title: Endonuclease-independent LINE-1 retrotransposition at mammalian telomeres.
abstract: Long interspersed element-1 (LINE-1 or L1) elements are abundant, non-long-terminal-repeat (non-LTR) retrotransposons that comprise approximately 17% of human DNA. The average human genome contains approximately 80-100 retrotransposition-competent L1s (ref. 2), and they mobilize by a process that uses both the L1 endonuclease and reverse transcriptase, termed target-site primed reverse transcription. We have previously reported an efficient, endonuclease-independent L1 retrotransposition pathway (EN(i)) in certain Chinese hamster ovary (CHO) cell lines that are defective in the non-homologous end-joining (NHEJ) pathway of DNA double-strand-break repair. Here we have characterized EN(i) retrotransposition events generated in V3 CHO cells, which are deficient in DNA-dependent protein kinase catalytic subunit (DNA-PKcs) activity and have both dysfunctional telomeres and an NHEJ defect. Notably, approximately 30% of EN(i) retrotransposition events insert in an orientation-specific manner adjacent to a perfect telomere repeat (5'-TTAGGG-3'). Similar insertions were not detected among EN(i) retrotransposition events generated in controls or in XR-1 CHO cells deficient for XRCC4, an NHEJ factor that is required for DNA ligation but has no known function in telomere maintenance. Furthermore, transient expression of a dominant-negative allele of human TRF2 (also called TERF2) in XRCC4-deficient XR-1 cells, which disrupts telomere capping, enables telomere-associated EN(i) retrotransposition events. These data indicate that L1s containing a disabled endonuclease can use dysfunctional telomeres as an integration substrate. The findings highlight similarities between the mechanism of EN(i) retrotransposition and the action of telomerase, because both processes can use a 3' OH for priming reverse transcription at either internal DNA lesions or chromosome ends. Thus, we propose that EN(i) retrotransposition is an ancestral mechanism of RNA-mediated DNA repair associated with non-LTR retrotransposons that may have been used before the acquisition of an endonuclease domain.
authors.affiliation: Department of Human Genetics, University of Michigan Medical School, Ann Arbor, Michigan 48109-0618, USA. morrisht@jhmi.edu
authors.forename: Tammy A
authors.initials: TA
authors.lastname: Morrish
authors.forename: José Luis
authors.initials: JL
authors.lastname: Garcia-Perez
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Stamato
authors.forename: Guillermo E
authors.initials: GE
authors.lastname: Taccioli
authors.forename: JoAnn
authors.initials: J
authors.lastname: Sekiguchi
authors.forename: John V
authors.initials: JV
authors.lastname: Moran
mesh_terms: D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D043171:Chromosomal Instability / Q000235:genetics; D006224:Cricetinae; D003412:Cricetulus; D004720:Endonucleases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D006801:Humans; D020084:Long Interspersed Nucleotide Elements / Q000235:genetics*; D016254:Mutagenesis, Insertional / Q000235:genetics*; D016133:Polymerase Chain Reaction / Q000379:methods; D018626:Retroelements / Q000235:genetics*; D016615:Telomere / Q000235:genetics*
chemical_list: D018626:Retroelements; D004720:Endonucleases