Abstract
Telomeres are repetitive non-coding nucleotide sequences (TTAGGGn) capping the ends of chromosomes. Progressive telomere shortening with increasing age has been associated with shifts in gene expression through models such as the telomere position effect (TPE), which suggests reduced interference of the telomere with transcriptional activity of increasingly more distant genes. A modification of the TPE model, referred to as Telomere Position Effects over Long Distance (TPE-OLD), explains why some genes 1–10 MB from a telomere are still affected by TPE, but genes closer to the telomere are not. Here, we describe an imaging approach to systematically examine the occurrence of TPE-OLD at the single cell level. Compared to existing methods, the pipeline allows rapid analysis of hundreds to thousands of cells, which is necessary to establish TPE-OLD as an acceptable mechanism of gene expression regulation. We examined two human genes, ISG15 and TERT, for which TPE-OLD has been described before. For both genes, we found less interaction with the telomere on the same chromosome in old cells compared to young cells; and experimentally elongated telomeres in old cells rescued the level of telomere interaction for both genes. However, the dependency of the interactions on the age progression from young to old cells varied. One model for the differences between ISG15 and TERT may relate to the markedly distinct interstitial telomeric sequence arrangement in the two genes. Overall, this provides a strong rationale for the role of telomere length shortening in the regulation of gene expression.
Highlights
Telomeres are repetitive nucleotide sequences (TTAGGGn) capping the end of chromosomes
The relation between telomere length shortening and gene expression regulation under the telomere position effect (TPE)-OLD mechanism is thought to rely on the interaction between telomere and gene locus, which is mediated by the shelterin protein (Kim et al 2016; Kim and Shay 2018; Robin et al 2014, 2015; Stadler et al 2013) (Fig. 1a)
To measure how the interaction is affected by telomere length shortening in individual cells, we first prepared human fibroblasts (BJ cells) with different ranges of population doubling (PD)
Summary
Telomeres are repetitive nucleotide sequences (TTAGGGn) capping the end of chromosomes. The length of telomeres becomes progressively shorter after each cell division (Harley et al 1990), which appears to serve as a clock or replicometer of human cellular lifespan. Telomeres prevent chromosome ends from fusion, degradation, and being recognized as double-strand DNA breaks (O'Sullivan and Karlseder 2010; Webb et al 2013). Cells undergo replicative senescence when a single or perhaps a few telomeres become very short and unprotected, which results in DNA damage at telomeres (Zou et al 2004). Cells lose a small amount of telomeric sequences after each cell division due to incomplete DNA lagging strand synthesis known as the end replication problem (Olovnikov 1971; Watson 1972). Cells will gradually reach a senescence checkpoint
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