p300-mediated acetylation of TRF2 is required for maintaining functional telomeres

Non-canonical telomere protection role of FOXO3a of human skeletal muscle cells regulated by the TRF2-redox axis

Telomeric repeat binding factor 2 (TRF2) binds to telomeres and protects chromosome ends against the DNA damage response and senescence. Although the expression of TRF2 is downregulated upon cellular senescence and in various aging tissues, including skeletal muscle tissues, very little is known about the contribution of this decline to aging. We previously showed that TRF2 loss in myofibers does not trigger telomere deprotection but mitochondrial dysfunction leading to an increased level of reactive oxygen species. We show here that this oxidative stress triggers the binding of FOXO3a to telomeres where it protects against ATM activation, revealing a previously unrecognized telomere protective function of FOXO3a, to the best of our knowledge. We further showed in transformed fibroblasts and myotubes that the telomere properties of FOXO3a are dependent on the C-terminal segment of its CR2 domain (CR2C) but independent of its Forkhead DNA binding domain and of its CR3 transactivation domain. We propose that these non-canonical properties of FOXO3a at telomeres play a role downstream of the mitochondrial signaling induced by TRF2 downregulation to regulate skeletal muscle homeostasis and aging.

Genome Instability in Multiple Myeloma: Facts and Factors

Multiple myeloma (MM) is a malignant neoplasm of terminally differentiated immunoglobulin-producing B lymphocytes called plasma cells. MM is the second most common hematologic malignancy, and it poses a heavy economic and social burden because it remains incurable and confers a profound disability to patients. Despite current progress in MM treatment, the disease invariably recurs, even after the transplantation of autologous hematopoietic stem cells (ASCT). Biological processes leading to a pathological myeloma clone and the mechanisms of further evolution of the disease are far from complete understanding. Genetically, MM is a complex disease that demonstrates a high level of heterogeneity. Myeloma genomes carry numerous genetic changes, including structural genome variations and chromosomal gains and losses, and these changes occur in combinations with point mutations affecting various cellular pathways, including genome maintenance. MM genome instability in its extreme is manifested in mutation kataegis and complex genomic rearrangements: chromothripsis, templated insertions, and chromoplexy. Chemotherapeutic agents used to treat MM add another level of complexity because many of them exacerbate genome instability. Genome abnormalities are driver events and deciphering their mechanisms will help understand the causes of MM and play a pivotal role in developing new therapies.

The Intra- and Extra-Telomeric Role of TRF2 in the DNA Damage Response

Telomere repeat binding factor 2 (TRF2) has a well-known function at the telomeres, which acts to protect the telomere end from being recognized as a DNA break or from unwanted recombination. This protection mechanism prevents DNA instability from mutation and subsequent severe diseases caused by the changes in DNA, such as cancer. Since TRF2 actively inhibits the DNA damage response factors from recognizing the telomere end as a DNA break, many more studies have also shown its interactions outside of the telomeres. However, very little has been discovered on the mechanisms involved in these interactions. This review aims to discuss the known function of TRF2 and its interaction with the DNA damage response (DDR) factors at both telomeric and non-telomeric regions. In this review, we will summarize recent progress and findings on the interactions between TRF2 and DDR factors at telomeres and outside of telomeres.

Structural Features of Nucleoprotein CST/Shelterin Complex Involved in the Telomere Maintenance and Its Association with Disease Mutations

Telomere comprises the ends of eukaryotic linear chromosomes and is composed of G-rich (TTAGGG) tandem repeats which play an important role in maintaining genome stability, premature aging and onsets of many diseases. Majority of the telomere are replicated by conventional DNA replication, and only the last bit of the lagging strand is synthesized by telomerase (a reverse transcriptase). In addition to replication, telomere maintenance is principally carried out by two key complexes known as shelterin (TRF1, TRF2, TIN2, RAP1, POT1, and TPP1) and CST (CDC13/CTC1, STN1, and TEN1). Shelterin protects the telomere from DNA damage response (DDR) and regulates telomere length by telomerase; while, CST govern the extension of telomere by telomerase and C strand fill-in synthesis. We have investigated both structural and biochemical features of shelterin and CST complexes to get a clear understanding of their importance in the telomere maintenance. Further, we have analyzed ~115 clinically important mutations in both of the complexes. Association of such mutations with specific cellular fault unveils the importance of shelterin and CST complexes in the maintenance of genome stability. A possibility of targeting shelterin and CST by small molecule inhibitors is further investigated towards the therapeutic management of associated diseases. Overall, this review provides a possible direction to understand the mechanisms of telomere borne diseases, and their therapeutic intervention.

