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George SL, Parmar V, Lorenzi F, Marshall LV, Jamin Y, Poon E, Angelini P, Chesler L. Novel therapeutic strategies targeting telomere maintenance mechanisms in high-risk neuroblastoma. J Exp Clin Cancer Res 2020; 39:78. [PMID: 32375866 PMCID: PMC7201617 DOI: 10.1186/s13046-020-01582-2] [Citation(s) in RCA: 28] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2020] [Accepted: 04/22/2020] [Indexed: 12/13/2022] Open
Abstract
The majority of high-risk neuroblastomas can be divided into three distinct molecular subgroups defined by the presence of MYCN amplification, upstream TERT rearrangements or alternative lengthening of telomeres (ALT). The common defining feature of all three subgroups is altered telomere maintenance; MYCN amplification and upstream TERT rearrangements drive high levels of telomerase expression whereas ALT is a telomerase independent telomere maintenance mechanism. As all three telomere maintenance mechanisms are independently associated with poor outcomes, the development of strategies to selectively target either telomerase expressing or ALT cells holds great promise as a therapeutic approach that is applicable to the majority of children with aggressive disease.Here we summarise the biology of telomere maintenance and the molecular drivers of aggressive neuroblastoma before describing the most promising therapeutic strategies to target both telomerase expressing and ALT cancers. For telomerase-expressing neuroblastoma the most promising targeted agent to date is 6-thio-2'-deoxyguanosine, however clinical development of this agent is required. In osteosarcoma cell lines with ALT, selective sensitivity to ATR inhibition has been reported. However, we present data showing that in fact ALT neuroblastoma cells are more resistant to the clinical ATR inhibitor AZD6738 compared to other neuroblastoma subtypes. More recently a number of additional candidate compounds have been shown to show selectivity for ALT cancers, such as Tetra-Pt (bpy), a compound targeting the telomeric G-quadruplex and pifithrin-α, a putative p53 inhibitor. Further pre-clinical evaluation of these compounds in neuroblastoma models is warranted.In summary, telomere maintenance targeting strategies offer a significant opportunity to develop effective new therapies, applicable to a large proportion of children with high-risk neuroblastoma. In parallel to clinical development, more pre-clinical research specifically for neuroblastoma is urgently needed, if we are to improve survival for this common poor outcome tumour of childhood.
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Affiliation(s)
- S L George
- Paediatric Tumour Biology, Division of Clinical Studies, The Institute of Cancer Research, London, UK.
- Children and Young People's Unit, Royal Marsden NHS Foundation Trust, London, UK.
| | - V Parmar
- Children and Young People's Unit, Royal Marsden NHS Foundation Trust, London, UK
| | - F Lorenzi
- Paediatric Tumour Biology, Division of Clinical Studies, The Institute of Cancer Research, London, UK
| | - L V Marshall
- Paediatric Tumour Biology, Division of Clinical Studies, The Institute of Cancer Research, London, UK
- Children and Young People's Unit, Royal Marsden NHS Foundation Trust, London, UK
| | - Y Jamin
- Division of Radiotherapy and Imaging, The Institute of Cancer Research, London, UK
| | - E Poon
- Paediatric Tumour Biology, Division of Clinical Studies, The Institute of Cancer Research, London, UK
| | - P Angelini
- Children and Young People's Unit, Royal Marsden NHS Foundation Trust, London, UK
| | - L Chesler
- Paediatric Tumour Biology, Division of Clinical Studies, The Institute of Cancer Research, London, UK
- Children and Young People's Unit, Royal Marsden NHS Foundation Trust, London, UK
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Kjeldsen E. Characterization of an acquired jumping translocation involving 3q13.31-qter in a patient with de novo acute monocytic leukemia. Exp Mol Pathol 2017. [PMID: 28625614 DOI: 10.1016/j.yexmp.2017.06.002] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
