ABCC2 haplotype is not associated with drug-resistant epilepsy

Integrative analysis of rs717620 polymorphism in therapeutic response to anti-seizure medications

Background

Previous studies have shown that the rs717620 polymorphism in ABCC2, the gene encoding multidrug resistance protein 2, influences the therapeutic response to anti-seizure medications (ASMs). However, this result is not consistent, and the mechanism by which rs717620 influences ASM responses is unclear.

Aims

The present study evaluated the association between rs717620 genotype and ASM efficacy, and examined the potential mechanisms.

Main

methods: We conducted a literature search of five electronic databases, Embase, Medline, Web of Science, China National Knowledge Infrastructure, and Wanfang, to identify relevant studies on response to ASM therapy among rs717620 genotypes. Expression quantitative trait loci analysis and drug-gene interaction analysis were also performed to assess the underlying mechanisms.

Key findings

The pooled results for 18 studies revealed a significant association between rs717620 genotype and ASM resistance under the recessive model (TT vs. CT + CC: OR = 1.68, 95 % CI = 1.27-2.21, I2 = 3.1 %). A significant association was also found in the Asian population under the recessive model (TT vs. CT + CC: OR = 1.70, 95 % CI = 1.26-2.29, I2 = 29.3 %). Further analysis revealed that rs717620 regulates the expression of ABCC2 in human brain, while drug-gene interaction analysis suggested that ABCC2 interacts with oxcarbazepine and carbamazepine.

Significance

The rs717620 polymorphism influences ASM therapeutic responses by altering brain expression levels of ABCC2.

Impact of ABCC2 1249G>A and -24C>T Polymorphisms on Lacosamide Efficacy and Plasma Concentrations in Uygur Pediatric Patients With Epilepsy in China

PURPOSE

We aimed to evaluate the effect of the ABCC2 1249G>A (rs2273697) and -24C>T (rs717620) polymorphisms on lacosamide (LCM) plasma concentrations and the efficacy of LCM in Uygur pediatric patients with epilepsy.

METHODS

We analyzed 231 pediatric patients with epilepsy, among which 166 were considered to be LCM responsive. For drug assays, 2-3 mL of venous blood was collected from each patient just before the morning LCM dose was administered (approximately 12 hours after the evening dose, steady-state LCM concentrations). The remaining samples after routine therapeutic drug monitoring were used for genotyping analysis. The χ 2 test and Fisher exact test were utilized for comparative analysis of the allelic and genotypic distribution of ABCC2 polymorphisms between the LCM-resistant and LCM-responsive groups. The Student t test or Mann-Whitney U test was conducted to analyze differences in plasma LCM concentration among pediatric patients with epilepsy with different genotypes.

RESULTS

Patients with the ABCC2 1249G>A GA genotype (0.7 ± 0.3 mcg/mL per kg/mg) and AA genotype (0.5 ± 0.3 mcg/mL per kg/mg) showed significantly ( P < 0.001) lower LCM concentration-to-dose (CD) ratios than patients with the GG genotype (1.0 ± 0.4 mcg/mL per kg/mg). Moreover, patients with the ABCC2 -24C>T CT genotype (0.6 ± 0.2 mcg/mL per kg/mg) and TT genotype (0.6 ± 0.3 mcg/mL per kg/mg) presented a significantly ( P < 0.001) lower LCM CD ratio than patients with the CC genotype (1.1 ± 0.4 mcg/mL per kg/mg).

CONCLUSIONS

The ABCC2 1249G>A (rs2273697) and ABCC2 -24C>T (rs717620) polymorphisms can affect plasma LCM concentrations and treatment efficacy among a population of Uygur pediatric patients with epilepsy, causing these patients to become resistant to LCM. In clinical practice, ABCC2 polymorphisms should be identified before LCM treatment, and then, the dosage should be adjusted for pediatric patients with epilepsy accordingly.

