Variants inCHRNB2andCHRNA4Identified in Patients with Insular Epilepsy

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Abstract
Purpose: Our purpose was to determine the role ofCHRNA4andCHRNB2in insular epilepsy.Method: We identified two patients with drug-resistant predominantly sleep-related hypermotor seizures, one harboring a heterozygous missense variant (c.77C>T; p. Thr26Met) in theCHRNB2gene and the other a heterozygous missense variant (c.1079G>A; p. Arg360Gln) in theCHRNA4gene. The patients underwent electrophysiological and neuroimaging studies, and we performed functional characterization of the p. Thr26Met (c.77C>T) in theCHRNB2gene.Results: We localized the epileptic foci to the left insula in the first case (now seizure-free following epilepsy surgery) and to both insulae in the second case. Based on tools predicting the possible impact of amino acid substitutions on the structure and function of proteins (sorting intolerant from tolerant and PolyPhen-2), variants identified in this report could be deleterious. Functional expression in human cell lines of α4β2 (wild-type), α4β2-Thr26Met (homozygote), and α4β2/β2-Thr26Met (heterozygote) nicotinic acetylcholine receptors revealed that the mutant subunit led to significantly higher whole-cell nicotinic currents. This feature was observed in both homo- and heterozygous conditions and was not accompanied by major alterations of the current reversal potential or the shape of the concentration-response relation.Conclusions: This study suggests that variants inCHRNB2andCHRNA4, initially linked to autosomal dominant nocturnal frontal lobe epilepsy, are also found in patients with predominantly sleep-related insular epilepsy. Although the reported variants should be considered of unknown clinical significance for the moment, identification of additional similar cases and further functional studies could eventually strengthen this association.

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