Ink4a/Arf and oncogene-induced senescence prevent tumor progression during alternative colorectal tumorigenesis

Colonic cancers with a serrated morphology have been proposed to comprise a molecularly distinct tumor entity following an alternative pathway of genetic alterations independently of APC mutations. We demonstrate that intestinal epithelial cell specific expression of oncogenic K-ras^G12D in mice induces serrated hyperplasia, which is characterized by p16^ink4a overexpression and induction of senescence. Deletion of Ink4a/Arf in K-ras^G12D expressing mice prevents senescence and leads to invasive, metastasizing carcinomas with morphological and molecular alterations comparable to human KRAS mutated serrated tumors. Thus, we suggest that oncogenic K-ras represents a key player during an alternative, serrated pathway to colorectal cancer and hence propose RAS-RAF-MEK signaling apart from APC as an additional gatekeeper in colorectal tumor development.