Posttranscriptional depletion of ribosome biogenesis factors engenders therapeutic vulnerabilities in NPM1-mutant AML

NPM1 is a multifunctional phosphoprotein with key roles in ribosome biogenesis amongst its many functions. NPM1 gene mutations drive 30% of acute myeloid leukemia (AML) cases. The mutations disrupt a nucleolar localization signal (NoLS) and create a novel nuclear export signal (NES), leading to cytoplasmic displacement of the protein (NPM1c). NPM1c mutations prime hematopoietic progenitors to leukemic transformation, but their precise molecular consequences remain elusive. Here, we first examine the effects of isolated NPM1c mutations on the global proteome of pre-leukemic hematopoietic stem and progenitor cells (HSPCs) using conditional knock-in Npm1cA/+ mice. We discover that many proteins involved in ribosome biogenesis are significantly depleted in these murine HSPCs, but also importantly in human NPM1-mutant AMLs. In line with this, we found that pre-leukemic Npm1cA/+ HSPCs display higher sensitivity to RNA polymerase I inhibitors, including Actinomycin D (ActD), compared to Npm1+/+ cells. Combination treatment with ActD and Venetoclax inhibited the growth and colony forming ability of pre-leukemic and leukemic NPM1c+ cells, whilst low-dose ActD treatment was able to re-sensitize resistant NPM1c+ cells to Venetoclax. Furthermore, using data from CRISPR dropout screens, we identified and validated TSR3, a 40S ribosomal maturation factor whose knock-out preferentially inhibited the proliferation of NPM1c+ AML cells by activating a p53-dependent apoptotic response. Similarly to low-dose ActD treatment, TSR3 depletion could partially restore sensitivity to Venetoclax in therapy-resistant NPM1c+ AML models. Our findings propose that targeted disruption of ribosome biogenesis should be explored as a therapeutic strategy against NPM1-mutant AML.