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 1
. Among the six shelterin components TRF2 and POT1 (Protection Of Telomeres 1) have predominantly been implicated in chromosome end protection by preventing ATM- and ATR (Ttaxia Telangiectasia and Rad3 related)-dependent checkpoint activation 2–5
. 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 (p53 Binding Protein) with the chromosome ends. The co-localization of DNA-damage response factors and chromosome ends can be visualized as telomere dysfunction-induced foci (TIF) 6
. TIF have also been intimately linked to replicative senescence 7
and shown to occur spontaneously in cancer cell lines 8
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 9
. Mitotic slippage occurs through the degradation of Cyclin B1 in the presence of the active spindle assembly checkpoint (SAC) 10
. Cells that exit from prolonged mitotic arrest or progress through mitotic slippage exhibit various fates, including apoptosis or p53-dependent cell cycle arrest 9,11
. 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 12–14
. However, despite intense research, the molecular mechanisms that trigger growth arrest or death in mitotically arrested cultures have not yet been identified.
We set out to explore putative telomeric functions for cohesin and 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, but independent of 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 potentially serves as a mitotic duration checkpoint, responsible for eliminating cells that fail to progress through mitosis properly.