Oncogene-induced senescence underlies the mutual exclusive nature of oncogenic KRAS and BRAF
Menée à l'aide de modèles murins, cette étude met en évidence des mécanismes permettant d'expliquer pourquoi les gènes KRAS et BRAF ne sont jamais mutés ensemble dans une même cellule tumorale
KRAS and BRAF are among the most commonly mutated oncogenes in human cancer that contribute to tumorigenesis in both distinct and overlapping tissues. However, KRAS and BRAF mutations are mutually exclusive; they never occur in the same tumor cell. The reason for the mutual exclusivity is unknown, but there are several possibilities. The two mutations could be functionally redundant and not create a selective advantage to tumor cells. Alternatively, they could be deleterious for the tumor cell and induce apoptosis or senescence. To distinguish between these possibilities, we activated the expression of BRAFV600E and KRASG12D from their endogenous promoters in mouse lungs. Although the tumor-forming ability of BRAFV600E was higher than KRASG12D, KRASG12D tumors were larger and more advanced. Coactivation of BRAFV600E and KRASG12D markedly reduced lung tumor numbers and overall tumor burden compared with activation of BRAFV600E alone. Moreover, several tumors expressed only one oncogene, suggesting negative selection against expression of both. Similarly, expression of both oncogenes in mouse embryonic fibroblasts essentially stopped proliferation. The expression of both oncogenes hyperactivated the MEK-ERK-cyclin D pathway but reduced proliferation by increasing the production of p15, p16 and p19 proteins encoded by the Ink4/Arf locus and thereby increased senescence-associated [beta]-galactosidase-positive cells. The data suggest that coexpression of BRAFV600E and KRASG12D in early tumorigenesis leads to negative selection due to oncogene-induced senescence.