We studied an adult with de novo acute monocytic leukemia and a dismal outcome where her leukemic cells harbored an acquired rare jumping translocation (JT). We used oligo-based array CGH (oaCGH) analysis, fluorescence in situ hybridization (FISH), and 24-color karyotyping to enhance the characterization of the JT. G-banding detected a JT involving the 3q13.3-qter chromosomal segment and the recipient chromosomal regions 17p, 8q, and 15q. Each clone with JT was associated with trisomy 8. oaCGH analysis revealed an additional submicroscopic deletion in 3q13.31 as well as small subtelomeric duplications on several chromosomes. Locus-specific FISH with BAC-based probes from the 3q13.31-q13.32 region showed great heterogeneity. Telomere FISH revealed significantly reduced telomeric content in the aberrant cells with JT compared with cytogenetically normal cells at diagnosis and in normal cells at complete remission. A literature search revealed two previous de novo AML-M5 cases of JT involving the 3q13.3-qter chromosomal segment and concomitant trisomy 8. In addition, a case with an unbalanced der(Y)t(Y;3)(q12;q13.31) and additional trisomy 8 was previously reported in a patient with de novo AML-M5. All of these cases had a dismal outcome. In the present case, and in the der(Y)t(Y;3) case, a concurrent submicroscopic deletion at 3q13.31 was observed affecting the TUSC7 gene. Duplication of 3q13.31-qter might be a non-random chromosomal abnormality with concomitant submicroscopic deletion at 3q13.31 occurring in rare cases of acute monocytic leukemia, being associated with adverse prognosis. The impact of shortened telomeres in forming the JT is reviewed.
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MESH Headings
- Aged
- Chromosome Deletion
- Chromosome Duplication
- Chromosomes, Human, Pair 3/genetics
- Chromosomes, Human, Pair 8/genetics
- Cloning, Molecular
- Comparative Genomic Hybridization
- DNA Copy Number Variations
- Female
- Humans
- In Situ Hybridization, Fluorescence
- Karyotyping
- Leukemia, Monocytic, Acute/diagnosis
- Leukemia, Monocytic, Acute/genetics
- Prognosis
- Translocation, Genetic
- Trisomy/genetics
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Affiliation(s)
- Eigil Kjeldsen
- Cancercytogenetic Section, Hemodiagnostic Laboratory, Department of Hematology, Center for Cancer and Inflammation, Aarhus University Hospital, Tage Hansens Gade 2, Ent. 4A, DK-8000 Aarhus C, Denmark.
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3
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Habibi L, Pedram M, AmirPhirozy A, Bonyadi K. Mobile DNA Elements: The Seeds of Organic Complexity on Earth. DNA Cell Biol 2015. [DOI: 10.1089/dna.2015.2938] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Affiliation(s)
- Laleh Habibi
- Department of Pharmaceutics, School of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran
- Cellular and Molecular Nutrition Department, School of Nutritional Science and Dietetics, Tehran University of Medical Sciences, Tehran, Iran
| | - Mehrdad Pedram
- Department of Genetics and Molecular Medicine, School of Medicine, Zanjan University of Medical Sciences, Zanjan, Iran
| | - Akbar AmirPhirozy
- Department of Medical Genetics, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
| | - Khadijeh Bonyadi
- Department of Medical Genetics, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
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Popuri V, Tadokoro T, Croteau DL, Bohr VA. Human RECQL5: guarding the crossroads of DNA replication and transcription and providing backup capability. Crit Rev Biochem Mol Biol 2013; 48:289-99. [PMID: 23627586 DOI: 10.3109/10409238.2013.792770] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
DNA helicases are ubiquitous enzymes that catalyze unwinding of duplex DNA and function in all metabolic processes in which access to single-stranded DNA is required, including DNA replication, repair, recombination and RNA transcription. RecQ helicases are a conserved family of DNA helicases that display highly specialized and vital roles in the maintenance of genome stability. Mutations in three of the five human RecQ helicases, BLM, WRN and RECQL4 are associated with the genetic disorders Bloom syndrome, Werner syndrome and Rothmund-Thomson syndrome that are characterized by chromosomal instability, premature aging and predisposition to cancer. The biological role of human RECQL5 is only partially understood and RECQL5 has not yet been associated with any human disease. Illegitimate recombination and replication stress are hallmarks of human cancers and common instigators for genomic instability and cell death. Recql5 knockout mice are cancer prone and show increased chromosomal instability. Recql5-deficient mouse embryonic fibroblasts are sensitive to camptothecin and display elevated levels of sister chromatid exchanges. Unlike other human RecQ helicases, RECQL5 is recruited to single-stranded DNA breaks and is also proposed to play an essential role in RNA transcription. Here, we review the established roles of RECQL5 at the cross roads of DNA replication, recombination and transcription, and propose that human RECQL5 provides important backup functions in the absence of other DNA helicases.
