Cited by 519Chromosomal and Genetic VariationsDNA Repair MechanismsGenomic variations and chromosomal abnormalities

The Human LINE-1 Retrotransposon Creates DNA Double-strand Breaks

Open access
Gasior, Stephen L., Wakeman, Timothy P., Xu, Bo, Deininger, Prescott L.·2006·Journal of Molecular Biology

Abstract

Long interspersed element-1 (L1) is an autonomous retroelement that is active in the human genome. The proposed mechanism of insertion for L1 suggests that cleavage of both strands of genomic DNA is required. We demonstrate that L1 expression leads to a high level of double-strand break (DSB) formation in DNA using immunolocalization of gamma-H2AX foci and the COMET assay. Similar to its role in mediating DSB repair in response to radiation, ATM is required for L1-induced gamma-H2AX foci and for L1 retrotransposition. This is the first characterization of a DNA repair response from expression of a non-long terminal repeat (non-LTR) retrotransposon in mammalian cells as well as the first demonstration that a host DNA repair gene is required for successful integration. Notably, the number of L1-induced DSBs is greater than the predicted numbers of successful insertions, suggesting a significant degree of inefficiency during the integration process. This result suggests that the endonuclease activity of endogenously expressed L1 elements could contribute to DSB formation in germ-line and somatic tissues.

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AI snapshot

Written from the paper’s full text.

TL;DR

L1 expression induces DNA double-strand breaks (DSBs) in mammalian cells, detected by γ-H2AX foci and neutral COMET, and this requires L1 endonuclease activity. ATM is required for L1-induced γ-H2AX foci and for L1 retrotransposition.

Key findings

  • L1.3 transfection in HeLa produced γ-H2AX foci in 60–70% of cells, with 20–200 foci per focus-positive cell (mean 68), similar to 6 Gy ionizing radiation (mean 86); an endonuclease mutant (EN–) essentially abolished foci.
  • Neutral COMET showed pronounced tail moments at 24 h after L1.3, with no tails at 48 h; L1-induced foci were largely repaired by 48 h.
  • L1.3 cotransfection reduced neoR colony yield by 60%; EN– showed no significant toxicity, while RT– showed toxicity similar to wild-type L1.3. L1.3 reduced the mitotic index, whereas empty vector or EN– did not induce checkpoint responses.
  • In ATM+ YZ-5 cells, synL1_optORF1_neo produced 62±6 and 24±10 L1 colonies; in ATM– EBS-7 cells, 0 colonies, while neoR plasmid controls were 207±38, 133±10, 115±12, and 91±4.7. L1-induced DSBs exceed insertions; the paper estimates conservatively tenfold more DSBs than insertions, with 100-fold within reason.

Methods

  • Transient transfections of L1.3, L1 EN–, L1 RT–, ORF2, and synL1_optORF1_neo into HeLa, mouse 3T3, MCF7, and isogenic YZ-5 (ATM+) / EBS-7 (ATM–) cells.
  • DSBs assessed by anti-γ-H2AX immunofluorescence and neutral single-cell gel electrophoresis (COMET); retrotransposition by G418 colony formation; toxicity by cotransfected neoR colony yield; cell-cycle by mitotic index; apoptosis by annexin V FACS/TUNEL.
  • Main comparisons were between wild-type L1 and endonuclease/reverse transcriptase mutants, empty vector, and ATM+ versus ATM– cells; colony counts reported with SD.

Limitations

  • Stated: the mechanism of second-strand cleavage is unresolved; whether ORF2 makes both breaks, uses another enzyme, or homes to existing nicks remains to be determined.
  • Stated: poor understanding of L1 RNA/protein nuclear transport; overexpression may skew the DSB:insertion ratio by titrating repair factors or producing ORF2 activity unrelated to integration; a quantitative assay of L1 integration intermediates without selectable cassettes is needed.
  • Stated/evident: fixed-cell γ-H2AX counts underestimate total breaks (the paper says slightly underestimate expected by about two- to threefold) and do not integrate frequency over time; most experiments use transformed cell lines and overexpressed L1, so relevance to endogenous L1 in normal tissues is not directly established (evident).

