76
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Patterson-Fortin J, Shao G, Bretscher H, Messick TE, Greenberg RA. Differential regulation of JAMM domain deubiquitinating enzyme activity within the RAP80 complex. J Biol Chem 2010; 285:30971-81. [PMID: 20656689 PMCID: PMC2945588 DOI: 10.1074/jbc.m110.135319] [Citation(s) in RCA: 59] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2010] [Revised: 07/16/2010] [Indexed: 11/06/2022] Open
Abstract
BRCC36 is a JAMM (JAB1/MPN/Mov34 metalloenzyme) domain, lysine 63-ubiquitin (K63-Ub)-specific deubiquitinating enzyme (DUB) and a member of two protein complexes: the DNA damage-responsive BRCA1-RAP80 complex, and the cytoplasmic BRCC36 isopeptidase complex (BRISC). The presence of several identical constituents in both complexes suggests common regulatory mechanisms and potential competition between K63-Ub-related signaling in cytoplasmic and nuclear compartments. Surprisingly, we discover that BRCC36 DUB activity requires different interactions within the context of each complex. Abraxas and BRCC45 were essential for BRCC36 DUB activity within the RAP80 complex, whereas KIAA0157/Abro was the only interaction required for DUB activity within the BRISC. Poh1 also required protein interactions for activity, suggesting a common regulatory mechanism for JAMM domain DUBs. Finally, BRISC deficiency enhanced formation of the BRCA1-RAP80 complex in vivo, increasing BRCA1 levels at DNA double strand breaks. These findings reveal that JAMM domain DUB activity and K63-Ub levels are regulated by multiple mechanisms within the cell.
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77
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Shanbhag NM, Greenberg RA. Neighborly DISCourse: DNA double strand breaks silence transcription. Cell Cycle 2010; 9:3635-6. [PMID: 20855969 PMCID: PMC5830114 DOI: 10.4161/cc.9.18.13171] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
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78
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Shanbhag NM, Rafalska-Metcalf IU, Balane-Bolivar C, Janicki SM, Greenberg RA. ATM-dependent chromatin changes silence transcription in cis to DNA double-strand breaks. Cell 2010; 141:970-81. [PMID: 20550933 DOI: 10.1016/j.cell.2010.04.038] [Citation(s) in RCA: 550] [Impact Index Per Article: 39.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2009] [Revised: 01/27/2010] [Accepted: 04/06/2010] [Indexed: 12/01/2022]
Abstract
DNA double-strand breaks (DSBs) initiate extensive local and global alterations in chromatin structure, many of which depend on the ATM kinase. Histone H2A ubiquitylation (uH2A) on chromatin surrounding DSBs is one example, thought to be important for recruitment of repair proteins. uH2A is also implicated in transcriptional repression; an intriguing yet untested hypothesis is that this function is conserved in the context of DSBs. Using a novel reporter that allows for visualization of repair protein recruitment and local transcription in single cells, we describe an ATM-dependent transcriptional silencing program in cis to DSBs. ATM prevents RNA polymerase II elongation-dependent chromatin decondensation at regions distal to DSBs. Silencing is partially dependent on E3 ubiquitin ligases RNF8 and RNF168, whereas reversal of silencing relies on the uH2A deubiquitylating enzyme USP16. These findings give insight into the role of posttranslational modifications in mediating crosstalk between diverse processes occurring on chromatin.
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79
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Abstract
The intimate relationship between DNA double-strand break (DSB) repair and cancer susceptibility has sparked profound interest in how transactions on DNA and chromatin surrounding DNA damage influence genome integrity. Recent evidence implicates a substantial commitment of the cellular DNA damage response machinery to the synthesis, recognition, and hydrolysis of ubiquitin chains at DNA damage sites. In this review, we propose that, in order to accommodate parallel processes involved in DSB repair and checkpoint signaling, DSB-associated ubiquitin structures must be nonuniform, using different linkages for distinct functional outputs. We highlight recent advances in the study of nondegradative ubiquitin signaling at DSBs, and discuss how recognition of different ubiquitin structures may influence DNA damage responses.
