Mammalian telomeres are nucleoprotein complexes at the end of chromosomes containing a specific protein complex, called shelterin. Shelterin protects chromosome ends from the DNA damage response (DDR), by facilitating the formation of a telomeric capping structure, called the T-loop. During their elongation in S phase, telomeres become transiently uncapped and can be sensed as DNA damage in G2 phase. This leads to the recruitment of DDR factors, such as phosphorylated histone H2AX (γH2AX), to the telomeres forming the so-called, telomere dysfunction-induced foci (TIFs). My PhD work described here, indicates that DNA damage occurring during interphase can persist after entry into mitosis, indicated by the detection of γH2AX at a subset of mitotic telomeres in human and mouse cells. This accumulation of γH2AX to mitotic telomeres is ATM-dependent and the γH2AX-labelled uncapped telomeres that persist, are shorter than the average telomere length for the entire cell population. Most importantly, my work suggests that telomere uncapping, naturally occurring or artificially induced, is detected by two parallel ATM/ATR-dependent pathways at the G2/M transition: a p53/p21-dependent pathway through the ATM/ATR-mediated phosphorylation of p53 at Ser15 and a CHK1/CHK2-dependent pathway that acts through negative regulation of CDC25 phosphatases. In particular, telomere uncapping triggered by TRF2 depletion leads to CHK2-dependent CDC25A degradation, while POT1 depletion results in CHK1-mediated CDC25A and CDC25C degradation. Both pathways act as sensors of unprotected telomeres at the G2/M transition and block cell cycle progression through inhibition of CDK1/Cyclin B complex, allowing telomere re-capping before entry into mitosis. This mechanism protects telomere integrity by the maintenance of a cell cycle stage conducive for capping reactions and thereby prevents genomic instability induced by telomere dysfunction. Finally, I studied the cellular functions of 3 poorly characterised shelterin components, TRF1, RAP1 and TPP1, in telomere protection. TRF1 and to a lesser extent RAP1 were shown to be important for telomere protection by suppressing DDR at the telomeres, while TPP1 was shown to be mainly responsible for the recruitment of the catalytic subunit of telomerase, TERT , to the chromatin, contributing to telomere maintenance. In conclusion, my work on both human and mouse models, reveals an important part of the DDR pathways activated by dysfunctional telomeres, as well as the molecular mechanisms underlying the cell cycle specific regulation of telomere capping, which ensures that only cells with intact telomeres enter mitosis.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:558533 |
| Date | January 2012 |
| Creators | Thanasoula, Maria |
| Contributors | Tarsounas, Madalena |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:b9f806e3-88e5-4dc4-b2e9-8ecf854249d1 |
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