Human LINE-1 retrotransposon induces DNA damage and apoptosis in cancer cells
© Belgnaoui et al; licensee BioMed Central Ltd. 2006
Received: 01 March 2006
Accepted: 02 May 2006
Published: 02 May 2006
Long interspersed nuclear elements (LINEs), Alu and endogenous retroviruses (ERVs) make up some 45% of human DNA. LINE-1 also called L1, is the most common family of non-LTR retrotransposons in the human genome and comprises about 17% of the genome. L1 elements require the integration into chromosomal target sites using L1-encoded endonuclease which creates staggering DNA breaks allowing the newly transposed L1 copies to integrate into the genome. L1 expression and retrotransposition in cancer cells might cause transcriptional deregulation, insertional mutations, DNA breaks, and an increased frequency of recombinations, contributing to genome instability. There is however little evidence on the mechanism of L1-induced genetic instability and its impact on cancer cell growth and proliferation.
We report that L1 has genome-destabilizing effects indicated by an accumulation of γ-H2AX foci, an early response to DNA strand breaks, in association with an abnormal cell cycle progression through a G2/M accumulation and an induction of apoptosis in breast cancer cells. In addition, we found that adjuvant L1 activation may lead to supra-additive killing when combined with radiation by enhancing the radiation lethality through induction of apoptosis that we have detected through Bax activation.
L1 retrotransposition is sensed as a DNA damaging event through the creation DNA breaks involving L1-encoded endonuclease. The apparent synergistic interaction between L1 activation and radiation can further be utilized for targeted induction of cancer cell death. Thus, the role of retrotransoposons in general, and of L1 in particular, in DNA damage and repair assumes larger significance both for the understanding of mutagenicity and, potentially, for the control of cell proliferation and apoptosis.
Retrotransposons are mobile retroelements that utilize reverse transcriptase and RNA intermediates to relocate within the cellular genome. Retrotransposons are subdivided into two subclasses: LTR-(long terminal repeats) and non-LTR-retrotransposons. LINE-1 (Long Interspersed Nuclear Element type 1, or L1), is the most common family of non-LTR retrotransposons in the human genome; with about 500,000 copies, it comprises about 17% of the genome [1, 2]. Only a fraction of L1 elements in the human genome are intact: most are truncated (usually at the 5' -end) and mutated (often at multiple sites). However, there are still about 80–100 retrotransposition-competent L1 elements (RC-L1s) in the genome. Most RC-L1 sequences are evidently silenced by methylation  and, possibly, also by the RNA interference pathway . Genomic demethylation after deleting DNA methyltransferase 1 can trigger L1 elements to become mobilized .
L1 elements encode proteins necessary for their own mobilization. L1 encodes a 40 kDa (p40) protein (ORF1p) with RNA-binding activity , and ORF2p produces a 150 kDa protein with endonuclease  and reverse transcriptase [8, 9] activities. L1 integrates into the genome by target-primed reverse transcription (TPRT) using the free 3'-OH at the endonuclease cut site on the genomic DNA as a primer and the L1 RNA as a template . ORF1p and ORF2p preferentially associate with their encoding transcript to form a ribonucleoprotein particle (RNP), which is a proposed retrotransposition intermediate.
The retrotransposition of L1 elements requires the integration into chromosomal target sites using L1-encoded endonuclease . L1 endonuclease creates staggering DNA breaks allowing the newly transposed L1 copies to integrate into the genome. Despite the small number of RC-L1s, and the constraints placed upon their movement by cis-preference , characterization of retrotransposition events using tagged RC-L1 clones in cultured cells indicate that about 10% of L1 insertions are accompanied by large chromosomal rearrangements, suggesting that active L1s could also lead to genomic instability [12, 13].