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80
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Messick TE, Greenberg RA. The ubiquitin landscape at DNA double-strand breaks. J Exp Med 2009. [DOI: 10.1084/jem20612oia29] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022] Open
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81
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Domchek SM, Greenberg RA. Breast cancer gene variants: separating the harmful from the harmless. J Clin Invest 2009; 119:2895-7. [PMID: 19770511 DOI: 10.1172/jci40577] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Individuals carrying a mutation in the breast cancer 1, early onset gene (BRCA1) are at increased risk of breast or ovarian cancer and thus are candidates for risk reduction strategies such as oophorectomy and mastectomy. A recurring problem in the clinic is that many detectable changes within the BRCA1 gene produce subtle alterations to the protein that are not easily recognized as either harmful (loss-of-function) alleles or harmless and thus inconsequential polymorphisms. In this issue of the JCI, Chang, Sharan, and colleagues describe a novel system to evaluate human BRCA1 alleles for in vivo function using BACs containing human BRCA1 vectors in mouse cells and embryos (see the related article beginning on page 3160). This strategy should provide new avenues for clinicians to interpret results of genetic testing of BRCA1 variants and for researchers to study the basic molecular mechanisms of BRCA1 function in in vivo model systems.
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82
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Solyom S, Patterson-Fortin J, Pylkäs K, Greenberg RA, Winqvist R. Mutation screening of the MERIT40 gene encoding a novel BRCA1 and RAP80 interacting protein in breast cancer families. Breast Cancer Res Treat 2009; 120:165-8. [PMID: 19572197 DOI: 10.1007/s10549-009-0453-7] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2009] [Accepted: 06/20/2009] [Indexed: 01/07/2023]
Abstract
MERIT40 is a recently identified BRCA1 and RAP80 interacting protein that is essential for protein-protein interactions of a BRCA1 complex also containing Abraxas, BRCC36 and BRCC45. It is a mediator of checkpoint functions and DNA damage signaling through a (de)ubiquitination cascade. Based on its interaction with BRCA1 and its role in genome integrity maintenance, MERIT40 is a novel candidate gene for being involved in hereditary susceptibility to breast cancer. Here, we report to our knowledge the first comprehensive mutation screening of this gene in affected cases of breast cancer families. Only a number of sequence variants were found, four of which are novel. None of the observed variants appeared to be disease related, suggesting that germline mutations in MERIT40 are rare or absent in familial breast cancer patients.
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83
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Shao G, Patterson-Fortin J, Messick TE, Feng D, Shanbhag N, Wang Y, Greenberg RA. MERIT40 controls BRCA1-Rap80 complex integrity and recruitment to DNA double-strand breaks. Genes Dev 2009; 23:740-54. [PMID: 19261746 DOI: 10.1101/gad.1739609] [Citation(s) in RCA: 129] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Rap80 targets the breast cancer suppressor protein BRCA1 along with Abraxas and the BRCC36 deubiquitinating enzyme (DUB) to polyubiquitin structures at DNA double-strand breaks (DSBs). These DSB targeting events are essential for BRCA1-dependent DNA damage response-induced checkpoint and repair functions. Here, we identify MERIT40 (Mediator of Rap80 Interactions and Targeting 40 kD)/(C19orf62) as a Rap80-associated protein that is essential for BRCA1-Rap80 complex protein interactions, stability, and DSB targeting. Moreover, MERIT40 is required for Rap80-associated lysine(63)-ubiquitin DUB activity, a critical component of BRCA1-Rap80 G2 checkpoint and viability responses to ionizing radiation. Thus, MERIT40 represents a novel factor that links BRCA1-Rap80 complex integrity, DSB recognition, and ubiquitin chain hydrolytic activities to the DNA damage response. These findings provide new molecular insights into how BRCA1 associates with independently assembled core protein complexes to maintain genome integrity.
