Long lived organisms such as humans have evolved several intrinsic tumor suppressor mechanisms to combat the slew of oncogenic somatic mutations that constantly arise in proliferating stem cell compartments. One of these anti-cancer barriers is the telomere, a specialized nucleoprotein that caps the ends of eukaryotic chromosome. Impaired telomere function activates the canonical DNA damage response pathway that engages p53 to initiate apoptosis or replicative senescence. Here, we discuss how p53-dependent senescence induced by dysfunctional telomeres may be as potent as apoptosis in suppressing tumorigenesis in vivo.More than 40 years ago, Hayflick and Moorhead discovered that normal human diploid fibroblasts (HDFs) cannot grow indefinitely in culture 1 . Rather, their proliferative capacities are intrinsically limited. After 60-80 population doublings in culture, HDFs stop dividing and adopt a phenotype characterized by large, flat cell size, a vacuolated morphology, inability to synthesize DNA, and the presence of the senescence-associated β-galactosidase (SA-β-gal) marker 2 . We now know that the endpoint of this proliferative limit, termed replicative senescence, is due largely to erosion of telomeres, protective structures that cap the end of all eukaryotic chromosomes. Confirmation of this came from studies in which telomerase, the enzyme that maintains telomeres, was ectopically expressed in normal human somatic cells 3,4 . Activation of telomerase results in telomere elongation, abrogation of replicative senescence, a normal karyotype and cellular immortalization. These results clearly demonstrate that telomere length determines the proliferative lifespan of HDFs, and that upregulation of telomerase activity (hence telomere length) restores proliferative capacity.A large body of experimental evidence has demonstrated that telomere attrition contributes to tumorigenesis by promoting genome instability 5 . However, in the setting of a competent p53 pathway, mouse models have recently shown that telomere shortening is also tumor suppressive by promoting replicative senescence to inhibit tumor formation. In this Perspective, we will discuss how telomeres shorten with cell replication and how this might initiate a DNA damage response to induce replicative senescence (and cell death) and ultimately prevent tumorigenesis.

The POT1 (protection of telomeres) protein binds the single-stranded G-rich overhang and is essential for both telomere end protection and telomere length regulation. Telomeric binding of POT1 is enhanced by its interaction with TPP1. In this study, we demonstrate that mouse Tpp1 confers telomere end protection by recruiting Pot1a and Pot1b to telomeres. Knockdown of Tpp1 elicits a p53-dependent growth arrest and an ATM-dependent DNA damage response at telomeres. In contrast to depletion of Trf2, which activates ATM, removal of Pot1a and Pot1b from telomeres initiates an ATR-dependent DNA damage response (DDR). Finally, we show that telomere dysfunction as a result of Tpp1 depletion promotes chromosomal instability and tumorigenesis in the absence of an ATMdependent DDR. Our results uncover a novel ATR-dependent DDR at telomeres that is normally shielded by POT1 binding to the single-stranded G-overhang. In addition, our results suggest that loss of ATM can cooperate with dysfunctional telomeres to promote cellular transformation and tumor formation in vivo.

Ink4a/Arf inactivation and epidermal growth factor receptor (EGFR) activation are signature lesions in high-grade gliomas. How these mutations mediate the biological features of these tumors is poorly understood. Here, we demonstrate that combined loss of p16(INK4a) and p19(ARF), but not of p53, p16(INK4a), or p19(ARF), enables astrocyte dedifferentiation in response to EGFR activation. Moreover, transduction of Ink4a/Arf(-/-) neural stem cells (NSCs) or astrocytes with constitutively active EGFR induces a common high-grade glioma phenotype. These findings identify NSCs and astrocytes as equally permissive compartments for gliomagenesis and provide evidence that p16(INK4a) and p19(ARF) synergize to maintain terminal astrocyte differentiation. These data support the view that dysregulation of specific genetic pathways, rather than cell-of-origin, dictates the emergence and phenotype of high-grade gliomas.