While the properties of L1-encoded enzymes have been studied extensively in vitro , the biological impact of retroelements on normal and cancer cells requires clarification and has been difficult to assess. We propose to test the ability of RC-L1 to induce targeted DNA strand breaks as a mechanism for inducing apoptosis in human cancer cells. Although several reports exist that L1 induces genomic instability, a precise mechanism of action and especially its impact on cell growth is still generally lacking. It is essential that a clear mechanistic model needs to be established to provide a clear understanding of how human L1 retrotransposition is sensed as a DNA damaging event. Here we report that L1 has genome-destabilizing effects indicated by an accumulation of γ-H2AX foci, an early response to DNA strand breaks, in association with induction of apoptosis in breast cancer cells.
RC-L1 expression and retrotransposition assay
RC-L1 ORFs are expressed in MCF-7 cells
RC-L1 expressing cells exhibit an abnormal cell cycle progression and a DNA damage recognition response
An early response to DSBs is phosphorylation of H2AX, a variant form of the histone H2AX. Phosphorylated H2AX, termed γ-H2AX, can be observed over several megabases flanking the DSB . The role of H2AX and the proteins that accumulate at the site of DSBs is promoting survival of the cells . The presence of γ-H2AX provides the platform for other damage proteins such as 53BP1, Mre11 and Brac1 to localize to the break site . To determine whether RC-L1 expression and retrotransposition is sensed as a DNA damaging event, we checked for the activation and accumulation of γ-H2AX in discrete sites known as DNA damage repair foci. The histone γ-H2AX is rapidly phosphorylated at the sites of DNA double-strand breaks (DSBs) . Interestingly, γ-H2AX is activated and appears as discrete nuclear foci in RC-L1-expressing cells, suggesting that integration had induced such breaks in DNA (Fig. 3D–F).
Induction of apoptosis in RC-L1-expressing cells
RC-L1 retrotransposition in p53 mutant cells
We then checked for RC-L1 ability to activate H2AX in these cells. We did not detect any significant activation of H2AX after RC-L1 expression and retrotransposition in comparison with the level of H2AX activation in MCF-7 cells expressing RC-L1 (figure 5, panel C).
Since T47D cells do not seem to allow similar RC-L1 retrotransposition level as MCF-7 cells with wild type p53, we then postulated that simultaneously inducing DSBs via γ-irradiation and L1 activation will possibly swamp the capacity of DNA repair pathways to process Radiation Induced DSBs (RIDSBs). We further postulated that RIDSBs and L1-induced DSBs will competitively sequester γH2AX (and the associated repair protein complexes), resulting in sub-optimal levels of the "repairsome" at each loci; which will delay the processing of the DSBs. In the case of RIDSBs this increased persistence may result in an increased conversion to chromosomal aberrations or may trigger apoptotic cell death. Thus the co-administration of RC-L1 may be a powerful adjuvant to radiation in tumors that are inherently resistant to radiation. Alternatively, the additional sequestration of γ-H2AX/NHEJ complex by RIDSBs may serve to enhance the L1-mediated signal for apoptosis. Our results showed that indeed RC-L1 enhances the radiation-induced apoptosis as measured by immunoblotting using anti-Bax antibody (figure 5, panel D).
Insertion of an L1 copy into the genome necessitates the creation and repair of broken DNA. After L1 integration, the DNA ends are sealed and filled in, forming the target site duplications that flank a typical L1 insertion. Reactivation of L1 retrotransposition may interfere with potential *symbiotic' effects of L1 sequences such as their contribution to the global and local organization of the genome and the provision of gene regulatory sequences. Increased L1 retrotransposition may instead have a deleterious effect on the cell. It is widely presumed that L1 integration is random, therefore, increasing its mobility will most likely have neutral or negative consequences for the host cell. Even simply upregulating the L1 endonuclease in the absence of successful integration could be toxic to the cell by promoting the formation of additional DSBs, fostering chromosomal rearrangements and translocations. Furthermore, following DNA damage, cells initiate a repair response, which depends upon the close coordination of cell cycle checkpoints and activated DNA repair . If the repair does not occur in a timely fashion or if the damage is massive, cell death by mechanisms involving apoptosis can occur [23, 24].