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Affiliation(s)
- Venkateswarlu Popuri
- Laboratory of Molecular Gerontology, National Institute on Aging, NIH, Baltimore, MD 21224, USA
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5
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Mendez-Bermudez A, Hidalgo-Bravo A, Cotton VE, Gravani A, Jeyapalan JN, Royle NJ. The roles of WRN and BLM RecQ helicases in the Alternative Lengthening of Telomeres. Nucleic Acids Res 2012; 40:10809-20. [PMID: 22989712 PMCID: PMC3510502 DOI: 10.1093/nar/gks862] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Approximately 10% of all cancers, but a higher proportion of sarcomas, use the recombination-based alternative lengthening of telomeres (ALT) to maintain telomeres. Two RecQ helicase genes, BLM and WRN, play important roles in homologous recombination repair and they have been implicated in telomeric recombination activity, but their precise roles in ALT are unclear. Using analysis of sequence variation present in human telomeres, we found that a WRN– ALT+ cell line lacks the class of complex telomere mutations attributed to inter-telomeric recombination in other ALT+ cell lines. This suggests that WRN facilitates inter-telomeric recombination when there are sequence differences between the donor and recipient molecules or that sister-telomere interactions are suppressed in the presence of WRN and this promotes inter-telomeric recombination. Depleting BLM in the WRN– ALT+ cell line increased the mutation frequency at telomeres and at the MS32 minisatellite, which is a marker of ALT. The absence of complex telomere mutations persisted in BLM-depleted clones, and there was a clear increase in sequence homogenization across the telomere and MS32 repeat arrays. These data indicate that BLM suppresses unequal sister chromatid interactions that result in excessive homogenization at MS32 and at telomeres in ALT+ cells.
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Affiliation(s)
- Aaron Mendez-Bermudez
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK
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Yuan SSF, Hou MF, Hsieh YC, Huang CY, Lee YC, Chen YJ, Lo S. Role of MRE11 in cell proliferation, tumor invasion, and DNA repair in breast cancer. J Natl Cancer Inst 2012; 104:1485-502. [PMID: 22914783 DOI: 10.1093/jnci/djs355] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
BACKGROUND Previous studies have shown that altered forms of MRE11, a protein known to play a vital role in DNA double-strand break repair, DNA replication, and telomere maintenance are associated with cancer outcomes. We investigated the role of MRE11 in breast cancer in both clinical and in vitro settings. METHODS We examined MRE11 expression in tumor tissues from invasive ductal carcinoma breast cancer patients (n = 254) by immunohistochemistry, and associations with clinicopathological characteristics and overall survival were assessed using Cox proportional hazards regression models and Kaplan-Meier survival curves. Effect of MRE11 overexpression and knockdown on cell proliferation, invasion, and radioresistance was assessed in vitro using breast cancer cell lines (MCF-7 and MDA-MB-231). We also investigated the mechanisms involved. Effect of MRE11 overexpression on tumor growth was assessed in an orthotopic xenograft model (n = 8 mice per group). All statistical tests were two-sided. RESULTS Of the 254 tissue samples, 69.3% and 30.7% showed high and low MRE11 expression, respectively. High MRE11 expression was statistically significantly associated with malignant cancer behavior compared with low MRE11 expression (eg, stages III and IV vs stage I, P = .004; poor overall survival, P = .005). MRE11 overexpression in breast cancer cell lines promoted cell proliferation through STAT3, cell cycle entry, invasion and migration, and radioresistance via enhanced DNA repair activity and also inhibited apoptosis; knockdown of MRE11 had the opposite effect. In xenograft tumor-bearing mice (n = 8 per group), increased tumor growth was observed in the MRE11-overexpressing group compared with the control group (tumor volume at week 8, control vs MRE11-overexpressing tumor originating from MCF-7 cells, mean = 280.4 mm(3), 95% confidence interval [CI] = 62.4 to 498.4 mm(3) vs mean = 631.0 mm(3), 95% CI = 296.9 to 965.0 mm(3), P = .043). CONCLUSION High MRE11 expression was associated with a more malignant behavior in breast cancer. MRE11 may be a novel oncoprotein and may therefore serve as a new therapeutic intervention against breast cancer.
