FLT3 is genetically essential for ITD-mutated leukemic stem cells but dispensable for human HSCs

Leukemic stem cells (LSCs) fuel relapse in acute myeloid leukemia (AML), but therapies tailored at eradicating LSCs without harming healthy hematopoietic stem cells (HSCs) are lacking. FLT3 is frequently mutated in AML and associated with relapse; but FLT3 targeting has met with limited clinical success. This raises questions of whether more potent inhibitors would increase effectiveness but whether toxicity to HSC would become limiting. Here, we tested the consequence of complete FLT3 ablation using CRISPR/Cas9 FLT3 knock-out (FLT3-KO) in human HSCs and LSCs followed by functional xenograft assays to test their ability to regenerate human hematopoiesis and leukemia, respectively. FLT3-KO in HSCs from human fetal liver, cord blood and adult bone marrow showed no impairment in multilineage hematopoiesis in primary and secondary xenografts. By contrast, FLT3-KO LSCs from 6 of 7 FLT3-ITD mutated AMLs were able to generate short-term engraftment but were completely exhausted by 12 weeks. Thus, FLT3 is essential for LSC long-term propagation. This dependency was unique to FLT3-ITD AML samples as non-FLT3-ITD AML samples generated leukemic grafts upon FLT3-KO. Transcriptomic analysis revealed that FLT3-KO induced downregulation of DNA repair and cell cycle checkpoints, uniquely in FLT3-ITD AML, but not in healthy HSCs or other AMLs. Our research highlights a critical distinction between healthy HSCs and LSCs: whereas healthy hematopoiesis proceeds unperturbed upon FLT3-KO, FLT3-ITD leukemogenesis is impaired through elimination of LSCs. This evidence underscores the necessity for more potent FLT3-targeting and places FLT3 as an ideal therapeutic target to selectively eradicate LSCs, while sparing HSC.