Abstract SY23-02: A telomere-dependent DNA damage checkpoint induced by prolonged mitotic arrest
Authors: Hayashi Makoto , Cesare Anthony , Fitzpatrick James , Denchi Eros Lazzerini , Karlseder Jan 
Published: August 2012

Abstract
Telomere shortening and disruption of telomeric components are pathways that induce telomere deprotection. Here we describe another pathway, where prolonged mitotic arrest induces damage signals at telomeres. Exposure to microtubule drugs, kinesin inhibitors, proteasome inhibitors or the disruption of proper chromosome alignment resulted in the formation of damage-foci at telomeres. Induction of mitotic telomere deprotection coincided with dissociation of TRF2 from telomeres, telomeric 3′-overhang degradation and ATM activation, and could be suppressed by TRF2 overexpression or inhibition of Aurora B kinase. Normal cells that escape from prolonged mitotic arrest halted in the following G1 phase, whereas cells lacking p53 continued to cycle and became aneuploid. We propose a telomere dependent mitotic duration checkpoint that ensures elimination of cells, which fail to progress through mitosis.
To avoid unwanted checkpoint activation by natural chromosome ends, cells have evolved telomeres. Human telomeres are composed of double stranded TTAGGG repeats and a single stranded G rich 3′ overhang, which are covered and protected by shelterin. Among the six shelterin components TRF2 and POT1 have predominantly been implicated in chromosome end protection by preventing ATM- and ATR-dependent checkpoint activation. Upon disruption of TRF2 or POT1 telomeres are recognized as sites of DNA damage, resulting in phosphorylation of histone H2AX (γ-H2AX) within the telomeric and sub-telomeric chromatin and association of 53BP1 with the chromosome ends. The co-localization of DNA-damage response factors and chromosome ends can be visualized as telomere dysfunction-induced foci (TIF). TIF have also been intimately linked to replicative senescence and shown to occur spontaneously in cancer cell lines.
Cells arrested in mitosis are known to either die during mitotic arrest, or skip cytokinesis and “slip” into the subsequent G1 phase of the cell cycle. Mitotic slippage occurs through the degradation of Cyclin B1 in the presence of the active spindle assembly checkpoint (SAC). Cells that exit from prolonged mitotic arrest or progress through mitotic slippage exhibit various fates, including apoptosis or p53-dependent cell cycle arrest. In both normal and cancer cells, cell death during mitotic arrest, or apoptosis or senescence after escape from prolonged mitotic arrest are crucial for preventing chromosome instability. A failure to remove cells from the cycling population following prolonged mitotic arrest may allow cells to continue propagating with an abnormal number of chromosomes. However, despite intense research, the molecular mechanisms that trigger growth arrest or death in mitotically arrested cultures have not yet been identified.
In the course of experimentation to explore putative telomeric functions for cohesin, we found that mitotic arrest per se induces telomere deprotection in primary and transformed human cells. Telomere deprotection during mitotic arrest associated with loss of the telomeric 3′-overhangs, led to ATM activation and was ATM dependent. TRF2 was dissociated from telomeres during prolonged mitotic arrest, providing the molecular basis for overhang loss and ATM activation, which was emphasized by the finding that TRF2 overexpression protected telomeres from the damage machinery during mitotic arrest. Inhibition of Aurora B kinase suppressed the telomere deprotection phenotype, suggesting the involvment of the SAC. Cells suffering from mitotic telomere deprotection underwent p53 dependent cell cycle arrest in the following G1 phase after mitotic release, while cells lacking p53 function continued to cycle and became aneuploid. Our findings provide a molecular mechanism explaining the induction of DNA damage signaling, cell cycle arrest or apoptosis following prolonged mitotic arrest, and explain the mechanism of action of therapeutic drugs, such as Taxol, Vinblastine and Velcade, which all inhibit mitotic progression. We propose that telomeric destabilization during mitotic arrest induces DNA damage signaling and serves as a mitotic duration checkpoint, responsible for eliminating cells that fail to progress through mitosis properly.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr SY23-02. doi:1538-7445.AM2012-SY23-02