A role for WRN in telomere-based DNA damage responses

Abstract

Telomeres cap the ends of eukaryotic chromosomes and prevent them from being recognized as DNA breaks. We have shown that certain DNA damage responses induced during senescence and, at times of telomere uncapping, also can be induced by treatment of cells with small DNA oligonucleotides homologous to the telomere 3 single-strand overhang (T-oligos), implicating this overhang in generation of these telomere-based damage responses. Here, we show that T-oligo-treated fibroblasts contain ␥H2AX foci and that these foci colocalize with telomeres. T-oligos with nuclease-resistant 3 ends are inactive, suggesting that a nuclease initiates T-oligo responses. We therefore examined WRN, a 3 3 5 exonuclease and helicase mutated in Werner syndrome, a disorder characterized by aberrant telomere maintenance, premature aging, chromosomal rearrangements, and predisposition to malignancy. Normal fibroblasts and U20S osteosarcoma cells rendered deficient in WRN showed reduced phosphorylation of p53 and histone H2AX in response to T-oligo treatment. Together, these data demonstrate a role for WRN in processing of telomeric DNA and subsequent activation of DNA damage responses. The T-oligo model helps define the role of WRN in telomere maintenance and initiation of DNA damage responses after telomere disruption.exonuclease ͉ ␥-H2AX foci ͉ Werner syndrome ͉ senescence ͉ oligonucleotide

Shelterin Telomere Protection Protein 1 Reduction Causes Telomere Attrition and Cellular Senescence via Sirtuin 1 Deacetylase in Chronic Obstructive Pulmonary Disease

“…Therefore, CSE-induced telomere uncapping may initiate a cellular senescence response, irrespective of telomere shortening, whereas persistent telomere deprotection and DNA degradation may induce its shortening. This is in agreement with the finding that telomere uncapping induces telomere attrition and TIF formation (27). Telomere deprotection induced by changes in its capping proteins is a critical phenomenon in regulating cellular senescence (12).…”

Enhanced Cytotoxicity from Deoxyguanosine-Enriched T-oligo in Prostate Cancer Cells

“…Specifically, a T-oligo bearing four tandem repeats of GGTT, identified in this report as (GGTT) 4 , demonstrated equitable cytotoxic effects at half the concentration observed from exposure of cells to a sequence with identical length, but containing a sequence largely homologous with human telomeric DNA. This striking observation questions the currently accepted model for T-oligo function; namely, that T-oligo elicits its cytotoxic effects by interfering in an unknown manner with telomeric structure, or possibly serving as a destabilized telomere mimic Li et al, 2004;Eller et al, 2006). It is quite possible, though untested to date, that the deoxyguanosine-rich T-oligo may share mechanistic similarities to other cytotoxic and Grich oligonucleotides developed independently of T-oligo (Saijo et al, 1997).…”

“…Cellular responses elicited by exposure to T-oligo include the upregulation of a DNA-damage-like response characterized by phosphorylation and activation of the DNA damage response proteins g-H 2 A.X, ATM, and chk2. (Eller et al, 2006;Rankin et al, 2012) In cancer cells containing wild-type p53, this signaling cascade results in p53 stabilization and the transcriptional upregulation of the cyclin-dependent kinase inhibitor p21 waf1/cip1 , followed by cell death (Li et al, 2004). Interestingly, certain cancer cell lines with mutated p53 have also demonstrated significant cytotoxic responses following exposure to T-oligo (Longe et al, 2009;Rankin et al, 2012), although the underlying molecular mechanism in this case is not yet elucidated.…”

Cellular mechanisms regulating human melanogenesis

“…This is intriguing in light of the recent recognition that telomere disruption or "uncapping" leads to DNA damage signaling [22 -24], including increased melanogenesis [25]. Indeed, treatment of cultured human cells with telomere homolog oligonucleotides (T-oligos) leads to the formation of DNA damage foci at telomeres [26], presumably at sites of single stranded DNA [26], leading to p53 activation and its multiple downstream events. These observations have led us to speculate [27] that the telomere sequence, TTAGGG in all mammalian species, has evolved to contain thymidine dinucleotides precisely because this is a preferred UV target (with GGG constituting the preferred target for oxidative DNA damage and chemical carcinogens).…”