Genetic Disruption of Rac1 but Not Rac2 Is Sufficient To Ameliorate the Development of Juvenile Myeloproliferative Myeloid Leukemia in Nf1−/− Mice.
Authors: Yang Fengchun , Allen Jayme , Chen Shi , Li Yan , Yuan Jin , Bessler Waylan , Ingram David A. , Clapp D. Wade 
Published: October 2019

Abstract
Neurofibromin, the protein encoded by the NF1 tumor-suppressor gene, negatively regulates the output of p21ras proteins by accelerating the hydrolysis of active Ras-guanosine triphosphate to inactive Ras-guanosine diphosphate. Children with neurofibromatosis type 1 (NF1) are predisposed to juvenile myelomonocytic leukemia (JMML) associated with loss of the normal NF1 allele. Genetically engineered mice containing nullizygous loss of Nf1 in the hematopoietic system develop a leukemia with complete penetrance that is similar to JMML, including hepatosplenomegaly, elevated peripheral blood cell counts, and elevated numbers of myeloid progenitors that are hypersensitive to multiple cytokines, particularly GM-CSF. Though we and others have found that p21ras is hyperactivated in Nf1−/− myeloid progenitors, inhibiting Ras specifically as a therapeutic target has been challenging. Therefore, identification of alterations in distinct p21ras effector pathways that control leukemia progression in Nf1-deficient cells is critical for understanding disease pathogenesis and identifying therapeutic targets. Here we intercrossed MxCre; Nf1flox/flox mice with mice that are deficient in the small Rho GTPases Rac1 or Rac2 to generate syngeneic progeny that were MxCre; Nf1flox/flox, MxCre; Nf1flox/flox;Rac1flox flox or MxCre; Nf1flox/flox; Rac2 −/−. Consistent with previous studies (Le, Blood 2004) MxCre; Nf1flox/flox mice develop a progressive myeloproliferative disease with 100% penetrance 6 months following interferon inducible induction of the MxCre transgene to disrupt the Nf1flox alleles. Eighty percent of MxCre; Nf1flox/flox die by 9 months after inactivation. Genetic disruption of Rac2 was not sufficient to diminish the onset or severity of the characteristic myeloproliferative disease of MxCre; Nf1flox/flox mice. In contrast, MxCre; Nf1flox/flox; Rac1flox/flox mice followed for 11 months all survived and had normal bone marrow cellularity, spleen weight and splenic architecture. MxCre; Nf1flox/flox mice have elevated numbers of both HPP-CFC and LPP-CFC in the bone marrow and spleen. In contrast, MxCre; Nf1flox/flox; Rac1flox/flox mice had myeloid progenitor numbers that were comparable to wildtype, age-matched controls. Further, though myeloid progenitors from MxCre; Nf1flox/flox mice are hypersensitive to GM-CSF, myeloid progenitors from MxCre; Nf1flox/flox; Rac1flox/flox have a sensitivity to GM-CSF comparable to wildtype controls. The correction in the myeloproliferative phenotype isolated from MxCre; Nf1flox/flox; Rac1flox/flox mice was associated with the reduction in Rac-GTP and prolonged Erk phosphorylation, a MAPK effector that is characteristically elevated in Nf1−/− myeloid progenitors. Collectively, these genetic data identify Ras-Rac1 signaling pathway as a key axis in the genesis of juvenile myelomonocytic leukemia and provide evidence that Rac1 is a therapeutic molecular target for this myeloproliferative disease that currently has no effective therapies.