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Zhang W, Tian Y, Chen JJ, Zhao W, Yu X. A postulated role of p130 in telomere maintenance by human papillomavirus oncoprotein E7. Med Hypotheses 2012; 79:178-80. [PMID: 22595804 DOI: 10.1016/j.mehy.2012.04.028] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2011] [Revised: 04/18/2012] [Accepted: 04/18/2012] [Indexed: 11/28/2022]
Abstract
High-risk human papillomaviruses (HR-HPVs) infections is highly associated with the development of cervical cancer. It is now recognized that telomere length maintenance or extension is indispensable for carcinogenesis. The early oncoproteins E6 and E7 are the main malignant transformation factors of HR-HPVs and they maintain telomeres by different mechanisms, of which E6 protein activating telomerase is well documented. Reports showed that E7 protein utilized an alternative lengthen of telomere (ALT) mechanism to restore telomere length, yet the underlying molecular basis remains largely unknown. We propose that degradation of tumor suppressor pRb family member p130 plays an essential role in E7-regulated telomere extension by ALT. ALT is a mechanism based on homologous recombination (HR) between telomere sister chromatids, and a number of proteins involved in the HR pathway, such as MRN [MRE11 (meiotic recombination 11)-Rad50-NBS1 (Nijmegen breakage syndrome 1)] complex are required for the ALT pathway. Rb family member p130 could inhibit ALT by interacting with Rad50, while HPV E7 could activate ALT by degrading p130. We will make E7 mutants which are defective in p130 degradation to test whether these cells have a limited life span. Besides, immunofluorescence assay will show an ALT-related promyelocytic leukemia (PML) body (APBs) in E7-expressing cells. Although cervical cancer usually has high telomerase activities since the expressing of HPV E6, the anti-telomerase therapy will be unavailable for cervical cancer since it may activate E7-induced ALT. Our hypothesis not only enrich the knowledge of the regulation of ALT, but also indicate that p130 may serve as a potential suppressor of ALT, and gene therapy of p130 may be used in cervical cancers.
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Affiliation(s)
- WeiFang Zhang
- Department of Pathogenic Microbiology, Shandong University School of Medicine, Jinan, Shandong 250012, China.
