title: Accumulation of single-strand breaks is the major cause of telomere shortening in human fibroblasts.
abstract: Telomere shortening triggers replicative senescence in human fibroblasts. The inability of DNA polymerases to replicate a linear DNA molecule completely (the end replication problem) is one cause of telomere shortening. Other possible causes are the formation of single-stranded overhangs at the end of telomeres and the preferential vulnerability of telomeres to oxidative stress. To elucidate the relative importance of these possibilities, amount and distribution of telomeric single-strand breaks, length of the G-rich overhang, and telomere shortening rate in human MRC-5 fibroblasts were measured. Treatment of nonproliferating cells with hydrogen peroxide increases the sensitivity to S1 nuclease in telomeres preferentially and accelerates their shortening by a corresponding amount as soon as the cells proliferate. A reduction of the activity of intracellular peroxides using the spin trap alpha-phenyl-t-butyl-nitrone reduces the telomere shortening rate and increases the replicative life span. The length of the telomeric single-stranded overhang is independent of DNA damaging stresses, but single-strand breaks accumulate randomly all along the telomere after alkylation. The telomere shortening rate and the rate of replicative aging can be either accelerated or decelerated by a modification of the amount of oxidative stress. Quantitatively, stress-mediated telomere damage contributes most to telomere shortening under standard conditions.
authors.affiliation: Institute of Pathology, Charité, Humboldt-University Berlin, Germany. thomas.von_zglinicki@charite.de
authors.forename: T
authors.initials: T
authors.lastname: von Zglinicki
authors.forename: R
authors.initials: R
authors.lastname: Pilger
authors.forename: N
authors.initials: N
authors.lastname: Sitte
mesh_terms: D000478:Alkylation; D002453:Cell Cycle; D002478:Cells, Cultured; D016922:Cellular Senescence / Q000502:physiology*; D004249:DNA Damage; D004261:DNA Replication; D004277:DNA, Single-Stranded / Q000378:metabolism*; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure*; D006801:Humans; D006861:Hydrogen Peroxide / Q000633:toxicity*; D016877:Oxidants / Q000633:toxicity*; D018384:Oxidative Stress; D017382:Reactive Oxygen Species / Q000378:metabolism; D013113:Spin Labels; D016615:Telomere / Q000378:metabolism / Q000648:ultrastructure*
chemical_list: D004277:DNA, Single-Stranded; D016877:Oxidants; D017382:Reactive Oxygen Species; D013113:Spin Labels; D006861:Hydrogen Peroxide