Altres ajuts: We thank CERCA/Generalitat de Catalunya and Fundació Josep Carreras-Obra Social la Caixa for their institutional support. Financial support for this work was obtained from [...], the Fundación Uno entre Cienmil, the Obra Social La Caixa (ID 100010434, under agreement LCF/PR/HR19/52160011), the Josep Carreras Foundation, the Leo Messi Foundation, and the Banco Santander Foundation to P.M.; and the Spanish Association against cancer (AECC-CI-2015) to C.B. E.A. acknowledges support form "Fundación Hay Esperanza". P.M. is an investigator of the Spanish Cell Therapy cooperative network (TERCEL).

The cell-of-origin of NPM1- and FLT3-mutated acute myeloid leukemia (AML) is still a matter of debate. Here, we combined in vitro clonogenic assays with targeted sequencing to gain further insights into the cell-of-origin of NPM1 and FLT3-ITD-mutated AML in diagnostic bone marrow (BM) from nine NPM1+/FLT3-ITD (+/−) AMLs. We reasoned that individually plucked colony forming units (CFUs) are clonal and reflect the progeny of a single stem/progenitor cell. NPM1 and FLT3-ITD mutations seen in the diagnostic blasts were found in only 2/95 and 1/57 individually plucked CFUs, suggesting that BM clonogenic myeloid progenitors in NPM1-mutated and NPM1/FLT3-ITD-mutated AML patients do not harbor such molecular lesions. This supports previous studies on NPM1 mutations as secondary mutations in AML, likely acquired in an expanded pool of committed myeloid progenitors, perhaps CD34-, in line with the CD34 phenotype of NPM1-mutated AMLs. This study has important implications on the cell-of-origin of NPM1+ AML, and reinforces that therapeutic targeting of either NPM1 or FLT3-ITD mutations might only have a transient clinical benefit in debulking the leukemia, but is unlikely to be curative since will not target the AML-initiating/preleukemic cells. The absence of NPM1 and FLT3-ITD mutations in normal clonogenic myeloid progenitors is in line with their absence in clonal hematopoiesis of indeterminate potential.