T. Von Zglinicki et al., Accumulation of single-strand breaks is the major cause of telomere shortening in human fibroblasts, FREE RAD B, 28(1), 2000, pp. 64-74
Telomere shortening triggers replicative senescence in human fibroblasts. T
he inability of DNA polymerases to replicate a linear DNA molecule complete
ly (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 s
tress. To elucidate the relative importance of these possibilities, amount
and distribution of telomeric single-strand breaks, length of the G-rich ov
erhang, and telomere shortening rate in human MRC-5 fibroblasts were measur
ed. Treatment of nonproliferating cells with hydrogen peroxide increases th
e sensitivity to S1 nuclease in telomeres preferentially and accelerates th
eir 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 incre
ases 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. Quanti
tatively, stress-mediated telomere damage contributes most to telomere shor
tening under standard conditions. (C) 2000 Elsevier Science Inc.