Cell-intrinsic Depletion of Aml1-ETO-positive Pre-leukemic HSCs by K-Ras Activating Mutation

Somatic mutations in acute myeloid leukemia (AML) are acquired sequentially and hierarchically. First, pre-leukemic mutations, such as t(8;21) that encodes AML1-ETO, are acquired within the hematopoietic stem cell (HSC) compartment, while signaling pathway mutations, including K-RAS activating mutations, are late events acquired during transformation of leukemic progenitor cells and rarely detectable in HSCs. This raises the possibility that signaling pathway mutations are detrimental to clonal expansion of pre-leukemic HSCs. To address this hypothesis we here used conditional genetics to introduce Aml1-ETO and K-RasG12D into murine HSCs, either individually or in combination. In the absence of activated Ras, Aml1-ETO expression conferred a competitive advantage to HSCs. However, activated K-Ras had a marked detrimental effect on Aml1-ETO expressing HSCs, leading to loss of both phenotypic and functional HSCs. Cell cycle analysis revealed a loss of quiescence in HSCs co-expressing Aml1-ETO and K-RasG12D, accompanied by an enrichment in E2F and Myc target gene expression and depletion of HSC self-renewal-associated gene expression. These findings provide a mechanistic basis for the observed absence of KRAS, and potentially other, signaling mutations in the pre-malignant HSC compartment.