Oncogenic BRAF and KRAS Promote Global DNA Hypomethylation Through a Directed Pathway That Upregulates TET3

Aberrant epigenetic modification is one of the characteristics of the cancer genome. DNA hypermethylation of cytosine-phospho-guanine (CpG) islands, a hallmark of cancer cells, is well-studied and contributes to cancer development by silencing tumor suppressor genes. However, the mechanisms and biological significance of global DNA hypomethylation in cancer are still unclear. Here, using the v-Raf murine sarcoma viral oncogene homolog B1 (BRAF) V600E knock-in mouse models, we demonstrate that endogenous expression of oncogenic BRAFV600E in non-transformed cells promotes global DNA hypomethylation by increasing the levels of ten-eleven translocation 3 (TET3), which converts 5-methylcytosine (5-mC) into 5-hydroxymethylcytosine (5-hmC). Furthermore, TET3 is targeted for proteasomal degradation by F-box and WD repeat domain containing 7 (FBXW7). BRAFV600E increases TET3 levels by inhibiting glycogen synthase kinase 3β (GSK3β), which phosphorylates TET3 and leads to its ubiquitination and proteasomal degradation. We further found elevated levels of TET3 and 5-hmC in BRAFV600E-induced mouse lung tumors and show that TET3 enhances the ability of BRAFV600E to induce the formation of lung tumors. Notably, endogenous expression of oncogenic Kirsten rat sarcoma virus (KRAS) G12D also promotes global DNA hypomethylation and induces lung tumors through a similar TET3-mediated mechanism. Our findings elucidate one of the unknown mechanisms of global DNA hypomethylation promoted by oncogenic BRAF and KRAS and establish a role for TET3 to promote transformation in cooperation with BRAF and KRAS at an early stage of tumorigenesis.