L1-encoded endonuclease creates staggered DNA breaks, which enables newly-transposed L1 copies to integrate into the genome . The outcome of single-strand breaks introduced by the endonuclease in a cell depends on several factors. A first factor is the cell cycle phase. Nicks in S-phase are most problematic, because they can be converted into double-strand breaks by the replication complex. A second factor is the DNA repair competency and capacity of the cell which may differ between normal and cancer cells. Thirdly, the presence of L1 RNA and other proteins at the nicked site may influence the type and efficiency of repair.
In a recent study, Goodier et al., have mapped a functional nucleolar localization signal in L1 ORF2. They showed that L1 ORF1 is localized in the cytoplasm with a speckled pattern and colocalized with ORF2 in nucleoli in a subset of cells . However, although wild-type ORF2 expression was repeatedly observed, detectable levels remained prohibitively low. One cause of poor detection could be cell toxicity induced by nicking of genomic DNA by the endonuclease . Similarly, early events in retroviral replication include entry of the viral capsid with the accompanying enzymes reverse transcriptase and integrase (IN) followed by synthesis of a DNA copy of the viral RNA genome to form a preintegration complex. This complex then enters the nucleus, and integration is first detected at approximately 3–4 h postinfection . Retroviral integration is catalyzed by integrase acting on specific sequences at the ends of the viral DNA and via a concerted cleavage-ligation reaction that is mechanistically similar to that catalyzed by RAG proteins during V(D)J recombination [26, 27]. As a consequence of integrase-mediated joining, the host cell DNA suffers a DSB, but the ends are held together by single strand links to viral DNA. Postintegration repair of this intermediate is essential for the maintenance of host DNA integrity as well as the stable association of retroviral DNA with host chromosomes. Numerous lines of evidence [28–30] indicate that retroviral DNA elicits a DNA damage response and that the integration intermediate is repaired primarily viacomponents of the non-homologous end-joining (NHEJ) pathway. It is noteworthy that Daniel et al.,  provided direct confirmation that cultured cells respond to retroviral DNA integration in the same way that they respond to DSBs produced by a variety of genotoxic agents or normal programmed events, namely, by massive phosphorylation of histone H2AX in the vicinity of the damage site. The second finding is that H2AX appears to be dispensable for postintegration repair. These observations lend independent support to a model in which the anchoring of broken DNA ends to facilitate their repair is a critical function of γ-H2AX . Severe DNA damage can result in cell cycle arrest and apoptosis . Both cell cycle arrest and apoptosis have been seen to accompany retrotransposition in severely stressed cells .
This is the first demonstration that human L1 retrotransposition induces DNA damage, as indicated by γ-H2AX accumulation. It also reveals a correlation between L1 expression/retrotransposition and induction of apoptosis. Taken together, the data imply that DNA nicks created by RC-L1 expression and retrotransposition are sensed as a DNA damaging event, which leads to apoptosis in cancer cells. Obviously further studies are needed to test whether additional components in the DNA damage recognition response, in particular NMR complex (Nbs1, MRE11, Rad50) and/or ATM are involved in signaling RC-L1 retrotransposition effects on these cells. In addition, while we realize that it is most likely the impact of the "active" form of L1, or RC-L1 that induces this DNA damage response, it will be of interest to clarify whether it is because of DNA double strand breaks or other intermediates in the retrotransposition cycle. While this manuscript was under review two other independent reports have been published by two different groups leading to the same conclusion indicating the activation of γ-H2AX in response to L1 expression and retrotransposition through the creation of DNA double strand breaks [34, 35]
Cells, plasmids, and antibodies
MCF-7 and T47D human carcinoma cells were grown in DMEM (Gibco-BRL) supplemented with 10% fetal calf serum and 1% penicillin-streptomycin. L1-EGFP construct was obtained from E. Luning Prak (University of Pennsylvania, PA, USA). The anti-L1 ORF1 rabbit polyclonal antibody was a gift from Gerald Schumann (Paul-Ehrlich-Institut, Langen, Germany). The anti-L1-ORF2 rabbit polyclonal antibody was a gift from John Goodier (University of Pennsylvania, PA, USA). Anti-α-tubulin mouse monoclonal antibody from Sigma. Anti-γ-H2AX rabbit polyclonal antibody from Cell Signaling Technologies and FITC-conjugated secondary antibody from ICN Biomedicals, Inc.