Loss of RNA-binding protein HuR facilitates cellular senescence through posttranscriptional regulation of TIN2 mRNA

Cellular senescence can be induced by high levels of reactive oxygen species (ROS) produced by mitochondria. However, the mechanism by which elevated mitochondrial ROS levels are produced during replicative senescence is not yet fully understood. Here, we report that loss of the RNA-binding protein, human antigen R (HuR), during replicative senescence leads to an increase in ROS levels through enhanced mitochondrial localization of the telomeric protein TIN2. HuR binds to the 3′ untranslated region of TIN2 mRNA. This association decreases TIN2 protein levels by both destabilizing TIN2 mRNA and reducing its translation. Conversely, depletion of HuR levels enhances TIN2 expression, leading to increased mitochondrial targeting of TIN2. Mitochondrial localization of TIN2 increases ROS levels, which contributes to induction and maintenance of cellular senescence. Our findings provide compelling evidence for a novel role of HuR in controlling the process of cellular senescence by regulating TIN2-mediated mitochondrial ROS production, and for a useful therapeutic route for modulating intracellular ROS levels in treating both aging-related complications and cancer.

Epigenetic features of human telomeres

Although subtelomeric regions in humans are heterochromatic, the epigenetic nature of human telomeres remains controversial. This controversy might have been influenced by the confounding effect of subtelomeric regions and interstitial telomeric sequences (ITSs) on telomeric chromatin structure analyses. In addition, different human cell lines might carry diverse epigenetic marks at telomeres. We have developed a reliable procedure to study the chromatin structure of human telomeres independently of subtelomeres and ITSs. This procedure is based on the statistical analysis of multiple ChIP-seq experiments. We have found that human telomeres are not enriched in the heterochromatic H3K9me3 mark in most of the common laboratory cell lines, including embryonic stem cells. Instead, they are labeled with H4K20me1 and H3K27ac, which might be established by p300. These results together with previously published data argue that subtelomeric heterochromatin might control human telomere functions. Interestingly, U2OS cells that exhibit alternative lengthening of telomeres have heterochromatic levels of H3K9me3 in their telomeres.

Cancer cells induce immune escape via glycocalyx changes controlled by the telomeric protein TRF2

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells with strong immunosuppressive activity that promote tumor growth. In this study, we describe a mechanism by which cancer cells control MDSCs in human cancers by upregulating TRF2, a protein required for telomere stability. Specifically, we showed that the TRF2 upregulation in cancer cells has extratelomeric roles in activating the expression of a network of genes involved in the biosynthesis of heparan sulfate proteoglycan, leading to profound changes in glycocalyx length and stiffness, as revealed by atomic force microscopy. This TRF2-dependent regulation facilitated the recruitment of MDSCs, their activation via the TLR2/MyD88/IL-6/STAT3 pathway leading to the inhibition of natural killer recruitment and cytotoxicity, and ultimately tumor progression and metastasis. The clinical relevance of these findings is supported by our analysis of cancer cohorts, which showed a correlation between high TRF2 expression and MDSC infiltration, which was inversely correlated with overall patient survival.

Targeting Cancer through the Epigenetic Features of Telomeric Regions

The integrity of the chromatin associated with telomeric regions, which include telomeres and subtelomeres, is essential for telomeres function and cell viability. Whereas human subtelomeres are heterochromatic, telomeres are labeled with euchromatic marks like H4K20me1 and H3K27ac in most commonly studied human cell lines. The epigenetic marks of human telomeric regions influence oncogenic processes. Indeed, different drugs that decrease their genome-wide levels are currently being used or tested in specific cancer therapies. These drugs can challenge cancer by altering the function of key cellular proteins. However, they should also compromise oncogenic processes by modifying the epigenetic landscape of telomeric regions. We believe that studies of telomeric chromatin structure and telomeres dysfunction should help to design epigenetic therapies for cancer treatment.

Opposite effects of GCN5 and PCAF knockdowns on the alternative mechanism of telomere maintenance

Cancer cells can use a telomerase-independent mechanism, known as alternative lengthening of telomeres (ALT), to elongate their telomeres. General control non-derepressible 5 (GCN5) and P300/CBP-associated factor (PCAF) are two homologous acetyltransferases that are mutually exclusive subunits in SAGA-like complexes. Here, we reveal that down regulation of GCN5 and PCAF had differential effects on some phenotypic characteristics of ALT cells. Our results suggest that GCN5 is present at telomeres and opposes telomere recombination, in contrast to PCAF that may indirectly favour them in ALT cells.

