Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P16104 (H2AX)
 3,930 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We report cytologic and genetic data indicating that telomere dysfunction induces a DNA damage response in mammalian cells. Dysfunctional, uncapped telomeres, created through inhibition of TRF2, became associated with DNA damage response factors, such as 53BP1, gamma-H2AX, Rad17, ATM, and Mre11. We refer to the domain of telomere-associated DNA damage factors as a Telomere Dysfunction-Induced Focus (TIF). The accumulation of 53BP1 on uncapped telomeres was reduced in the presence of the PI3 kinase inhibitors caffeine and wortmannin, which affect ATM, ATR, and DNA-PK. By contrast, Mre11 TIFs were resistant to caffeine, consistent with previous findings on the Mre11 response to ionizing radiation. A-T cells had a diminished 53BP1 TIF response, indicating that the ATM kinase is a major transducer of this pathway. However, in the absence of ATM, TRF2 inhibition still induced TIFs and senescence, pointing to a second ATM-independent pathway. We conclude that the cellular response to telomere dysfunction is governed by proteins that also control the DNA damage response. TIFs represent a new tool for evaluating telomere status in normal and malignant cells suspected of harboring dysfunctional telomeres. Furthermore, induction of TIFs through TRF2 inhibition provides an opportunity to study the DNA damage response within the context of well-defined, physically marked lesions.
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PMID:DNA damage foci at dysfunctional telomeres. 1295 59

Replicative senescence is a natural barrier to cellular proliferation that is triggered by telomere erosion and dysfunction. Here, we demonstrate that ATM activation and H2AX-gamma nuclear focus formation are sensitive markers of telomere dysfunction in primary human fibroblasts. Whereas the activated form of ATM and H2AX-gamma foci were rarely observed in early-passage cells, they were readily detected in late-passage cells. The ectopic expression of telomerase in late-passage cells abrogated ATM activation and H2AX-gamma focus formation, suggesting that these stress responses were the consequence of telomere dysfunction. ATM activation was induced in quiescent fibroblasts by inhibition of TRF2 binding to telomeres, indicating that telomere uncapping is sufficient to initiate the telomere signaling response; breakage of chromosomes with telomeric associations is not required for this activation. Although ATM activation and H2AX-gamma foci were readily observed in late-passage cells, they disappeared once cells became fully senescent, indicating that constitutive signaling from dysfunctional telomeres is not required for the maintenance of senescence.
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PMID:Disappearance of the telomere dysfunction-induced stress response in fully senescent cells. 1517 78

Telomere attrition and other forms of telomere damage can activate the ATM kinase pathway. What generates the DNA damage signal at mammalian chromosome ends or at other double-strand breaks is not known. Telomere dysfunction is often accompanied by disappearance of the 3' telomeric overhang, raising the possibility that DNA degradation could generate the structure that signals. Here we address these issues by studying telomere structure after conditional deletion of mouse TRF2, the protective factor at telomeres. Upon removal of TRF2 from TRF2(F/-) p53-/- mouse embryo fibroblasts, a telomere damage response is observed at most chromosome ends. As expected, the telomeres lose the 3' overhang and are processed by the non-homologous end-joining pathway. Non-homologous end joining of telomeres was abrogated in DNA ligase IV-deficient (Lig4-/-) cells. Unexpectedly, the telomeres of TRF2-/- Lig4-/- p53-/- cells persisted in a free state without undergoing detectable DNA degradation. Notably, the telomeres retained their 3' overhangs, but they were recognized as sites of DNA damage, accumulating the DNA damage response factors 53BP1 and gamma-H2AX, and activating the ATM kinase. Thus, activation of the ATM kinase pathway at chromosome ends does not require overhang degradation or other overt DNA processing.
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PMID:DNA processing is not required for ATM-mediated telomere damage response after TRF2 deletion. 1596 70

Telomeres are specialized structures at the ends of chromosomes that consist of tandem repeats of the DNA sequence TTAGGG and several proteins that protect the DNA and regulate the plasticity of the telomeres. The telomere-associated protein TRF2 (telomeric repeat binding factor 2) is critical for the control of telomere structure and function; TRF2 dysfunction results in the exposure of the telomere ends and activation of ATM (ataxia telangiectasin mutated)-mediated DNA damage response. Recent findings suggest that telomere attrition can cause senescence or apoptosis of mitotic cells, but the function of telomeres in differentiated neurons is unknown. Here, we examined the impact of telomere dysfunction via TRF2 inhibition in neurons (primary embryonic hippocampal neurons) and mitotic neural cells (astrocytes and neuroblastoma cells). We demonstrate that telomere dysfunction induced by adenovirus-mediated expression of dominant-negative TRF2 (DN-TRF2) triggers a DNA damage response involving the formation of nuclear foci containing phosphorylated histone H2AX and activated ATM in each cell type. In mitotic neural cells DN-TRF2 induced activation of both p53 and p21 and senescence (as indicated by an up-regulation of beta-galactosidase). In contrast, in neurons DN-TRF2 increased p21, but neither p53 nor beta-galactosidase was induced. In addition, TRF2 inhibition enhanced the morphological, molecular and biophysical differentiation of hippocampal neurons. These findings demonstrate divergent molecular and physiological responses to telomere dysfunction in mitotic neural cells and neurons, indicate a role for TRF2 in regulating neuronal differentiation, and suggest a potential therapeutic application of inhibition of TRF2 function in the treatment of neural tumors.
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PMID:TRF2 dysfunction elicits DNA damage responses associated with senescence in proliferating neural cells and differentiation of neurons. 1653 55