A systematic review and meta-analysis of the association of ABCC2/ABCG2 polymorphisms with antiepileptic drug responses in epileptic patients

PURPOSE

Accumulating evidence indicates that genetic polymorphisms in ATP-binding cassette superfamily members, such asABCC2 and ABCG2, alter responses to antiepileptic drugs (AEDs); however, this evidence is controversial and inconclusive. To provide strong evidence of the association between common polymorphisms in ABCC2 and ABCG2 and AED responses in patients with epilepsy, we performed a systematic review and meta-analysis.

METHODS

A literature search of electronic databases (PubMed, EBSCO, Ovid and the China National Knowledge Infrastructure) was performed. To evaluate the association of genetic polymorphisms inABCC2 and ABCG2 and risk of AED treatment, we calculated pooled odds ratios (ORs) and 95 % confidence intervals (CIs) using a fixed- or random-effect model.

RESULTS

A significant association of theABCC2 rs717620 polymorphism with resistance to AEDs was found in the overall pooled populations (homozygous comparison: OR = 1.77, 95 % CI, 1.27-2.48; dominant model: OR = 1.23, 95 % CI, 1.06-1.43; recessive model: OR = 1.75, 95 % CI, 1.28-2.40) and Asians (dominant model: OR = 1.21, 95 % CI, 1.03-1.42; recessive model: OR = 1.80, 95 % CI, 1.30-2.50). Using a recessive model, a similarly significant association of ABCC2 rs3740066 with AED resistance was observed in the overall pooled populations (OR = 2.29, 95 % CI, 1.44-3.64) and Asians (OR = 2.53, 95 % CI, 1.56-4.08). However, ABCC2 rs2273697, ABCG2 rs2231137 and rs2231142 were not found to be associated with AED responsiveness.

CONCLUSION

This meta-analysis suggests thatABCC2 rs717620 and rs3740066 are risk factors that predict responses to AEDs in epileptic patients.

Effects of MTHFR and ABCC2 gene polymorphisms on antiepileptic drug responsiveness in Jordanian epileptic patients

Background

Epilepsy is one of the most common neurological diseases with unclear etiology where its genetic background and treatment regime still need further exploration.

Objectives

This study designed to evaluate the pharmacogenomics of MTHFR and ABCC2 genes, and their association with epilepsy susceptibility among Jordanian population.

Methods

A case-control study was conducted on Jordanian cohort of 296 epileptic patients and 299 healthy individuals. Custom platform array was used to genotype the genetic polymorphisms within MTHFR (rs1801133) and ABCC2 (rs717620, rs3740066, rs2273697) genes.

Results

This study revealed a significant genetic association of MTHFR rs1801133 polymorphism with susceptibility to generalized in general and generalized tonic-clonic epilepsy (GTCE)(p=0.018 and 0.01, respectively). Regarding ABCC2 gene, rs717620 was of linkage with generalized and GTCE subtypes (p=0.045 and 0.048, respectively), while rs717620 was associated with poor responder patients (p=0.036) with no linkage of the ABCC2 haplotypes.

Conclusions

MTHFR and ABCC2 polymorphisms showed an association with either epilepsy types in general or subtypes and treatment response among Jordanian population. This study also suggested that these gene polymorphisms have an important role in epilepsy development and drug effectiveness and could be of a great impact in the era of epilepsy diagnosis and treatment.

Clinical reappraisal of the influence of drug-transporter polymorphisms in epilepsy

INTRODUCTION

Although novel antiepileptic drugs (AEDs) have been recently released, the issue of drug resistance in epileptic patients remains unsolved and largely unpredictable. Areas covered: We aim to assess the clinical impact of genetic variations that may influence the efficacy of medical treatment in epilepsy patients. Indeed, many genes, including genes encoding drug transporters (ABCB1), drug targets (SCN1A), drug-metabolizing enzymes (CYP2C9, CYP2C19), and human leucocyte antigen (HLA) proteins, may regulate the mechanisms of drug resistance in epilepsy. This review specifically focuses on the ABC genes, which encode multidrug resistance-associated proteins (MRPs) and may reduce the blood-brain barrier penetration of anticonvulsant AEDs. Expert opinion: Drug resistance remains a crucial problem in epilepsy patients. Pharmacogenomic studies may improve our understanding of drug responses and drug resistance by exploring the impact of gene variants and predicting drug responses and tolerability.

