The Importance of Isoprenylation and Nf1 Deficiency in K-RAS-induced Cancer

Abstract: The RAS and RHO family proteins contribute to tumorigenesis and metastasis and belong to a family of so called CAAX proteins. The membrane targeting and proper function of CAAX proteins are dependent on posttranslational isoprenylation by farnesyltransferase (FTase) or geranylgeranyltransferase type I (GGTase-I). Inhibitors of FTase and GGTase-I have been developed to block RAS-induced cancer, but their utility has been difficult to evaluate because of off-target effects, drug resistance, and toxicity. One aim of this thesis was to use genetic strategies in mice to define the physiologic importance of CAAX protein isoprenylation and to evaluate FTase and GGTase-I as potential anti-cancer drug targets. Oncogenic mutations in RAS are common in cancer and result in hyperactive RAS signaling. However, a RAS mutation alone is not sufficient for cancer development in humans. Rather, cancer arises as a consequence of cooperation between several mutational events. The tumor suppressor gene neurofibromatosis type I (NF1) is a RAS-inactivating protein. Thus, loss of NF1 also results in hyperactive RAS signaling and this occurs in some types of cancer. It has been proposed that NF1 deficiency is functionally equivalent to an oncogenic RAS; but NF1 may operate in other pathways. It is not clear if NF1 deficiency would be redundant in RAS-induced cancer development or if the two mutations would cooperate. A second aim of this thesis was to define the impact of Nf1 deficiency on the development of K-RAS–induced cancer in mice. To approach these aims, Cre/loxP gene targeting techniques in mice were used, to simultaneously activate an oncogenic K-RAS allele, to induce lung cancer or myeloid leukemia, and inactivate the genes encoding FTase and GGTase-I, or Nf1. Inactivating the gene encoding the b-subunit of GGTase-I eliminated enzyme activity, blocked proliferation and reduced motility of fibroblasts. Moreover, inactivation of GGTase-I reduced tumor formation and increased survival of mice with K-RAS–induced lung cancer. Finally, several cell types, including lung tumor cells and macrophages remained viable in the absence of GGTase-I. Inactivating the gene encoding the b-subunit of FTase eliminated farnesylation of HDJ2 and H-RAS, prevented H-RAS targeting to the plasma membrane, and blocked proliferation of fibroblasts. FTase inactivation reduced tumor formation and increased survival of mice with K-RAS–induced cancer to a similar extent as the inactivation of GGTase-I. The simultaneous inactivation of FTase and GGTase-I markedly reduced lung tumors and improved survival. These data suggest that inhibition of FTase and/or GGTase-I could be useful in the treatment of K-RAS–induced cancer. In mice, expression of oncogenic K-RAS or inactivation of Nf1 in hematopoietic cells results in myeloproliferative disorders (MPDs) that do not progress to acute myeloid leukemia (AML). However, the simultaneous inactivation of Nf1 and activation of oncogenic K-RAS in hematopoietic cells induced AML in mice. The levels of active RAS were not increased in mice with AML, raising the possibility that Nf1 deficiency may contribute to AML by non-RAS pathways. This result points to a strong cooperation between Nf1 deficiency and oncogenic K-RAS and sheds new light on mechanisms of RAS-induced leukemia development.