Genetic and Epigenetic Alterations of Genomes, de Duve Institute, Catholic University of Louvain, Brussels 1200, Belgium and Telomeres and Cancer Laboratory, Equipe Labellisée Ligue, UMR3244-UPMC-Institut Curie, Paris 75248, France.
Nucleic Acids Res. 2014 Apr;42(7):4391-405. doi: 10.1093/nar/gku114. Epub 2014 Feb 5.
Proper telomeric chromatin configuration is thought to be essential for telomere homeostasis and stability. Previous studies in mouse suggested that loss of heterochromatin marks at telomeres might favor onset of Alternative Lengthening of Telomeres (ALT) pathway, by promoting homologous recombination. However, analysis of chromatin status at human ALT telomeres has never been reported. Here, using isogenic human cell lines and cellular hybrids, which rely either on telomerase or ALT to maintain telomeres, we show that chromatin compaction is reduced at ALT telomeres and this is associated with a global decrease in telomeric H3K9me3. This, subsequently, leads to upregulation of telomere transcription. Accordingly, restoration of a more condensed telomeric chromatin through telomerase-dependent elongation of short ALT telomeres reduces telomere transcription. We further show that loss of ATRX chromatin remodeler function, a frequent characteristic of ALT cells, is not sufficient to decrease chromatin condensation at telomeres nor to increase the expression of telomeric RNA species. These results offer new insight on telomeric chromatin properties in ALT cells and support the hypothesis that telomeric chromatin decondensation is important for ALT pathway.
适当的端粒染色质构象被认为对端粒的动态平衡和稳定性至关重要。先前在小鼠中的研究表明，端粒处异染色质标记的丢失可能通过促进同源重组而有利于端粒的替代延长（ALT）途径的起始。然而，人类 ALT 端粒的染色质状态分析从未被报道过。在这里，我们使用依赖端粒酶或 ALT 维持端粒的同基因人细胞系和细胞杂种，表明 ALT 端粒处的染色质紧缩减少，这与端粒 H3K9me3 的整体减少有关。这随后导致端粒转录的上调。因此，通过端粒酶依赖性伸长短的 ALT 端粒来恢复更浓缩的端粒染色质可降低端粒转录。我们进一步表明，ATRX 染色质重塑酶功能的丧失（ALT 细胞的常见特征）不足以降低端粒处的染色质紧缩，也不能增加端粒 RNA 种类的表达。这些结果为 ALT 细胞中端粒染色质特性提供了新的见解，并支持端粒染色质去凝聚对于 ALT 途径很重要的假设。
