Abstract
Telomeric repeats preserve genome integrity by stabilizing chromosomes, a function that appears to be important for both cancer and aging. In view of this critical role in genomic integrity, the telomere's own integrity should be of paramount importance to the cell. Ultraviolet light (UV), the preeminent risk factor in skin cancer development, induces mainly cyclobutane pyrimidine dimers (CPD) which are both mutagenic and lethal. The human telomeric repeat unit (5′TTAGGG/CCCTAA3′) is nearly optimal for acquiring UV-induced CPD, which form at dipyrimidine sites. We developed a ChIP–based technique, immunoprecipitation of DNA damage (IPoD), to simultaneously study DNA damage and repair in the telomere and in the coding regions of p53, 28S rDNA, and mitochondrial DNA. We find that human telomeres in vivo are 7-fold hypersensitive to UV-induced DNA damage. In double-stranded oligonucleotides, this hypersensitivity is a property of both telomeric and non-telomeric repeats; in a series of telomeric repeat oligonucleotides, a phase change conferring UV-sensitivity occurs above 4 repeats. Furthermore, CPD removal in the telomere is almost absent, matching the rate in mitochondria known to lack nucleotide excision repair. Cells containing persistent high levels of telomeric CPDs nevertheless proliferate, and chronic UV irradiation of cells does not accelerate telomere shortening. Telomeres are therefore unique in at least three respects: their biophysical UV sensitivity, their prevention of excision repair, and their tolerance of unrepaired lesions. Utilizing a lesion-tolerance strategy rather than repair would prevent double-strand breaks at closely-opposed excision repair sites on opposite strands of a damage-hypersensitive repeat.
Highlights
Telomeric DNA consists, in all eukaryotes examined to date, of a tandemly repeated sequence located at each end of each chromosome
We developed a novel technique, the immunoprecipitation of DNA damage (IPoD), to study DNA damage induction and repair in the telomere and in coding regions (p53, 28S rDNA, and mitochondrial DNA)
We find that human telomeres are hypersensitive to UVinduced DNA photoproducts and that the removal of those DNA photoproducts is almost absent
Summary
Telomeric DNA consists, in all eukaryotes examined to date, of a tandemly repeated sequence located at each end of each chromosome. In humans, it is constituted of 5–10 kb of a repeated hexamer (59TTAGGG/59CCCTAA). Telomeres are hypersensitive to single-strand DNA damage induced by oxidative stress. This is thought to be due to the fact that sequences containing guanine triplets are highly sensitive to oxidation [2,3]. It was proposed that the telomere enters DNA replication with greater oxidative DNA damage than the rest of the genome and this elevated damage contributes to telomere shortening [6]. Contrasting with this hypothesis, it has been shown that telomere shortening induced by oxidative DNA damage can be replication independent [3]
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