Clinical, genetic, neurophysiological and functional study of new mutations in episodic ataxia type 1

Abstract

Background and objective: Heterozygous mutations in KCNA1 cause episodic ataxia type 1 (EA1), an ion channel disorder characterised by brief paroxysms of cerebellar dysfunction and persistent neuromyotonia. This paper describes four previously unreported families with EA1, with the aim of understanding the phenotypic spectrum associated with different mutations.

Methods: 15 affected individuals from four families underwent clinical, genetic and neurophysiological evaluation. The functional impact of new mutations identified in the KCNA1 gene was investigated with in vitro electrophysiology and immunocytochemistry.

Results: Detailed clinical documentation, dating back to 1928 in one family, indicates that all patients manifested episodic ataxia of varying severity. Four subjects from three families reported hearing impairment, which has not previously been reported in association with EA1. New mutations (R167M, C185W and I407M) were identified in three out of the four families. When expressed in human embryonic kidney cells, all three new mutations resulted in a loss of K(v)1.1 channel function. The fourth family harboured a previously reported A242P mutation, which has not been previously described in association with ataxia.

Conclusions: The genetic basis of EA1 in four families is established and this report presents the earliest documented case from 1928. All three new mutations caused a loss of K(v)1.1 channel function. The finding of deafness in four individuals raises the possibility of a link between K(v)1.1 dysfunction and hearing impairment. Our findings broaden the phenotypic range associated with mutations in KCNA1.

Keywords: Cerebellar Ataxia; Epilepsy; Neurogenetics; Neuromuscular; Neurophysiol, Clinical.

Figure 1

Pedigree for families A, B, C and D. Black shapes denote affected family members. Asterisk indicates proband.

Figure 2

Clinic letter dated 1928 describing assessment of subject II:6 in family C.

Figure 3

(i) Electrophoretograms showing new mutations for families A, B and C. Red=thymine, green=adenosine, black=guanine, blue=cytosine. (ii) Conservation of KCNA1 amino acid sequence across species. KCNA1 amino acids 152–196 are shown with the S1 segment outlined in grey. Arrows A, R167; B, C185. (iii) Amino acids 381–413 with S6 segment outlined in grey. Arrow C: I407 in S6.

Figure 4

(A) Voltage step protocol and representative current traces elicited by applying a voltage step from a holding potential of −80 mV. (B) Peak current density measured following depolarisation to ±10 mV (SEM). (C) Voltage dependence of activation estimated from tail currents measured at −80 mV following depolarising steps to different potentials obtained in cells transfected with wt (n=7), I407 M (n=8), wt:I407M (n=5), wt:R167M (n=7) and wt:C185W (n=5). Symbols as in panel A. Tail currents were sampled 1 ms after return to −80 mV. Error bars are SEM. Recordings were made using low potassium internal solution.

Figure 5

The Kv1.1 antibody detects the channel and the mutants, R167M and C185W in transfected human embryonic kidney (HEK) cells. The antibody does not detect any Kv1.1 channel in the HEK cells transfected with green fluorescent protein only (bottom panel).