Wild-Type Kras Inhibits Nras^Q61R/+-Induced Leukemias

Ras proteins belong to the super family of small GTPases. Although RAS mutations are oncogenic, it remains highly controversial whether wild-type (WT) RAS facilitates or inhibits tumorigenesis in the presence of oncogenic RAS. Previous studies suggest that this might depend on cell type, specific RAS isoforms, and/or stage of tumorigenesis (e.g. tumor initiation versus tumor progression). Recently, we found that deletion of WT Kras in oncogenic Kras mice promotes myeloidproliferative neoplasm (MPN) in a cell-autonomous manner. This is through promoting activation of all Ras isoforms and thus enhancing cytokine signaling in vivo. However, it remains unknown whether WT Kras plays a similar role in oncogenic Nras-induced leukemogenesis.

To address this issue, we conditionally down-regulated WT Kras expression in a novel leukemia model induced by endogenous Nras^Q61R/+. Detailed characterization of Nras^Q61R allele reveals that Nras^Q61R/+ displays an intermediate leukemogenic activity between Nras^G12D/+ and Nras^G12D/G12D. Nras^Q61R/+ mice developed MPN (100%) and acute T-cell lymphoblastic leukemia/lymphoma (T-ALL) (~10%) at the moribund stage with a median survival of 197 days. Therefore, Nras^Q61R/+ mice provide an excellent experimental system to test genetic lesions promoting or inhibiting oncogenic Nras-induced leukemogenesis. We observed that 3-weeks after activation of Nras^Q61R/+ and deletion of WT Kras, the double mutant mice displayed more severe MPN phenotypes than Nras^Q61R/+ mice, as demonstrated by more prominent hepatosplenomegaly and further expanded myeloid compartment in bone marrow and spleen. Consistent with these MPN phenotypes, Nras^Q61R/+; Kras^-/- myeloid progenitor and precursor cells showed prolonged and hyperactivated GM-CSF signaling compared to Nras^Q61R/+ cells. In addition, Nras^Q61R/+; Kras^-/- mice displayed further expanded common lymphoid progenitor compartment and a less differentiated phenotype in thymic T-cells than Nras^Q61R/+ mice. In Nras^Q61R/+; Kras^-/- mice, early thymic T cell progenitors demonstrated enhanced cell proliferation and stronger ERK1/2 signaling compared to Nras^Q61R/+ cells. Consequently, Nras^Q61R/+; Kras^-/- mice survived significantly shorter than Nras^Q61R/+ mice (median survival: 97 days; P < 0.001) and 100% of them developed both MPN and T-ALL at the moribund stage. Preliminary results from bone marrow transplantation assay indicate that loss of WT Kras expression promoted Nras^Q61R/+-induced MPN and T-ALL in a cell-autonomous manner. Taken together, our results suggest that loss of WT Kras promotes leukemogenesis in Nras^Q61R/+ mice. We are currently studying on the underlying mechanisms.