A newly identified chromosomal microdeletion and an N-box mutation of the AChRε gene cause a congenital myasthenic syndrome

Abstract

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 patients.