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1 Centro de Estudios Cientificos (CECS), Valdivia, Chile
* To whom correspondence should be addressed. E-mail: pcid{at}cecs.cl.
The ClC-2 Cl- channel has been postulated to play a role in the inhibitory GABA response in neurons or to participate in astrocyte-dependent extracellular electrolyte homeostasis. Three different mutations in the CLCN2 gene, encoding the voltage-dependent homodimeric ClC-2 channel, have been associated with idiopathic generalized epilepsy (IGE). We study their function in vitro by patch-clamp and confocal microscopy in transiently transfected HEK-293 cells. A first mutation predicts a premature stop codon (M200fsX231). An altered splicing, due to an 11 bp deletion in intron 2 (IVS2-14del11), predicts exon 3 skipping (
74-117). A third is a missense mutation (G715E). M200fsX231 and (74-117 are non-functional and do not affect the function of the normal (WT) channel. Neither M200fsX231 nor 74-117 reach the plasma membrane. Concerning the IVS2-14del11 mutation, we find no difference in the proportion of exon-skipped to normally spliced mRNA using a minigene approach and, on this basis, predict no alteration in channel expression in affected individuals. G715E has voltage- and intracellular Cl--dependence indistinguishable from WT channels. ClC-2 channels are shown to be sensitive to intracellular replacement of ATP by AMP, which accelerates the opening and closing kinetics. This effect is diminished in the G715E mutant and not significant in WT+G715E co-expression. We do not know if in a situation of cellular ATP depletion, this might become pathological in individuals carrying the mutation. We postulate that loss of function mutation M200fsX231 of ClC-2 might contribute to the IGE phenotype through a haploinsufficieny mechanism.
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