Identification of Novel Regulators of LINE-1 Expression via CRISPR/Cas9 Screening
Background Long Interspersed Nuclear Elements-1 (LINE-1, L1) are transposable elements that make up roughly 17% of the human genome. These elements can copy and insert themselves into new genomic locations [1]. Typically, LINE-1 is repressed in healthy tissues but may become activated in various human diseases. LINE-1 expression has been associated with aging [2–4], neurodegenerative disorders [5–7], cancer [8–10], and autoimmune diseases [11,12]. Despite the strong association between LINE-1 expression and disease, the regulatory mechanisms controlling the expression of LINE-1-encoded ORF1p and ORF2p and the link between LINE-1 activity and cancer cell survival remain poorly understood. Gaining insights into these regulatory pathways may help elucidate how LINE-1 contributes to disease pathogenesis.  Results To identify upstream regulators of LINE-1 and genes associated with LINE-1 activity-dependent lethality, we developed a dual-reporter system that simultaneously monitors the protein levels of LINE-1-encoded ORF1p and ORF2p (wild-type or catalytically inactive EN/RT mutant). Using genome-wide CRISPR/Cas9-based screens with this system, we identified candidate genes that may influence LINE-1 regulation at multiple levels, including RNA and protein expression. Alongside known factors such as the HUSH complex, the screens revealed additional genes not previously linked to LINE-1 regulation, suggesting possible new regulatory mechanisms for ORF1p and ORF2p expression. We also identified genes whose loss correlated with reduced viability in a manner dependent on LINE-1 activity. These findings collectively provide a broad resource for exploring cellular factors that may modulate LINE-1 expression and activity.  Conclusion This study provides a resource for investigating the cellular regulation of LINE-1, highlighting distinct candidate factors that may modulate ORF1p and ORF2p expression and influence LINE-1 activity-associated cytotoxicity. While functional validation of these candidate regulators remains necessary, the findings offer a foundation for future studies aimed at experimentally confirming their roles and elucidating the molecular mechanisms underlying LINE-1 regulation and its potential contributions to disease contexts. Overall design: HCT116-Cas9 cells lacking a reporter, or stably expressing the L1 WT or L1 EN/RT mutant reporters, were maintained in RPMI-1640 medium supplemented with 10% fetal bovine serum (FBS), 1% penicillin–streptomycin, and 1 mM L-glutamine at 37 °C in a humidified incubator with 5% CO2. Cells were harvested at 3 × 106 cells per tube for RNA-seq and submitted to Azenta Life Sciences for downstream RNA isolation, library preparation, and sequencing.