Loss of Sos1 Inhibits Oncogenic Kras-Induced Hyperactivation of Wild-Type Ras during Leukemogenesis

As members of small GTPase super family, the functional output of Ras proteins depends on their GTP binding status, which is regulated by the interactions with guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). Activating mutations in NRAS and KRAS isoforms are identified in various types of hematopoietic malignancies. Interestingly, the same oncogenic mutation (G12D) at the endogenous Kras locus displays much more potent leukemogenic activity than that at the endogenous Nras locus in vivo. Moreover, combined inhibition of MEK and ERK provides long-term disease-free survival in Nras^G12D/G12D mice but had much less effect in Kras^G12D/+ mice. During our investigation to understand the potent leukemogenic activity of oncogenic Kras, we found that in total bone marrow cells, oncogenic Kras, but not oncogenic Nras, induces hyperactivation of wild-type (WT) Hras and Nras. We hypothesize that the hyperactivated WT Ras significantly contributes to oncogenic Kras-mediated leukemogenesis and inhibition of this process might improve the sensitivity of oncogenic Kras cells towards combined therapy.

Because Sos1, a RAS GEF, has been implicated in oncogenic Ras-mediated activation of WT Ras in human cancer cell lines, we investigated whether Sos1 plays an essential role in this process in vivo. We find that Sos1 is overexpressed in Kras^G12D/+ bone marrow cells. Genetic deletion of Sos1 indeed significantly decreases the GTP-bound active form of WT Nras and Hras without affecting the activation status of oncogenic Kras. Consequently, Sos1 deficiency-mediated downregulation of ERK activation rescues oncogenic Kras mediated depletion of hematopoietic stem cells (HSCs). HSCs, multipotent progenitors (MPPs) and LSKs (Lin-Sca-1+c-Kit+) in Kras^G12D/+;Sos1-/- mice are much more quiescent than those in Kras^G12D/+ mice. Moreover, Sos1 deficiency significantly inhibits granulocyte-macrophage colony stimulating factor (GM-CSF) evoked ERK signaling in Kras^G12D/+ myeloid progenitor and precursor cells. Consistent with these biochemical data, we show that myeloproliferative neoplasm (MPN) phenotypes are significantly alleviated in Kras^G12D/+;Sos1-/- mice and these animals survived significantly longer than Kras^G12D/+ mice.

However, we find that in differentiated myeloid cells (e.g. neutrophils), loss of Sos1 does not affect GM-CSF-evoked ERK activation. This result is consistent with our previous finding that Ras-mediated ERK activation in differentiated myeloid cells is predominantly through Kras but not Hras or Nras. Together, our results demonstrate that Sos1 mediates oncogenic Kras-induced hyperactivation of WT Ras. Inhibition of Sos1 thus blocks this process and attenuates the leukemogenic activity of oncogenic Kras. In contrast, Sos1 deficiency does not affect the unique signaling mediated by oncogenic Kras itself. Therefore, we hypothesize that targeting Sos1 alone will not effectively treat Kras^G12D-associated leukemias but it might increase the sensitivity of Kras^G12D cells to other therapies, such as combined inhibition of MEK and JAK. We are currently testing this hypothesis in vivo.