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84
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Urtishak KA, Smith KD, Chanoux RA, Greenberg RA, Johnson FB, Brown EJ. Timeless Maintains Genomic Stability and Suppresses Sister Chromatid Exchange during Unperturbed DNA Replication. J Biol Chem 2008; 284:8777-85. [PMID: 19112184 DOI: 10.1074/jbc.m806103200] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
Genome integrity is maintained during DNA replication by coordination of various replisome-regulated processes. Although it is known that Timeless (Tim) is a replisome component that participates in replication checkpoint responses to genotoxic stress, its importance for genome maintenance during normal DNA synthesis has not been reported. Here we demonstrate that Tim reduction leads to genomic instability during unperturbed DNA replication, culminating in increased chromatid breaks and translocations (triradials, quadriradials, and fusions). Tim deficiency led to increased H2AX phosphorylation and Rad51 and Rad52 foci formation selectively during DNA synthesis and caused a 3-4-fold increase in sister chromatid exchange. The sister chromatid exchange events stimulated by Tim reduction were largely mediated via a Brca2/Rad51-dependent mechanism and were additively increased by deletion of the Blm helicase. Therefore, Tim deficiency leads to an increased reliance on homologous recombination for proper continuation of DNA synthesis. Together, these results indicate a pivotal role for Tim in maintaining genome stability throughout normal DNA replication.
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85
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Gordan JD, Lal P, Dondeti VR, Letrero R, Parekh KN, Oquendo CE, Greenberg RA, Flaherty KT, Rathmell WK, Keith B, Simon MC, Nathanson KL. HIF-alpha effects on c-Myc distinguish two subtypes of sporadic VHL-deficient clear cell renal carcinoma. Cancer Cell 2008; 14:435-46. [PMID: 19061835 PMCID: PMC2621440 DOI: 10.1016/j.ccr.2008.10.016] [Citation(s) in RCA: 392] [Impact Index Per Article: 24.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/14/2008] [Revised: 09/30/2008] [Accepted: 10/24/2008] [Indexed: 01/16/2023]
Abstract
von Hippel-Lindau (VHL) tumor suppressor loss results in hypoxia-inducible factor alpha (HIF-alpha) stabilization and occurs in 70% of sporadic clear cell renal carcinomas (ccRCCs). To determine whether opposing influences of HIF-1alpha and HIF-2alpha on c-Myc activity regulate human ccRCC progression, we analyzed VHL genotype and HIF-alpha expression in 160 primary tumors, which segregated into three groups with distinct molecular characteristics. Interestingly, ccRCCs with intact VHL, as well as pVHL-deficient HIF-1alpha/HIF-2alpha-expressing ccRCCs, exhibited enhanced Akt/mTOR and ERK/MAPK signaling. In contrast, pVHL-deficient ccRCCs expressing only HIF-2alpha displayed elevated c-Myc activity, resulting in enhanced proliferation and resistance to replication stress. These reproducible distinctions in ccRCC behavior delineate HIF-alpha effects on c-Myc in vivo and suggest molecular criteria for selecting targeted therapies.
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86
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Greenberg RA. Recognition of DNA double strand breaks by the BRCA1 tumor suppressor network. Chromosoma 2008; 117:305-17. [PMID: 18369654 DOI: 10.1007/s00412-008-0154-8] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2007] [Revised: 02/20/2008] [Accepted: 02/21/2008] [Indexed: 12/17/2022]
Abstract
DNA double-strand breaks (DSBs) occur in response to both endogenous and exogenous genotoxic stress. Inappropriate repair of DSBs can lead to either loss of viability or to chromosomal alterations that increase the likelihood of cancer development. In strong support of this assertion, many cancer predisposition syndromes stem from germline mutations in genes involved in DNA DSB repair. Among the most prominent of such tumor suppressor genes are the Breast Cancer 1 and Breast Cancer 2 genes (BRCA1 and BRCA2), which are mutated in familial forms of breast and ovarian cancer. Recent findings implicate BRCA1 as a central component of several distinct macromolecular protein complexes, each dedicated to distinct elements of DNA DSB repair and tumor suppression. Emerging evidence has shed light on some of the molecular recognition processes that are responsible for targeting BRCA1 and its associated partners to DNA and chromatin directly flanking DSBs. These events are required for BRCA1-dependent DNA repair and tumor suppression. Thus, a detailed temporal and spatial knowledge of how breaks are recognized and repaired has profound implications for understanding processes related to the genesis of malignancy and to its treatment.
