Identification of a New de Novo Mutation Underlying Regressive Episodic Ataxia Type I

Abstract

Episodic ataxia type 1 (EA1), a Shaker-like K⁺channelopathy, is a consequence of genetic anomalies in the KCNA1 gene that lead to dysfunctions in the voltage-gated K⁺ channel Kv1. 1. Generally, KCNA1 mutations are inherited in an autosomal dominant manner. Here we report the clinical phenotype of an EA1 patient characterized by ataxia attacks that decrease in frequency with age, and eventually leading to therapy discontinuation. A new de novo mutation (c.932G>A) that changed a highly conserved glycine residue into an aspartate (p.G311D) was identified by using targeted next-generation sequencing. The conserved glycine is located in the S4-S5 linker, a crucial domain controlling Kv1.1 channel gating. In silico analyses predicted the mutation deleterious. Heterologous expression of the mutant (Kv1.1-G311D) channels resulted in remarkably decreased amplitudes of measured current, confirming the identified variant is pathogenic. Collectively, these findings corroborate the notion that EA1 also results from de novo variants and point out that regardless of the mutation-induced deleterious loss of Kv1.1 channel function the ataxia phenotype may improve spontaneously.

Keywords: G311D; KCNA1; diplopia; episodic ataxia type 1(EA1); myokymia.

Figure 1

(A) Membrane topology model of the Kv1.1 α subunit. The G311D mutation occurred at a highly conserved residue located in the S4-S5 linker domain. (B) Protein multisequence alignment of the Kv1.1 region of interest. The alignment, obtained by using MUSCLE 3.6, showed the Glycine at position 311 is highly conserved among species.

Figure 2

Heterologous expression of Kv1.1-WT and G311D mutant channels in Xenopus oocytes. (A) Averaged current amplitudes computed at +60 mV for Kv1.1-WT, G311D, and WT+G311D channels at 24, 48, and 72 h post cRNA injections. Each data point is the mean±SE of 8 cells.(B) Bar graph current amplitudes recorded 48 h after injection. The mutant channel shows ~3-fold smaller current than the WT (*p < 0.05, **p < 0.005, Student t-test). (C) Representative Kv1.1 current traces recorded at +60 mV from a holding potential of−80 mV showing reduction in oocytes expressing G311D cRNA.