Rapid induction of alternative lengthening of telomeres by depletion of the histone chaperone ASF1

Abstract

The mechanism of activation of the Alternative Lengthening of Telomeres (ALT) pathway of mammalian chromosome end maintenance has remained an unresolved issue. We have discovered that co-depletion of the histone chaperones ASF1a and ASF1b in human cells induced all hallmarks of ALT in both primary and cancer cells. These included the formation of ALT associated PML bodies (APBs), extra-chromosomal telomeric DNA species an elevated frequency of telomeric sister chromatid exchanges (t-SCE) events and inter-telomeric exchange of an integrated tag. The induction of ALT characteristics in this setting led to the simultaneous suppression of telomerase. We identified that ALT induction is positively regulated by RAD17 and BLM, while negatively regulated by EXO1 and DNA2. The induction of ALT phenotypes as a consequence of ASF1 depletion strongly support the hypothesis that ALT is a consequence of a histone management dysfunction.

Genetic Inactivation of ATRX Leads to a Decrease in the Amount of Telomeric Cohesin and Level of Telomere Transcription in Human Glioma Cells

“…Our present finding that ATRX controls telomeric cohesin, together with the recent finding by Deng et al (16) that telomeric cohesin regulates TERRA transcription, dictates a necessary orientation of future research toward understanding these relationships. It might also be pertinent to try to find functional interactions between ASF1 and ATRX (and to know whether cohesin might play a role in such interactions), as ASF1, a major regulator of chromatin organization, was recently found to negatively control the ALT pathway (52). Finally, recent data showing, in mice, that ATRX regulates the expression of genes in intragenic G-rich regions and allows optimal RNAP II function at these sites (32) leave open the possibility that loss of ATRX function in ALT tumors correlates with the acquisition of a particular transcriptional program uniquely compatible with the survival of cells that have undergone telomeric recombination.…”

Histone H3.3 maintains genome integrity during mammalian development

“…Changes in local histone marks can compromise the functions of telomeres and centromeres (Jones et al 2008;O'Sullivan et al 2014). Therefore, we also examined changes in the characteristic histone marks, including dimethylation at H3K9 and H3K79 and trimethylation at H3K9 and H3K36 (Jones et al 2008;Chantalat et al 2011), at pericentromeric (major satellite) and centromeric (minor satellite) regions and telomere repeats with ChIP-qPCR assays.…”

Noncanonical views of homology-directed DNA repair

“…Rad51-dependent telomere clustering was also reported in ALT cells undergoing replication stress after the depletion of annealing helicase SMARCAL1 (Cox et al 2016). Additionally, induction of replication-induced damage by depletion of histone chaperon ASF1 resulted in the induction of ALT activity even in immortalized human cells with longer telomeres (O'Sullivan et al 2014). Intertelomere recombination during ALT may also be assisted by other unique and alternative mechanisms.…”