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87
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Pujana MA, Han JDJ, Starita LM, Stevens KN, Tewari M, Ahn JS, Rennert G, Moreno V, Kirchhoff T, Gold B, Assmann V, Elshamy WM, Rual JF, Levine D, Rozek LS, Gelman RS, Gunsalus KC, Greenberg RA, Sobhian B, Bertin N, Venkatesan K, Ayivi-Guedehoussou N, Solé X, Hernández P, Lázaro C, Nathanson KL, Weber BL, Cusick ME, Hill DE, Offit K, Livingston DM, Gruber SB, Parvin JD, Vidal M. Network modeling links breast cancer susceptibility and centrosome dysfunction. Nat Genet 2007; 39:1338-49. [PMID: 17922014 DOI: 10.1038/ng.2007.2] [Citation(s) in RCA: 499] [Impact Index Per Article: 29.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2007] [Accepted: 08/02/2007] [Indexed: 12/29/2022]
Abstract
Many cancer-associated genes remain to be identified to clarify the underlying molecular mechanisms of cancer susceptibility and progression. Better understanding is also required of how mutations in cancer genes affect their products in the context of complex cellular networks. Here we have used a network modeling strategy to identify genes potentially associated with higher risk of breast cancer. Starting with four known genes encoding tumor suppressors of breast cancer, we combined gene expression profiling with functional genomic and proteomic (or 'omic') data from various species to generate a network containing 118 genes linked by 866 potential functional associations. This network shows higher connectivity than expected by chance, suggesting that its components function in biologically related pathways. One of the components of the network is HMMR, encoding a centrosome subunit, for which we demonstrate previously unknown functional associations with the breast cancer-associated gene BRCA1. Two case-control studies of incident breast cancer indicate that the HMMR locus is associated with higher risk of breast cancer in humans. Our network modeling strategy should be useful for the discovery of additional cancer-associated genes.
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88
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Schaetzlein S, Kodandaramireddy N, Ju Z, Lechel A, Stepzynska A, Lilli DR, Clark AB, Rudolph C, Wei K, Schlegelberger B, Schirmacher P, Kunkel TA, Greenberg RA, Edelmann W, Rudolph KL. Exonuclease-1 deletion impairs DNA damage signaling and prolongs lifespan of telomere-dysfunctional mice. Cell 2007; 130:863-77. [PMID: 17803909 PMCID: PMC2658812 DOI: 10.1016/j.cell.2007.08.029] [Citation(s) in RCA: 123] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2006] [Revised: 05/17/2007] [Accepted: 08/20/2007] [Indexed: 12/26/2022]
Abstract
Exonuclease-1 (EXO1) mediates checkpoint induction in response to telomere dysfunction in yeast, but it is unknown whether EXO1 has similar functions in mammalian cells. Here we show that deletion of the nuclease domain of Exo1 reduces accumulation of DNA damage and DNA damage signal induction in telomere-dysfunctional mice. Exo1 deletion improved organ maintenance and lifespan of telomere-dysfunctional mice but did not increase chromosomal instability or cancer formation. Deletion of Exo1 also ameliorated the induction of DNA damage checkpoints in response to gamma-irradiation and conferred cellular resistance to 6-thioguanine-induced DNA damage. Exo1 deletion impaired upstream induction of DNA damage responses by reducing ssDNA formation and the recruitment of Replication Protein A (RPA) and ATR at DNA breaks. Together, these studies provide evidence that EXO1 contributes to DNA damage signal induction in mammalian cells, and deletion of Exo1 can prolong survival in the context of telomere dysfunction.
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89
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Sobhian B, Shao G, Lilli DR, Culhane AC, Moreau LA, Xia B, Livingston DM, Greenberg RA. RAP80 targets BRCA1 to specific ubiquitin structures at DNA damage sites. Science 2007; 316:1198-202. [PMID: 17525341 PMCID: PMC2706583 DOI: 10.1126/science.1139516] [Citation(s) in RCA: 535] [Impact Index Per Article: 31.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Mutations affecting the BRCT domains of the breast cancer-associated tumor suppressor BRCA1 disrupt the recruitment of this protein to DNA double-strand breaks (DSBs). The molecular structures at DSBs recognized by BRCA1 are presently unknown. We report the interaction of the BRCA1 BRCT domain with RAP80, a ubiquitin-binding protein. RAP80 targets a complex containing the BRCA1-BARD1 (BRCA1-associated ring domain protein 1) E3 ligase and the deubiquitinating enzyme (DUB) BRCC36 to MDC1-gammaH2AX-dependent lysine(6)- and lysine(63)-linked ubiquitin polymers at DSBs. These events are required for cell cycle checkpoint and repair responses to ionizing radiation, implicating ubiquitin chain recognition and turnover in the BRCA1-mediated repair of DSBs.
