LINE retrotransposition and host DNA repair machinery

Long interspersed elements (LINEs), or non-long-terminal repeat (LTR) retrotransposons, are mobile genetic elements that exist in the genomic DNA of most eukaryotes, comprising a considerable portion of the host chromosomes. LINEs constitute endogenous mutagens that cause insertional mutations in host chromosomes and have a large impact on host genome evolution. Despite their importance, however, the molecular mechanism of LINE retrotransposition is not fully understood. Several studies suggest that host proteins that participate in the repair of DNA breaks modulate LINE retrotransposition. Recently, we provided evidence that there are 2 distinct pathways-annealing and direct-that join the 5'-end of LINEs to host chromosomal DNA. These pathways appear to be used distinctively by zebrafish LINEs and the human L1 in DT40 cells. In HeLa cells, only the annealing pathway appears to be used. This implies that different characteristics of the 2 LINEs and also host factors dictate which pathway is selected. Here, we discuss the 5'-end-joining pathways of LINE retrotransposition and propose that the pathways of LINE integration adopt certain host repair factors.