Correlation of MCT1 and ABCC2 gene polymorphisms with valproic acid resistance in patients with epilepsy on valproic acid monotherapy

Valproic acid (VPA) is used as one of the first-line antiepileptic drugs to control seizure in epilepsy patients. However, one third of patients do not respond to VPA. This study is to investigate the influence of single nucleotide polymorphisms (SNPs) in multidrug transporters on VPA responses in Han Chinese epilepsy patients on VPA monotherapy. Twelve SNPs involved in VPA transport pathways, including ABCC2, ABCC4, ABCG2, MCT1, MCT2 and OATP2B1 were genotyped in 153 Han Chinese epilepsy patients. We found that among all the patients, MCT1 rs60844753 CC carriers have higher incidence of VPA-resistance than CG carriers (P = 0.05), and in subgroup of generalized seizure, ABCC2 rs3740066 CC carriers had higher frequency of VPA resistance than TC + TT carriers (P = 0.03). Although other SNPs were not correlated with VPA resistance, significant ethnic difference was found in minor allele frequency of these SNPs, indicating that the influence of these SNPs on VPA efficacy should be broadly investigated in other ethnic populations. This study provides nominal evidence that SNPs of genes involved in the transport of VPA contribute to interpatient variation in VPA response. Although the associations were abolished after Bonferroni correction, the results provide an incentive for further research in sufficiently large samples.

Relationship between ABCB1 3435TT genotype and antiepileptic drugs resistance in Epilepsy: updated systematic review and meta-analysis

Background

Antiepileptic drugs (AEDs) are effective medications available for epilepsy. However, many patients do not respond to this treatment and become resistant. Genetic polymorphisms may be involved in the variation of AEDs response. Therefore, we conducted an updated systematic review and a meta-analysis to investigate the contribution of the genetic profile on epilepsy drug resistance.

Methods

We proceeded to the selection of eligible studies related to the associations of polymorphisms with resistance to AEDs therapy in epilepsy, published from January 1980 until November 2016, using Pubmed and Cochrane Library databases. The association analysis was based on pooled odds ratios (ORs) and 95% confidence intervals (CIs).

Results

From 640 articles, we retained 13 articles to evaluate the relationship between ATP-binding cassette sub-family C member 1 (ABCB1) C3435T polymorphism and AEDs responsiveness in a total of 454 epileptic AEDs-resistant cases and 282 AEDs-responsive cases. We found a significant association with an OR of 1.877, 95% CI 1.213–2.905. Subanalysis by genotype model showed a more significant association between the recessive model of ABCB1 C3435T polymorphism (TT vs. CC) and the risk of AEDs resistance with an OR of 2.375, 95% CI 1.775–3.178 than in the dominant one (CC vs. TT) with an OR of 1.686, 95% CI 0.877–3.242.

Conclusion

Our results indicate that ABCB1 C3435T polymorphism, especially TT genotype, plays an important role in refractory epilepsy. As genetic screening of this genotype may be useful to predict AEDs response before starting the treatment, further investigations should validate the association.

SCN1A, ABCC2 and UGT2B7 gene polymorphisms in association with individualized oxcarbazepine therapy

AIM

Associations between the effects of SCN1A, SCN2A, ABCC2 and UGT2B7 genetic polymorphisms and oxcarbazepine (OXC) maintenance doses in Han Chinese epileptic patients were investigated.

PATIENTS & METHODS

Genetic polymorphisms were detected in 184 epileptic patients receiving OXC monotherapy by high-resolution melting curve and TaqMan method.

RESULTS

Carriers of the SCN1A IVS5-91G>A, UGT2B7 c.802T>C and ABCC2 c.1249G>A variant alleles required significantly higher OXC maintenance doses than noncarriers (p < 0.05). Corresponding relative ln (concentration-dose ratios) values for SCN1A IVS5-91 variants differed by the genotypic order GG > GA > AA.

