RAS Transformation Requires CUX1-Dependent Repair of Oxidative DNA Damage

The Cut homeobox1 (CUX1) gene is a target of loss-of- heterozygosityin many cancers, yet elevated CUX1 expression is frequently observed and is associated with shorter disease-free survival. The dual role of CUX1 in cancer is illustrated by the fact that most cell lines with CUX1 LOH display amplification of the remaining allele, suggesting that decreased CUX1 expression facilitates tumor development while increased CUX1 expression is needed in tumorigenic cells. Indeed, CUX1 was found in a genome-wide RNAi screen to identify synthetic lethalinteractions with oncogenic RAS. Here we show that CUX1 functions in base excision repairas an ancillary factor for the 8-oxoG- DNA glycosylase, OGG1. Single cell gel electrophoresis ( comet assay) reveals that Cux1+/− MEFs are haploinsufficientfor the repair of oxidative DNA damage, whereas elevated CUX1 levels accelerate DNA repair. In vitro base excision repairassays with purified components demonstrate that CUX1 directly stimulates OGG1's enzymatic activity. Elevated reactive oxygen species (ROS) levels in cells with sustained RAS pathway activation can cause cellular senescence. We show that elevated expression of either CUX1 or OGG1 prevents RAS-induced senescence in primary cells, and that CUX1 knockdown is synthetic lethalwith oncogenic RAS in human cancer cells. Elevated CUX1 expression in a transgenic mouse model enables the emergence of mammary tumors with spontaneous activating Kras mutations. We confirmed cooperation between KrasG12V and CUX1 in a lung tumor model. Cancer cells can overcome the antiproliferative effects of excessive DNA damage by inactivating a DNA damage response pathway such as ATM or p53 signaling. Our findings reveal an alternate mechanism to allow sustained proliferation in RAS-transformed cells through increased DNA base excision repaircapability. The heightened dependency of RAS-transformed cells on base excision repairmay provide a therapeutic windowthat could be exploited with drugs that specifically target this pathway.