Telomere uncapping in progenitor cells with critical telomere shortening is coupled to S-phase progression in vivo

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 104; no. 45; pp. 17747 - 17752
• Main Authors: Rajaraman, Sripriya, Choi, Jinkuk, Cheung, Peggie, Beaudry, Veronica, Moore, Helen, Artandi, Steven E
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 06-11-2007; National Acad Sciences
• Subjects: Animals; Apoptosis; Biological Sciences; Bromodeoxyuridine; Cell cycle; Cells; Cellular senescence; Chromosomes; Crosses, Genetic; Crypts; DNA Damage; G2 Phase; Half lives; In Situ Nick-End Labeling; Intestinal Mucosa - cytology; Intestinal Mucosa - physiology; Mice; Mice, Knockout; Models, Genetic; Progenitor cells; Rodents; S Phase - physiology; Stem cells; Stem Cells - cytology; Stem Cells - physiology; Telomerase - deficiency; Telomerase - genetics; Telomere - ultrastructure; Telomeres; Tumor Suppressor Protein p53 - genetics; Tumor Suppressor Protein p53 - physiology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0706485104

Telomeres protect chromosome ends and serve as a substrate for telomerase, a reverse transcriptase that adds DNA repeats to the telomere terminus. In the absence of telomerase, telomeres progressively shorten, ultimately leading to telomere uncapping, a structural change at the telomere that activates DNA damage responses and leads to ligation of chromosome ends. Telomere uncapping has been implicated in aging and cancer, yet the precise mechanism of uncapping and its relationship to cell cycle remain to be defined. Here, we show that telomeres uncap in an S-phase-dependent manner in gastrointestinal progenitors of TERT⁻/⁻ mice. We develop an in vivo assay that allows a quantitative kinetic assessment of telomere dysfunction-induced apoptosis and its relationship to cell cycle. By exploiting the mathematical relationship between rates of generation and clearance of apoptotic cells, we show that 86.2 ± 8.8% of apoptotic gastrointestinal cells undergo programmed cell death either late in S-phase or in G₂. Apoptosis is primarily triggered via a signaling cascade from newly uncapped telomeres to the tumor suppressor p53, rather than by chromosome fusion-bridge breakage, because mitotic blockade did not alter the rate of newly generated apoptotic bodies. These data support a model in which rapidly dividing progenitor cells within a tissue with short telomeres are vulnerable to telomere uncapping during or shortly after telomere replication.