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

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

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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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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).

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Integrity signals

Registry

Crossref · 10.1016/j.jmb.2006.01.089

Cites

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    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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Metadata

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 › Biochemistry, Genetics and Molecular Biology › Genetics › 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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