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1016/j.jmb.2006.01.089

Cites

88 of 90 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 10 cited sources checked · 0 supported · 0 flagged

  • Findingown data
    § Abstract

    ATM is required for L1 retrotransposition.

    “Similar to its role in mediating DSB repair in response to radiation, ATM is required for L1-induced γ-H2AX foci and for L1 retrotransposition.”
    • Also stated in: § Results · plain
      “As shown in Table 1, synL1_optORF1_neo produced colonies efficiently in ATM+ cells. No colonies were detected in ATM– cells.”
    Quote in the paperSubstantial claim
  • Findingown data
    § Abstract

    ATM is required for L1-induced γ-H2AX foci.

    “Similar to its role in mediating DSB repair in response to radiation, ATM is required for L1-induced γ-H2AX foci and for L1 retrotransposition.”
    • Also stated in: § Results · plain
      “L1.3 expression induced γ-H2AX foci in ATMC but not in ATMK cells with exposure times normalized to observation of IR-induced foci in ATM+ (Figure 5).”
    Quote in the paperSubstantial claim
  • Findingreasoning
    § Abstract

    The number of L1-induced DSBs is greater than the predicted number of successful insertions, conservatively estimated at tenfold more and possibly 100-fold more.

    “Notably, the number of L1-induced DSBs is greater than the predicted numbers of successful insertions, suggesting a significant degree of inefficiency during the integration process.”
    • Also stated in: § Results · hedged
      “Therefore, it is conservative to estimate that tenfold more L1-induced DSBs are produced than insertions, although a 100-fold difference is within reason (our γ-H2AX counts slightly underestimate the expected by about twoto threefold39,40).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [39] Rothkamm K, Lobrich M. Evidence for a lack of DNA double-strand break repair in human cells exposed to very low x-ray doses. Proc. Natl Acad. Sci. USA. 2003;100:5057–5062. doi: 10.1073/pnas.0830918100. [DOI] [PMC free article] [PubMed] [Google Scholar] source

      Not checked in the free run.

    • cites [40] Sedelnikova OA, Rogakou EP, Panyutin IG, Bonner WM. Quantitative detection of (125)IdU-induced DNA double-strand breaks with gamma-H2AX antibody. Radiat. Res. 2002;158:486–492. doi: 10.1667/0033-7587(2002)158[0486:qdoiid]2.0.co;2. [DOI] [PubMed] [Google Scholar] d'Adda d. F., Reaper PM, Clay-Farrace L, Fiegler H, Carr P, Von Zglinicki T, et al. A DNA damage checkpoint response in telomere-initiated senescence. Nature. 2003;426:194–198. doi: 10.1038/nature02118. [DOI] [PubMed] [Google Scholar] source

      Not checked in the free run.

  • Findingown data
    § Results

    L1-induced γ-H2AX foci are largely repaired by 48 h post transfection.

    “The L1-induced foci are largely repaired by 48 h post transfection, as evidenced by the decreasing percentage of cells with γ-H2AX foci and the decreasing number of γ-H2AX foci per cell (Figure 2(b)).”
    Quote in the paperNot a substantial claim
  • Findingown data
    § Results

    Mutations in the endonuclease domain of L1 ORF2 result in essentially complete loss of γ-H2AX foci in HeLa cells.

    “Mutations in the endonuclease domain of the L1 ORF2 resulted in essentially complete loss of the γ-H2AX foci in HeLa cells (Figure 2(a)), consistent with the inability of the endonuclease mutant of L1 to support retrotransposition.44”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [44] Moran JV, Holmes SE, Naas TP, DeBerardinis RJ, Boeke JD, Kazazian HH., Jr High frequency retrotransposition in cultured mammalian cells. Cell. 1996;87:917–927. doi: 10.1016/s0092-8674(00)81998-4. [DOI] [PubMed] [Google Scholar] source

      Not checked in the free run.

  • May combine two claims. 9
  • Classified as probably not a substantial claim. 6
  • May combine two claims.

    Findingown data
    § Abstract

    L1 expression leads to a high level of double-strand break (DSB) formation in DNA, as shown by γ-H2AX foci and the COMET assay.