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8
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Durant ST. Telomerase-independent paths to immortality in predictable cancer subtypes. J Cancer 2012; 3:67-82. [PMID: 22315652 PMCID: PMC3273709 DOI: 10.7150/jca.3965] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2011] [Accepted: 01/28/2012] [Indexed: 01/17/2023] Open
Abstract
The vast majority of cancers commandeer the activity of telomerase - the remarkable enzyme responsible for prolonging cellular lifespan by maintaining the length of telomeres at the ends of chromosomes. Telomerase is only normally active in embryonic and highly proliferative somatic cells. Thus, targeting telomerase is an attractive anti-cancer therapeutic rationale currently under investigation in various phases of clinical development. However, previous reports suggest that an average of 10-15% of all cancers lose the functional activity of telomerase and most of these turn to an Alternative Lengthening of Telomeres pathway (ALT). ALT-positive tumours will therefore not respond to anti-telomerase therapies and there is a real possibility that such drugs would be toxic to normal telomerase-utilising cells and ultimately select for resistant cells that activate an ALT mechanism. ALT exploits certain DNA damage response (DDR) components to counteract telomere shortening and rapid trimming. ALT has been reported in many cancer subtypes including sarcoma, gastric carcinoma, central nervous system malignancies, subtypes of kidney (Wilm's Tumour) and bladder carcinoma, mesothelioma, malignant melanoma and germ cell testicular cancers to name but a few. A recent heroic study that analysed ALT in over six thousand tumour samples supports this historical spread, although only reporting an approximate 4% prevalence. This review highlights the various methods of ALT detection, unravels several molecular ALT models thought to promote telomere maintenance and elongation, spotlights the DDR components known to facilitate these and explores why certain tissues are more likely to subvert DDR away from its usually protective functions, resulting in a predictive pattern of prevalence in specific cancer subsets.
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Affiliation(s)
- Stephen T Durant
- AstraZeneca - DNA Damage Response, Bioscience, Oncology iMed, Alderley Park, Cheshire, SK10 4TG, England, UK
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9
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Gladych M, Wojtyla A, Rubis B. Human telomerase expression regulation. Biochem Cell Biol 2011; 89:359-76. [DOI: 10.1139/o11-037] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Since telomerase has been recognized as a relevant factor distinguishing cancer cells from normal cells, it has become a very promising target for anti-cancer therapy. A correlation between short telomere length and increased mortality was revealed in many studies. The telomerase expression/activity appears to be one of the most crucial factors to study to improve cancer therapy and prevention. However, this multisubunit enzymatic complex can be regulated at various levels. Thus, several strategies have been proposed to control telomerase in cancer cells such as anti-sense technology against TR and TERT, ribozymes against TERT, anti-estrogens, progesterone, vitamin D, retinoic acid, quadruplex stabilizers, telomere and telomerase targeting agents, modulation of interaction with other proteins involved in the regulation of telomerase and telomeres, etc. However, the transcription control of key telomerase subunits seems to play the crucial role in whole complexes activity and cancer cells immortality. Thus, the research of telomerase regulation can bring significant insight into the knowledge concerning stem cells metabolism but also ageing. This review summarizes the current state of knowledge of numerous telomerase regulation mechanisms at the transcription level in human that might become attractive anti-cancer therapy targets.
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Affiliation(s)
- Marta Gladych
- Poznan University of Medical Sciences, Department of Clinical Chemistry and Molecular Diagnostics, Przybyszewskiego 49 St., 60-355 Poznan, Poland
| | - Aneta Wojtyla
- Poznan University of Medical Sciences, Department of Clinical Chemistry and Molecular Diagnostics, Przybyszewskiego 49 St., 60-355 Poznan, Poland
| | - Blazej Rubis
- Poznan University of Medical Sciences, Department of Clinical Chemistry and Molecular Diagnostics, Przybyszewskiego 49 St., 60-355 Poznan, Poland
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Recombination can either help maintain very short telomeres or generate longer telomeres in yeast cells with weak telomerase activity. EUKARYOTIC CELL 2011; 10:1131-42. [PMID: 21666075 DOI: 10.1128/ec.05079-11] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Yeast mutants lacking telomerase are able to elongate their telomeres through processes involving homologous recombination. In this study, we investigated telomeric recombination in several mutants that normally maintain very short telomeres due to the presence of a partially functional telomerase. The abnormal colony morphology present in some mutants was correlated with especially short average telomere length and with a requirement for RAD52 for indefinite growth. Better-growing derivatives of some of the mutants were occasionally observed and were found to have substantially elongated telomeres. These telomeres were composed of alternating patterns of mutationally tagged telomeric repeats and wild-type repeats, an outcome consistent with amplification occurring via recombination rather than telomerase. Our results suggest that recombination at telomeres can produce two distinct outcomes in the mutants we studied. In occasional cells, recombination generates substantially longer telomeres, apparently through the roll-and-spread mechanism. However, in most cells, recombination appears limited to helping to maintain very short telomeres. The latter outcome likely represents a simplified form of recombinational telomere maintenance that is independent of the generation and copying of telomeric circles.
