Human Molecular Genetics, Vol 5, 1217-1227, Copyright © 1996 by Oxford University Press
AG Engel, K Ohno, M Milone, HL Wang, S Nakano, C Bouzat, JN Pruitt 2nd, DO Hutchinson, JM Brengman, N Bren, JP Sieb and SM Sine
Mutations in genes encoding the epsilon, delta, beta and alpha subunits of
the end plate acetylcholine (ACh) receptor (AChR) are described and
functionally characterized in three slow-channel congenital myasthenic
syndrome patients. All three had prolonged end plate currents and AChR
channel opening episodes and an end plate myopathy with loss of AChR from
degenerating junctional folds. Genetic analysis revealed heterozygous
mutations: epsilon L269F and delta Q267E in Patient 1, beta V266M in
Patient 2, and alpha N217K in Patient 3 that were not detected in 100
normal controls. Patients 1 and 2 have no similarly affected relatives; in
Patient 3, the mutation cosegregates with the disease in three generations.
epsilon L269F, delta Q267E and beta V266M occur in the second and alpha
N217K in the first transmembrane domain of AChR subunits; all have been
postulated to contribute to the lining of the upper half of the channel
lumen and all but delta Q267E are positioned toward the channel lumen, and
introduce an enlarged side chain. Expression studies in HEK cells indicate
that all of the mutations express normal amounts of AChR. epsilon L269F,
beta V266M, and alpha N217K slow the rate of channel closure in the
presence of ACh and increase apparent affinity for ACh; epsilon L269F and
alpha N217K enhance desensitization, and epsilon L269F and beta V266M cause
pathologic channel openings in the absence of ACh, rendering the channel
leaky, delta Q267E has none of these effects and is therefore a rare
polymorphism or a benign mutation. The end plate myopathy stems from
cationic overloading of the postsynaptic region. The safety margin of
neuromuscular transmission is compromised by AChR loss from the junctional
folds and by a depolarization block owing to temporal summation of
prolonged end plate potentials at physiologic rates of stimulation.
ARTICLES
New mutations in acetylcholine receptor subunit genes reveal heterogeneity in the slow-channel congenital myasthenic syndrome
Department of Neurology, Mayo Clinic and Foundation, Rochester, MN 55905, USA.
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