Cell transfection and selection
Cells were seeded in 6-well plates with about 4 × 105 cells/well and grown to 70% confluency in DMEM complete medium. Cells were transfected with the L1-EGFP construct using Lipofectamin 2000 transfection reagent (Invitrogen) following the manufacturer's protocol. Each transfection well received 2 μg plasmid DNA, 6 μl transfection reagent and 2 ml DMEM complete medium. Antibiotic selection was begun 24 h after transfection. Puromycin-resistant cells (purR) were selected by growth in DMEM complete medium containing 10 μg/ml puromycin.
Isolation of genomic DNA and PCR analysis
Genomic DNA was isolated using Qiagen Blood & Cell Culture DNA Mini Kit Kit following the manufacturer's protocol. The oligonucleotides used for PCR were GFP968F (5' GCACCATCTTCTTCAAGGACGAC-3') and GFP1013R (5'-TCTTTGCTCAGGGCGGACTG-3'). Amplifications were performed in 50 μl containing 1.25 U AmpliTaq Gold polymerase (Roche), 2.5 mM MgCl2, 1 × GeneAmp PCR Gold buffer (Roche), 0.2 mM each dNTP, 200 ng of each oligonucleotide primer and ~500 ng genomic DNA or 70 ng plasmid DNA template. After an initial step at 94°C (15 min), 35 cycles of amplification were performed (30 s at 94°C, 30 s at 59°C, 2 min at 72°C), followed by a final step at 72°C (10 min).
This procedure was performed as previously described . Primary antibodies used were: anti-L1 ORF1 rabbit polyclonal (1:500 dilution), anti-L1 ORF2 rabbit polyclonal (1:200 dilution), anti-γ-H2AX rabbit polyclonal (1:1000 dilution), or anti-γ-tubulin mouse monoclonal (1:1000). HRP-conjugated secondary antibodies (Bio-Rad) were used. The detection was performed using the Immun-Star HRP Detection kit (Bio-Rad).
This procedure was performed as described in Haoudi et al . Primary antibodies used were: anti-L1 ORF1 rabbit polyclonal (1:50 dilution), anti-L1 ORF2 rabbit polyclonal (1:100 dilution), anti-γ-H2AX rabbit polyclonal (1:500 dilution). The secondary antibody Alex-Fluor anti-rabbit (Molecular Probes) was used. The cells were viewed using a Zeiss LSM510 confocal microscope outfitted with Metamorph software. For quantitative analysis, foci were counted by eye during the imaging process using a 63 objective. In a single experiment, cell counting was performed until at least 40 cells and 40 foci were registered/sample. For data points that were derived from a single experiment, the error bars represent the SE from the analysis of the number of cells analyzed. For data points that were derived from more than one experiment, the error bars represent either the SE from the number of cells analyzed in the single experiments or the SE between the different experiments (whichever is highest).
This procedure was performed as previously described . To analyze the cell cycle profile, MCF-7 cells were either mock transfected or transfected with GFP or RC-L1. Forty eight hours later, RC-L1 transfected cells were subjected to puromycin selection, then DNA flow analysis was conducted on a BD Biosciences FACScan and analyzed with MODFIT software.
Caspase 3 assay
Caspase 3 assay was performed using BD ApoAlert Caspase colorimetric assay kit following the manufacturer's recommendations.
