MEK Inhibition Modulates the Growth of Nf1 mutant Hematopoietic Cells and Induces Clinical Improvement in a Murine Model of Juvenile Myelomonocytic Leukemia (JMML)

Abstract 797^FN2

Neurofibromatosis type 1 (NF1) is a dominant genetic disorder that is caused by germ line mutations in the NF1 tumor suppressor gene. Children with NF1 have a 200- to 500-fold increase in the risk of developing malignant myeloid disorders, particularly JMML, an aggressive myeloproliferative neoplasm (MPN) characterized by over-production of myeloid lineage cells that show extensive tissue infiltration, anemia, and thrombocytopenia. The only curative therapy for JMML is hematopoietic stem cell transplantation, and 50% of children relapse after HSCT and die of recurrent JMML or of transformation to acute myeloid leukemia (AML). Somatic inactivation of Nf1 in hematopoietic cells causes a progressive MPN in Mx1-Cre, Nf1^flox/flox mice that closely models JMML. We administered the MEK inhibitor PD0325901 (901) to Mx1-Cre, Nf1^flox/flox mice with MPN to investigate the therapeutic effects of inhibiting Raf/MEK/ERK signaling in vivo and to interrogate the underlying mechanism of any observed response. Mx1-Cre, Nf1^flox/flox mice (n = 35) and WT littermates (n = 38) were randomized to receive 901 (5 mg/kg/day) or control vehicle at 6 months of age. Treatment was continued for 10 weeks or until the mice became moribund. Mx1-Cre, Nf1^flox/flox mice that received 901 demonstrated a rapid reduction in leukocyte counts and enhanced erythropoiesis. In addition, spleen size was markedly reduced with correction of aberrant proliferation and differentiation of progenitor cells. Flow cytometric analysis showed that 901 treatment restored a normal pattern of erythroid differentiation and greatly reduced splenic hematopoiesis. Remarkably, genetic studies revealed persistence of Nf1-deficient hematopoietic cells despite dramatic improvement in hematologic abnormalities. Based on these data, we conclude that treatment with 901 rebalances the proliferation and differentiation of Nf1 mutant cells in vivo rather than eliminating them. These preclinical data and a recent study in Kras mutant mice with MPN (Lyubynska N, et al. Science Trans Med 2011; 3: 76ra27) support clinical trials of MEK inhibitors in JMML and chronic myelomonocytic leukemia and suggest differentiation of malignant cell as a potential mechanism of response to this class of targeted agents.