Mutation in the AChR ion channel gate underlies a fast channel congenital myasthenic syndrome.
BACKGROUND
Most congenital myasthenic syndromes (CMS) have postsynaptic defects from mutations within the muscle acetylcholine receptor (AChR). Mutations underlying the slow channel syndrome cause a "gain of function" and usually show dominant inheritance, whereas mutations underlying AChR deficiency or the fast channel syndrome cause a "loss of function" and show recessive inheritance.


OBJECTIVE
To characterize the disease mechanism underlying an apparently dominantly inherited CMS that responds to IV edrophonium.


METHODS
DNA from CMS patients was analyzed for mutations by single-strand conformation polymorphism analysis, DNA sequence analysis, and restriction endonuclease digestion. Functional analysis of mutations was by alpha-bungarotoxin binding studies and by patch clamp analysis of mutant AChR expressed in human embryonic kidney cells.


RESULTS
Analysis of muscle biopsies from father and son in an affected kinship showed normal endplate morphology and AChR number but severely reduced miniature endplate potentials. DNA analysis revealed that each harbors a single missense mutation in the AChR alpha-subunit gene, alphaF256L. Expression studies demonstrate this mutation underlies a fast channel phenotype with fewer and shorter ion channel activations. The major effect of alphaF256L, located within the M2 transmembrane domain, is on channel gating, both reducing the opening and increasing the closure rate.


CONCLUSIONS
Mutation alphaF256L results in fast channel kinetics. Expression studies suggest a dominant-negative effect within the AChR pentamer, severely compromising receptor function.
Webster
R
R
Neurosciences Group, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Headington, Oxford, UK.
Brydson
M
M
Croxen
R
R
Newsom-Davis
J
J
Vincent
A
A
Beeson
D
D
D000293:Adolescent; D000595:Amino Acid Sequence; D001706:Biopsy; D002460:Cell Line; D004252:DNA Mutational Analysis; D004491:Edrophonium; D004568:Electrodiagnosis / Q000379:methods; D018014:Gene Transfer Techniques; D005799:Genes, Dominant; D006801:Humans; D015640:Ion Channel Gating / Q000235:genetics*; D007473:Ion Channels / Q000235:genetics*; D007668:Kidney / Q000166:cytology / Q000378:metabolism; D008297:Male; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D020294:Myasthenic Syndromes, Congenital / Q000175:diagnosis / Q000235:genetics* / Q000503:physiopathology*; D018408:Patch-Clamp Techniques; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D011950:Receptors, Cholinergic / Q000235:genetics* / Q000378:metabolism; D013577:Syndrome
D007473:Ion Channels; D021122:Protein Subunits; D011950:Receptors, Cholinergic; D004491:Edrophonium