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90
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91
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Greenberg RA, Sobhian B, Pathania S, Cantor SB, Nakatani Y, Livingston DM. Multifactorial contributions to an acute DNA damage response by BRCA1/BARD1-containing complexes. Genes Dev 2006; 20:34-46. [PMID: 16391231 PMCID: PMC1356099 DOI: 10.1101/gad.1381306] [Citation(s) in RCA: 248] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2005] [Accepted: 11/02/2005] [Indexed: 01/28/2023]
Abstract
The BRCA1 gene product and its stoichiometric binding partner, BARD1, play a vital role in the cellular response to DNA damage. However, how they acquire specific biochemical functions after DNA damage is poorly understood. Following exposure to genotoxic stress, DNA damage-specific interactions were observed between BRCA1/BARD1 and the DNA damage-response proteins, TopBP1 and Mre11/Rad50/NBS1. Two distinct DNA damage-dependent super complexes emerged; their activation was dependent, in part, on the actions of specific checkpoint kinases, and each super complex contributed to a distinctive aspect of the DNA damage response. The results support a new, multifactorial model that describes how genotoxic stress enables BRCA1 to execute a diverse set of DNA damage-response functions.
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92
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Abstract
Eukaryotic chromosomes terminate in specialized nucleic acid-protein complexes known as telomeres. Disruption of telomere structure by erosion of telomeric DNA or loss of telomere binding protein function activates a signal transduction program that closely resembles the cellular responses generated upon DNA damage. Telomere dysfunction in turn induces a permanent proliferation arrest known as senescence. Senescence is postulated to perform a tumor suppressor function by limiting cellular proliferative capacity, thus imposing a barrier to cellular immortalization. Genetic or epigenetic silencing of components of the DNA damage pathway, allows cells to proliferate beyond senescence limits. However, these cells eventually reach a stage of extreme telomere dysfunction known as crisis that is characterized by cell death and the concomitant appearance of cytogenetic abnormalities. Telomeric crisis produces significant chromosomal instability, a hallmark of human cancer, and may thus be relevant to carcinogenesis by increasing the occurrence of genetic alterations that would favor neoplastic transformation. The following review examines the relationship of telomere function during crisis in accelerating chromosomal instability and cancer.
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93
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Greenberg RA, Rudolph KL. Telomere structural dynamics in genome integrity control and carcinogenesis. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2005; 570:311-341. [PMID: 18727506 DOI: 10.1007/1-4020-3764-3_11] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
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94
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Satyanarayana A, Greenberg RA, Schaetzlein S, Buer J, Masutomi K, Hahn WC, Zimmermann S, Martens U, Manns MP, Rudolph KL. Mitogen stimulation cooperates with telomere shortening to activate DNA damage responses and senescence signaling. Mol Cell Biol 2004; 24:5459-74. [PMID: 15169907 PMCID: PMC419883 DOI: 10.1128/mcb.24.12.5459-5474.2004] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Replicative senescence is induced by critical telomere shortening and limits the proliferation of primary cells to a finite number of divisions. To characterize the activity status of the replicative senescence program in the context of cell cycle activity, we analyzed the senescence phenotypes and signaling pathways in quiescent and growth-stimulated primary human fibroblasts in vitro and liver cells in vivo. This study shows that replicative senescence signaling operates at a low level in cells with shortened telomeres but becomes fully activated when cells are stimulated to enter the cell cycle. This study also shows that the dysfunctional telomeres and nontelomeric DNA lesions in senescent cells do not elicit a DNA damage signal unless the cells are induced to enter the cell cycle by mitogen stimulation. The amplification of senescence signaling and DNA damage responses by mitogen stimulation in cells with shortened telomeres is mediated in part through the MEK/mitogen-activated protein kinase pathway. These findings have implications for the further understanding of replicative senescence and analysis of its role in vivo.
