Tracking LINE1 retrotransposition in the germline.

Nearly half of the mammalian genome is occupied by repetitive transposon elements, including longinterspersed nuclear elements (LINEs), short-interspersed nuclear elements, and long terminal repeat retrotransposons. The ability of these transposable elements to propagate and insert randomly throughout the genome plays an important role in genome evolution (1). However, as transposon-mediated insertional mutagenesis has been shown to play a causative role in >65 human genetic diseases (2), it is critical that this process be tightly controlled, particularly in the germline. Remarkably, LINEs account for about one-fifth of the human or mouse genome (3), the majority of which are LINE1 (∼600,000 copies in the murine genome). LINE1 is an active and autonomous transposable element that propagates in the genome through retrotransposition, whereby a LINE1 transcript is reversetranscribed and inserted into the genome at a different location. The full-length LINE1 encodes two proteins: ORF1p and ORF2p, both of which are needed for its mobilization. Because of its repetitive and highly abundant nature, it is nearly impossible to track the activity of individual endogenous LINE1 elements in animals. As reported in PNAS, Newkirk et al. have overcome this hurdle by generating a new LINE1 reporter transgene in mouse (4). Although a number of LINE1 transgenic mouse models have been reported, these transgenes used either human LINE1 elements or non-LINE1 promoters (5–8), and therefore may not fully recapitulate the expression and activity of endogenous mouse LINE1 elements. Newkirk et al. (4) generated new LINE1 transgenes in which expression was driven under the control of the endogenous mouse LINE1 promoter encoded within its own 5′UTR. This new transgene consists of codon-optimized mouse ORF1 and ORF2 for improved translation and an intron-containing split EGFP for assaying retrotransposition. Newkirk et al. (4) next asked whether regulation of these novel LINE1 transgenes mirrors that of endogenous LINE1 elements undergoing epigenetic reprogramming in embryonic gonocytes. Specifically, primordial germ cells (PGCs) undergo genome-wide demethylation. Soon after, PGCs differentiate into embryonic gonocytes, the genomes of which undergo genome-wide de novo DNA remethylation. At this point, LINE1s become methylated at their promoters and thus are silenced (Fig. 1). Newkirk et al. observed that a singlecopy LINE1 transgene, referred to as SN1, exhibited the typical hypomethylation and remethylation characteristics of endogenous LINE1 in embryonic gonocytes and could thus be used to study mechanisms regulating LINE1 retrotransposition in vivo. Because maintenance of genome integrity in the germline is particularly crucial, LINE1 retrotransposons are silenced by multiple mechanisms: DNA methylation, histonemodification, and piRNAs (piwi-interacting RNAs) (Fig. 1) (9, 10). The piRNA pathway is a highly conserved process in metazoans required for silencing Fig. 1. Dynamics of piRNAs, DNA methylation, histone modification, and LINE1 activity during mouse male germ-cell development. Two populations of piRNAs (prepachytene and pachytene) are shown. Prepachytene piRNAs are mostly derived from transposable elements, whereas the majority of pachytene piRNAs are derived from nonrepetitive regions. Crosses on the lines mark the time point of meiotic arrest in Mov10l1−/− and Mili−/− males (13, 20). Data from Newkirk et al. (4) support that H3K9me2 expression pattern inMov10l1 testis is similar to that in Mili testis (depicted here) (9). The figure is modified from figure 1 in ref. 15.