A newly identified chromosomal microdeletion and an N-box mutation of the AChR epsilon gene cause a congenital myasthenic syndrome.
Congenital myasthenic syndromes (CMSs) are frequently caused by mutations of the coding region of the acetylcholine receptor epsilon subunit (AChRepsilon) gene leading to a reduced expression of the acetylcholine receptor (AChR) at the postsynaptic membrane. Two recent observations have linked two different N-box mutations of the human AChRepsilon promoter to a clinical CMS phenotype. N-boxes are regulatory sequence elements of mammalian promoters that confer synapse-specific expression of several genes, including the AChR subunit genes. Here, we report on a novel point mutation (epsilon-154G-->A) in the N-box of the AChRepsilon promoter in a German CMS pedigree. Semiquantitative analysis of AChRepsilon mRNA levels in the patient's muscle indicated significantly impaired AChRepsilon expression. We provide additional evidence of a pathogenic role for this mutation using the mutated promoter (epsilon-154G-->A) driving a heterologous gene (luciferase) in rat skeletal muscle. We show that agrin-induced gene expression is significantly reduced by the N-box mutant (mt) compared with the wild-type (wt) promoter. Refined haplotype analysis and direct sequencing revealed maternal inheritance of the mutant AChRepsilon promoter (epsilon-154G-->A) together with paternal inheritance of a chromosomal microdeletion (Delta1290 bp) encompassing the promoter and the first two exons of the AChRepsilon gene in the index patient. In conclusion, we provide genetic and functional evidence that a mutation of the AChRepsilon subunit promoter (epsilon-154G-->A) causes CMS due to the reduction of gene expression in skeletal muscle. Moreover, this is the first report of a chromosomal microdeletion affecting an AChR gene. This type of mutation may be missed in standard screening techniques of CMS patients.
Abicht
Angela
A
Genzentrum and Friedrich-Baur-Institut, Ludwig-Maximilians-University Munich, Germany.
Stucka
Rolf
R
Schmidt
Carolin
C
Briguet
Alexandre
A
Höpfner
Sebastian
S
Song
In-Ho
IH
Pongratz
Dieter
D
Müller-Felber
Wolfgang
W
Ruegg
Markus A
MA
Lochmüller
Hanns
H
D000328:Adult; D000818:Animals; D001483:Base Sequence; D002872:Chromosome Deletion; D005260:Female; D005786:Gene Expression Regulation; D005822:Genetic Vectors; D006801:Humans; D008297:Male; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism; D009154:Mutation / Q000235:genetics*; D020294:Myasthenic Syndromes, Congenital / Q000235:genetics* / Q000378:metabolism; D010375:Pedigree; D011110:Polymorphism, Genetic / Q000235:genetics; D011401:Promoter Regions, Genetic; D051381:Rats; D011950:Receptors, Cholinergic / Q000096:biosynthesis / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D024363:Transcription Initiation Site
D011950:Receptors, Cholinergic