SIRT6 interacts with TRF2 and promotes its degradation in response to DNA damage

Telomere repeat binding factor 2 (TRF2) has been increasingly recognized to be involved in telomere maintenance and DNA damage response. Here, we show that TRF2 directly binds SIRT6 in a DNA independent manner and that this interaction is increased upon replication stress. Knockdown of SIRT6 up-regulates TRF2 protein levels and counteracts its down-regulation during DNA damage response, leading to cell survival. Moreover, we report that SIRT6 deactetylates in vivo the TRFH domain of TRF2, which in turn, is ubiquitylated in vivo activating the ubiquitin-dependent proteolysis. Notably, overexpression of the TRF2cT mutant failed to be stabilized by SIRT6 depletion, demonstrating that the TRFH domain is required for its post-transcriptional modification. Finally, we report an inverse correlation between SIRT6 and TRF2 protein expression levels in a cohort of colon rectal cancer patients. Taken together our findings describe TRF2 as a novel SIRT6 substrate and demonstrate that acetylation of TRF2 plays a crucial role in the regulation of TRF2 protein stability, thus providing a new route for modulating its expression level during oncogenesis and damage response.

HULC cooperates with MALAT1 to aggravate liver cancer stem cells growth through telomere repeat-binding factor 2

The dysregulation of lncRNAs has increasingly been linked to many human diseases, especially in cancers. Our results demonstrate HULC, MALAT1 and TRF2 are highly expressed in human hepatocellular carcinoma tissues, and HULC plus MALAT1 overexpression drastically promotes the growth of liver cancer stem cells. Mechanistically, both HULC and MALAT1 overexpression enhanced RNA polII, P300, CREPT to load on the promoter region of telomere repeat-binding factor 2(TRF2), triggering the overexpression, phosphorylation and SUMOylation of TRF2. Strikingly, the excessive TRF2 interacts with HULC or MALAT1 to form the complex that loads on the telomeric region, replacing the CST/AAF and recruiting POT1, pPOT1, ExoI, SNM1B, HP1 α. Accordingly, the telomere is greatly protected and enlonged. Furthermore, the excessive HULC plus MALAT1 reduced the methylation of the TERC promoter dependent on TRF2, increasing the TERC expression that causes the increase of interplay between TRET and TERC. Ultimately, the interaction between RFC and PCNA or between CDK2 and CyclinE, the telomerase activity and the microsatellite instability (MSI) are significantly increased in the liver cancer stem cells. Our demonstrations suggest that haploinsufficiency of HULC/MALAT1 plays an important role in malignant growth of liver cancer stem cell.

Telomerase activates transcription of cyclin D1 gene through an interaction with NOL1

Telomerase is a ribonucleoprotein enzyme that is required for the maintenance of telomere repeats. Although overexpression of telomerase in normal human somatic cells is sufficient to overcome replicative senescence, the ability of telomerase to promote tumorigenesis requires additional activities that are independent of its role in telomere extension. Here, we identify proliferation-associated nucleolar antigen 120 (NOL1, also known as NOP2) as a telomerase RNA component (TERC)-binding protein that is found in association with catalytically active telomerase. Although NOL1 is highly expressed in the majority of human tumor cells, the molecular mechanism by which NOL1 contributes to tumorigenesis remained unclear. We show that NOL1 binds to the T-cell factor (TCF)-binding element of the cyclin D1 promoter and activates its transcription. Interestingly, telomerase is also recruited to the cyclin D1 promoter in a TERC-dependent manner through the interaction with NOL1, further enhancing transcription of the cyclin D1 gene. Depletion of NOL1 suppresses cyclin D1 promoter activity, thereby leading to induction of growth arrest and altered cell cycle distributions. Collectively, our findings suggest that NOL1 represents a new route by which telomerase activates transcription of cyclin D1 gene, thus maintaining cell proliferation capacity.

RETRACTED: SET1A Cooperates With CUDR to Promote Liver Cancer Growth and Hepatocyte-like Stem Cell Malignant Transformation Epigenetically

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the editor-in-chief. Similarities were found between images in this article and an article in Oncotarget (Pu et al., 2015, Oncotarget 6, 40775-40798, https://doi.org/10.18632/oncotarget.5905) previously published by the same authors. Similarities were also found between images within this article. Image analysis performed by the editorial office confirmed findings of image reuse in Figures 2E and 6E of the Molecular Therapy article. Some of the original data provided by the authors do not match the published article. This reuse (and in part misrepresentation) of data without appropriate attribution represents a severe abuse of the scientific publishing system. No authors responded to the notice of retraction. The original email was undeliverable to 6 authors (not including the corresponding author; T.L., Q.Z., J.A., M.W., H.L., X.G.).

