV peak-to-peak amplitude in at least 5 of 10 consecutive trials. Complete muscle relaxation was monitored via audiovisual feedback. A step width of 1% of maximal stimulator output was used for determination of the RMT. The program MLTH (maximum likelihood threshold hunting procedure for TMS©; Dr. Friedemann Awiszus, Magdeburg, Germany) was used for determination of the RMT.

SICI and ICF were obtained during paired-pulse TMS. A conditioning and a test stimulus were applied with different fixed interstimulus intervals (ISIs). The conditioning stimulus was set to an intensity of 75% of RMT, which produces no excitability changes in the spinal cord.[22] The intensity of the following suprathreshold test stimulus (TS) was adjusted to produce MEPs of approximately 1.5 mV peak-to-peak amplitude if delivered without preceding conditioning stimuli. SICI was obtained at short ISIs of 3 msec, leading to a decreased MEP as compared to an MEP induced by a nonconditioned test stimulus. ICF was obtained using ISIs of 10 msec, leading to an increased MEP.[20, 22] Fifteen trials of single, nonconditioned test stimuli and 15 paired stimuli of each ISI were recorded, each generated in random order by the software. The average of the 15 trials was used to define the amplitude of the peak-to-peak MEP for each condition. The conditioned response was defined as the mean amplitude of the conditioned responses belonging to each ISI, expressed as percentage of the mean amplitude of the unconditioned test response. For better comparability, this percentage was subtracted from 100% for SICI (SICI: 100% – [conditioned response/unconditioned response × 100%]; ICF: conditioned response/unconditioned response × 100% as previously suggested).[23]

The CSP was measured in 20 trials at a stimulus intensity of 110% of the RMT. The subjects were instructed to hold a voluntary muscle contraction of approximately 30% of the maximal force, monitored by audiovisual feedback. The CSP duration was defined in individual trials as the time interval from the beginning of the stimulus-induced MEP to the first recurrence of voluntary EMG activity displayed.

To reduce the duration of the TMS session necessary for determination of RMT, TS, SICI, ICF, and CSP and the risk of head movements, SICI and ICF measures were determined only at a stimulus intensity of 75% of RMT and the CSP at a stimulus intensity of 110% of the RMT.

One single value per person and test session was calculated by averaging separately for RMT, TS, SICI, ICF, and CSP.

Five hours after intake of either placebo or CBZ, the serum level of CBZ was determined in order to account for possible differences between the two genotypes. In addition, subjects were asked to complete a questionnaire concerning the side effects of the study drug (Table S1).

Female subjects were investigated in the follicular phase to exclude confounding of the results by hormonal fluctuations in the course of the menstrual cycle.[24, 25]

Statistical analysis

Statistical analysis was computed with IBM SPSS Statistics 20 (SPSS; IBM Company, Chicago, IL, U.S.A.). Chi-square tests were applied to categorical variables and t-tests for independent samples to metric variables, respectively. t-Tests for paired samples were used to compare overall treatment effects of CBZ versus placebo irrespective of the genotype. Baseline measurements from sessions 1 and 2 were averaged individually for further analysis.

We included the two TMS parameters RMT (reflecting mainly modulation of voltage-gated sodium channels) and CSP (reflecting mainly modulation of GABA-ergic inhibition) in the analysis of a possible selective effect of CBZ for the genotypes AA and GG, as these two parameters were previously shown to be influenced by CBZ.[16, 18, 26] A multivariate analysis of covariance (MANCOVA) with the two-stage-factor genotype (AA vs. GG), the differences in change in RMT and CSP from baseline to postmedication (CBZ-PL) as dependent variables, and the covariates gender, CBZ-level, and TS after intake of CBZ were computed. TS, SICI, and ICF were evaluated in an exploratory analysis using t-tests for independent samples.

Pearson's correlation coefficient was determined for the association between CBZ level, gender, and TMS parameters as well as side effects. Statistical significance was set to p < 0.05 (two-tailed).

Results

Subject characteristics

Of the 271 subjects who underwent genotyping, 140 (51%) had either the AA (77 subjects, 28.4%) or the GG (63 subjects, 23.2%) genotype of rs3812718. The distribution of the genotypes did not deviate significantly from that predicted by the Hardy-Weinberg equilibrium (p = 0.61).