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95
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Ganesan S, Silver DP, Greenberg RA, Avni D, Drapkin R, Miron A, Mok SC, Randrianarison V, Brodie S, Salstrom J, Rasmussen TP, Klimke A, Marrese C, Marahrens Y, Deng CX, Feunteun J, Livingston DM. BRCA1 supports XIST RNA concentration on the inactive X chromosome. Cell 2002; 111:393-405. [PMID: 12419249 DOI: 10.1016/s0092-8674(02)01052-8] [Citation(s) in RCA: 229] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
BRCA1, a breast and ovarian tumor suppressor, colocalizes with markers of the inactive X chromosome (Xi) on Xi in female somatic cells and associates with XIST RNA, as detected by chromatin immunoprecipitation. Breast and ovarian carcinoma cells lacking BRCA1 show evidence of defects in Xi chromatin structure. Reconstitution of BRCA1-deficient cells with wt BRCA1 led to the appearance of focal XIST RNA staining without altering XIST abundance. Inhibiting BRCA1 synthesis in a suitable reporter line led to increased expression of an otherwise silenced Xi-located GFP transgene. These observations suggest that loss of BRCA1 in female cells may lead to Xi perturbation and destabilization of its silenced state.
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96
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Artandi SE, Alson S, Tietze MK, Sharpless NE, Ye S, Greenberg RA, Castrillon DH, Horner JW, Weiler SR, Carrasco RD, DePinho RA. Constitutive telomerase expression promotes mammary carcinomas in aging mice. Proc Natl Acad Sci U S A 2002; 99:8191-6. [PMID: 12034875 PMCID: PMC123043 DOI: 10.1073/pnas.112515399] [Citation(s) in RCA: 249] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
Telomerase is up-regulated in the vast majority of human cancers and serves to halt the progressive telomere shortening that ultimately blocks would-be cancer cells from achieving a full malignant phenotype. In contrast to humans, the laboratory mouse possesses long telomeres and, even in early generation telomerase-deficient mice, the level of telomere reserve is sufficient to avert telomere-based checkpoint responses and to permit full malignant progression. These features in the mouse provide an opportunity to determine whether enforced high-level telomerase activity can serve functions that extend beyond its ability to sustain telomere length and function. Here, we report the generation and characterization of transgenic mice that express the catalytic subunit of telomerase (mTERT) at high levels in a broad variety of tissues. Expression of mTERT conferred increased telomerase enzymatic activity in several tissues, including mammary gland, splenocytes, and cultured mouse embryonic fibroblasts. In mouse embryonic fibroblasts, mTERT overexpression extended telomere lengths but did not prevent culture-induced replicative arrest, thus reinforcing the view that this phenomenon is not related to occult telomere shortening. Robust telomerase activity, however, was associated with the spontaneous development of mammary intraepithelial neoplasia and invasive mammary carcinomas in a significant proportion of aged females. These data indicate that enforced mTERT expression can promote the development of spontaneous cancers even in the setting of ample telomere reserve.
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97
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Abstract
BACKGROUND Diabetes mellitus is estimated to have a worldwide prevalence of 4.6% and afflict 200 million people. The prevalence is accelerating rapidly and the disease has reached epidemic proportions. While type 1 diabetes usually has a dramatic clinical onset, almost half of all those individuals with type 2 diabetes have not been diagnosed. This observation, coupled with the presence of complications at diagnosis and the significant reduction of microvascular complications obtained with tight glycemic control, has led to the recommendation that screening for diabetes be instituted. The American Diabetes Association recommends that adults aged 45 years or more should be evaluated for diabetes by measuring fasting plasma glucose concentrations. CONCLUSIONS The rationale for screening, evidence that tight glycemic control reduces complications, and the possible roles of Hb A(1c) and autoantibodies in screening strategies are addressed in this review.
