Activation of Young LINE-1 Elements by CRISPRa.

Long interspersed element-1 (LINE-1; L1s) are mobile genetic elements that comprise nearly 20% of the human genome. L1s have been shown to have important functions in various biological processes, and their dysfunction is thought to be linked with diseases and cancers. However, the roles of the repetitive elements are largely not understood. While the CRISPR activation (CRISPRa) system based on catalytically deadCas9 (dCas9) is widely used for genome-wide interrogation of gene function and genetic interaction, few studies have been conducted on L1s. Here, we report using the CRISPRa method to efficiently activate L1s in human L02 cells, a derivative of the HeLa cancer cell line. After CRISPRa, the young L1 subfamilies such as L1HS/L1PA1 and L1PA2 are found to be expressed at higher levels than the older L1s. The L1s with high levels of transcription are closer to full-length and are more densely occupied by the YY1 transcription factor. The activated L1s can either be mis-spliced to form chimeric transcripts or act as alternative promoters or enhancers to facilitate the expression of neighboring genes. The method described here can be used for studying the functional roles of young L1s in cultured cells of interest.