Genetic variation of the human μ-opioid receptor and susceptibility to idiopathic absence epilepsy
Introduction
Idiopathic absence epilepsy (IAE) comprises two common syndromes of idiopathic generalized epilepsy, childhood (CAE) and juvenile absence epilepsy (JAE), which are characterized by typical absence seizures (Berkovic et al., 1987, Duncan, 1997, Janz, 1997). Absence seizures are invariably accompanied by an ictal EEG signature of generalized spike-wave discharges (GSW-EEG), which reflect a hyperexcitability state of thalamo-cortico-thalamic circuits involving the reticular thalamic nucleus (Hosford, 1995, Avanzini et al., 1996). Despite extensive research efforts, the molecular mechanisms underlying the epileptogenesis of IAE remain elusive. Hereditary factors play a predominant role in the etiology of IAE, but the complex inheritance pattern of the IAE syndromes suggests that several genetic factors contribute to the genetic pathogenesis (Beck-Mannagetta and Janz, 1991, Greenberg et al., 1992, Sander, 1996).
Animal models that develop spontaneous absences offer a valuable tool to elucidate key sites of excitability mechanisms involved in the epileptogenesis of human IAE (Noebels, 1996, Puranam and McNamara, 1999). Absence-like seizures in rats may be provoked by the administration of endogenous and exogenous opioids, and prevented by opiate antagonists, implying a role for endogenous opioids in the pathogenesis of absence epilepsies (Lason et al., 1994, Prevett et al., 1994, Przewlocka et al., 1998, Vathy et al., 1998). The WAG/Rij strain of rats exhibits non-convulsive epileptic seizures that resembles human absence seizures of IAE (Coenen et al., 1992). An autoradiographic study in WAG/Rij rats demonstrated a high density of μ-opioid receptors (OPRM) in brain areas involved in the generation of GSW-discharges (Przewlocka et al., 1998). Furthermore, the high increase in the number of GSW-discharges after intrathalamic administration of the OPRM agonist DAMGO (d-Ala2-N-methyl-Phe4-Gly5-ol-enkephalin) suggests the involvement of OPRM in the generation of GSW-activity and the pathogenesis of absence seizures (Przewlocka et al., 1998).
The human gene encoding the OPRM receptor has been cloned and physically mapped to the chromosomal region 6q24-q25 (Wang et al., 1993). Mutation screening of the coding region of the OPRM gene revealed several DNA sequence variants (Bergen et al., 1997, Bond et al., 1998, Wendel and Hoehe, 1998). The most prevalent single-nucleotide polymorphism is a nucleotide exchange at position 118 (A118G), predicting an amino acid substitution of asparagine to aspartic acid (Asn40Asp) in the extracellular N-terminal domain at a highly conserved N-glycosylation site (Bergen et al., 1997, Bond et al., 1998). This Asn40Asp substitution polymorphism may be of functional significance, because the Asp40 variant exhibits a three times higher affinity to β-endorphin than the Asn40 variant, and the agonist-induced activation of G protein-coupled potassium channels is three times higher for the Asp40 variant compared to the Asn40 variant (Bond et al., 1998). Thus, the Asn40Asp substitution polymorphism is a plausible candidate gene polymorphism that might explain individual differences in the liability to epileptogenesis. The present association study was designed to test the hypothesis that the Asn40Asp substitution polymorphism of the OPRM gene confers genetic susceptibility to IAE.
Section snippets
Subjects
The study protocol has been approved by the Ethics Committee of the University Hospital Rudolf Virchow at the Humboldt University of Berlin. Written informed consent was obtained from all participants who were unrelated individuals of German descent.
In total, 72 unrelated IAE patients with either CAE (n=26) or JAE (n=46) were included in the present study. Fourteen of the JAE subjects and three of the CAE subjects evolved subsequently to juvenile myoclonic epilepsy. The diagnoses of the
Results
The frequency of the Asp40 allele in 680 chromosomes of German controls was 7.8% with an observed heterozygosity of 14.4%. The genotype and allele frequencies in 340 controls and 72 IAE patients are shown in Table 1. None of genotype counts deviated significantly from those expected according to the Hardy–Weinberg equilibrium (P>0.70).
The Asp40 allele frequency was increased significantly in the IAE patients [f(Asp40)=0.139] compared to the controls [f(Asp40)=0.078; χ2=5.467, df=1, P=0.019;
Discussion
Although there is strong evidence that the common human IAE syndromes are genetically determined, the underlying genetic factors remain still unknown. Two principal challenges confront the molecular genetic dissection of human susceptibility genes of the common IAE syndromes: complex inheritance and genetic heterogeneity (Lander and Schork, 1994, Sander, 1996). Several genetic factors are likely to be involved in the etiology of the IAE syndromes, and the effect size of each single genetic
Acknowledgements
Supported by the Deutsche Forschungsgemeinschaft (Sa 434/2-1 and 2-2) and the Stiftung Michael.
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