Cell division promotes efficient retrotransposition in a stable L1 reporter cell line
1 School of Molecular Biosciences and Center for Reproductive Biology, Washington State University, Pullman, WA, 99164, USA
2 Max Delbrück Center for Molecular Medicine, Berlin, Germany
3 Division of Medical Biotechnology, Paul Ehrlich Institute, Langen, 63225, Germany
4 Department of Cell and Developmental Biology, University of Colorado School of Medicine, Aurora, CO, 80045, USA
5 Current address: Department of Genetic Medicine, Weill Cornell Medical College, New York, NY, 10021, USA
6 Current address: Institute of Genetics, Biological Research Centre, Szeged, Hungary
Mobile DNA 2013, 4:10 doi:10.1186/1759-8753-4-10Published: 6 March 2013
Long interspersed element type one (L1) actively modifies the human genome by inserting new copies of itself. This process, termed retrotransposition, requires the formation of an L1 ribonucleoprotein (RNP) complex, which must enter the nucleus before retrotransposition can proceed. Thus, the nuclear import of L1 RNP presents an opportunity for cells to regulate L1 retrotransposition post-translationally. The effect of cell division on L1 retrotransposition has been investigated by two previous studies, which observed varied degrees of inhibition in retrotransposition when primary cell strains or cancer cell lines were experimentally arrested in different stages of the cell cycle. However, seemingly divergent conclusions were reached. The role of cell division on retrotransposition remains highly debated.
To monitor both L1 expression and retrotransposition quantitatively, we developed a stable dual-luciferase L1 reporter cell line, in which a bi-directional tetracycline-inducible promoter drives the expression of both a firefly luciferase-tagged L1 element and a Renilla luciferase, the latter indicative of the level of promoter induction. We observed an additional 10-fold reduction in retrotransposition in cell-cycle arrested cells even after retrotransposition had been normalized to Renilla luciferase or L1 ORF1 protein levels. In synchronized cells, cells undergoing two mitoses showed 2.6-fold higher retrotransposition than those undergoing one mitosis although L1 expression was induced for the same amount of time.
Our data provide additional support for an important role of cell division in retrotransposition and argue that restricting the accessibility of L1 RNP to nuclear DNA could be a post-translational regulatory mechanism for retrotransposition.