Cell-intrinsic depletion of Aml1-ETO-expressing pre-leukemic hematopoietic stem cells by K-Ras activating mutation

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