Loss Of p53 Promotes Transformation Of Oncogenic Nras-Induced Chronic Myelomonocytic Leukemia To An Acute Phase

Chronic myelomonocytic leukemia (CMML) primarily occurs in the elderly with the median age ranging from 65 to 75 years. As defined by WHO, CMML is characterized by persistent monocytosis in peripheral blood, hepatosplenomegaly, and the absence of BCR-ABL fusion gene. CMML is a devastating cancer for multiple reasons, one of which is that approximately 20% of CMML cases evolve into acute myeloid leukemia (AML) soon after their first diagnosis. However, little is known about the cellular and molecular mechanisms underlying this malignant transformation.

Recently, our lab developed a CMML mouse model induced by oncogenic Nras^G12D/+ expressed from its endogenous locus. Above 90% of recipient mice with Nras^G12D/+ bone marrow cells developed CMML-like phenotypes with a median survival of ∼56 weeks. Interestingly, none of these mice spontaneously transform to AML. To identify the pathogenetic origins underlying CMML transformation to AML, we further deleted p53 expression in Nras^G12D/+ bone marrow cells using p53^fl/fl allele and Mx1-Cre because deletion of p53 is a common genetic event observed in oncogenic Ras-driven cancers. We found that ERK1/2 is significantly hyperactivated in Nras^G12D/+; p53^-/- hematopoietic stem/progenitor cells (enriched for myeloid progenitors) in the absence of cytokines or in the presence of low concentration of GM-CSF. Concomitantly, the mutant myeloid progenitors show significantly increased self-renewal in a serial replating assay in vitro.

We transplanted Nras^G12D/+, p53^-/-, or Nras^G12D/+; p53^-/- bone marrow cells into lethally irradiated mice. Unlike recipients with p53^-/- cells that died of a T-cell disease with 100% penetrance and a median survival of 24 weeks, ∼70% of recipients with Nras^G12D/+; p53^-/- cells died of AML or acute myeloid sarcoma with a median survival of 16 weeks. These malignant myeloid diseases are transplantable in secondary recipients. Interestingly, only mutant hematopoietic stem cells (HSCs) could initiate and maintain leukemia phenotypes in the Nras^G12D/+ induced CMML model, whereas both Nras^G12D/+; p53^-/- HSCs and myeloid progenitors could initiate AML or acute myeloid sarcoma.

Our results indicate that deletion of p53 cooperates with Nras^G12D/+ mutation to transform CMML into an acute phase. This malignant transformation is initiated by mutant myeloid progenitors, which show increased self-renewal and potentially serve as leukemia initiating cells.