Lalioti M D, Scott H S, Antonarakis S E
Division of Medical Genetics, University of Geneva Medical School, 1 rue Michel-Servet, 1211 Geneva 4, Switzerland.
Hum Mol Genet. 1999 Sep;8(9):1791-8. doi: 10.1093/hmg/8.9.1791.
Progressive myoclonus epilepsy of the Unverricht-Lundborg type (EPM1; MIM 254800) is an autosomal recessive disorder characterized by seizures, myoclonus and progression to cerebellar ataxia. EPM1 arises due to mutations in the cystatin B (CSTB) gene which encodes a cysteine proteinase inhibitor. Only a minority of EPM1 alleles carry point mutations, while the majority contain large expansions of the dodecamer CCCCGCCCCGCG repeat which is present at two to three copies in normal individuals. The dodecamer repeat is located in the 5' flanking region of the CSTB gene, presumably in its promoter. The pathological repeat expansion results in a reduction in CSTB mRNA, which may be cell specific. To elucidate the mechanism of this reduction of gene expression, we have studied the putative CSTB promoter in vitro. A 3.8 kb fragment, containing the putative promoter with a 600 bp repeat expansion, showed a 2- to 4-fold reduction in luciferase activity compared with an identical fragment with a normal repeat; this reduction was observed only in certain cell types. Introduction of heterologous DNA fragments of 730 and 1000 bp into the normal promoter, instead of the repeat expansion, showed similarly reduced activity. Terminal deletions of the promoter implicate a putative AP-1 binding site, upstream of the repeat, in CSTB transcription activation. We propose that a novel mechanism of pathogenesis, the altering of the spacing of transcription factor binding sites from each other and/or the transcription initiation site due to repeat expansion, is among the causes of reduction in CSTB expression and thus EPM1.
昂韦里希特 - 伦德伯格型进行性肌阵挛癫痫（EPM1；MIM 254800）是一种常染色体隐性疾病，其特征为癫痫发作、肌阵挛以及逐渐发展为小脑共济失调。EPM1是由编码半胱氨酸蛋白酶抑制剂的胱抑素B（CSTB）基因突变引起的。只有少数EPM1等位基因携带点突变，而大多数含有十二聚体CCCCGCCCCGCG重复序列的大量扩增，该重复序列在正常个体中以两到三个拷贝存在。十二聚体重复序列位于CSTB基因的5'侧翼区域，可能在其启动子中。病理性重复扩增导致CSTB mRNA减少，这可能具有细胞特异性。为了阐明这种基因表达减少的机制，我们在体外研究了推定的CSTB启动子。一个3.8 kb的片段，包含带有600 bp重复扩增的推定启动子，与具有正常重复的相同片段相比，荧光素酶活性降低了2至4倍；这种降低仅在某些细胞类型中观察到。将730和1000 bp的异源DNA片段引入正常启动子而非重复扩增区域，显示出类似的活性降低。启动子的末端缺失表明在CSTB转录激活中，重复序列上游存在一个推定的AP - 1结合位点。我们提出，一种新的发病机制，即由于重复扩增导致转录因子结合位点彼此之间和/或与转录起始位点的间距改变，是CSTB表达减少从而导致EPM1的原因之一。