    “We demonstrate that L1 expression leads to a high level of double-strand break (DSB) formation in DNA using immunolocalization of γ-H2AX foci and the COMET assay.”
    • Also stated in: § Results · plain
      “γ-H2AX foci were present in L1.3-transfected HeLa cells in high numbers and high focus intensity 24 h after transfection (Figure 2(a)).”
    • Also stated in: § Results · plain
      “Transfection with L1.3 produced pronounced tail-moments at 24 h post transfection in the majority of cells (Figure 3).”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data
    § Results

    L1 EN- shows no significant toxicity, while L1.3 RT- shows toxicity levels similar to wild-type L1.3.

    “L1 EN– showed no significant toxicity, while L1.3 RT– showed toxicity levels similar to that of the wild-type L1.3.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationown data
    § Discussion

    Although most full-length L1 elements in the human genome are defective, the data suggest that more may be capable of causing genomic damage than the limited number capable of active retrotransposition.

    “Although most full-length L1 elements in the human genome are defective, our data suggest that more may be capable of causing genomic damage than the limited number capable of active retro-transposition.78”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [78] Brouha B, Schustak J, Badge RM, Lutz-Prigge S, Farley AH, Moran JV, Kazazian HH., Jr Hot L1s account for the bulk of retrotransposition in the human population. Proc. Natl Acad. Sci. USA. 2003;100:5280–5285. doi: 10.1073/pnas.0831042100. [DOI] [PMC free article] [PubMed] [Google Scholar] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationown data
    § Results

    ATM is specifically required for retrotransposition and not survival of cells after L1 expression.

    “Notably, a modified toxicity assay shows no L1-induced toxicity in the ATM– cell line (data not shown), suggesting that ATM is specifically required for retrotransposition and not the survival of cells after L1 expression.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationown data
    § Results

    ORF2 expression alone is sufficient for generating DSBs, but it is probably facilitated by the presence of ORF1.

    “Thus, ORF2 expression alone is sufficient for generating DSBs, but it is probably facilitated by the presence of ORF1.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim.

    Premisecitation
    § Introduction

    The current model for non-LTR element insertion is target-primed reverse transcription (TPRT), based on in vitro analysis of R2Bm.

    “On the basis of in vitro analysis of the R2 element from the silkworm Bombyx mori (R2Bm), the current model for non-long terminal repeat (non-LTR) element insertion is target-primed reverse transcription (TPRT) (Figure 1).6,7”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [6] 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. doi: 10.1016/0092-8674(93)90078-5. [DOI] [PubMed] [Google Scholar] source

      Not checked in the free run.

    • cites [7] Luan DD, Eickbush TH. RNA template requirements for target DNA-primed reverse transcription by the R2 retrotransposable element. Mol. Cell. Biol. 1995;15:3882–3891. doi: 10.1128/mcb.15.7.3882. [DOI] [PMC free article] [PubMed] [Google Scholar] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning
    § Discussion

    Non-LTR retrotransposons encode protein machinery that initiates insertion into DNA but largely requires host factors for many subsequent steps.

    “This last report and our results demonstrate that non-LTR retrotransposons encode protein machinery that initiates insertion into DNA but largely requires host factors for many of the subsequent steps.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning
    § Discussion

    The ATM requirement for L1 integration could have potential consequences for human health in ATM-compromised individuals.

    “The ATM requirement for L1 integration could have potential consequences for human health in ATM-compromised individuals.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Interpretationown data
    § Discussion

    This work demonstrates that the L1 integration process produces DSBs and requires a DSB-repair protein.

    “This work demonstrates that the L1 integration process produces DSBs and requires a DSB-repair protein.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationreasoning
    § Discussion

    Because non-LTR insertion proceeds through a different integration mechanism in which cDNA synthesis precedes endonuclease activity, this dependence represents a novel understanding of the L1 life cycle.

    “Because non-LTR insertion proceeds through a different integration mechanism in which cDNA synthesis precedes endonuclease activity, this dependence represents a novel understanding of the L1 life cycle.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductcitation
    § Materials and Methods

    The COMET assay used in this study is based on the method outlined by Singh et al.