CONCLUSION

SCN1A, UGT2B7 and ABCC2 genetic polymorphisms are associated with OXC maintenance doses and may be useful for the personalization of OXC therapy in epileptic patients. Further studies are needed. Original submitted 6 June 2014; Revision submitted 5 September 2014.

Association between PK/PD-involved gene polymorphisms and carbamazepine-individualized therapy

Aim: To evaluate the association between the major genetic variants involved in the pharmacokinetic/pharmacodynamic (PK/PD) properties of carbamazepine (CBZ) and its maintenance doses and concentrations. Patients & methods: The genotypes of 166 patients receiving CBZ monotherapy were detected using high-resolution melting curve (HRM) and TaqMan methods. Results: Both univariate and multiple regression analyses revealed that carriers of the SCN1A IVS5–91G>A or EPHX1 c.337T>C allele tended to require a higher CBZ dose and a lower CBZ natural logarithmic concentration–dose ratio (lnCDR) than noncarriers (p < 0.05). Furthermore, two interactions between these genes were associated with the lnCDR and the maintenance dosage of CBZ, respectively. Conclusion: SCN1A IVS5–91G>A gene polymorphism is potential genetic biomarker associated with the PK of CBZ.

Association of SCN1A, SCN2A and ABCC2 gene polymorphisms with the response to antiepileptic drugs in Chinese Han patients with epilepsy

AIM

The purpose of this study was to investigate the potential impact of SCN1A, SCN2A and ABCC2 gene polymorphisms on the response to antiepileptic drugs in Chinese Han patients with epilepsy.

PATIENTS & METHODS

Genetic polymorphisms in the candidate genes were detected in 453 Chinese epileptic patients by high-resolution melting curve and TaqMan methods.

RESULTS

The SCN1A IVS5-91G>A AA genotype and the ABCC2 c.1249G>A GA genotype were significantly associated with carbamazepine/oxcarbamazepine (CBZ/OXC)-resistant epilepsy (p =0.002 and p = 0.036, respectively). The frequencies of haplotypes AA (SCN1A gene) and AC (ABCC2 gene) in drug-resistant patients were significantly higher than those in responsive patients (p = 0.002 and p = 0.005, respectively).

CONCLUSION

This study suggested that SCN1A and ABCC2 polymorphisms may be associated with the response to CBZ/OXC in the Chinese Han population, indicating that they could serve as predictors of drug response. Original submitted 29 January 2014; Revision submitted 30 May 2014.

ABCC2 rs2273697 and rs3740066 polymorphisms and resistance to antiepileptic drugs in Asia Pacific epilepsy cohorts

AIM

To examine the relevance of ABCC2 polymorphisms to drug responsiveness in epilepsy cohorts from the Asia Pacific region.

MATERIALS & METHODS

The rs2273697 and rs3740066 polymorphisms were genotyped in 2056 Malaysian (55%), Hong Kong (32%) and Japanese (13%) epilepsy patients.

RESULTS

Significant allele association of rs2273697 was observed in Chinese females with epilepsy, Malaysian Chinese patients with generalized seizure and Japanese patients with partial seizure for the AA versus GG genotype model and Malaysian Chinese patients with generalized seizure for the GA versus GG and autosomal dominant models. Significant association of the rs3740066 allele was observed in Malaysian females of Malay origin with cryptogenic epilepsy and Chinese patients with partial seizure and for genotypes in Malay patients with cryptogenic epilepsy for the CT versus CC and autosomal dominant genotype models. Significant results were observed for all haplotypes, but following Bonferroni correction, only the GT haplotype in Chinese patients remained significant.

CONCLUSION

This study suggests that the GT haplotype might be a risk factor for resistance to medication in Chinese patients.

