Expanded GAA repeats impede transcription elongation through the FXN gene and induce transcriptional silencing that is restricted to the FXN locus
Expanded GAA repeats impede transcription elongation through the FXN gene and induce transcriptional silencing that is restricted to the FXN locus
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DOI：
10.1093/hmg/ddv397
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发表时间：
2015-12-15
影响因子：
3.5
通讯作者：
Napierala, Marek
中科院分区：
文献类型：
--
作者：
Li, Yanjie;Lu, Yue;Napierala, Marek
弗里德赖希共济失调（FRDA）是一种严重的神经退行性疾病，由FXN基因内含子1中鸟嘌呤-腺嘌呤-腺嘌呤（GAA）重复序列的纯合扩增引起，导致共济失调蛋白表达的转录抑制。代表异染色质的翻译后组蛋白修饰在重复序列附近富集，而该区域的活性染色质标记在FRDA样品中代表性不足。然而，扩展的重复序列通过FXN对转录进展的直接影响及其对周围基因组环境的长期影响是FRDA分子发病机制中尚未回答的两个关键问题。为了解决这些问题，我们对FRDA和对照原代成纤维细胞的大队列进行了下一代RNA测序。这种全面的分析表明，GAA诱导的沉默效应不会影响FXN上游或下游邻近基因的表达。此外，在9号染色体的大部分区域未检测到长距离沉默效应。此外，染色质免疫沉淀研究的结果证实，与抑制转录相关的组蛋白修饰仅限于FXN基因座。最后，FXN前mRNA分子的深度测序显示，与对照组相比，FRDA细胞中的转录延长率存在明显缺陷。这些结果表明，旨在重新激活共济失调蛋白表达的方法应同时解决FXN基因座处转录起始和延伸的缺陷。
Friedreich's ataxia (FRDA) is a severe neurodegenerative disease caused by homozygous expansion of the guanine-adenine-adenine (GAA) repeats in intron 1 of the FXN gene leading to transcriptional repression of frataxin expression. Post-translational histone modifications that typify heterochromatin are enriched in the vicinity of the repeats, whereas active chromatin marks in this region are underrepresented in FRDA samples. Yet, the immediate effect of the expanded repeats on transcription progression through FXN and their long-range effect on the surrounding genomic context are two critical questions that remain unanswered in the molecular pathogenesis of FRDA. To address these questions, we conducted next-generation RNA sequencing of a large cohort of FRDA and control primary fibroblasts. This comprehensive analysis revealed that the GAA-induced silencing effect does not influence expression of neighboring genes upstream or downstream of FXN. Furthermore, no long-range silencing effects were detected across a large portion of chromosome 9. Additionally, results of chromatin immunoprecipitation studies confirmed that histone modifications associated with repressed transcription are confined to the FXN locus. Finally, deep sequencing of FXN pre-mRNA molecules revealed a pronounced defect in the transcription elongation rate in FRDA cells when compared with controls. These results indicate that approaches aimed to reactivate frataxin expression should simultaneously address deficits in transcription initiation and elongation at the FXN locus.