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Mladenov E, Iliakis G. Induction and repair of DNA double strand breaks: the increasing spectrum of non-homologous end joining pathways. Mutat Res 2011; 711:61-72. [PMID: 21329706 DOI: 10.1016/j.mrfmmm.2011.02.005] [Citation(s) in RCA: 287] [Impact Index Per Article: 22.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2010] [Revised: 01/31/2011] [Accepted: 02/06/2011] [Indexed: 04/13/2023]
Abstract
A defining characteristic of damage induced in the DNA by ionizing radiation (IR) is its clustered character that leads to the formation of complex lesions challenging the cellular repair mechanisms. The most widely investigated such complex lesion is the DNA double strand break (DSB). DSBs undermine chromatin stability and challenge the repair machinery because an intact template strand is lacking to assist restoration of integrity and sequence in the DNA molecule. Therefore, cells have evolved a sophisticated machinery to detect DSBs and coordinate a response on the basis of inputs from various sources. A central function of cellular responses to DSBs is the coordination of DSB repair. Two conceptually different mechanisms can in principle remove DSBs from the genome of cells of higher eukaryotes. Homologous recombination repair (HRR) uses as template a homologous DNA molecule and is therefore error-free; it functions preferentially in the S and G2 phases. Non-homologous end joining (NHEJ), on the other hand, simply restores DNA integrity by joining the two ends, is error prone as sequence is only fortuitously preserved and active throughout the cell cycle. The basis of DSB repair pathway choice remains unknown, but cells of higher eukaryotes appear programmed to utilize preferentially NHEJ. Recent work suggests that when the canonical DNA-PK dependent pathway of NHEJ (D-NHEJ), becomes compromised an alternative NHEJ pathway and not HRR substitutes in a quasi-backup function (B-NHEJ). Here, we outline aspects of DSB induction by IR and review the mechanisms of their processing in cells of higher eukaryotes. We place particular emphasis on backup pathways of NHEJ and summarize their increasing significance in various cellular processes, as well as their potential contribution to carcinogenesis.
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Affiliation(s)
- Emil Mladenov
- Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Hufelandstr. 55, 45122 Essen, Germany
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12
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Grach AA. Alternative telomere-lengthening mechanisms. CYTOL GENET+ 2011. [DOI: 10.3103/s0095452711020046] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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13
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Hyper telomere recombination accelerates replicative senescence and may promote premature aging. Proc Natl Acad Sci U S A 2010; 107:15768-73. [PMID: 20798040 DOI: 10.1073/pnas.1006338107] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Werner syndrome and Bloom syndrome result from defects in the RecQ helicases Werner (WRN) and Bloom (BLM), respectively, and display premature aging phenotypes. Similarly, XFE progeroid syndrome results from defects in the ERCC1-XPF DNA repair endonuclease. To gain insight into the origin of cellular senescence and human aging, we analyzed the dependence of sister chromatid exchange (SCE) frequencies on location [i.e., genomic (G-SCE) vs. telomeric (T-SCE) DNA] in primary human fibroblasts deficient in WRN, BLM, or ERCC1-XPF. Consistent with our other studies, we found evidence of elevated T-SCE in telomerase-negative but not telomerase-positive backgrounds. In telomerase-negative WRN-deficient cells, T-SCE-but not G-SCE-frequencies were significantly increased compared with controls. In contrast, SCE frequencies were significantly elevated in BLM-deficient cells irrespective of genome location. In ERCC1-XPF-deficient cells, neither T- nor G-SCE frequencies differed from controls. A theoretical model was developed that allowed an in silico investigation into the cellular consequences of increased T-SCE frequency. The model predicts that in cells with increased T-SCE, the onset of replicative senescence is dramatically accelerated even though the average rate of telomere loss has not changed. Premature cellular senescence may act as a powerful tumor-suppressor mechanism in telomerase-deficient cells with mutations that cause T-SCE levels to rise. Furthermore, T-SCE-driven premature cellular senescence may be a factor contributing to accelerated aging in Werner and Bloom syndromes, but not XFE progeroid syndrome.
