Telomeres avoid end detection by severing the checkpoint signal transduction pathway

Area of Science:

• Molecular Biology
• Cell Biology
• Epigenetics

Background:

• Telomeres protect chromosome ends from DNA damage recognition.
• Proteins involved in DNA repair and checkpoints are recruited to telomeres, yet permanent cell cycle arrest is avoided.
• The mechanism preventing telomeres from activating DNA damage responses remains unclear.

Purpose of the Study:

• To investigate how telomeres prevent DNA damage responses from causing cell cycle arrest in fission yeast.
• To identify the molecular mechanisms underlying telomere-specific chromatin regulation.
• To understand the role of epigenetic modifications in telomere function.

Main Methods:

• Analysis of fission yeast (Schizosaccharomyces pombe) mutants lacking Taz1 (an orthologue of human TRF1/TRF2).
• Recruitment of DNA repair proteins (Rad22/RAD52, Rhp51/RAD51) and checkpoint sensors (RPA, Rad9, Rad26/ATRIP, Cut5/Rad4/TOPBP1) to telomeres was assessed.
• Investigated the role of histone modifications (H4K20me2, γH2A) and telomere-binding proteins (Pot1, Ccq1) in checkpoint activation.

Main Results:

• Cells lacking Taz1 recruited DNA repair and checkpoint proteins to telomeres but failed to accumulate Crb2 (53BP1), thus not activating Chk1-dependent cell cycle arrest.
• Artificial recruitment of Crb2 (53BP1) to taz1Δ telomeres induced a full checkpoint response and cell cycle arrest.
• Telomeres lack H4K20me2, a modification required for stable Crb2 (53BP1) association, unlike internal chromosome regions.
• Loss of Pot1 or Ccq1 from telomeres led to Crb2 (53BP1) foci, Chk1 activation, and cell cycle arrest.
• Telomeres are chromatin-privileged regions lacking essential epigenetic markers for DNA damage response amplification.

Conclusions:

• Fission yeast telomeres are epigenetically distinct, lacking H4K20me2, which prevents amplification of DNA damage signals.
• Exclusion of Crb2 (53BP1) from telomeres, mediated by Pot1 and Ccq1, is crucial for preventing cell cycle arrest.
• This chromatin privilege ensures telomere maintenance and function by preventing inappropriate checkpoint activation, which is vital for recruiting telomerase via ATM/ATR kinases.