T47D breast cancer cells were transfected with L1 plasmid as described above. Cells were then irradiated with 5 Gy using a 137Cesium source. Cells were returned to culture then selected for puromycin resistance. Whole cell lysates were then collected and subjected to immunoblotting using anti-Bax antibody, as described above.
long interspersed nuclear element 1
long terminal repeats
open reading frame
target-primed reverse transcription
polymerase chain reaction
Dulbecco's modified Eagle's Medium
phosphate buffered saline
fluorescence-activated cell sorting
Enhanced green fluorescence protein
double strand breaks
We are very grateful to Nina Luning Prak (University of Pennsylvania, School of Medicine, PA) for providing L1-EGFP construct. Anti-L1 ORF1 and anti-L1-ORF2 antibodies were generous gifts from Gerald Schumann (Paul-Ehrlich-Institut, Langen Germany) and John Goodier (University of Pennsylvania, School of Medicine, PA). We thank Julie Kerry and Edward Johnson for critical reading of the manuscript. This work was supported by the Elsa U. Pardee Foundation grant to AH.
- Lander ES, Linton LM, Birren B, Nusbaum C, Zody MC, Baldwin J, Devon K, Dewar K, Doyle M, FitzHugh W, Funke R, Gage D, Harris K, Heaford A, Howland J, Kann L, Lehoczky J, LeVine R, McEwan P, McKernan K, Meldrim J, Mesirov JP, Miranda C, Morris W, Naylor J, Raymond C, Rosetti M, Santos R, Sheridan A, Sougnez C, Stange-Thomann N, Stojanovic N, Subramanian A, Wyman D, Rogers J, Sulston J, Ainscough R, Beck S, Bentley D, Burton J, Clee C, Carter N, Coulson A, Deadman R, Deloukas P, Dunham A, Dunham I, Durbin R, French L, Grafham D, Gregory S, Hubbard T, Humphray S, Hunt A, Jones M, Lloyd C, McMurray A, Matthews L, Mercer S, Milne S, Mullikin JC, Mungall A, Plumb R, Ross M, Shownkeen R, Sims S, Waterston RH, Wilson RK, Hillier LW, McPherson JD, Marra MA, Mardis ER, Fulton LA, Chinwalla AT, Pepin KH, Gish WR, Chissoe SL, Wendl MC, Delehaunty KD, Miner TL, Delehaunty A, Kramer JB, Cook LL, Fulton RS, Johnson DL, Minx PJ, Clifton SW, Hawkins T, Branscomb E, Predki P, Richardson P, Wenning S, Slezak T, Doggett N, Cheng JF, Olsen A, Lucas S, Elkin C, Uberbacher E, Frazier M, Gibbs RA, Muzny DM, Scherer SE, Bouck JB, Sodergren EJ, Worley KC, Rives CM, Gorrell JH, Metzker ML, Naylor SL, Kucherlapati RS, Nelson DL, Weinstock GM, Sakaki Y, Fujiyama A, Hattori M, Yada T, Toyoda A, Itoh T, Kawagoe C, Watanabe H, Totoki Y, Taylor T, Weissenbach J, Heilig R, Saurin W, Artiguenave F, Brottier P, Bruls T, Pelletier E, Robert C, Wincker P, Smith DR, Doucette-Stamm L, Rubenfield M, Weinstock K, Lee HM, Dubois J, Rosenthal A, Platzer M, Nyakatura G, Taudien S, Rump A, Yang H, Yu J, Wang J, Huang G, Gu J, Hood L, Rowen L, Madan A, Qin S, Davis RW, Federspiel NA, Abola AP, Proctor MJ, Myers RM, Schmutz J, Dickson M, Grimwood J, Cox DR, Olson MV, Kaul R, Raymond C, Shimizu N, Kawasaki K, Minoshima S, Evans GA, Athanasiou