A functional screen identifies hDRIL1 as an oncogene that rescues RAS-induced senescence

Abstract

Primary fibroblasts respond to activated H-RASV12 by undergoing premature arrest, which resembles replicative senescence1. This irreversible 'fail-safe mechanism' requires p19ARF, p53 and the Retinoblastoma (Rb) family: upon their disruption, RASV12-expressing cells fail to undergo senescence and continue to proliferate1, 2, 3, 4, 5, 6, 7. Similarly, co-expression of oncogenes such as c-MYC or E1A rescues RASV12-induced senescence. To identify novel genes that allow escape from RASV12-induced senescence, we designed an unbiased, retroviral complementary DNA library screen. We report on the identification of DRIL1, the human orthologue of the mouse Bright and Drosophila dead ringer transcriptional regulators. DRIL1 renders primary murine fibroblasts unresponsive to RASV12-induced anti-proliferative signalling by p19ARF/p53/p21CIP1, as well as by p16INK4a. In this way, DRIL1 not only rescues RASV12-induced senescence but also causes these fibroblasts to become highly oncogenic. Furthermore, DRIL1 immortalizes mouse fibroblasts, in the presence of high levels of p16INK4a. Immortalization by DRIL1, whose product binds the pRB-controlled transcription factor E2F1 (ref. 8), is correlated with induction of E2F1 activity. Correspondingly, DRIL1 induces the E2F1 target Cyclin E1, overexpression of which is sufficient to trigger escape from senescence. Thus, DRIL1 disrupts cellular protection against RASV12-induced proliferation downstream of the p19ARF/p53 pathway.