ALT Telomeres Borrow from Meiosis to Get Moving

Telomere clustering is required for the homologous recombination events that maintain chromosome ends in cells relying on alternative lengthening of telomeres (ALT). New data demonstrate that damage signaling at telomeres, a likely step in activating maintenance mechanisms, induces directional movement and synapsis driven by the machinery responsible for recombination in meiosis

Uncoupling of Longevity and Telomere Length in C. elegans

The nematode Caenorhabditis elegans, after completing its developmental stages and a brief reproductive period, spends the remainder of its adult life as an organism consisting exclusively of post-mitotic cells. Here we show that telomere length varies considerably in clonal populations of wild-type worms, and that these length differences are conserved over at least ten generations, suggesting a length regulation mechanism in cis. This observation is strengthened by the finding that the bulk telomere length in different worm strains varies considerably. Despite the close correlation of telomere length and clonal cellular senescence in mammalian cells, nematodes with long telomeres were neither long lived, nor did worm populations with comparably short telomeres exhibit a shorter life span. Conversely, long-lived daf-2 and short-lived daf-16 mutant animals can have either long or short telomeres. Telomere length of post-mitotic cells did not change during the aging process, and the response of animals to stress was found independent of telomere length. Collectively, our data indicate that telomere length and life span can be uncoupled in a post-mitotic setting, suggesting separate pathways for replication-dependent and -independent aging

Human Telomeres Are Tethered to the Nuclear Envelope during Postmitotic Nuclear Assembly

SummaryTelomeres are essential for nuclear organization in yeast and during meiosis in mice. Exploring telomere dynamics in living human cells by advanced time-lapse confocal microscopy allowed us to evaluate the spatial distribution of telomeres within the nuclear volume. We discovered an unambiguous enrichment of telomeres at the nuclear periphery during postmitotic nuclear assembly, whereas telomeres were localized more internally during the rest of the cell cycle. Telomere enrichment at the nuclear rim was mediated by physical tethering of telomeres to the nuclear envelope, most likely via specific interactions between the shelterin subunit RAP1 and the nuclear envelope protein Sun1. Genetic interference revealed a critical role in cell-cycle progression for Sun1 but no effect on telomere positioning for RAP1. Our results shed light on the dynamic relocalization of human telomeres during the cell cycle and suggest redundant pathways for tethering telomeres to the nuclear envelope

Moderate expression of TRF2 in the hematopoietic system increases development of large cell blastic T-cell lymphomas

The telomeric repeat binding factor 2 (TRF2) plays a central role in the protection of chromosome ends by inhibiting telomeres from initiating a DNA damage cascade. TRF2 overexpression has been suggested to induce tumor development in the mouse, and TRF2 levels have been found increased in human tumors. Here we tested whether moderate expression of TRF2 in the hematopoietic system leads to cancer development in the mouse. TRF2 and a GFP-TRF2 fusion protein were introduced into hematopoietic precursors, and tested for function. TRF2 overexpressing cells were integrated into the hematopoietic system of C57BL/6J recipient mice, and animals were put on tumor watch. An increase in the development of T-cell lymphomas was observed in secondary recipient animals, however, overexpression of the TRF2 transgene was not detectable anymore in the tumors. The tumors were characterized as large cell blastic T-cell lymphomas and displayed signs of genome instability as evidenced by chromosome fusions. However, the rate of lymphoma development in TRF2-overexpressing animals was low, suggesting the TRF2 does not serve as a dominant oncogene in the system used

The Telomeric Protein TRF2 Binds the ATM Kinase and Can Inhibit the ATM-Dependent DNA Damage Response

The telomeric protein TRF2 is required to prevent mammalian telomeres from activating DNA damage checkpoints. Here we show that overexpression of TRF2 affects the response of the ATM kinase to DNA damage. Overexpression of TRF2 abrogated the cell cycle arrest after ionizing radiation and diminished several other readouts of the DNA damage response, including phosphorylation of Nbs1, induction of p53, and upregulation of p53 targets. TRF2 inhibited autophosphorylation of ATM on S1981, an early step in the activation of this kinase. A region of ATM containing S1981 was found to directly interact with TRF2 in vitro, and ATM immunoprecipitates contained TRF2. We propose that TRF2 has the ability to inhibit ATM activation at telomeres. Because TRF2 is abundant at chromosome ends but not elsewhere in the nucleus, this mechanism of checkpoint control could specifically block a DNA damage response at telomeres without affecting the surveillance of chromosome internal damage

Telomere disruption results in non-random formation of de novo dicentric chromosomes involving acrocentric human chromosomes

