Alternative Lengthening of Telomeres is characterized by reduced compaction of telomeric chromatin

Abstract

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.

PML induces compaction, TRF2 depletion and DNA damage signaling at telomeres and promotes their alternative lengthening

“…1E; Table 2). This method is well established to detect and quantify ECTRs and has been used in a number of previous studies (Episkopou et al, 2014;Hande et al, 2001;Kamranvar et al, 2013;Kamranvar and Masucci, 2011;Tokutake et al, 1998). Note that we refer here to those ECTRs that are detected by a peptide nucleic acid (PNA) FISH probe against the G-rich telomere repeat sequence.…”

Telomere elongation chooses TERRA ALTernatives

“…TERRA increase in ALT is mainly due to augmented telomere transcription, as TERRA half-life in ALT cells appears not to be markedly different than in telomerase-positive cells. 22,24 Consistently, in ALT cells, subtelomeric TERRA CpG island promoters are hypomethylated and more avidly bound by RNAPII while the density of the transcriptionally repressive histone H3 trimethylated at lysine 9 (H3K9me3) mark is decreased. [24][25][26] Moreover, contrary to what is observed in telomerase positive cancer cells, TERRA levels do not decline during progression from S-phase to G2/M, possibly due to a lack of ATRX activity.…”

“…22,24 Consistently, in ALT cells, subtelomeric TERRA CpG island promoters are hypomethylated and more avidly bound by RNAPII while the density of the transcriptionally repressive histone H3 trimethylated at lysine 9 (H3K9me3) mark is decreased. [24][25][26] Moreover, contrary to what is observed in telomerase positive cancer cells, TERRA levels do not decline during progression from S-phase to G2/M, possibly due to a lack of ATRX activity. 27,28 Thus increased transcription and loss of its cell cycle regulation account for the overall increase of TERRA cellular levels characteristic of ALT lines.…”

“…22,23 Successive work employing additional ALT cells has indeed confirmed that TERRA is generally up-regulated in ALT. 12,24,25 Moreover, TERRA largely localizes to APBs. 25 Thus TERRA levels and localization can now be used as additional markers for ALT tumor screening.…”

A comprehensive anticancer molecular study for genistein the promising anticancer drug

“…These two protein-coding genes play an essential role in DNA replication. 22,23 Moreover, only one gene was down-regulated in the cell cycle/proliferation family. It is an important regulator of cell cycle transitions, Cyclin F (CCNF), through its ability to activate and bind to cyclin-dependent protein kinases.…”