We report that mouse liver cells are highly resistant to extensive telomere dysfunction. In proliferating cells, telomere dysfunction results in chromosome end fusions, a DNA damage signal, and apoptosis or senescence. To determine the consequences of telomere dysfunction in noncycling cells, we used conditional deletion of the telomeric protein TRF2 in hepatocytes. TRF2 loss resulted in telomeric accumulation of gamma-H2AX and frequent telomere fusions, indicating telomere deprotection. However, there was no induction of p53 or apoptosis, and liver function appeared unaffected. Furthermore, the loss of TRF2 did not compromise liver regeneration after partial hepatectomy. Remarkably, liver regeneration occurred without cell division involving endoreduplication and cell growth, thereby circumventing the chromosome segregation problems associated with telomere fusions. We conclude that nondividing hepatocytes can maintain and regenerate liver function despite substantial loss of telomere integrity.
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PMID:Hepatocytes with extensive telomere deprotection and fusion remain viable and regenerate liver mass through endoreduplication. 1701 29

Telomestatin is a potent G-quadruplex ligand that interacts with the 3' telomeric overhang, leading to its degradation, and induces a delayed senescence and apoptosis of cancer cells. POT1 and TRF2 were recently identified as specific telomere-binding proteins involved in telomere capping and t-loop maintenance and whose interaction with telomeres is modulated by telomestatin. We show here that the treatment of HT1080 human tumor cells by telomestatin induces a rapid decrease of the telomeric G-overhang and of the double-stranded telomeric repeats. Telomestatin treatment also provokes a strong decrease of POT1 and TRF2 from their telomere sites, suggesting that the ligand triggers the uncapping of the telomere ends. The effect of the ligand is associated with an increase of the gamma-H2AX foci, one part of them colocalizing at telomeres, thus indicating the occurrence of a DNA damage response at the telomere, but also the presence of additional DNA targets for telomestatin. Interestingly, the expression of GFP-POT1 in HT1080 cells increases both telomere and G-overhang length. As compared with HT1080 cells, HT1080GFP-POT1 cells presented a resistance to telomestatin treatment characterized by a protection to the telomestatin-induced growth inhibition and the G-overhang shortening. This protection is related to the initial G-overhang length rather than to its degradation rate and is overcome by increased telomestatin concentration. Altogether these results suggest that telomestatin induced a telomere dysfunction in which G-overhang length and POT1 level are important factors but also suggest the presence of additional DNA sites of action for the ligand.
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PMID:Telomestatin-induced telomere uncapping is modulated by POT1 through G-overhang extension in HT1080 human tumor cells. 1705 May 46

Here we document the role of MDC1 (mediator of DNA damage checkpoint 1) in the detection and repair of human and mouse telomeres rendered dysfunctional through inhibition of TRF2. Consistent with its role in promoting DNA damage foci, MDC1 knockdown affected the formation of telomere dysfunction-induced foci (TIFs), diminishing the accumulation of phosphorylated ATM, 53BP1, Nbs1, and to a lesser extent, gamma-H2AX. In addition to this effect on TIFs, the rate of nonhomologous end-joining (NHEJ) of dysfunctional telomeres was significantly decreased when MDC1 itself or its recruitment to chromatin was inhibited. MDC1 appeared to promote a step in the NHEJ pathway after the removal of the 3' telomeric overhang. The acceleration of NHEJ was unlikely to be due to increased presence of 53BP1 and Mre11 in TIFs, since knockdown of neither factor affected telomere fusions. Furthermore, relevant cell cycle effectors (Chk2, p53, and p21) of the ATM kinase pathway were unaffected and there was no change in the rate of cell cycle progression. We propose that the binding of MDC1 to gamma-H2AX directly affects NHEJ in a manner that is independent of the ATM-dependent cell cycle arrest pathway.
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PMID:MDC1 accelerates nonhomologous end-joining of dysfunctional telomeres. 1715 42