Involvement of SRSF11 in cell cycle-specific recruitment of telomerase to telomeres at nuclear speckles

Telomerase, a unique ribonucleoprotein complex that contains the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC) and the TERC-binding protein dyskerin, is required for continued cell proliferation in stem cells and cancer cells. Here we identify SRSF11 as a novel TERC-binding protein that localizes to nuclear speckles, subnuclear structures that are enriched in pre-messenger RNA splicing factors. SRSF11 associates with active telomerase enzyme through an interaction with TERC and directs it to nuclear speckles specifically during S phase of the cell cycle. On the other hand, a subset of telomeres is shown to be constitutively present at nuclear speckles irrespective of cell cycle phase, suggesting that nuclear speckles could be the nuclear sites for telomerase recruitment to telomeres. SRSF11 also associates with telomeres through an interaction with TRF2, which facilitates translocation of telomerase to telomeres. Depletion of SRSF11 prevents telomerase from associating with nuclear speckles and disrupts telomerase recruitment to telomeres, thereby abrogating telomere elongation by telomerase. These findings suggest that SRSF11 acts as a nuclear speckle-targeting factor that is essential for telomerase association with telomeres through the interactions with TERC and TRF2, and provides a potential target for modulating telomerase activity in cancer.

Transcriptional outcome of telomere signalling

Telomeres protect chromosome ends from degradation and inappropriate DNA damage response activation through their association with specific factors. Interestingly, these telomeric factors are able to localize outside telomeric regions, where they can regulate the transcription of genes involved in metabolism, immunity and differentiation. These findings delineate a signalling pathway by which telomeric changes control the ability of their associated factors to regulate transcription. This mechanism is expected to enable a greater diversity of cellular responses that are adapted to specific cell types and telomeric changes, and may therefore represent a pivotal aspect of development, ageing and telomere-mediated diseases.

Methylated TRF2 associates with the nuclear matrix and serves as a potential biomarker for cellular senescence

Methylation of N-terminal arginines of the shelterin component TRF2 is important for cellular proliferation. While TRF2 is found at telomeres, where it plays an essential role in maintaining telomere integrity, little is known about the cellular localization of methylated TRF2. Here we report that the majority of methylated TRF2 is resistant to extraction by high salt buffer and DNase I treatment, indicating that methylated TRF2 is tightly associated with the nuclear matrix. We show that methylated TRF2 drastically alters its nuclear staining as normal human primary fibroblast cells approach and enter replicative senescence. This altered nuclear staining, which is found to be overwhelmingly associated with misshapen nuclei and abnormal nuclear matrix folds, can be suppressed by hTERT and it is barely detectable in transformed and cancer cell lines. We find that dysfunctional telomeres and DNA damage, both of which are potent inducers of cellular senescence, promote the altered nuclear staining of methylated TRF2, which is dependent upon the ATM-mediated DNA damage response. Collectively, these results suggest that the altered nuclear staining of methylated TRF2 may represent ATM-mediated nuclear structural alteration associated with cellular senescence. Our data further imply that methylated TRF2 can serve as a potential biomarker for cellular senescence.

Beyond Telomerase: Telomere Instability as a Novel Target for Cancer Therapy

Telomeres are areas of heterochromatin composed of TTAGGG repeats located at the ends of linear chromosomes. They play a critical role in keeping genome stable and preventing premature aging diseases and the development of cancer. Characterizing mechanisms of telomere maintenance and understanding how their deregulation contributes to human diseases are therefore important for developing novel therapies. A key mechanism driving telomere maintenance and replicative immortality in cancer cells is telomere elongation by telomerase, and many emerging potential telomere-based therapies have focused on targeting telomerase components. By contrast, recent studies on telomere maintenance mechanism suggest that disrupting telomere stability by interfering with alternative mechanisms of telomere synthesis or protection may also yield new strategies for the treatment of cancer. This review will focus on emerging regulators of telomere synthesis or maintenance, such as G4 telomeric DNA, the CST complex, the t-loop, and shelterins, and discuss their potential as targets for anti-cancer chemotherapeutic intervention in the future.