DNA damage and L1 retrotransposition

Abstract

Barbara McClintock was the first to suggest that transposons are a source of genome instability and that genotoxic stress assisted in their mobilization. The generation of double-stranded DNA breaks (DSBs) is a severe form of genotoxic stress that threatens the integrity of the genome, activates cell cycle checkpoints, and, in some cases, causes cell death. Applying McClintock's stress hypothesis to humans, are L1 retrotransposons, the most active autonomous mobile elements in the modern day human genome, mobilized by DSBs? Here, evidence that transposable elements, particularly retrotransposons, are mobilized by genotoxic stress is reviewed. In the setting of DSB formation, L1 mobility may be affected by changes in the substrate for L1 integration, the DNA repair machinery, or the L1 element itself. The review concludes with a discussion of the potential consequences of L1 mobilization in the setting of genotoxic stress.

Figure 1

DNA damage can affect multiple stages of the L1 life cycle. (1) Transcription of the L1 element is controlled by epigenetic factors and transcription factors. (2) L1 RNA is exported to the cytoplasm, where its copy number influences retrotransposition frequency. (3) Translation of ORF1 and ORF2 proteins. (4) L1 protein and mRNA are imported into the nucleus, where ORF2 endonuclease creates a DNA double-strand break. Induced breaks may be able to serve as alternative substrates for insertion. (5) ORF2 reverse transcribes a cDNA copy of L1 at the insertion site. Host factors are thought to inhibit or assist in resolution of the insertion. The dark square represents the cell nucleus, and the lighter surrounding square represents the cytoplasm.

Figure 2

Potential ways in which DNA damage could influence L1 retrotransposition. (a) Endonuclease-dependent insertion under normal conditions. The L1 endonuclease (○) makes staggered nicks at the target site, creating 3′ overhangs. Filling in generates 7–20 base pair target site duplications (➡) flanking the insertion. (b) Endonuclease-independent insertion at the site of a double-strand break. The preexisting double-strand break shown here lacks staggered nicks or overhangs. L1 entry into this site would therefore also lack target site duplications. Genomic deletions may occur due to processing by cellular DNA repair processes (∗). (c) Endonuclease-dependent insertion potentiated by DNA damage. DNA damage may upregulate cellular cofactors of reverse transcription and integration (●). Insertion via pathway c is endonuclease-dependent, but occurs at an increased efficiency.

Figure 2

Potential ways in which DNA damage could influence L1 retrotransposition. (a) Endonuclease-dependent insertion under normal conditions. The L1 endonuclease (○) makes staggered nicks at the target site, creating 3′ overhangs. Filling in generates 7–20 base pair target site duplications (➡) flanking the insertion. (b) Endonuclease-independent insertion at the site of a double-strand break. The preexisting double-strand break shown here lacks staggered nicks or overhangs. L1 entry into this site would therefore also lack target site duplications. Genomic deletions may occur due to processing by cellular DNA repair processes (∗). (c) Endonuclease-dependent insertion potentiated by DNA damage. DNA damage may upregulate cellular cofactors of reverse transcription and integration (●). Insertion via pathway c is endonuclease-dependent, but occurs at an increased efficiency.

Figure 2

Potential ways in which DNA damage could influence L1 retrotransposition. (a) Endonuclease-dependent insertion under normal conditions. The L1 endonuclease (○) makes staggered nicks at the target site, creating 3′ overhangs. Filling in generates 7–20 base pair target site duplications (➡) flanking the insertion. (b) Endonuclease-independent insertion at the site of a double-strand break. The preexisting double-strand break shown here lacks staggered nicks or overhangs. L1 entry into this site would therefore also lack target site duplications. Genomic deletions may occur due to processing by cellular DNA repair processes (∗). (c) Endonuclease-dependent insertion potentiated by DNA damage. DNA damage may upregulate cellular cofactors of reverse transcription and integration (●). Insertion via pathway c is endonuclease-dependent, but occurs at an increased efficiency.