Copyright: © 2010 Stimpson et al.Genome rearrangement often produces chromosomes with two centromeres (dicentrics) that are inherently unstable because of bridge formation and breakage during cell division. However, mammalian dicentrics, and particularly those in humans, can be quite stable, usually because one centromere is functionally silenced. Molecular mechanisms of centromere inactivation are poorly understood since there are few systems to experimentally create dicentric human chromosomes. Here, we describe a human cell culture model that enriches for de novo dicentrics. We demonstrate that transient disruption of human telomere structure non-randomly produces dicentric fusions involving acrocentric chromosomes. The induced dicentrics vary in structure near fusion breakpoints and like naturally-occurring dicentrics, exhibit various inter-centromeric distances. Many functional dicentrics persist for months after formation. Even those with distantly spaced centromeres remain functionally dicentric for 20 cell generations. Other dicentrics within the population reflect centromere inactivation. In some cases, centromere inactivation occurs by an apparently epigenetic mechanism. In other dicentrics, the size of the alpha-satellite DNA array associated with CENP-A is reduced compared to the same array before dicentric formation. Extrachromosomal fragments that contained CENP-A often appear in the same cells as dicentrics. Some of these fragments are derived from the same alpha-satellite DNA array as inactivated centromeres. Our results indicate that dicentric human chromosomes undergo alternative fates after formation. Many retain two active centromeres and are stable through multiple cell divisions. Others undergo centromere inactivation. This event occurs within a broad temporal window and can involve deletion of chromatin that marks the locus as a site for CENP-A maintenance/replenishment.This work was supported by the Tumorzentrum Heidelberg/Mannheim grant (D.10026941)and by March of Dimes Research Foundation grant #1-FY06-377 and NIH R01 GM069514

Loss of p16Ink4a Function Rescues Cellular Senescence Induced by Telomere Dysfunction

p16Ink4a is a tumor suppressor and a marker for cellular senescence. Previous studies have shown that p16Ink4a plays an important role in the response to DNA damage signals caused by telomere dysfunction. In this study, we crossed Wrn−/− and p16Ink4a−/− mice to knock out the p16Ink4a function in a Wrn null background. Growth curves showed that loss of p16Ink4a could rescue the growth barriers that are observed in Wrn−/− mouse embryonic fibroblasts (MEFs). By challenging the MEFs with the global genotoxin doxorubicin, we showed that loss of p16Ink4a did not dramatically affect the global DNA damage response of Wrn−/− MEFs induced by doxorubicin. However, in response to telomere dysfunction initiated by the telomere damaging protein TRF2ΔBΔM, loss of p16Ink4a could partially overcome the DNA damage response by disabling p16Ink4a up-regulation and reducing the accumulation of γ-H2AX that is observed in Wrn−/− MEFs. Furthermore, in response to TRF2ΔBΔM overexpression, Wrn−/− MEFs senesced within several passages. In contrast, p16Ink4a−/− and p16Ink4a−/−Wrn−/− MEFs could continuously grow and lose expression of the exogenous TRF2ΔBΔM in their late passages. In summary, our data suggest that in the context of telomere dysfunction, loss of p16Ink4a function could prevent cells from senescence. These results shed light on the anti-aging strategy through regulation of p16Ink4a expression

The Myb/SANT domain of the telomere-binding protein TRF2 alters chromatin structure

Eukaryotic DNA is packaged into chromatin, which regulates genome activities such as telomere maintenance. This study focuses on the interactions of a myb/SANT DNA-binding domain from the telomere-binding protein, TRF2, with reconstituted telomeric nucleosomal array fibers. Biophysical characteristics of the factor-bound nucleosomal arrays were determined by analytical agarose gel electrophoresis (AAGE) and single molecules were visualized by atomic force microscopy (AFM). The TRF2 DNA-binding domain (TRF2 DBD) neutralized more negative charge on the surface of nucleosomal arrays than histone-free DNA. Binding of TRF2 DBD at lower concentrations increased the radius and conformational flexibility, suggesting a distortion of the fiber structure. Additional loading of TRF2 DBD onto the nucleosomal arrays reduced the flexibility and strongly blocked access of micrococcal nuclease as contour lengths shortened, consistent with formation of a unique, more compact higher-order structure. Mirroring the structural results, TRF2 DBD stimulated a strand invasion-like reaction, associated with telomeric t-loops, at lower concentrations while inhibiting the reaction at higher concentrations. Full-length TRF2 was even more effective at stimulating this reaction. The TRF2 DBD had less effect on histone-free DNA structure and did not stimulate the t-loop reaction with this substrate, highlighting the influence of chromatin structure on the activities of DNA-binding proteins