Pharmacogenetics of adverse reactions to antiepileptic drugs

INTRODUCTION

Adverse drug reactions (ADRs) are a major public health concern and a leading cause of morbidity and mortality in the world. In the case of antiepileptic drugs (AEDs), ADRs constitute a barrier to successful treatment since they decrease treatment adherence and impact patients' quality of life of patients. Pharmacogenetics aims to identify genetic polymorphisms associated with drug safety. This article presents a review of genes coding for drug metabolising enzymes and drug transporters, and HLA system genes that have been linked to AED-induced ADRs.

DEVELOPMENT

To date, several genetic variations associated with drug safety have been reported: CYP2C9*2 and *3 alleles, which code for enzymes with decreased activity, have been linked to phenytoin (PHT)-induced neurotoxicity; GSTM1 null alleles with hepatotoxicity induced by carbamazepine (CBZ) and valproic acid (VPA); EPHX1 polymorphisms with teratogenesis; ABCC2 genetic variations with CBZ- and VPA-induced neurological ADRs; and HLA alleles (e.g. HLA-B*15:02, -A*31:01, -B*15:11, -C*08:01) with cutaneous ADRs.

CONCLUSIONS

Published findings show that there are ADRs with a pharmacogenetic basis and a high interethnic variability, which indicates a need for future studies in different populations to gather more useful results for larger number of patients. The search for biomarkers that would allow predicting ADRs to AEDs could improve pharmacotherapy for epilepsy.

The recessive model of MRP2 G1249A polymorphism decrease the risk of drug-resistant in Asian Epilepsy: a systematic review and meta-analysis

ABCC2 gene polymorphisms have been shown to be associated with drug-resistant epilepsy. However, the published results were controversial. To comprehensively re-evaluate the association between ABCC2 gene polymorphisms and drug-resistant epilepsy in Asian, we carried out this meta-analysis, which included eight related studies. Studies were selected using PUBMED, Web of science, the Cochrane database of system reviews and Embase. Pooled odds ratio (OR) with 95% confidence interval (CI) was used to assess the association. Studies with 1302 drug-resistant cases and 1563 drug-sensitive controls were included. No significant association was detected by combined analyses for C-24T, G-1774delG, C3972T and G2934A. However, significant association was found in recessive model for G1249A polymorphism (GG vs. GA+AA: OR=0.72, 95%CI=0.53-0.96, P=0.03), indicating the recessive model of G1249A in MRP2/ABCC2 might decrease the risk of drug resistance in Asian epilepsy.

A comprehensive functional and clinical analysis of ABCC2 and its impact on treatment response to carbamazepine

At the blood-brain barrier, overexpression of the drug efflux transporter ABCC2 (also known as MRP2) has been proposed as a mechanism for impaired carbamazepine (CBZ) treatment response in epilepsy. However, investigation of the impact of ABCC2 polymorphisms on CBZ treatment efficacy has produced conflicting and inconclusive results. A series of in vitro cell efflux and plasma membrane vesicle uptake assays were undertaken to investigate whether CBZ was an ABCC2 substrate. In addition, the effect of three common ABCC2 polymorphisms, -24C>T, c.1249G>A and c.3972C>T, on the efficacy of CBZ in epilepsy (assessed using the clinical end points time to first seizure and time to 12-month remission from the SANAD (Standard and New Antiepileptic Drugs) trial) was determined. CBZ was found not to be a substrate for human ABCC2 in vitro. Clinically, no significant association was observed for the ABCC2 genetic variants and CBZ treatment outcomes. This comprehensive analysis does not support a role for ABCC2 in CBZ treatment efficacy.

The effects of ABCC2 G1249A polymorphism on the risk of resistance to antiepileptic drugs: a meta-analysis of the literature

AIMS

The G1249A variant of the multidrug resistance-associated protein 2 (ABCC2) gene may be associated with the development of antiepileptic drug (AED) resistance. Although numerous studies have investigated the association between the G1249A variant and the risk of drug resistance in epilepsy, the results of these studies have been inconclusive. To assess the role of G1249A polymorphism in drug resistance in epilepsy, a meta-analysis was performed.

MATERIALS AND METHODS

We systematically reviewed relevant studies retrieved by the PubMed and Embase. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated based on the date extracted from the studies to evaluate the strength of association. We also analyzed the heterogeneity and sensitivity of each report and the publication bias of the studies.

