PARP1 is required for preserving telomeric integrity but is dispensable for A-NHEJ

Shaping DNA damage responses: Therapeutic potential of targeting telomeric proteins and DNA repair factors in cancer

Shelterin proteins regulate genomic stability by preventing inappropriate DNA damage responses (DDRs) at telomeres. Unprotected telomeres lead to persistent DDR causing cell cycle inhibition, growth arrest, and apoptosis. Cancer cells rely on DDR to protect themselves from DNA lesions and exogenous DNA-damaging agents such as chemotherapy and radiotherapy. Therefore, targeting DDR machinery is a promising strategy to increase the sensitivity of cancer cells to existing cancer therapies. However, the success of these DDR inhibitors depends on other mutations, and over time, patients develop resistance to these therapies. This suggests the need for alternative approaches. One promising strategy is co-inhibiting shelterin proteins with DDR molecules, which would offset cellular fitness in DNA repair in a mutation-independent manner. This review highlights the associations and dependencies of the shelterin complex with the DDR proteins and discusses potential co-inhibition strategies that might improve the therapeutic potential of current inhibitors.

PARP1 allows proper telomere replication through TRF1 poly (ADP-ribosyl)ation and helicase recruitment

Telomeres are nucleoprotein structures at eukaryotic chromosome termini. Their stability is preserved by a six-protein complex named shelterin. Among these, TRF1 binds telomere duplex and assists DNA replication with mechanisms only partly clarified. Here we found that poly (ADP-ribose) polymerase 1 (PARP1) interacts and covalently PARylates TRF1 in S-phase modifying its DNA affinity. Therefore, genetic and pharmacological inhibition of PARP1 impairs the dynamic association of TRF1 and the bromodeoxyuridine incorporation at replicating telomeres. Inhibition of PARP1 also affects the recruitment of WRN and BLM helicases in TRF1 containing complexes during S-phase, triggering replication-dependent DNA-damage and telomere fragility. This work unveils an unprecedented role for PARP1 as a "surveillant" of telomere replication, which orchestrates protein dynamics at proceeding replication fork.

An inducible CRISPR/Cas9 screen identifies DTX2 as a transcriptional regulator of human telomerase

Most tumor cells reactivate telomerase to ensure unlimited proliferation, whereas the expression of human telomerase reverse transcriptase (hTERT) is tightly regulated and rate-limiting for telomerase activity maintenance. Several general transcription factors (TFs) have been found in regulating hTERT transcription; however, a systematic study is lacking. Here we performed an inducible CRISPR/Cas9 KO screen using an hTERT core promoter-driven reporter. We identified numerous positive regulators including an E3 ligase DTX2. In telomerase-positive cancer cells, DTX2 depletion downregulated hTERT transcription and telomerase activity, contributing to progressive telomere shortening, growth arrest, and increased apoptosis. Utilizing BioID, we characterized multiple TFs as DTX2 proximal proteins, among which NFIC functioned corporately with DTX2 in promoting hTERT transcription. Further analysis demonstrated that DTX2 mediated K63-linked ubiquitination of NFIC, which facilitated NFIC binding to the hTERT promoter and enhanced hTERT expression. These findings highlight a new hTERT regulatory pathway that may be exploited for potential cancer therapeutics.

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An inducible CRISPR/Cas9 screen identifies regulators for hTERT transcription

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DTX2 deficiency leads to telomere shortening and cell growth arrest

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DTX2 mediates ubiquitination on NFIC, stabilizing NFIC binding on hTERT promoter

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DTX2-NFIC functions corporately to promote hTERT transcription and tumorigenesis

Why structure and chain length matter: on the biological significance underlying the structural heterogeneity of poly(ADP-ribose)

Poly(ADP-ribosyl)ation (PARylation) is a multifaceted post-translational modification, carried out by poly(ADP-ribosyl)transferases (poly-ARTs, PARPs), which play essential roles in (patho-) physiology, as well as cancer therapy. Using NAD+ as a substrate, acceptors, such as proteins and nucleic acids, can be modified with either single ADP-ribose units or polymers, varying considerably in length and branching. Recently, the importance of PAR structural heterogeneity with regards to chain length and branching came into focus. Here, we provide a concise overview on the current knowledge of the biochemical and physiological significance of such differently structured PAR. There is increasing evidence revealing that PAR's structural diversity influences the binding characteristics of its readers, PAR catabolism, and the dynamics of biomolecular condensates. Thereby, it shapes various cellular processes, such as DNA damage response and cell cycle regulation. Contrary to the knowledge on the consequences of PAR's structural diversity, insight into its determinants is just emerging, pointing to specific roles of different PARP members and accessory factors. In the future, it will be interesting to study the interplay with other post-translational modifications, the contribution of natural PARP variants, and the regulatory role of accessory molecules. This has the exciting potential for new therapeutic approaches, with the targeted modulation and tuning of PARPs' enzymatic functions, rather than their complete inhibition, as a central premise.

