FREQUENT LOSS-OF-FUNCTION MUTATIONS IN MLK4 SUPPRESS SIGNALING IN THE MKK7-JNK-CJUN-P21/P15 PATHWAY IN COLON CANCER.

Anna Marusiak, The University of Manchester, UK

Anna Marusiak, Natalie Stephenson, Hayeon Baik, Eleanor Trotter, Yaoyong Li, Karen Blyth, Susan Mason, Lorena Puto, Ewelina Testoni, Crispin Miller, Tony Hunter, Owen Sansom, John Brognard

MLK4 is a member of the mixed lineage family of kinases activated by multiple stimuli which regulate the JNK, p38 and ERK pathways. Mutations in MLK4 have been identified in various human cancers, including colon cancer at a high frequency. Here we evaluate the impact of mutations in MLK4 on protein function and the process of tumorigenesis in colorectal cancers. Biochemical data imply that a majority of MLK4 mutations are loss-of-function (LOF), which is consistent with structural analysis, and these mutants can act in a dominant negative manner. Furthermore, we used the colon cancer cell lines harboring these inactivating mutations to determine if they are critical for maintenance of tumorigenic phenotypes. Reintroduction of wild-type MLK4 into colon cancer cells with LOF mutations in MLK4 reduced viability, proliferation and ability to form colonies as well as significantly slowed the tumor growth in mouse xenografts. Additionally, restoring the function of MLK4 induced selective activation of the JNK pathway and its downstream targets, cJUN, ATF3 and p21/p15. In summary our data convey a complex picture where a majority of MLK4 mutations identified in colon cancer alter kinetic activity to modify cancer-signaling pathways dependent upon the presence of other oncogenic mutations.