Telomere capping conceals chromosome ends from exonucleases and checkpoints, but the full range of capping mechanisms is not well defined. Telomeres have the potential to form G-quadruplex (G4) DNA, although evidence for telomere G4 DNA function in vivo is limited. In budding yeast, capping requires the Cdc13 protein and is lost at nonpermissive temperatures in cdc13-1 mutants. Here, we use several independent G4 DNA–stabilizing treatments to suppress cdc13-1 capping defects. These include overexpression of three different G4 DNA binding proteins, loss of the G4 DNA unwinding helicase Sgs1, or treatment with small molecule G4 DNA ligands. In vitro, we show that protein-bound G4 DNA at a 3′ overhang inhibits 5′→3′ resection of a paired strand by exonuclease I. These findings demonstrate that, at least in the absence of full natural capping, G4 DNA can play a positive role at telomeres in vivo.

Summary The identification of the cellular mechanisms responsible for the wide differences in species lifespan remains one of the major unsolved problems of the biology of aging. We measured the capacity of nuclear protein to recognize DNA double strand breaks (DSB) and telomere length of skin fibroblasts derived from mammalian species that exhibit wide differences in longevity. Our results indicate DNA DSB recognition increases exponentially with longevity. Further, an analysis of the level of Ku80 protein in human, cow, and mouse suggests that Ku levels vary dramatically between species and these levels are strongly correlated with longevity. In contrast mean telomere length appears to decrease with increasing longevity of the species, although not significantly. These findings suggest that an enhanced ability to bind to DNA-ends may be important for longevity. A number of possible roles for increased levels of Ku and DNA-PKcs are discussed.

Certain guanine-rich sequences are capable of forming higher order structures known as Gquadruplexes. Moreover, particular genomic regions in a number of highly divergent organisms are enriched for such sequences, raising the possibility that G-quadruplexes form in vivo and affect cellular processes. While G-quadruplexes have been rigorously studied in vitro, whether these structures actually form in vivo and what their roles might be in the context of the cell have remained largely unanswered questions. Recent studies suggest that G-quadruplexes participate in the regulation of such varied processes as telomere maintenance, transcriptional regulation and ribosome biogenesis. Here we review studies aimed at elucidating the in vivo functions of quadruplex structures, with a particular focus on findings in yeast. In addition, we discuss the utility of yeast model systems in the study of the cellular roles of G-quadruplexes.

Purpose: The therapeutic effect of allogeneic hematopoietic stem cell transplantation (HSCT) for patients with myeloid malignancies has been attributed in part to a graftversus-leukemia effect that is dependent on donor T lymphocytes. CD8 + T-cell responses to MHC class I-restricted tumor epitopes, not just allogeneic antigens, may help mediate antileukemia effects after HSCT, but the specificity and function of such cells are not completely understood. Experimental Design: We examined the diversity, phenotype, and functional potential of leukemia-associated antigen-specific CD8 + T cells in patients with myeloid leukemia following allogeneic HSCT. Screening for antigen-specific T cells was accomplished with a peptide/MHC tetramer library. Results: Patients with acute myelogenous leukemia or chronic myelogenous leukemia in remission following HSCT exhibited significant numbers of peripheral blood CD8 + T cells that recognized varying combinations of epitopes derived from leukemiaassociated antigens. However, these cells failed to proliferate, release cytokines, or degranulate in response to antigen-specific stimuli. As early as 2 months after HSCT, CD8 + T cells from patients were predominantly CD28 -CD57 + and had relatively short telomeres, consistent with cellular senescence. Conclusions: Circulating leukemia-specific CD8 + T cells are prominent in myeloid leukemia patients after HSCT, but such cells are largely functionally unresponsive, most likely due to replicative senescence. These findings carry important implications for the understanding of the graft-versus-leukemia effect and for the rational design of immunotherapeutic strategies for patients with myeloid leukemias.The curative potential of allogeneic hematopoietic stem cell transplantation (HSCT) for myeloid leukemias has been attributed in part to a graft-versus-leukemia (GVL) effect independent of the conditioning regimen and dependent on donor T cells (1). Recent studies show the existence of leukemia-associated antigens capable of eliciting specific CD8 +