ABCC2 Polymorphisms and Haplotype are Associated with Drug Resistance in Chinese Epileptic Patients

Epilepsy is a chronic neurological illness, in which abnormal electrical activity in the brain causes involuntary changes in body movement, function, sensation, or behavior [1]. The World Health Organization estimates that epilepsy affects approximately 50 million people worldwide. Now‐a‐days, we recognize that genetic factors play an even more important role in the pathogenesis of epilepsy and drug efficacy than previously appreciated [1, 2]. Approximately, every second epileptic patient became resistant to the initial drugs [3]. Why so many epileptic patients show resistance against the antiepileptic drugs (AEDs) is not well‐understood. Some potential mechanisms have been suggested with regard to the targets and transporters of the drugs. The voltage‐gated ion channels are targets of some AEDs; alternations in the composition and functionality of them may lead to resistance in epileptic patients [4, 5]. The overexpression of ATP‐binding cassette (ABC) efflux transporters in the blood‐brain barrier (BBB), causing low‐drug concentration at their target, may be a potential mechanism [6]. Seizures, exposure to some AEDs such as carbamazepine, and genetic factors may influence the expression of transporters [7, 8, 9]. Some ABC transporters such as multidrug resistance 1 (MDR1, p‐glycoprotein, ABCB1), and a group of multidrug resistance proteins (MRPs, ABCCs), have been shown to mediate AEDs in brain [7, 9, 10]. These transporters are expressed in endothelial cells, astrocytes, and neurons of human brain tissues [11]. Therefore, these transporters may influence the efficacy of AEDs. Initial observation of the relationship between ABCB1 3435C>T and drug resistant epilepsy was followed by a number of studies to evaluate the single nucleotide polymorphism (SNP) as a predictor of AED resistance [12, 13]. However, some studies have conversely suggested no association between ABCB1 polymorphisms and the response to AEDs [14, 15]. Leschziner et al. studied a cohort of 503 epilepsy patients and found no evidence to support that ABCB1 common variation influences drug withdrawal outcomes [16]. ABCC2 is one of the ABC efflux transporters, whose role in human brain tissues is not fully understood due to its low expression in normal brain tissues [17]. But some studies found that the expression of ABCC2 was upregulated in the brain tissues of epileptic patients [18, 19]. Moreover, in the ABCC2‐deficient rat model, it is found that ABCC2 is involved in carbamazepine efficacy [20]. Recently, a study found a significant association between ABCC2 V417I polymorphism and reduced oral bioavailability of talinolol [21]. Kin et al. found that the ABCC2 1249G>A, and ABCB1 3435C>T, 1236C>T, 2677G>T/A polymorphisms were not associated with AED resistance in Japanese epileptic patients [22]. The study of Ufer et al. showed that a higher risk of AED failure in ABCC2−24T allele carriers is possible due to upregulation of ABCB1, but the mechanism is unknown [23]. Overall, whether the polymorphisms of these genes are associated with AED resistance is still not clear. To clarify whether ABCC2 and ABCB1 genes are involved in drug resistance epilepsy, we investigated the effects of ABCB1 rs1045642 and ABCC2 rs717620, rs3740066, and rs2273697 polymorphisms on AED resistance in Chinese epileptic patients.