Pharmacogenetics of antiepileptic drug efficacy in childhood absence epilepsy

Abstract

Objective: To determine whether common polymorphisms in CACNA1G, CACNA1H, CACNA1I, and ABCB1 are associated with differential short-term seizure outcome in childhood absence epilepsy (CAE).

Methods: Four hundred forty-six CAE children in a randomized double-blind trial of ethosuximide, lamotrigine, and valproate had short-term seizure outcome determined. Associations between polymorphisms (minor allele frequency ≥ 15%) in 4 genes and seizure outcomes were assessed. In vitro electrophysiology on transfected CACNA1H channels determined impact of 1 variant on T-type calcium channel responsiveness to ethosuximide.

Results: Eighty percent (357 of 446) of subjects had informative short-term seizure status (242 seizure free, 115 not seizure free). In ethosuximide subjects, 2 polymorphisms (CACNA1H rs61734410/P640L, CACNA1I rs3747178) appeared more commonly among not-seizure-free participants (p = 0.011, odds ratio [OR] = 2.63, 95% confidence limits [CL] = 1.25-5.56; p = 0.026, OR = 2.38, 95% CL = 1.11-5.00). In lamotrigine subjects, 1 ABCB1 missense polymorphism (rs2032582/S893A; p = 0.015, OR = 2.22, 95% CL = 1.16-4.17) was more common in not-seizure-free participants, and 2 CACNA1H polymorphisms (rs2753326, rs2753325) were more common in seizure-free participants (p = 0.038, OR = 0.52, 95% CL = 0.28-0.96). In valproate subjects, no common polymorphisms were associated with seizure status. In vitro electrophysiological studies showed no effect of the P640L polymorphism on channel physiology in the absence of ethosuximide. Ethosuximide's effect on rate of decay of Ca_V 3.2 was significantly less for P640L channel compared to wild-type channel.

Interpretation: Four T-type calcium channel variants and 1 ABCB1 transporter variant were associated with differential drug response in CAE. The in vivo P640L variant's ethosuximide effect was confirmed by in vitro electrophysiological studies. This suggests that genetic variation plays a role in differential CAE drug response. Ann Neurol 2017;81:444-453.

Figure 1. Biophysical and Pharmacological Properties of wild type (WT) Ca_V3.2 and rs61734410 (P640L) variant of Ca_V3.2

A. Current-voltage relationships of whole cells currents are similar between cells transiently transfected with WT (black) or P640L (gray). Currents were activated by 5–10 mV voltage steps to between −80 to +40 mV for 150 ms from a holding potential of −110 mV. Currents were normalized to the maximal current attained from each cell. The insets are representative families of whole cell currents from WT (black) and P640L (gray) obtained by stepping from a holding potential of −110 mV to various test potentials. Scale bars for the insets are 200 pA and 10 ms B. Voltage dependence of activation (open symbols, dotted lines) and steady-state inactivation (closed symbols, solid lines) for WT (black) and P640L (gray) are the same. The voltage dependence of activation was estimated by measuring the calcium current during a test potential from a holding current of −110 mV using the voltage steps in A. The current at each membrane potential was divided by the driving force and normalized to the maximal conductance. The voltage dependence of inactivation was obtained from 2-second steps to various voltages (ranging from −130 to +30 mV in 5–10 mV increments) followed by a 90 ms test pulse to −30 mV. The lines were fitted using the Boltzmann equation. C. Low concentration of ESM accelerates the decay of Ca_V3.2 in wild type (black, n=9) but not P640L (gray, n=9). Voltage dependence of the time constant of inactivation in the absence of (solid symbols, solid lines) and presence of (open symbols, dashed lines) 3 mM ethosuximide demonstrating that ethosuximide increased the rate inactivation in the WT but not in P640L. Inactivation rates (⊤) were determined by fitting the current-voltage traces with two exponentials; one for activation and the other for inactivation. The insets represent representative current traces at −30 mV normalized to the peak current. The dotted lines represent the currents in the presence of 3 mM ethosuximide and the solid lines are currents without ESM. Scale bar is 5 ms. D. Dose-Dependent acceleration of Ca_V3.2 decay occurs at lower concentrations in WT (left) than P640L (right). The time constant of inactivation at −30 mV at various concentrations of ESM. The inactivation time constant was determined as in C. * p<0.05; ** p<0.01 compared to responses without ESM (paired Student’s t-test with Bonferroni correction for multiple comparisons); (for WT n=6 at 1 mM; n=10 at 3 mM; n=9 at 10 mM; n=8 at 30 mM; for P640L n=5 at 1 mM; n=9 at 3 mM; n=8 at 10 mM; n=8 at 30 mM)