Long Interspersed Element-1 (LINE-1 or L1) retrotransposons have dramatically impacted the human genome. L1s must retrotranspose in the germ-line or during early development to ensure their evolutionary success; yet the extent to which this process impacts somatic cells is poorly understood. We previously demonstrated that engineered human L1s can retrotranspose in adult rat hippocampus progenitor cells (NPCs) in vitro and in the mouse brain in vivo 1 . Here, we demonstrate that NPCs isolated from human fetal brain and NPCs derived from human embryonic stem cells (hESCs) support the retrotransposition of engineered human L1s in vitro. Furthermore, we developed a quantitative multiplex polymerase chain reaction that detected an increase in the copy number of endogenous L1s in the hippocampus and in several regions of adult human brains when compared to the copy number of endogenous L1s in heart or liver genomic DNAs from the same donor. These data suggest that de novo L1 retrotransposition events may occur in the human brain and, in principle, have the potential to contribute to individual somatic mosaicism.The human nervous system is complex, containing approximately 10 15 synapses with a vast diversity of neuronal cell types and connections that are influenced by complex and incompletely understood environmental and genetic factors 2 . Neural progenitor cells (NPCs) give rise to the three main lineages of the nervous system: neurons, astrocytes, and oligodendrocytes. To determine if human NPCs can support L1 retrotransposition, we transfected human fetal brain stem cells (hCNS-SCns) (Fig. 1A) 3 with an expression construct containing a retrotransposition-competent human L1 driven from its native promoter (RC-L1; L1 RP ). The RC-L1 also contains a retrotransposition indicator cassette in its 3′ UTR, consisting of a reversed copy of the enhanced green fluorescent protein (EGFP) expression cassette, which is interrupted by an intron in the same transcriptional orientation as the RC-L1 4-7 . The

ASCs induce a distinct regulatory activation state of macrophages which possess potent immunomodulatory ability and therapeutic potential in inflammatory bowel diseases and sepsis.

Long INterspersed Element-1 (LINE-1 or L1) retrotransposition continues to impact human genome evolution1,2. L1s can retrotranspose in the germline, during early development, and in select somatic cells3,4,5,6,7,8; however, the host response to L1 retrotransposition remains largely unexplored. Here, we show that reporter genes introduced into the genome of various human embryonic carcinoma-derived cell lines (ECs) by L1 retrotransposition are rapidly and efficiently silenced either during or immediately after their integration. Treating ECs with histone deacetylase inhibitors (IHDACs) rapidly reverses this silencing, and chromatin immunoprecipitation (ChIP) experiments revealed that reactivation of the reporter gene was correlated with changes in chromatin status at the L1 integration site. Under our assay conditions, rapid silencing also was observed when reporter genes were delivered into ECs by mouse L1s and a zebrafish LINE-2 element, but not when similar reporter genes were delivered into ECs by Moloney murine leukemia virus (MMLV) or human immunodeficiency virus (HIV), suggesting these integration events are silenced by distinct mechanisms. Finally, we demonstrate that subjecting ECs to culture conditions that promote differentiation attenuates the silencing of reporter genes delivered by L1 retrotransposition, but that differentiation, per se, is not sufficient to reactivate previously silenced reporter genes. Thus, our data suggest that ECs differ from many differentiated cells in their ability to silence reporter genes delivered by L1 retrotransposition.