title: Potential for genomic instability associated with retrotranspositionally-incompetent L1 loci.
abstract: Expression of the L1 retrotransposon can damage the genome through insertional mutagenesis and the generation of DNA double-strand breaks (DSBs). The majority of L1 loci in the human genome are 5'-truncated and therefore incapable of retrotransposition. While thousands of full-length L1 loci remain, most are retrotranspositionally-incompetent due to inactivating mutations. However, mutations leading to premature stop codons within the L1 ORF2 sequence may yield truncated proteins that retain a functional endonuclease domain. We demonstrate that some truncated ORF2 proteins cause varying levels of toxicity and DNA damage when chronically overexpressed in mammalian cells. Furthermore, transfection of some ORF2 constructs containing premature stop codons supported low levels of Alu retrotransposition, demonstrating the potential for select retrotranspositionally-incompetent L1 loci to generate genomic instability. This result suggests yet another plausible explanation for the relative success of Alu elements in populating the human genome. Our data suggest that a subset of retrotranspositionally-incompetent L1s, previously considered to be harmless to genomic integrity, may have the potential to cause chronic DNA damage by introducing DSBs and mobilizing Alu. These results imply that the number of known L1 loci in the human genome that potentially threaten its stability may not be limited to the retrotranspositionally active loci.
authors.affiliation: Department of Structural and Cellular Biology, Tulane School of Medicine, Tulane Cancer Center, and Tulane Center for Aging, New Orleans, LA 70112, USA.
authors.forename: Kristine J
authors.initials: KJ
authors.lastname: Kines
authors.affiliation: Department of Structural and Cellular Biology, Tulane School of Medicine, Tulane Cancer Center, and Tulane Center for Aging, New Orleans, LA 70112, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Sokolowski
authors.affiliation: Department of Structural and Cellular Biology, Tulane School of Medicine, Tulane Cancer Center, and Tulane Center for Aging, New Orleans, LA 70112, USA.
authors.forename: Dawn L
authors.initials: DL
authors.lastname: deHaro
authors.affiliation: Department of Structural and Cellular Biology, Tulane School of Medicine, Tulane Cancer Center, and Tulane Center for Aging, New Orleans, LA 70112, USA.
authors.forename: Claiborne M
authors.initials: CM
authors.lastname: Christian
authors.affiliation: Department of Structural and Cellular Biology, Tulane School of Medicine, Tulane Cancer Center, and Tulane Center for Aging, New Orleans, LA 70112, USA vperepe@tulane.edu.
authors.forename: Victoria P
authors.initials: VP
authors.lastname: Belancio
mesh_terms: D020087:Alu Elements; D000818:Animals; D018389:Codon, Nonsense; D004249:DNA Damage; D004720:Endonucleases / Q000235:genetics / Q000378:metabolism; D056426:Genetic Loci; D015894:Genome, Human; D042822:Genomic Instability; D006367:HeLa Cells; D006801:Humans; D020084:Long Interspersed Nucleotide Elements; D051379:Mice; D041681:NIH 3T3 Cells; D017434:Protein Structure, Tertiary; D012194:RNA-Directed DNA Polymerase / Q000235:genetics / Q000378:metabolism
chemical_list: D018389:Codon, Nonsense; C531246:ORF2 protein, human; D012194:RNA-Directed DNA Polymerase; D004720:Endonucleases