    “The COMET assay is based on the method outlined by Singh et al.53”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Singh et al.53 Singh NP, McCoy MT, Tice RR, Schneider EL. A simple technique for quantitation of low levels of DNA damage in individual cells. Expt. Cell Res. 1988;175:184–191. doi: 10.1016/0014-4827(88)90265-0. [DOI] [PubMed] [Google Scholar]

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation
    § Introduction

    L1 utilizes an APE-like endonuclease to insert into target DNA.

    “L1 utilizes an APE-like endonuclease to insert into the target DNA.5”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [5] Feng Q, Moran JV, Kazazian HH, Jr, Boeke JD. Human L1 retrotransposon encodes a conserved endonuclease required for retrotransposition. Cell. 1996;87:905–916. doi: 10.1016/s0092-8674(00)81997-2. [DOI] [PubMed] [Google Scholar] source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Interpretationreasoning
    § Introduction

    In TPRT models, cleavage of both strands of genomic DNA is required, implying an intermediate equivalent to a DSB is present during integration.

    “In these models of TPRT, cleavage of both strands of genomic DNA is required, which implies that an intermediate equivalent to a DSB is present during the integration process.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationreasoning
    § Discussion

    The L1 endonuclease may be responsible for insertion of the dispersed sites of homology, as well as creating the DSBs adjacent to them that facilitate instability.

    “Thus, the L1 endonuclease may be responsible for insertion of the dispersed sites of homology, as well as creating the DSBs adjacent to them that facilitate the instability.”
    Quote in the paperSubstantial claim
14 of 14
  • The paper relies on this without support.

    The DSB formation observed from transfected L1 expression reflects the behavior of endogenously expressed L1 elements.

    “This result suggests that the endonuclease activity of endogenously expressed L1 elements could contribute to DSB formation in germ-line and somatic tissues.”
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Authors
Gasior, S. L., Wakeman, T. P., Xu, B., & Deininger, P. L.
Affiliations
  • Tulane University (US)
  • Louisiana State University (US)
Corresponding author
Deininger, Prescott L.
Venue
Journal of Molecular Biology
Publisher
Elsevier BV
Published
2006-04
Volume
357
Issue
5
Pages
1383-1393
ISSN
0022-2836 (ISSN-L)
OpenAlex
W1987332588
Cited by
519
References
90
Citations by year
2026: 26 · 2025: 31 · 2024: 33 · 2023: 30 · 2022: 35 · 2021: 35 · 2020: 28 · 2019: 21 · 2018: 26 · 2017: 20 · 2016: 22 · 2015: 27 · 2014: 27 · 2013: 30 · 2012: 26
FWCI
25.43
Type
journal article
Language
en
Open access
Locations
  • Journal of Molecular Biology (publishedVersion)
  • PubMed (publishedVersion)
  • PubMed Central (acceptedVersion)
Funders
  • National Science Foundation (EPS-0346411)
  • U.S. Public Health Service (R01GM45668)
  • National Institutes of Health
  • U.S. Department of Defense (DAM 17-03-1-0709)
  • American Cancer Society
  • Brown Foundation
  • Louisiana Board of Regents (P20RR020152-01, R03ES013301-01)
Topics
  • Life Sciences › Agricultural and Biological Sciences › Plant Science › Chromosomal and Genetic Variations
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › DNA Repair Mechanisms
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Genetics › Genomic variations and chromosomal abnormalities
Keywords
LINE-1 retrotransposons, DNA double-strand breaks, retrotransposition, ATM, γH2AX foci, DNA repair, endonuclease activity, non-LTR retrotransposons, human genome
MeSH terms
Animals, Cell Cycle, DNA, DNA Damage, DNA Repair, DNA-Binding Proteins, HeLa Cells, Histones, Humans, Protein Serine-Threonine Kinases, Cell Cycle Proteins, Long Interspersed Nucleotide Elements, Comet Assay, Tumor Suppressor Proteins, DNA Fragmentation, Ataxia Telangiectasia Mutated Proteins

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