Loss-of-function EA2 mutations are associated with impaired neuromuscular transmission.
OBJECTIVE
To examine the functional consequences of episodic ataxia type 2 (EA2)-causing nonsense and missense mutations in vitro and to characterize the basis of fluctuating weakness in patients with E2A.


BACKGROUND
Mutations in CACNA1A encoding the Ca(v)2.1 calcium channel subunit cause EA2 through incompletely understood mechanisms. Although the Ca(v)2.1 subunit is important for neurotransmission at the neuromuscular junction, weakness has not been considered a feature of EA2.


METHODS
The disease-causing mutations in three unrelated patients with EA2 and fluctuating weakness were identified by mutation screening and sequencing. Mutant constructs harboring mutations R1281X, F1406C, R1549X were transfected into COS7 cells and expressed for patch clamp studies. Single-fiber electromyography (SFEMG) was performed in patients to examine synaptic transmission at the neuromuscular junction.


RESULTS
Functional studies in COS7 cells of nonsense and missense EA2 mutants demonstrated markedly decreased current densities compared with wild type. SFEMG demonstrated jitter and blocking in these patients with EA2, compared with normal subjects and three patients with SCA-6.


CONCLUSIONS
EA2-causing missense and nonsense mutations in CACNA1A produced mutant channels with diminished whole cell calcium channel activity in vitro due to loss of function. Altered biophysical properties or reduced efficiency of plasma membrane targeting of mutant channels may contribute to abnormal neuromuscular transmission, manifesting as myasthenic syndrome.
Jen
J
J
Departments of Neurology, UCLA School of Medicine, Los Angeles, CA 90095-1769, USA. jjen@ucla.edu
Wan
J
J
Graves
M
M
Yu
H
H
Mock
A F
AF
Coulin
C J
CJ
Kim
G
G
Yue
Q
Q
Papazian
D M
DM
Baloh
R W
RW
D015220:Calcium Channels / Q000235:genetics*; D020864:Calcium Channels, N-Type / Q000235:genetics; D018389:Codon, Nonsense / Q000235:genetics*; D004252:DNA Mutational Analysis; D006801:Humans; D008297:Male; D008564:Membrane Potentials / Q000235:genetics / Q000502:physiology; D008875:Middle Aged; D020125:Mutation, Missense / Q000235:genetics*; D020294:Myasthenic Syndromes, Congenital / Q000235:genetics / Q000503:physiopathology; D009460:Neurologic Examination; D009469:Neuromuscular Junction / Q000235:genetics / Q000503:physiopathology; D020511:Neuromuscular Junction Diseases / Q000235:genetics* / Q000503:physiopathology; D010375:Pedigree; D020754:Spinocerebellar Ataxias / Q000235:genetics* / Q000503:physiopathology; D009435:Synaptic Transmission / Q000235:genetics / Q000502:physiology
D015220:Calcium Channels; D020864:Calcium Channels, N-Type; D018389:Codon, Nonsense; C105656:CACNA1A protein, human