M, Schultz R, Roe BA, Chen F, Pan H, Ramser J, Lehrach H, Reinhardt R, McCombie WR, de la Bastide M, Dedhia N, Blocker H, Hornischer K, Nordsiek G, Agarwala R, Aravind L, Bailey JA, Bateman A, Batzoglou S, Birney E, Bork P, Brown DG, Burge CB, Cerutti L, Chen HC, Church D, Clamp M, Copley RR, Doerks T, Eddy SR, Eichler EE, Furey TS, Galagan J, Gilbert JG, Harmon C, Hayashizaki Y, Haussler D, Hermjakob H, Hokamp K, Jang W, Johnson LS, Jones TA, Kasif S, Kaspryzk A, Kennedy S, Kent WJ, Kitts P, Koonin EV, Korf I, Kulp D, Lancet D, Lowe TM, McLysaght A, Mikkelsen T, Moran JV, Mulder N, Pollara VJ, Ponting CP, Schuler G, Schultz J, Slater G, Smit AF, Stupka E, Szustakowski J, Thierry-Mieg D, Thierry-Mieg J, Wagner L, Wallis J, Wheeler R, Williams A, Wolf YI, Wolfe KH, Yang SP, Yeh RF, Collins F, Guyer MS, Peterson J, Felsenfeld A, Wetterstrand KA, Patrinos A, Morgan MJ, de Jong P, Catanese JJ, Osoegawa K, Shizuya H, Choi S, Chen YJL: International Human Genome Sequencing Consortium. Initial sequencing and analysis of the human genome. Nature. 2001, 409: 860-921. 10.1038/35057062.View ArticlePubMedGoogle Scholar
- Kazazian HH: Mobile elements: drivers of genome evolution. Science. 2004, 303: 1626-1632. 10.1126/science.1089670.View ArticlePubMedGoogle Scholar
- Hata K, Sakaki Y: Identification of critical CpG sites for repression of L1 transcription by DNA methylation. Gene. 1997, 189: 227-234. 10.1016/S0378-1119(96)00856-6.View ArticlePubMedGoogle Scholar
- Soifer HS, Zaragoza A, Peyvan M, Behlke MA, Rossi JJ: A potential role for RNA interference in controlling the activity of the human LINE-1 retrotransposon. Nucleic Acids Res. 2005, 33: 846-856. 10.1093/nar/gki223.PubMed CentralView ArticlePubMedGoogle Scholar
- Jackson-Grusby L, Beard C, Possemato R, Tudor M, Fambrough D, Csankovszki G, Dausman J, Lee P, Wilson C, Lander E, Jaenisch R: Loss of genomic methylation causes p53-dependent apoptosis and epigenetic deregulation. Nat Genet. 2001, 27: 31-39. 10.1038/83730.View ArticlePubMedGoogle Scholar
- Hohjoh H, Singer MF: Sequence-specific single-strand RNA binding protein encoded by the human LINE-1 retrotransposon. EMBO J. 1997, 16: 6034-6043. 10.1093/emboj/16.19.6034.PubMed CentralView ArticlePubMedGoogle Scholar
- Feng Q, Moran JV, Kazazian HH, Boeke JD: Human L1 retrotransposon encodes a conserved endonuclease required for retrotransposition. Cell. 1996, 87: 905-916. 10.1016/S0092-8674(00)81997-2.View ArticlePubMedGoogle Scholar
- Mathias SL, Scott AF, Kazazian HH, Boeke JD, Gabriel A: Reverse transcriptase encoded by a human transposable element. Science. 1991, 254: 1808-1810.View ArticlePubMedGoogle Scholar
- Goodier JL, Ostertag EM, Engleka KA, Seleme MC, Kazazian HH: A potential role for the nucleolus in L1 retrotransposition. Hum Mol Genet. 2004, 13: 1041-1048. 10.1093/hmg/ddh118.View ArticlePubMedGoogle Scholar
