Cell fitness screens reveal a conflict between LINE-1 retrotransposition and DNA replication

LINE-1 retrotransposons are overexpressed in more than half of human cancers. We identified a colorectal cancer wherein a fast-growing tumor subclone downregulated LINE-1, prompting us to examine how LINE-1 expression affects cell growth. We find that non-transformed cells undergo a TP53-dependent growth arrest and activate interferon signaling in response to LINE-1. TP53 inhibition allows LINE-1(+) cells to grow, and genome wide knockout screens show that these cells require replication-coupled DNA repair pathways, replication stress signaling, and replication fork restart factors. Our findings demonstrate that LINE-1 expression creates specific molecular vulnerabilities and reveal a retrotransposition-replication conflict that may be an important determinant of cancer growth.