RESULTS

A total of 6 published studies, involving 2213 patients (1100 patients with drug-resistant epilepsy and 1113 controls with drug-responsive epilepsy) were reviewed in the present meta-analysis. The overall results indicated that the variant genotypes were associated with a significantly decreased risk of AED resistance (AA vs. GG: OR=0.372, 95% CI=0.182-0.762; recessive model: OR=0.399, 95% CI=0.200-0.795) (fixed-effects model). A stratified analysis by ethnicity showed similar findings for Caucasians in an additive model (A vs. G: OR=0.700, 95% CI=0.494-0.992).

CONCLUSIONS

The meta-analysis suggests that the ABCC2 G1249A polymorphism is significantly associated with a decreased risk of AED resistance. However, further functional investigations are warranted to validate the association.

Polymorphism of ORM1 is associated with the pharmacokinetics of telmisartan

BACKGROUND

The pharmacokinetics (PKs) and pharmacodynamics (PDs) of telmisartan varies among the individuals, and the main causes remain unknown. The aim of this study was to evaluate the impact of ORM1, as well as ABCC2, ABCB1, ABCG2 and SLCO1B3 polymorphisms, on the disposition of the drug and BP change after taking 40 mg telmisartan in 48 healthy Chinese males.

METHOD

A total of 48 healthy males were included in this trial. Every volunteer ingested a single dose of 40 mg telmisartan, and the plasma drug concentration and blood pressure (BP) were measured up to 48 h.

RESULT

In this study, the area under the plasma concentration-time curve (AUC) in the heterozygotes of ORM1 113AG was higher than that in the wild-type homozygotes, AUC(0-48) (113AA vs. 113AG, 1,549.18±859.84 ng·h/ml vs. 2,313.54±1,257.71 ng·h/ml, P = 0.033), AUC(0-∞) (113AA vs. 113AG, 1,753.13±1,060.60 ng·h/ml vs. 2,686.90±1,401.87 ng·h/ml, P = 0.016), and the change(%) of the diastolic blood pressure (DBP) from the baseline BP value also showed a significant difference between the ORM1 113AG and 113AA genotypes at 5 h after taking telmisartan (P = 0.026). This study also showed that the allele of ABCC2 C3972T would affected the disposition of telmsiartan and the DBP change significantly after taking the drug. However, the common SNPs of ABCG2 C421, ABCB1 C3435T, and SLCO1B3 T334G showed no impacts on the PKs of telmisartan or BP change(%) in our trial.

CONCLUSION

The ORM1 A113G polymorphism was associated with the PKs variability after taking telmsiartan, as well as ABCC2 C3972T. The heterozygotes of ORM1 113AG showed a larger AUC and a notable BP change(%) from the baseline compared with the wild-type.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR-TNC-10000898.

Genetic association analysis of transporters identifies ABCC2 loci for seizure control in women with epilepsy on first-line antiepileptic drugs

OBJECTIVE

The ATP-binding cassette (ABC) superfamily of transporters is known to efflux antiepileptic drugs (AEDs) primarily in the brain, gastrointestinal tract, liver, and kidneys. In addition, they are also known to be involved in estrogen disposition and may modulate seizure susceptibility and drug response. The objective of the present study was to investigate the role of genetic variants from ABC transporters in seizure control in epilepsy patients treated with monotherapy of first-line AEDs for 12 months.

METHODS

On the basis of gene coverage and functional significance, a total of 98 single nucleotide polymorphisms from ABCB1, ABCC1, and ABCC2 were genotyped in 400 patients from North India. Of these, 216 patients were eligible for therapeutic assessment. Genetic variants were compared between the 'no-seizures' and the 'recurrent-seizures' groups. Bonferroni corrections for multiple comparisons and adjustment for covariates were performed before assessment of associations.