Relationship between genetically determined telomere length and glioma risk

Background

Telomere maintenance is increasingly recognized as being fundamental to glioma oncogenesis with longer leukocyte telomere length (LTL) reported to increase risk of glioma. To gain further insight into the relationship between telomere genetics and risk of glioma, we conducted several complementary analyses, using genome-wide association studies data on LTL (78 592 individuals) and glioma (12 488 cases and 18 169 controls).

Methods

We performed both classical and summary Mendelian randomization (SMR), coupled with heterogeneity in dependent instruments tests, at genome-wide significant LTL loci to examine if an association was mediated by the same causal variant in glioma. To prioritize genes underscoring glioma-LTL associations, we analyzed gene expression and DNA methylation data.

Results

Genetically increased LTL was significantly associated with increased glioma risk, random-effects inverse variance weighted ORs per 1 SD unit increase in the putative risk factor (odds ratio [OR]_SD) 4.79 (95% confidence interval: 2.11-10.85; P = 1.76 × 10⁻⁴). SMR confirmed the previously reported LTL associations at 3q26.2 (TERC; P_SMR = 1.33 × 10⁻⁵), 5p15.33 (TERT; P_SMR = 9.80 × 10⁻²⁷), 10q24.33 (STN1 alias OBFC1; P_SMR = 4.31 × 10⁻⁵), and 20q13.3 (STMN3/RTEL1; P_SMR = 2.47 × 10⁻⁴) glioma risk loci. Our analysis implicates variation at 1q42.12 (P_SMR = 1.55 × 10⁻²), 6p21.3 (P_SMR = 9.76 × 10⁻³), 6p22.2 (P_SMR = 5.45 × 10⁻³), 7q31.33 (P_SMR = 6.52 × 10⁻³), and 11q22.3 (P_SMR = 8.89 × 10⁻⁴) as risk factors for glioma risk. While complicated by patterns of linkage disequilibrium, genetic variation involving PARP1, PRRC2A, CARMIL1, POT1, and ATM-NPAT1 was implicated in the etiology of glioma.

Conclusions

These observations extend the role of telomere-related genes in the development of glioma.

Effect of DNA repair inhibitor AsiDNA on the incidence of telomere fusion in crisis

Telomere fusions lead to a state of genomic instability, and are thought to drive clonal evolution and tumorigenesis. Telomere fusions occur via both Classical and Alternative Non-Homologous End Joining repair pathways. AsiDNA is a DNA repair inhibitor that acts by mimicking a DNA double strand break (DSB) and hijacking the recruitment of proteins involved in various DNA repair pathways. In this study, we investigated whether the inhibition of DSB-repair pathways by AsiDNA could prevent telomere fusions during crisis.

The present study showed that AsiDNA decreased the frequency of telomere fusions without affecting the rate of telomere erosion. Further, it indicated that AsiDNA does not impact the choice of the repair pathway used for the fusion of short dysfunctional telomeres.

AsiDNA is thought to prevent short telomeres from fusing by inhibiting DNA repair. An alternative, non-mutually exclusive possibility is that cells harbouring fusions preferentially die in the presence of AsiDNA, thus resulting in a reduction in fusion frequency.

This important work could open the way for investigating the use of AsiDNA in the treatment of tumours that have short dysfunctional telomeres and/or are experiencing genomic instability.

Intimate Relations-Mitochondria and Ageing

Mitochondrial dysfunction is associated with ageing, but the detailed causal relationship between the two is still unclear. We review the major phenomenological manifestations of mitochondrial age-related dysfunction including biochemical, regulatory and energetic features. We conclude that the complexity of these processes and their inter-relationships are still not fully understood and at this point it seems unlikely that a single linear cause and effect relationship between any specific aspect of mitochondrial biology and ageing can be established in either direction.

Role of PARP1 regulation in radiation-induced rescue effect

Radiation-induced rescue effect (RIRE) in cells refers to the phenomenon where irradiated cells (IRCs) receive help from feedback signals produced by partnered bystander unirradiated cells (UIRCs) or from the conditioned medium (CM) that has previously conditioned the UIRCs. In the present work, we explored the role of poly (ADP-ribose) polymerase 1 (PARP1) regulation in RIRE and the positive feedback loop between PARP1 and nuclear factor-kappa-light-chain-enhancer of activated B cell (NF-κB) in RIRE using various cell lines, including HeLa, MCF7, CNE-2 and HCT116 cells. We first found that when the IRCs (irradiated with 2 Gy X-ray) were treated with CM, the relative mRNA expression levels of both tumor suppressor p53-binding protein 1 (53BP1) and PARP1, the co-localization factor between 53BP1 and γH2AX as well as the fluorescent intensity of PARP1 were reduced. We also found that IRCs treated with the PARP1 inhibitor, Olaparib (AZD2281) had a higher 53BP1 expression. These results illustrated that PARP1 was involved in RIRE transcriptionally and translationally. We further revealed that treatment of IRCs with CM together with Olaparib led to significantly lower mRNA expression levels and fluorescent intensities of NF-κB, while treatment of IRCs with CM together the NF-κB inhibitor BAY-11-7082 led to significantly lower mRNA expression levels as well as fluorescent intensities of PARP1. These results illustrated that PARP1 and NF-κB were involved in the positive feedback loop transcriptionally and translationally. Thus, the results supported the occurrence of a PARP1-NF-κB positive feedback loop in RIRE. The present work provided insights into potential exploitation of inhibition of PARP1 and/or the PARP1-NF-κB positive feedback loop in designing adjuncts to cancer radiotherapeutics.