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98
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Lee KH, Rudolph KL, Ju YJ, Greenberg RA, Cannizzaro L, Chin L, Weiler SR, DePinho RA. Telomere dysfunction alters the chemotherapeutic profile of transformed cells. Proc Natl Acad Sci U S A 2001; 98:3381-6. [PMID: 11248087 PMCID: PMC30662 DOI: 10.1073/pnas.051629198] [Citation(s) in RCA: 86] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Telomerase inhibition has been touted as a novel cancer-selective therapeutic goal based on the observation of high telomerase levels in most cancers and the importance of telomere maintenance in long-term cellular growth and survival. Here, the impact of telomere dysfunction on chemotherapeutic responses was assessed in normal and neoplastic cells derived from telomerase RNA null (mTERC(-/-)) mice. Telomere dysfunction, rather than telomerase per se, was found to be the principal determinant governing chemosensitivity specifically to agents that induced double-stranded DNA breaks (DSB). Enhanced chemosensitivity in telomere dysfunctional cells was linked to therapy-induced fragmentation and multichromosomal fusions, whereas telomerase reconstitution restored genomic integrity and chemoresistance. Loss of p53 function muted the cytotoxic effects of DSB-inducing agents in cells with telomere dysfunction. Together, these results point to the combined use of DSB-inducing agents and telomere maintenance inhibition as an effective anticancer therapeutic approach particularly in cells with intact p53-dependent checkpoint responses.
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99
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Greenberg RA, Chin L, Femino A, Lee KH, Gottlieb GJ, Singer RH, Greider CW, DePinho RA. Short dysfunctional telomeres impair tumorigenesis in the INK4a(delta2/3) cancer-prone mouse. Cell 1999; 97:515-25. [PMID: 10338215 DOI: 10.1016/s0092-8674(00)80761-8] [Citation(s) in RCA: 268] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
Maintenance of telomere length is predicted to be essential for bypass of senescence and crisis checkpoints in cancer cells. The impact of telomere dysfunction on tumorigenesis was assessed in successive generations of mice doubly null for the telomerase RNA (mTR) and the INK4a tumor suppressor genes. Significant reductions in tumor formation in vivo and oncogenic potential in vitro were observed in late generations of telomerase deficiency, coincident with severe telomere shortening and associated dysfunction. Reintroduction of mTR into cells significantly restored the oncogenic potential, indicating telomerase activation is a cooperating event in the malignant transformation of cells containing critically short telomeres. The results described here demonstrate that loss of telomere function in a cancer-prone mouse model possessing intact DNA damage responses impairs, but does not prevent, tumor formation.
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100
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Greenberg RA, O'Hagan RC, Deng H, Xiao Q, Hann SR, Adams RR, Lichtsteiner S, Chin L, Morin GB, DePinho RA. Telomerase reverse transcriptase gene is a direct target of c-Myc but is not functionally equivalent in cellular transformation. Oncogene 1999; 18:1219-26. [PMID: 10022128 DOI: 10.1038/sj.onc.1202669] [Citation(s) in RCA: 295] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The telomerase reverse transcriptase component (TERT) is not expressed in most primary somatic human cells and tissues, but is upregulated in the majority of immortalized cell lines and tumors. Here, we identify the c-Myc transcription factor as a direct mediator of telomerase activation in primary human fibroblasts through its ability to specifically induce TERT gene expression. Through the use of a hormone inducible form of c-Myc (c-Myc-ER), we demonstrate that Myc-induced activation of the hTERT promoter requires an evolutionarily conserved E-box and that c-Myc-ER-induced accumulation of hTERT mRNA takes place in the absence of de novo protein synthesis. These findings demonstrate that the TERT gene is a direct transcriptional target of c-Myc. Since telomerase activation frequently correlates with immortalization and telomerase functions to stabilize telomers in cycling cells, we tested whether Myc-induced activation of TERT gene expression represents an important mechanism through which c-Myc acts to immortalize cells. Employing the rat embryo fibroblast cooperation assay, we show that TERT is unable to substitute for c-Myc in the transformation of primary rodent fibroblasts, suggesting that the transforming activities of Myc extend beyond its ability to activate TERT gene expression and hence telomerase activity.
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