RESULTS

Functionally relevant promoter polymorphisms from ABCC2: c.-1549G>A and c.-1019A>G either considered alone or in haplotype and diplotype combinations were observed for a significant association with seizure control in women (odds ratio>3.5, P<10, power>95%). Further, low protein-expressing CGT and TGT (c.-24C>T, c.1249G>A, c.3972C>T) haplotypes were always observed to be present in combination with the AG (c.-1549G>A, c.-1019A>G) haplotype that was over-represented in women with 'no seizures'.

CONCLUSION

The distribution of the associated variants supports the involvement of ABCC2 in controlling seizures in women possibly by lowering of its expression. The biological basis of this finding could be an altered interaction of ABCC2 with AEDs and estrogens. These results necessitate replication in a larger pool of patients.

Lack of association between ABCC2 gene variants and treatment response in epilepsy

AIM

The aim of this study was to replicate a previously reported association between drug resistance in epilepsy patients and the 24C>T variant of the ABCC2 gene that codes for the drug efflux transporter MRP2.

PATIENTS & METHODS

We genotyped 381 Caucasian epileptic patients (337 drug resistant and 44 drug responsive) and 247 healthy controls for the ABCC2 gene -24C>T polymorphism (rs717620) and two other nearby SNPs in linkage disequilibrium (1249G>A and 3972C>T). Genotype, allele and three-SNP-haplotype frequencies were compared between groups. Patients were further stratified into four groups according to their degree of drug resistance (as measured by seizure frequency under medication) to perform regression analysis against genotypes and haplotpyes.

RESULTS

We detected no significant differences in the distribution of any of the tested alleles, genotypes or haplotypes between the investigated groups. Neither was there an association between genotypes or haplotypes and degree of drug resistance. This study was adequately powered to detect genotype relative risks of above two.

CONCLUSION

Although adequately powered to detect the previously reported effect size and although our definition of drug resistance, following the International League Against Epilepsy guidelines, was slightly stricter than in the original study, we failed to confirm an association between the 24C>T variant in the ABCC2 gene and drug resistance in epilepsy.

Transporter hypothesis of drug-resistant epilepsy: challenges for pharmacogenetic approaches

Drug resistance in epilepsy is considered a complex and multifactorial problem. Overexpression of efflux transporters at the blood-brain barrier is discussed as one factor that might limit brain penetration and efficacy of antiepileptic drugs. Whereas experimental data render support for this hypothesis, there is still a lack of sufficient clinical evidence indicating a functional role of efflux transporters. Pharmacogenetic analysis has been considered as one approach in the evaluation of a putative link between transporters and drug-resistant epilepsy. However, the likelihood of a multifactorial nature of drug resistance and the complexity of the events regulating transporters pose a major challenge to any attempt at linking selected genetic polymorphisms to the outcome of drug therapy. In this article, the evidence for an impact of efflux transporters on the response to antiepileptic drugs is discussed, focusing in particular on the different issues presenting a challenge for pharmacogenetic approaches in this field.

Gene-wide tagging study of the association between ABCC2, ABCC5 and ABCG2 genetic polymorphisms and multidrug resistance in epilepsy

AIM

To determine the association between polymorphisms of the multidrug transporter genes ABCC2, ABCC5 and ABCG2, and drug resistance in epilepsy by genotyping comprehensive sets of tagging SNPs.

MATERIALS & METHODS

A total of 25 tagging SNPs from ABCC2, ABCC5 and ABCG2 genes were genotyped in a total of 590 Han Chinese epilepsy patients (262 drug resistant and 328 drug responsive). Genotype and allele distributions in drug-responsive and drug-resistant patients were compared.

RESULTS

Genotype distributions of all the selected SNPs were consistent with Hardy-Weinberg equilibrium. None of the polymorphisms, either genotype or allele distributions, were significantly associated with drug resistance. For each gene, no haplotypes of over 1% frequency, and that included all SNPs of the gene, were associated with drug resistance.

CONCLUSION

This gene-wide tagging study revealed no association between ABCC2, ABCC5 and ABCG2 genetic polymorphisms and multidrug resistance in epilepsy.