Overall dropout or removal from the study was 34.3% (48 of the 140 subjects with genotype AA or GG). Of the volunteers, 39 (16 GG, 23 AA) were not willing to complete the study after genotyping, and one additional subject (genotype GG) dropped out after the placebo visit. In addition, eight volunteers (three GG, five AA) were excluded from the statistical analysis (two due to technical problems during TMS measurements, one due to perinatal hypoxia not reported earlier, and five due to missing CBZ levels). The remaining sample comprised 92 healthy volunteers (49 with genotype AA and 43 with genotype GG).

Demographic data are displayed in Table 1. The genotype groups did not differ in age or gender frequency. Randomization was successful in distribution of treatment order, and the CBZ levels were comparable in the two groups.

Table 1. Demographic data of the two genotypes AA and GG; numbers or mean (SD)
VariableAA (n = 49)GG (n = 43)p-Valuea,b
  1. SD, standard deviation; m, male; f, female; PL, placebo; CBZ, carbamazepine.

  2. a

    t-Test for independent samples.

  3. b

    Chi-square test.

Gender (m/f)25/2427/160.256
Age23.5 (2.7)24.4 (3.9)0.224
Randomization (PL-CBZ/CBZ-PL)27/2217/260.136
CBZ-level (mg/L)4.5 (1.2)4.6 (1.0)0.545

Baseline differences in cortical excitability

Baseline measures of cortical excitability in both conditions were averaged and revealed no group difference in RMT between genotype GG and AA (p = 0.402, Table 2). Likewise, there were no statistically relevant dissimilarities between the genotypes on mean baseline TS, SICI, and ICF or CSP (all p > 0.05, Table 2). The mean baseline RMT was higher for women than for men (46.68 ± 9.09 vs. 42.75 ± 7.07; T = −2.33; p = 0.022). The genotype-dependent TMS results at baseline and after administration of CBZ and placebo are listed in Table 3.

Table 2. Mean (SD) of TMS measures at baseline between genotype AA and GG
VariableAA (n = 49)GG (n = 43)p-Valuea
  1. TS, test stimulus; RMT, resting motor threshold (% of max. stimulator output); SICI, short intracortical inhibition (100% – conditioned MEP/unconditioned MEP × 100%); ICF, intracortical facilitation (conditioned MEP/unconditioned MEP); CSP, cortical silent period (at 110% of RMT, in ms), baseline values of the two sessions were averaged for the analysis of baseline differences between the two genotypes.

  2. a

    t-test for independent samples.

TS57.57 (8.36)55.28 (7.59)0.174
RMT45.13 (8.41)43.69 (7.99)0.402
SICI16.50 (19.05)24.73 (31.95)0.132
ICF97.46 (19.47)97.53 (26.61)0.988
CSP107.91 (26.34)109.61 (34.06)0.788
Table 3. Means (SD) of the genotype dependent TMS measures
VariableBL_PLPLBL_CBZCBZ
AAGGAAGGAAGGAAGG
  1. TS, test stimulus; RMT, resting motor threshold (% of max. stimulator output); SICI, short intracortical inhibition (100% – conditioned MEP/unconditioned MEP × 100%); ICF, intracortical facilitation (conditioned MEP/unconditioned MEP × 100%); CSP, cortical silent period (at 110% of RMT, in ms); BL, baseline; PL, placebo; CBZ, carbamazepine; BL_PL, baseline before placebo; BL_CBZ, baseline before CBZ.

TS57.25 (8.96)55.07 (7.67)57.36 (1.27)55.70 (1.27)57.67 (8.27)55.49 (8.17)59.52 (1.40)57.90 (1.373)
RMT44.79 (9.04)43.84 (8.36)45.39 (1.26)43.45 (1.186)45.41 (8.56)43.53 (8.92)47.94 (1.36)45.53 (1.26)
SICI15.22 (21.09)26.66 (39.23)23.25 (3.573)31.22 (5.34)18.01 (27.83)22.80 (48.41)14.59 (4.56)23.05 (4.57)
ICF97.40 (22.14)99.69 (35.09)92.11 (3.26)95.30 (5.216)97.61 (30.72)95.38 (51.45)94.28 (4.63)92.21 (4.44)
CSP107.65 (32.33)117.70 (42.35)112.56 (5.279)109.40 (6.11)106.88 (27.49)101.45 (36.75)113.76 (4.71)113.87 (6.14)