Stilbene Compounds Inhibit Tumor Growth by the Induction of Cellular Senescence and the Inhibition of Telomerase Activity

Cellular senescence is a state of cell cycle arrest characterized by a distinct morphology, gene expression pattern, and secretory phenotype. It can be triggered by multiple mechanisms, including those involved in telomere shortening, the accumulation of DNA damage, epigenetic pathways, and the senescence-associated secretory phenotype (SASP), and so on. In current cancer therapy, cellular senescence has emerged as a potent tumor suppression mechanism that restrains proliferation in cells at risk for malignant transformation. Therefore, compounds that stimulate the growth inhibition effects of senescence while limiting its detrimental effects are believed to have great clinical potential. In this review article, we first review the current knowledge of the pro- and antitumorigeneic functions of senescence and summarize the key roles of telomerase in the regulation of senescence in tumors. Second, we review the current literature regarding the anticancer effects of stilbene compounds that are mediated by the targeting of telomerase and cell senescence. Finally, we provide future perspectives on the clinical utilization of stilbene compounds, especially resveratrol and pterostilbene, as novel cancer therapeutic remedies. We conclude and propose that stilbene compounds may induce senescence and may potentially be used as the therapeutic or adjuvant agents for cancers with high telomerase activity.

Both the classical and alternative non-homologous end joining pathways contribute to the fusion of drastically shortened telomeres induced by TRF2 overexpression

The double-stranded telomeric binding protein TRF2 is expressed in many human cancers at elevated levels. Moreover, experimental overexpression of TRF2 in human cells causes replication stalling in telomeric tracts, which leads to drastic telomere shortening and fusion of deprotected chromosome ends. To understand which end joining pathway is involved in mediating these chromosome fusions, we overexpressed TRF2 in human HCT116 cell lines that were deficient for the DNA Ligase 4 (Lig4)-dependent classical non-homologous end joining (C-NHEJ) or the DNA Ligase 3 (Lig3)-dependent alternative non-homologous end joining (A-NHEJ) pathway. Surprisingly, abrogation of either Lig4 or nuclear Lig3 significantly reduced inter-chromosomal fusion of drastically shortened telomeres, suggesting that both the C-NHEJ and A-NHEJ pathways are involved in mediating this type of fusion. Fusion between deprotected sister chromatids, however, only required the Lig3-dependent A-NHEJ pathway. Interestingly, a previous study reported similar end joining pathway requirements for the fusion of critically shortened telomeres during a telomere attrition-based cellular crisis. We speculate that, as in cellular crisis, the same repair pathway(s) may drive clonal and genomic evolution in human cancers containing elevated TRF2 levels.

PARP inhibition prevents escape from a telomere-driven crisis and inhibits cell immortalisation

Telomeric crisis is the final replicative barrier to cell immortalisation; it is characterised by genome instability and cell death and is triggered when telomeres become critically short and are subjected to fusion. Pre-cancerous lesions, or early stage cancers, often show signs of a telomere crisis, suggesting that escape from telomere crisis is a prerequisite for disease progression. Telomeric crisis therefore represents an attractive, and as yet unexplored, opportunity for therapeutic intervention. Here, we show that two clinically approved PARP inhibitors, selectively eliminate human cells undergoing a telomere-driven crisis. Clonal populations of a colorectal cancer cell line (HCT116), or the plasma cell leukaemia cell line (JJN-3), expressing a dominant-negative telomerase, entered a telomere-driven crisis at defined population doubling points and telomere lengths. The addition of the PARP inhibitors, olaparib or rucaparib prevented these cells from escaping crisis. PARP inhibition did not alter cellular proliferation prior to crisis, rates of telomere erosion or the telomere length at which crisis was initiated, but affected repair of eroded telomeres, resulting in an increased in intra-chromosomal telomere fusion. This was accompanied by enhanced DNA damage checkpoint activation and elevated levels of apoptosis. We propose that PARP inhibitors impair the repair of dysfunctional telomeres and/or induce replicative stress at telomeres to inhibit escape from a telomere crisis. This is the first demonstration that a drug can selectively kill cells experiencing telomeric crisis. We propose that this type of drug, which we term 'crisolytic', has the potential to eliminate pre-cancerous lesions and tumours exhibiting short dysfunctional telomeres.