ONTOGENY SHAPES THE ABILITY OF ETV6-RUNX1 TO ENHANCE HEMATOPOIETIC STEM CELL SELF-RENEWAL AND DISRUPT EARLY LYMPHOPOIESIS [in vivo]

The ETV6-RUNX1 (E/R) translocation, found in approximately 30% of pediatric B-cell acute lymphoblastic leukemia (ALL) cases, originates prenatally. Although hematopoietic stem cells (HSCs) have been proposed as the cell of origin for E/R-ALL, definitive evidence for this has been lacking. Here, we develop an E/R-inducible transgenic mouse model and observe that E/R rapidly expands the pool of phenotypic HSCs and block early B cell development. By leveraging the reversibility of the model, we demonstrate functional rather than merely phenotypic HSC expansion. At a molecular level, E/R activates immune checkpoint pathways, which is more pronounced in fetal cells. Comparison of fetal and adult HSCs reveals that although E/R disrupts HSC differentiation in both, fetal cells have a competitive advantage, especially in a setting of viral mimicry. These findings provide new insights into the nature of preleukemic clones associated with E/R and suggest potential benefits of immune checkpoint inhibition for their eradication. Overall design: RNA-seq was performed on HSCs (Lin- Kit+ Sca1+ CD48- CD150+) sorted from control (uninduced) or E/R-induced mice, n = 3 replicates/group