Overall effect of CBZ

Analysis of the overall effect of CBZ for all subjects revealed a greater increase in RMT from baseline values following administration of CBZ as compared to placebo (RMT: t(90) = −3.26, p = 0.002, Table 4). In addition, TS and CSP showed a differential reaction to CBZ (TS: t(90) = 3.80, p < 0.001; CSP: t(80) = 2.311, p = 0.023, Table 4). There was no significant overall effect of CBZ on the TMS parameters SICI and ICF (p > 0.05, Table 4). As expected, subjects experienced more side effects after intake of CBZ than after intake of placebo (3.70 ± 3.88 vs. 2.01 ± 2.67; t(87) = −4.36, p < 0.001). There was no correlation of side effects with gender, CBZ level, or genotype (all p > 0.1). The most common side effects of CBZ were described by the volunteers as slight effects, such as sleepiness, coordination difficulties, and vertigo.

Table 4. Overall effect of CBZ measured in differential reaction to CBZ (compared to placebo and the corresponding measures at baseline)
VariableDifference in reactionp
  1. TS, test stimulus; RMT, resting motor threshold (% of max. stimulator output); SICI, short intracortical inhibition (100% – conditioned MEP/unconditioned MEP × 100%); ICF, intracortical facilitation; CSP, cortical silent period (at 110% of RMT, in ms), bold, p < 0.05.

  2. a

    Paired samples t-test.

TS1.9560 <0.001
RMT2.1978 0.002
SICI0.07930.526
ICF−0.20000.299
CSP10.3986 0.023

Genotype-dependent effects of CBZ

A MANCOVA with the differences in change in RMT and CSP from baseline to post medication (CBZ-PL) as dependent variables, and the covariates gender, CBZ level, and TS at CBZ measure with the two-stage-factor genotype (AA vs. GG) revealed a significant main group effect (p = 0.029; Table 5). Univariate analysis showed that volunteers with genotype GG had a higher increase in CSP duration compared with genotype AA after intake of CBZ as compared to placebo (21.53 ± 6.31 msec vs. 0.56 ± 5.93 msec, p = 0.013, Fig. 1). There were no group differences in changes of RMT after intake of CBZ as compared to placebo (p = 0.813). Furthermore, SICI, ICF, and TS did not vary significantly by genotype (p > 0.05).

Table 5. Effects of MANCOVA with the group factor “genotype AA versus GG” of RMT and CSP with gender, CBZ-level and TS under CBZ as covariates
Source of variationDependent variableFd.f.1d.f.2pη2
  1. d.f., degrees of freedom; η2, effect size; TS, test stimulus; RMT, resting motor threshold; CSP, cortical silent period; CBZ, carbamazepine, bold, p < 0.05.

Main effect: “AA versus GG”Multivariate analysis3.73275 0.029 0.090
RMT0.061760.8130.001
CSP6.55176 0.013 0.079
Figure 1.

Differential reaction of the cortical silent period to carbamazepine depending on genotype. Mean and SD (error bars) for the significant (p = 0.013) difference in reaction to the two treatments (CBZ-Placebo); the group with genotype GG showed an increased CSP after CBZ versus placebo, whereas in the group with genotype AA, the CSP remained unchanged compared to the placebo treatment. CSP, cortical silent period (at 110% of RMT, in msec); GG, genotype GG; AA, genotype AA; CBZ, carbamazepine.

Discussion

This study demonstrates that changes in cortical excitability after intake of the sodium-channel blocker CBZ differ in subjects with different genotypes of the common SCN1A splice-site polymorphism rs3812718. In particular, CBZ induced a significant CSP prolongation in subjects with genotype GG, when compared to AA. This is the first demonstration of a pharmacogenetic functional link between this SCN1A SNP genotype and CBZ response, which may underlie the reported correlation of SNP genotype and antiepileptic drug efficacy in epilepsy patients.[8, 12] The expected genotype-independent increase in the TMS parameters RMT and CSP reflects effects on voltage-gated sodium channel function in general and is confirmatory of previous reports.[16, 26] No significant baseline differences in cortical excitability were observed.

Because the CSP depends on the stimulus intensity used as well as the intensity of voluntary muscle contraction of the probands, a reproduction of the results by further studies would be desirable.

