In vitro and in vivo effects of a farnesyltransferase inhibitor on Nf1-deficient hematopoietic cells.

Oncogenic RAS alleles encode proteins that accumulate in the guanosine triphosphate (GTP)-bound state. Because post-translational processing of Ras by farnesyltransferase is essential for biologic function, inhibitors of this enzyme have been developed as rational cancer therapeutics. We have investigated farnesyltransferase inhibitor (FTI) L-744,832 in an in vivo murine model of myeloid leukemia that is associated with inactivation of the Nf1 tumor suppressor gene. Nf1 encodes a GTPase activating protein for Ras, and Nf1-deficient (Nf1-/-) hematopoietic cells show hyperactive Ras signaling through the mitogen-activated protein (MAP) kinase pathway. L-744,832 inhibited H-Ras prenylation in cell lines and in primary hematopoietic cells and abrogated the in vitro growth of myeloid progenitor colonies in response to granulocyte-macrophage colony-stimulating factor (GM-CSF). This FTI also partially blocked GM-CSF-induced MAP kinase activation, but did not reduce constitutively elevated levels of MAP kinase activity in primary Nf1-/- cells. Injection of a single dose of 40 or 80 mg/kg of L-744, 832 increased the amount of unprocessed H-Ras in bone marrow cells, but had no detectable effect on N-Ras. Adoptive transfer of Nf1-/- hematopoietic cells into irradiated mice induces a myeloproliferative disorder that did not respond to L-744,832 treatment. We speculate that the lack of efficacy in this model is due to the resistance of N-Ras and K-Ras processing to inhibition by this FTI.

Myeloid malignancies induced by alkylating agents in Nf1 mice.

Therapy-related acute myeloid leukemia and myelodysplastic syndrome (t-AML and MDS) are severe late complications of treatment with genotoxic chemotherapeutic agents. Children with neurofibromatosis type 1 (NF1) are predisposed to malignant myeloid disorders that are associated with inactivation of the NF1 tumor suppressor gene in the leukemic clone. Recent clinical data suggest that NF1 might be also associated with an increased risk of t-AML after treatment with alkyating agents. To test this hypothesis, we administered cyclophosphamide or etoposide to cohorts of wild-type and heterozygous Nf1 knockout mice. Cyclophosphamide exposure cooperated strongly with heterozygous inactivation of Nf1 in myeloid leukemogenesis, while etoposide did not. Somatic loss of the normal Nf1 allele correlated with clinical disease and was more common in 129/Sv mice than in 129/Sv x C57BL/6 animals. Leukemic cells showing loss of heterozygosity at Nf1 retained a structural allele on each chromosome 11 homolog. These studies establish a novel in vivo model of alkylator-induced myeloid malignancy that will facilitate mechanistic and translational studies.