A newly identified chromosomal microdeletion and an N‐box mutation of the AChRϵ 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 (AChRϵ) 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 AChRϵ 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 (ϵ‐154G→A) in the N‐box of the AChRϵ promoter in a German CMS pedigree. Semiquantitative analysis of AChRϵ mRNA levels in the patient's muscle indicated significantly impaired AChRϵ expression. We provide additional evidence of a pathogenic role for this mutation using the mutated promoter (ϵ‐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 AChRϵ promoter (ϵ‐154G→A) together with paternal inheritance of a chromosomal microdeletion (Δ1290 bp) encompassing the promoter and the first two exons of the AChRϵ gene in the index patient. In conclusion, we provide genetic and functional evidence that a mutation of the AChRϵ subunit promoter (ϵ‐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 patient