- Luan DD, Korman MH, Jakubczak JL, Eickbush TH: Reverse transcription of R2Bm RNA is primed by a nick at the chromosomal target site: a mechanism for non-LTR retrotransposition. Cell. 1993, 72: 595-605. 10.1016/0092-8674(93)90078-5.View ArticlePubMedGoogle Scholar
- Wei W, Gilbert N, Ooi SL, Lawler JF, Ostertag EM, Kazazian HH, Boeke JD, Moran JV: Human L1 retrotransposition: cis preference versus trans complementation. Mol Cell Biol. 2001, 21: 1429-1439. 10.1128/MCB.21.4.1429-1439.2001.PubMed CentralView ArticlePubMedGoogle Scholar
- Symer DE, Connelly C, Szak ST, Caputo EM, Cost GJ, Parmigiani G, Boeke JD: Human l1 retrotransposition is associated with genetic instability in vivo. Cell. 2002, 110: 327-338. 10.1016/S0092-8674(02)00839-5.View ArticlePubMedGoogle Scholar
- Gilbert N, Lutz-Prigge S, Moran JV: Genomic deletions created upon LINE-1 retrotransposition. Cell. 2002, 110: 315-325. 10.1016/S0092-8674(02)00828-0.View ArticlePubMedGoogle Scholar
- Haoudi A, Semmes OJ, Mason JM, Cannon RE: Retrotransposition-Competent Human LINE-1 Induces Apoptosis in Cancer Cells With Intact p53. J Biomed Biotechnol. 2004, 4: 185-194. 10.1155/S1110724304403131.View ArticleGoogle Scholar
- Haoudi A, Daniels EC, Wong E, Kupfer G, Semmes OJ: Human T-cell leukemia virus-I tax oncoprotein functionally targets a subnuclear complex involved in cellular DNA damage-response. J Biol Chem. 2003, 278: 37736-37744. 10.1074/jbc.M301649200.View ArticlePubMedGoogle Scholar
- Moran JV, Holmes SE, Naas TP, DeBerardinis RJ, Boeke JD, Kazazian HH: High frequency retrotransposition in cultured mammalian cells. Cell. 1996, 87: 917-927. 10.1016/S0092-8674(00)81998-4.View ArticlePubMedGoogle Scholar
- Ostertag EM, Prak ET, DeBerardinis RJ, Moran JV, Kazazian HH: Determination of L1 retrotransposition kinetics in cultured cells. Nucleic Acids Res. 2000, 28: 1418-1423. 10.1093/nar/28.6.1418.PubMed CentralView ArticlePubMedGoogle Scholar
- Prak ET, Dodson AW, Farkash EA, Kazazian HH: Tracking an embryonic L1 retrotransposition event. Proc Natl Acad Sci U S A. 2003, 100: 1832-1837. 10.1073/pnas.0337627100.PubMed CentralView ArticlePubMedGoogle Scholar
- Rogakou EP, Pilch DR, Orr AH, Ivanova VS, Bonner WM: DNA double-stranded breaks induce histone H2AX phosphorylation on serine 139. J Biol Chem. 1998, 273: 5858-5868. 10.1074/jbc.273.10.5858.View ArticlePubMedGoogle Scholar
- Stiff T, O'Driscoll M, Rief N, Iwabuchi M, Lobrich M, Jeggo PA: ATM and DNA-PK function redundantly to phosphorylate H2AX after exposure to ionizing radiation. Cancer Res. 2004, 64: 2390-2396. 10.1158/0008-5472.CAN-03-3207.View ArticlePubMedGoogle Scholar
- Celeste A, Fernandez-Capetillo O, Kruhlak MJ, Pilch DR, Staudt DW, Lee A, Bonner RF, Bonner WM, Nussenzweig A: Histone H2AX phosphorylation is dispensable for the initial recognition of DNA breaks. Nat Cell Biol. 2003, 5: 675-679. 10.1038/ncb1004.View ArticlePubMedGoogle Scholar