Functional telomeres are required for the replicability of cancer cells. The G-rich strand of telomeric DNA can fold into a 4-stranded structure known as the G-quadruplex (G4), whose stabilization alters telomere function limiting cancer cell growth. Therefore, the G4 ligand RHPS4 may possess antitumor activity. Here, we show that RHPS4 triggers a rapid and potent DNA damage response at telomeres in human transformed fibroblasts and melanoma cells, characterized by the formation of several telomeric foci containing phosphorylated DNA damage response factors gamma-H2AX, RAD17, and 53BP1. This was dependent on DNA repair enzyme ATR, correlated with delocalization of the protective telomeric DNA-binding protein POT1, and was antagonized by overexpression of POT1 or TRF2. In mice, RHPS4 exerted its antitumor effect on xenografts of human tumor cells of different histotype by telomere injury and tumor cell apoptosis. Tumor inhibition was accompanied by a strong DNA damage response, and tumors overexpressing POT1 or TRF2 were resistant to RHPS4 treatment. These data provide evidence that RHPS4 is a telomere damage inducer and that telomere disruption selectively triggered in malignant cells results in a high therapeutic index in mice. They also define a functional link between telomere damage and antitumor activity and reveal the key role of telomere-protective factors TRF2 and POT1 in response to this anti-telomere strategy.
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PMID:Telomere damage induced by the G-quadruplex ligand RHPS4 has an antitumor effect. 1793 67

Here, we address the role of the MRN (Mre11/Rad50/Nbs1) complex in the response to telomeres rendered dysfunctional by deletion of the shelterin component TRF2. Using conditional NBS1/TRF2 double-knockout MEFs, we show that MRN is required for ATM signaling in response to telomere dysfunction. This establishes that MRN is the only sensor for the ATM kinase and suggests that TRF2 might block ATM signaling by interfering with MRN binding to the telomere terminus, possibly by sequestering the telomere end in the t-loop structure. We also examined the role of the MRN/ATM pathway in nonhomologous end joining (NHEJ) of damaged telomeres. NBS1 deficiency abrogated the telomere fusions that occur in G(1), consistent with the requirement for ATM and its target 53BP1 in this setting. Interestingly, NBS1 and ATM, but not H2AX, repressed NHEJ at dysfunctional telomeres in G(2), specifically at telomeres generated by leading-strand DNA synthesis. Leading-strand telomere ends were not prone to fuse in the absence of either TRF2 or MRN/ATM, indicating redundancy in their protection. We propose that MRN represses NHEJ by promoting the generation of a 3' overhang after completion of leading-strand DNA synthesis. TRF2 may ensure overhang formation by recruiting MRN (and other nucleases) to newly generated telomere ends. The activation of the MRN/ATM pathway by the dysfunctional telomeres is proposed to induce resection that protects the leading-strand ends from NHEJ when TRF2 is absent. Thus, the role of MRN at dysfunctional telomeres is multifaceted, involving both repression of NHEJ in G(2) through end resection and induction of NHEJ in G(1) through ATM-dependent signaling.
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PMID:Cell cycle-dependent role of MRN at dysfunctional telomeres: ATM signaling-dependent induction of nonhomologous end joining (NHEJ) in G1 and resection-mediated inhibition of NHEJ in G2. 1966 71

Agents stabilizing G-quadruplexes have the potential to destroy the functional structure of telomere and could therefore act as antitumor agents. We previously reported that SYUIQ-5 could stabilize G-quadruplex, induce senescence, and inhibit c-myc gene promoter activity. In this study, we showed that SYUIQ-5 inhibited proliferation of CNE2 and HeLa cancer cells, triggered a rapid and potent telomere DNA damage response characterized by the formation of telomeric foci gamma-H2AX, and obviously induced autophagy with the features of increased LC3-II and a punctuated pattern of YFP-LC3 fluorescence. These phenomena may primarily depend on the delocalization of TRF2 from telomere, which was further degraded by proteasomes. Furthermore, overexpression of TRF2 inhibited SYUIQ-5-induced gamma-H2AX expression. Also, ATM was activated following SYUIQ-5 treatment. The pretreatment with ATM inhibitor ku55933 and ATM siRNA effectively reduced the production of gamma-H2AX and LC3-II. ATM knockdown partially antagonized the anticancer effects of SYUIQ-5. Moreover, inhibition of autophagy by short hairpin RNA against the autophagy-related gene ATG5 attenuated the cytotoxicity of SYUIQ-5. These results indicated that SYUIQ-5 triggered potent telomere damage through TRF2 delocalization from telomeres, and eventually induced autophagic cell death in cancer cells. Our findings exhibit a novel mechanism that is responsible for the antitumor effects of SYUIQ-5.
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PMID:G-quadruplex ligand SYUIQ-5 induces autophagy by telomere damage and TRF2 delocalization in cancer cells. 1999 77


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