A nonsynonymous variation in MRP2/ABCC2 is associated with neurological adverse drug reactions of carbamazepine in patients with epilepsy

OBJECTIVES

Multidrug resistance protein 2 (MRP2, ABCC2) is involved in the transport of antiepileptic drugs and is upregulated in the brain tissues of patients with epilepsy. Therefore, genetic variations in the MRP2 gene may affect individual drug responses to the antiepileptic agent carbamazepine.

METHODS

Associations between MRP2 polymorphisms and the adverse drug reactions (ADRs) of carbamazepine were analyzed using an integrated population genetics and molecular functional approach. In the initial case-control study, five tag single nucleotide polymorphisms in the MRP2 gene were analyzed in 146 patients with epilepsy. Patients were divided into two groups: those who experienced ADRs of the central nervous system and those who did not. An independent replication study was performed using DNA samples from 279 patients.

RESULTS

A nonsynonymous polymorphism, c.1249G>A (p.V417I, rs2273697), showed a strong association with the neurological ADR caused by carbamazepine (P=0.005). Logistic regression analysis with multiple clinical variables indicated that the presence of A allele at the MRP2 c.1249G>A locus was an independent determinant of central nervous system ADR caused by carbamazepine. Moreover, the positive association of c.1249A was reproduced in the replication study (P=0.042, joint P value of the replication=0.001). The functional study using ATPase assay and FACScan flow cytometer indicated that carbamazepine was a substrate of MRP2 and that the 417I variation selectively reduced carbamazepine transport across the cell membrane.

CONCLUSION

These results strongly suggest that the A-allele of the MRP2 single nucleotide polymorphism c.1247G>A is associated with adverse neurological drug reactions to carbamazepine.

Gene polymorphisms and their role in epilepsy treatment and prognosis

The human genome carries an enormous number of genetic variants, many of them of functional consequence. In epilepsy, they are likely to be involved in drug-specific treatment efficacy, unwanted or even toxic drug reactions, teratogenic risks in pregnancy as well as in the long-term prognosis of patients with epilepsy. As in many other disorders with a complex genetic background, the associated genetic variants that could be verified successfully in replication studies are still only a few. However, new techniques and improved research strategies are likely to increase their number in the foreseeable future, although at a much slower pace as initially expected.

Epilepsy pharmacogenetics

Large interindividual variation in efficacy and adverse effects of anti-epileptic therapy presents opportunities and challenges in pharmacogenomics. Although the first true association of genetic polymorphism in drug-metabolizing enzymes with anti-epileptic drug dose was reported 10 years ago, most of the findings have had little impact on clinical practice so far. Most studies performed to date examined candidate genes and were focused on candidate gene selection. Genome-wide association and whole-genome sequencing technologies empower hypothesis-free comprehensive screening of genetic variation across the genome and now the main challenge remaining is to select and study clinically relevant phenotypes suitable for genetic studies. Here we review the current state of epilepsy pharmacogenetics focusing on phenotyping questions and discuss what characteristics we need to study to get answers.

Lack of association between ABCB1, ABCG2, and ABCC2 genetic polymorphisms and multidrug resistance in partial epilepsy

ATP-binding cassette (ABC) transporters participate in drug disposition and response in various conditions, and many polymorphisms in ABC transporter genes have been recognized in association with altered transporter functions of various drugs. Studies on epilepsy have focused on the C3435T polymorphism of the ABCB1 gene, but other ABC transporters are also thought to be involved in the transport of antiepileptic drugs. We have evaluated the functional polymorphisms of ABCB1, ABCG2, and ABCC2 genes with regard to epilepsy drug response in partial epilepsy, and have investigated the potential of combined effects of polymorphisms in more than one transporter gene. We studied 6 genetic polymorphisms in 3 transporter genes in 193 drug responders and 198 nonresponders. There was no significant difference between the two groups, and we did not observe any combined effects of ABCB1 and ABCG2 genetic polymorphisms. Our study suggests that genetic polymorphisms in ABC transporters may not be significant predictors of drug response in epilepsy patients.