NPM1-fusion proteins promote myeloid leukemogenesis  through XPO1-dependent HOX activation
Nucleophosmin (NPM1) is a nucleolar protein and one of the most frequently mutated genes in acute myeloid leukemia (AML). In addition to the commonly detected frameshift mutations in exon12 (NPM1c), previous studies have identified NPM1 gene rearrangements leading to the expression of NPM1-fusion proteins in pediatric AML. However, whether the NPM1-fusions are indeed oncogenic and how the NPM1-fusions cause AML have been largely unknown. In this study, we investigated the subcellular localization and leukemogenic potential of two rare NPM1-fusion proteins, NPM1-MLF1 and NPM1-CCDC28A. NPM1-MLF1 is present in both the nucleus and cytoplasm and occasionally induces AML in the mouse transplantation assay (2/12). NPM1-CCDC28A localizes predominantly to the cytoplasm, immortalizes mouse bone marrow cells in vitro and efficiently (5/6) induces AML in vivo. Mechanistically, both NPM1-fusions bind to the HOX gene cluster and, like NPM1c, cause aberrant upregulation of HOX genes in cooperation with XPO1. The XPO1 inhibitor selinexor suppressed HOX activation and colony formation driven by the NPM1-fusions. Thus, our study provides experimental evidence that both NPM1-MLF1 and NPM1-CCDC28A are oncogenes with functions similar to NPM1c. Inhibition of XPO1 may be a promising strategy for the NPM1-rearranged AML. Overall design: To examine the molecular changes induced by NPM1 fusions, we first examined the gene expression profiles of cells expressing vector, NPM1-MLF1 or NPM1-CCDC28A. We transplanted bone marrow cells transduced with retrovirus vector harboring, NPM1-MLF1 or NPM1-CCDC28A into recipient mice, and harvested GFP+ bone marrow cells two months after transplantation.