title: A tumor-promoting mechanism mediated by retrotransposon-encoded reverse transcriptase is active in human transformed cell lines.
abstract: LINE-1 elements make up the most abundant retrotransposon family in the human genome. Full-length LINE-1 elements encode a reverse transcriptase (RT) activity required for their own retrotranpsosition as well as that of non-autonomous Alu elements. LINE-1 are poorly expressed in normal cells and abundantly in cancer cells. Decreasing RT activity in cancer cells, by either LINE-1-specific RNA interference, or by RT inhibitory drugs, was previously found to reduce proliferation and promote differentiation and to antagonize tumor growth in animal models. Here we have investigated how RT exerts these global regulatory functions. We report that the RT inhibitor efavirenz (EFV) selectively downregulates proliferation of transformed cell lines, while exerting only mild effects on non-transformed cells; this differential sensitivity matches a differential RT abundance, which is high in the former and undetectable in the latter. Using CsCl density gradients, we selectively identify Alu and LINE-1 containing DNA:RNA hybrid molecules in cancer but not in normal cells. Remarkably, hybrid molecules fail to form in tumor cells treated with EFV under the same conditions that repress proliferation and induce the reprogramming of expression profiles of coding genes, microRNAs (miRNAs) and ultraconserved regions (UCRs). The RT-sensitive miRNAs and UCRs are significantly associated with Alu sequences. The results suggest that LINE-1-encoded RT governs the balance between single-stranded and double-stranded RNA production. In cancer cells the abundant RT reverse-transcribes retroelement-derived mRNAs forming RNA:DNA hybrids. We propose that this impairs the formation of double-stranded RNAs and the ensuing production of small regulatory RNAs, with a direct impact on gene expression. RT inhibition restores the 'normal' small RNA profile and the regulatory networks that depend on them. Thus, the retrotransposon-encoded RT drives a previously unrecognized mechanism crucial to the transformed state in tumor cells.
authors.affiliation: Istituto Superiore di Sanità, Viale Regina Elena 299, Rome, Italy.
authors.forename: Ilaria
authors.initials: I
authors.lastname: Sciamanna
authors.forename: Alberto
authors.initials: A
authors.lastname: Gualtieri
authors.forename: Cristina
authors.initials: C
authors.lastname: Cossetti
authors.forename: Emanuele Felice
authors.initials: EF
authors.lastname: Osimo
authors.forename: Manuela
authors.initials: M
authors.lastname: Ferracin
authors.forename: Gianfranco
authors.initials: G
authors.lastname: Macchia
authors.forename: Eleonora
authors.initials: E
authors.lastname: Aricò
authors.forename: Gianni
authors.initials: G
authors.lastname: Prosseda
authors.forename: Patrizia
authors.initials: P
authors.lastname: Vitullo
authors.forename: Tom
authors.initials: T
authors.lastname: Misteli
authors.forename: Corrado
authors.initials: C
authors.lastname: Spadafora
mesh_terms: D002454:Cell Differentiation / Q000235:genetics; D048708:Cell Growth Processes / Q000235:genetics; D002461:Cell Line, Transformed; D045744:Cell Line, Tumor; D004273:DNA, Neoplasm / Q000235:genetics; D006801:Humans; D020084:Long Interspersed Nucleotide Elements; D008545:Melanoma / Q000201:enzymology / Q000235:genetics; D035683:MicroRNAs / Q000235:genetics / Q000378:metabolism; D009369:Neoplasms / Q000201:enzymology / Q000235:genetics*; D012334:RNA, Neoplasm / Q000235:genetics; D012194:RNA-Directed DNA Polymerase / Q000235:genetics* / Q000378:metabolism; D018894:Reverse Transcriptase Inhibitors / Q000494:pharmacology; D059467:Transcriptome / Q000187:drug effects / Q000235:genetics
chemical_list: D004273:DNA, Neoplasm; D035683:MicroRNAs; D012334:RNA, Neoplasm; D018894:Reverse Transcriptase Inhibitors; D012194:RNA-Directed DNA Polymerase