Abicht Angela, Stucka Rolf, Schmidt Carolin, Briguet Alexandre, Höpfner Sebastian, Song In-Ho, Pongratz Dieter, Müller-Felber Wolfgang, Ruegg Markus A, Lochmüller Hanns
Genzentrum and Friedrich-Baur-Institut, Ludwig-Maximilians-University Munich, Germany.
Brain. 2002 May;125(Pt 5):1005-13. doi: 10.1093/brain/awf095.
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.
先天性肌无力综合征（CMSs）通常由乙酰胆碱受体ε亚基（AChRepsilon）基因编码区的突变引起，导致突触后膜上乙酰胆碱受体（AChR）表达减少。最近的两项观察结果将人类AChRepsilon启动子的两种不同N盒突变与临床CMS表型联系起来。N盒是哺乳动物启动子的调控序列元件，可赋予包括AChR亚基基因在内的多个基因突触特异性表达。在此，我们报告了一个德国CMS家系中AChRepsilon启动子N盒中的一个新的点突变（epsilon-154G→A）。对患者肌肉中AChRepsilon mRNA水平的半定量分析表明，AChRepsilon表达明显受损。我们利用驱动大鼠骨骼肌中异源基因（荧光素酶）的突变启动子（epsilon-154G→A），为该突变的致病作用提供了额外证据。我们表明，与野生型（wt）启动子相比，N盒突变体（mt）显著降低了聚集蛋白诱导的基因表达。精细的单倍型分析和直接测序显示，索引患者中突变的AChRepsilon启动子（epsilon-154G→A）为母系遗传，同时伴有包含AChRepsilon基因启动子和前两个外显子的染色体微缺失（Delta1290 bp）的父系遗传。总之，我们提供了遗传和功能证据，表明AChRepsilon亚基启动子的突变（epsilon-154G→A）由于骨骼肌中基因表达的减少而导致CMS。此外，这是关于影响AChR基因的染色体微缺失的首次报道。这种类型的突变在CMS患者的标准筛查技术中可能会被遗漏。