- Kastan MB, Bartek J: Cell-cycle checkpoints and cancer. Nature. 2004, 432: 316-23. 10.1038/nature03097.View ArticlePubMedGoogle Scholar
- Norbury CJ, Zhivotovsky B: DNA damage-induced apoptosis. Oncogene. 2004, 23: 2797-2808. 10.1038/sj.onc.1207532.View ArticlePubMedGoogle Scholar
- Motomaya N, Naka K: DNA damage tumor suppressor genes and genomic instability. Curr Opin Genet Dev. 2004, 14: 11-16. 10.1016/j.gde.2003.12.003.View ArticleGoogle Scholar
- Vandegraaff N, Kumar R, Burrell CJ: Kinetics of human immunodeficiency virus type 1 (HIV) DNA integration in acutely infected cells as determined using a novel assay for detection of integrated HIV DNA. J Virol. 2001, 75: 11253-11260. 10.1128/JVI.75.22.11253-11260.2001.PubMed CentralView ArticlePubMedGoogle Scholar
- Craig NL: V(D)J recombination and transposition: closer than expected. Science. 1996, 271: 1512-View ArticlePubMedGoogle Scholar
- Van Gent DC, Mizuuchi K, Gellert M: Similarities between initiation of V(D)J recombination and retroviral integration. Science. 1996, 271: 1592-1594.View ArticlePubMedGoogle Scholar
- Daniel R, Katz RA, Skalka AM: A role for DNA-PK in retroviral DNA integration. Science. 1999, 284: 644-647. 10.1126/science.284.5414.644.View ArticlePubMedGoogle Scholar
- Daniel R, Katz RA, Merkel G, Hittle JC, Yen TJ, Skalka AM: Wortmannin potentiates integrase-mediated killing of lymphocytes and reduces the efficiency of stable transduction by retroviruses. Mol Cell Biol. 2001, 21: 1164-1172. 10.1128/MCB.21.4.1164-1172.2001.PubMed CentralView ArticlePubMedGoogle Scholar
- Daniel R, Kao G, Taganov K, Greger J, Favorova O, Merkel G, Yen TJ, Katz RA, Skalka AM: Evidence that the retroviral DNA integration process triggers an ATR-dependent DNA damage response. Proc Natl Acad Sci U S A. 2003, 100: 4778-4783. 10.1073/pnas.0730887100.PubMed CentralView ArticlePubMedGoogle Scholar
- Daniel R, Ramcharan J, Rogakou E, Taganov KD, Greger JG, Bonner W, Nussenzweig A, Katz RA, Skalka AM: Histone H2AX is phosphorylated at sites of retroviral DNA integration but is dispensable for postintegration repair. J Biol Chem. 2004, 279: 45810-45814. 10.1074/jbc.M407886200.View ArticlePubMedGoogle Scholar
- Barzilai A, Yamamoto K: DNA damage responses to oxidative stress. DNA Repair (Amst). 2004, 8–9: 1109-1115. 10.1016/j.dnarep.2004.03.002.View ArticleGoogle Scholar
- Staleva Staleva L, Venkov P: Activation of Ty transposition by mutagens. Mutat Res. 2001, 474: 93-103.View ArticlePubMedGoogle Scholar
- Gasior SL, Wakeman TP, Xu B, Deininger PL: The Human LINE-1 Retrotransposon Creates DNA Double-strand Breaks. J Mol Biol. 2006, 357: 1383-1393. 10.1016/j.jmb.2006.01.089.PubMed CentralView ArticlePubMedGoogle Scholar
- Farkash EA, Kao GD, Horman SR, Luning Prak RT: Gamma radiation increases endonuclease-dependent L1 retrotransposition in a cultured cell assay. Nucleic Acids Res. 2006, 34: 1196-1204. 10.1093/nar/gkj522.PubMed CentralView ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.