Genotype-dependant effects of CBZ

The SCN1A polymorphism rs3812718 has been found to be associated with changes in electrophysiologic and pharmacologic properties of the sodium channel Nav 1.1.[3, 8, 10] By disrupting the splice donor consensus sequence directly following the neonatal splice variant of exon 5 (5N), the A allele leads to significantly decreased expression of the neonatal exon 5N relative to the adult exon 5A.[8, 9]

Heterologously expressed Nav1.1-5N channels have been shown to display enhanced tonic and use-dependent blocks by phenytoin and lamotrigine, when compared to Nav1.1-5A channels.[13] These data provide a pathophysiologic basis at the molecular level for the decreased effectiveness and higher required dose of sodium channel blocking anticonvulsant drugs in epilepsy patients with genotype AA, which has been reported in some,[8, 12] but not all, clinical studies.[14, 15] The present study extends the mechanism of this association to the network level. Our data support an altered effectiveness of sodium-channel blockers depending on the rs3812718 genotype.[8, 12] Using in vivo measurements in human subjects, we show that this differential effect may involve the GABAergic system, as the later components of the CSP are mainly modified by changes in GABAergic inhibition of pyramidal cells through interneurons.[16, 18, 27-29]

Although the most robust effect of CBZ in this and previous studies was on the RMT, which reflects changes in voltage-gated sodium channels in general,[16, 18, 28, 30] we unexpectedly did not find a differential effect of the rs3812718 genotype on CBZ-induced RMT change. However, in mouse models a mutation (R1648H) or knockout of SCN1A alters electrophysiologic properties and reduces sodium channel function in GABAergic interneurons, but not excitatory pyramidal cells.[1, 31, 32] This is likely due to the differential expression of the sodium channel Nav1.1[1] and suggests that mutations in the SCN1A gene in general, as well as the polymorphism rs3812718 studied here, mainly affect inhibitory interneurons expressing Nav1.1., which is consistent with the genotype-dependent changes in CSP found in this study. In contrast, the excitability of pyramidal cells determining the RMT remained unaffected by this variation in SCN1A.

SCN1A and pharmacoresistance

Pharmacoresistance is an important factor in the treatment of epilepsy, affecting about 30% of patients[33] and resulting in increased suffering, mortality, and treatment costs.[34] Two main hypotheses explaining pharmacoresistance have been proposed: (1) the transporter hypothesis, implying a lower brain concentration of AED due to altered transport across the blood–brain barrier, and (2) the substrate hypothesis, suggesting decreased responsiveness of the drug target, for example, due to structural alteration.[34] Our results provide further evidence for an association of the rs3812718 polymorphism with the pharmacoresponse and lend support for the substrate hypothesis.

Baseline differences

In addition to the altered pharmacologic properties of NAv1.1 related to the SCN1A polymorphism rs3812718, there are also reports of electrophysiologic changes in drug-naive physiologic conditions.[10] Such genotype-dependent electrophysiologic differences would suggest that rs3812718 may confer susceptibility to epilepsy. Accordingly, previous studies reported an association between the A allele of rs3812718 and febrile seizures,[3, 4] and a genome-wide association study revealed an association between chromosomal regions encompassing the SCN1A gene and GGE.[7] These findings support electrophysiologic changes associated with SCN1A polymorphisms and their possible role in the pathogenesis of genetically complex forms of generalized epilepsies.

However, other studies could not replicate an association between rs3812718 and febrile seizures.[5, 6] Furthermore, a recent patch-clamp investigation examining the electrophysiologic properties of Nav1.1 did not show significant electrophysiologic differences in drug-naive conditions including voltage dependence of activation, steady-state inactivation, and recovery from inactivation between the two genetic variants of Nav1.1.[13] Changes in electrophysiologic properties associated with rs3812718, therefore, remain controversial.

We investigated possible in vivo baseline differences in cortical excitability in healthy subjects with genotypes AA and GG using TMS. None of the TMS parameters applied revealed significant differences in cortical excitability before application of CBZ or placebo. Our study does not lend support to the idea of baseline differences in cortical excitability accompanying the different genotypes of rs3812718, at least not in a range or manner that can be measured by TMS.