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TCEAL7 inhibition of c-Myc activity in alternative lengthening of telomeres regulates hTERT expression. Neoplasia 2010; 12:405-14. [PMID: 20454512 DOI: 10.1593/neo.10180] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2010] [Revised: 03/03/2010] [Accepted: 03/05/2010] [Indexed: 11/18/2022] Open
Abstract
Replicative senescence forms a major barrier to tumor progression. Cancer cells bypass this by using one of the two known telomere maintenance mechanisms: telomerase or the recombination-based alternative lengthening of telomeres (ALT) mechanism. The molecular details of ALT are currently poorly understood. We have previously shown that telomerase is actively repressed through complex networks of kinase, gene expression, and chromatin regulation. In this study, we aimed to gain further understanding of the role of kinases in the regulation of telomerase expression in ALT cells. Using a whole human kinome small interfering RNA (siRNA) screen, we highlighted 106 kinases whose expression is linked to human telomerase reverse transcriptase (hTERT) promoter activity. Network modeling of transcriptional regulation implicated c-Myc as a key regulator of the 106 kinase hits. Given our previous observations of lower c-Myc activity in ALT cells, we further explored its potential to regulate telomerase expression in ALT. We found increased c-Myc binding at the hTERT promoter in telomerase-positive compared with ALT cells, although no expression differences in c-Myc, Mad, or Max were observed between ALT and telomerase-positive cells that could explain decreased c-Myc activity in ALT. Instead, we found increased expression of the c-Myc competitive inhibitor TCEAL7 in ALT cells and tumors and that alteration of TCEAL7 expression levels in ALT and telomerase-positive cells affects hTERT expression. Lower c-Myc activity in ALT may therefore be obtained through TCEAL7 regulation. Thus, TCEAL7 may present an interesting novel target for cancer therapy, which warrants further investigation.
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Abstract
Telomeres from most eukaryotes are composed of repeats of guanine-rich sequences whose main function is to preserve the end of the chromosomes. Telomeres are synthesised by a reverse transcriptase enzyme, telomerase (TERT), which forms part of a ribonucleoprotein complex containing also a RNA template molecule (TERC) and dyskerin. Exhaustion of telomeres during cell divisions triggers a DNA damage response that induces a senescence phenotype. The DNA damage machinery plays an essential role in maintaining the integrity of the genome and also detecting telomere shortening. However in some syndromes that involved mutations either in the telomerase complex genes or those involved in maintaining DNA secondary structure, such as the recQ helicase WRN, a higher frequency in the development of different types of malignancies is observed. We here describe the biology of some of these diseases, together with the molecular modifications in the telomerase complex genes and the impact of these alterations on the development of particular types of cancer.
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Abstract
In order to maintain genome integrity, it is essential that any DNA damage is repaired. This is achieved in diverse ways in all cells to ensure cellular survival. There is a large repertoire of proteins that remove and repair DNA damage. However, sometimes these processes do not function correctly, leading to genome instability. Studies of DNA repair and genome instability and their causes and cures were showcased in the 2008 Biochemical Society Annual Symposium. The present article provides a summary of the talks given and the subsequent papers in this issue of Biochemical Society Transactions.
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