Pot1 and cell cycle progression cooperate in telomere length regulation

Churikov, Dmitri; Price, Carolyn M.

doi:10.1038/nsmb1331

Cited by 60 publications

(55 citation statements)

References 48 publications

(6 reference statements)

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“…5D). This result was unexpected because depletion of POT1 from telomeres generally leads to extensive telomere elongation (Loayza and de Lange 2003;Hockemeyer et al 2005;Churikov and Price 2008).…”

Section: Complementation Assays For Tpp1 and Pot1 Coordinationmentioning

confidence: 81%

“…Depletion and overexpression experiments establish TPP1 and POT1 as negative regulators of telomere length (Loayza and de Lange 2003;Houghtaling et al 2004;Liu et al 2004;Ye et al 2004;Hockemeyer et al 2007;Churikov and Price 2008;Baumann and Price 2010). Recent studies using protein expression knockdown suggest that TPP1 also promotes telomerase activity at telomeres (Lue et al 2013;Nakashima et al 2013).…”

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confidence: 99%

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Genetic and molecular identification of three human TPP1 functions in telomerase action: recruitment, activation, and homeostasis set point regulation

et al. 2014

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Telomere length homeostasis is essential for the long-term survival of stem cells, and its set point determines the proliferative capacity of differentiated cell lineages by restricting the reservoir of telomeric repeats. Knockdown and overexpression studies in human tumor cells showed that the shelterin subunit TPP1 recruits telomerase to telomeres through a region termed the TEL patch. However, these studies do not resolve whether the TPP1 TEL patch is the only mechanism for telomerase recruitment and whether telomerase regulation studied in tumor cells is representative of nontransformed cells such as stem cells. Using genome engineering of human embryonic stem cells, which have physiological telomere length homeostasis, we establish that the TPP1 TEL patch is genetically essential for telomere elongation and thus long-term cell viability. Furthermore, genetic bypass, protein fusion, and intragenic complementation assays define two distinct additional mechanisms of TPP1 involvement in telomerase action at telomeres. We demonstrate that TPP1 provides an essential step of telomerase activation as well as feedback regulation of telomerase by telomere length, which is necessary to determine the appropriate telomere length set point in human embryonic stem cells. These studies reveal and resolve multiple TPP1 roles in telomere elongation and stem cell telomere length homeostasis.

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Section: Complementation Assays For Tpp1 and Pot1 Coordinationmentioning

confidence: 81%

mentioning

confidence: 99%

Genetic and molecular identification of three human TPP1 functions in telomerase action: recruitment, activation, and homeostasis set point regulation

et al. 2014

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“…Dysfunctional telomeres activate upstream checkpoint PI3 kinases, such as ATM (ataxiatelangiectasia mutated) or ATR (ataxia-telangiectasia and Rad3 related) 22,[27][28][29][30] (Figure 2). Once activated, these kinases phosphorylate downstream factors including CHK1 and CHK2 that in turn phosphorylates p53 29 .…”

Section: Telomere Dysfunction Initiates a Dna Damage Responsementioning

confidence: 99%

Telomere dysfunction and tumour suppression: the senescence connection

2008

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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.

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“…POT1 is highly conserved in fission yeast, plants, Caenorhabditis elegans, and vertebrates, and it is homologous to TEBP␣, the 3Ј-overhang binding protein of ciliates (1,4,7,21,23,28). Most vertebrates, including humans, possess a single POT1 gene.…”

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confidence: 99%

Functional Dissection of Human and Mouse POT1 Proteins

Palm

Hockemeyer

Kibe

et al. 2009

Molecular and Cellular Biology

119

144

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The single-stranded telomeric DNA binding protein POT1 protects mammalian chromosome ends from the ATR-dependent DNA damage response, regulates telomerase-mediated telomere extension, and limits 5-end resection at telomere termini. Whereas most mammals have a single POT1 gene, mice have two POT1 proteins that are functionally distinct. POT1a represses the DNA damage response, and POT1b controls 5-end resection. In contrast, as we report here, POT1a and POT1b do not differ in their ability to repress telomere recombination. By swapping domains, we show that the DNA binding domain of POT1a specifies its ability to repress the DNA damage response. However, no differences were detected in the in vitro DNA binding features of POT1a and POT1b. In contrast to the repression of ATR signaling by POT1a, the ability of POT1b to control 5-end resection was found to require two regions in the C terminus, one corresponding to the TPP1 binding domain and a second representing a new domain located between amino acids (aa) 300 and 350. Interestingly, the DNA binding domain of human POT1 can replace that of POT1a to repress ATR signaling, and the POT1b region from aa 300 to 350 required for the regulation of the telomere terminus is functionally conserved in human POT1. Thus, human POT1 combines the features of POT1a and POT1b.Mammalian chromosome ends associate with shelterin, the protein complex dedicated to telomere protection and homeostasis (reviewed in references 5 and 20). The DNA of mammalian telomeres consists of long arrays of doublestranded (ds) TTAGGG repeats ending in a single-stranded (ss) protrusion of the 3Ј end, referred to as the 3Ј overhang. The 3Ј-overhang strand can invade the duplex part of telomeres, forming a lariat structure called the t-loop (8). Three shelterin components interact with telomeric DNA. POT1 recognizes ss TTAGGG repeats and therefore can associate with the telomeric 3Ј overhang as well as with the displaced strand at the base of the t-loop. TRF1 and TRF2 cover the duplex part of telomeric DNA, and they recruit the other shelterin components Rap1, TIN2, and TPP1 to telomeres through protein-protein interactions. POT1 cannot localize to telomeres alone but instead requires heterodimerization with TPP1; this interaction is crucial for the association of POT1 with telomeres, as TPP1 tethers POT1 to TRF1 and TRF2 by a TPP1-TIN2 bridge and improves the DNA binding affinity of POT1 (12,19,26).Shelterin masks telomeres from the cellular machinery that recognizes and repairs DNA lesions. If shelterin function is perturbed, DNA damage response factors accumulate at telomeres, leading to the activation of the ATM and ATR kinase signaling pathways (2, 10, 15, 29). Distinct components of shelterin independently repress ATM and ATR signaling: TRF2 is required to prevent ATM activation at telomeres, while POT1 represses the activation of ATR (16). Dysfunctional telomeres trigger various DNA repair reactions and can be processed like dsDNA breaks through nonhomologous end joining or through homology-direc...

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Pot1 and cell cycle progression cooperate in telomere length regulation

Cited by 60 publications

References 48 publications

Genetic and molecular identification of three human TPP1 functions in telomerase action: recruitment, activation, and homeostasis set point regulation

Genetic and molecular identification of three human TPP1 functions in telomerase action: recruitment, activation, and homeostasis set point regulation

Telomere dysfunction and tumour suppression: the senescence connection

Functional Dissection of Human and Mouse POT1 Proteins

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