Conclusion and Open Questions

Our results provide evidence of increased cortical inhibition after intake of CBZ in subjects with genotype GG as compared to AA of the SCN1A polymorphism rs3812718 while showing no baseline differences in cortical excitability. The results imply that the higher doses of CBZ in patients with genotype AA that were described in earlier studies are not due to an increased baseline excitability, but rather a differential effect of CBZ that is dependent on the SCN1A genotype.

These data provide insight into the physiology underlying pharmacoresponse, supporting the substrate hypothesis and relating the altered effectiveness of CBZ to changes in the excitability of GABAergic interneurons. Furthermore, this study shows that TMS is a useful method to gain insight into the influence of genetic variants on cortical excitability and pharmacoresponse.

The present study only included subjects with the homozygous genotypes AA and GG and showed that these are related to changes in pharmacoresponse that can be measured by TMS. Considering these results, future studies should also include subjects with the heterozygous genotype AG to further characterize the effect of this polymorphism.

Acknowledgment

This work was supported by grants from the European Community (FP6 Integrated Project EPICURE, LSHM-CT-2006-037315), the German Federal Ministry of Education and Research (NGFNplus: EMINet, Grant 01GS08120), and the German Research Foundation (DFG) within the EUROCORES Programme EuroEPINOMICS-RES (Grant RO 3396/2-1) and EpiGENet (Grant SA 434/5-1) projects. KMK was supported by a research fellowship from the German Research Foundation (DFG. KL 2254/1-1). The funding sources were not involved in the conception or conduction of the research or preparation of the manuscript.

We thank the Center for Clinical Trials Marburg (KKS-Marburg) for help with obtaining approval of the local institutional review board (IRB) and the German Federal Institute of Drugs and Medical Devices (BfArM) as well as the randomization of the subjects. We also thank H.H. Müller, Institute of Medical Biometry and Epidemiology, Philipps-University Marburg, Germany for statistical advice and Braxton Norwood, Department of Neurology, Philipps-University Marburg, for his contribution as native speaker.

Disclosure

The authors declare the following: KM has received a travel grant and speaker honoraria from GlaxoSmithKline, which are unrelated to this study. CD has received speaker honoraria from the Epilepsy Center Hessen and a travel grant from Eisai, unrelated to this study. SS has nothing to disclose. She is a member of the Epilepsy Research United Kingdom Science Advisory Committee and the UCL council and has received a Royal Society Fellowship unrelated to this study. PSR has received travel grant from UCB. KMK has received a scholarship from the University of Melbourne and speaker honoraria from the Epilepsy Center Hessen. AHa has received honoraria for data analysis, manuscript preparation for an observational study from UCB, and for a survey study from Sanofi-Aventis. She was supported by a research fellowship from the German Research Foundation (DFG. Ha 6363/1-1).

In the past 2 years WHO has received speaker honoraria from Boehringer Ingelheim, Desitin, GlaxoSmithKline, Orion, Novartis, UCB Pharma/Schwarz Neuroscience, and Teva. He also has received honoraria as scientific advisory (consultant) from Desitin, Merck Sharp and Dome, Merck Serono, Novartis, Orion, UCB Pharma/Schwarz Neuroscience, and Teva.

HMH has received honoraria as advisory board member from Desitin, GSK, Eisai, UCB, and Pfizer, and research grants from Desitin, Janssen Cilag, and UCB. He has received speaker honoraria from several companies and institutions, such as University of Munich, University of Kiel, University of Saarbrücken, Desitin, Eisai, GSK, Pfizer, Novartis, Nihon Kohden, AdTech, and UCB. He has received payment for manuscript preparation from Pfizer and development of educational presentations from UCB. SK She has received speaker honoraria from UCB, Desitin, und Eisai, which are unrelated to this study.

Within the past 2 years, FR has received honoraria as scientific advisor from GSK, Eisai, UCB, and Pfizer. He has received speaker honoraria from UCB, GSK, Eisai, Desitin, and Medtronic, and educational grants from Nihon-Kohden, UCB, Medtronics, Cyberonics, and Cerbomed. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.

Biographies

  • Image of creator

    Dr. Katja Menzler is a researcher at the University Hospital Marburg in the Department of Neurology.

  • Image of creator

    Anke Hermsen is a neuropsychologist at the University Hospital Marburg in the Department of Neurology.

Ancillary