Abstract
DNA double-strand (DSBs) breaks activate the DNA damage checkpoint machinery to pause or halt the cell cycle. Telomeres, the specific DNA-protein complexes at linear eukaryotic chromosome ends, are capped DSBs that do not activate DNA damage checkpoints. This "checkpoint privileged" status of telomeres was previously investigated in the yeast Schizosaccharomyces pombe lacking the major double-stranded telomere DNA binding protein Taz1. Telomeric DNA repeats in cells lacking Taz1 are 10 times longer than normal and contain single-stranded DNA regions. DNA damage checkpoint proteins associate with these damaged telomeres, but the DNA damage checkpoint is not activated. This severing of the DNA damage checkpoint signaling pathway was reported to stem from exclusion of histone H4 lysine 20 dimethylation (H4K20me2) from telomeric nucleosomes in both wild type cells and cells lacking Taz1. However, experiments to identify the mechanism of this exclusion failed, prompting our re-evaluation of H4K20me2 levels at telomeric chromatin. In this short report, we used an extensive series of controls to identify an antibody specific for the H4K20me2 modification and show that the level of this modification is the same at telomeres and internal loci in both wild type cells and those lacking Taz1. Consequently, telomeres must block activation of the DNA Damage Response by another mechanism that remains to be determined.
Highlights
Genome instability is a potentially lethal event for a eukaryotic cell, and a mutational force for genetic diseases such as cancer
Immunofluorescence co-localization results from Carneiro et al indicated that the ortholog of the human DNA damage checkpoint protein 53BP1 (Crb2) found at double-strand breaks (DSBs) was not recruited to telomeres[5]
We found that H4K20me[2] levels are similar at all three loci in wild type and taz1∆ cells, and clearly distinguishable from the set9∆ and H4K20R negative controls (Figure 1B)
Summary
Genome instability is a potentially lethal event for a eukaryotic cell, and a mutational force for genetic diseases such as cancer. DNA double-strand breaks (DSBs) can drive genome instability and are sensed by the DNA damage checkpoint, a defined set of evolutionarily-conserved proteins that bind the DSB to signal a pause or arrest of the cell cycle[1] and recruit proteins to repair the DNA lesion[2,3]. Carneiro et al presented data that H4K20me[2] was depleted near telomeres in wild type and taz1∆ cells, suggesting a mechanism for checkpoint suppression[5]. Efforts to pursue this exciting result by ourselves and others failed. We carefully re-evaluated the presence of H4K20me[2] at different chromosomal loci, and found that H4K20me[2] is not depleted near telomeres, indicating that checkpoint suppression occurs by some other mechanism(s)
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