Mapping nucleophosmin interactions for the targeted treatment of acute myeloid leukemia

Nucleophosmin (NPM1) is a nucleolar protein that plays a key role in ribosome maturation and export, centrosome duplication, cell cycle control and response to stress stimuli. Mutations at the terminal exon of the NPM1 gene are the most frequent genetic alteration in acute myeloid leukemia (AML). Mutations lead to a variant protein that loses its nucleolar localization signal, acquires a new nuclear export signal and is therefore stably and aberrantly translocated in the cytosol of leukemic blasts. This causes the loss of nuclear functions but also the acquisition of new and unwanted cytosolic functions. Α wealth of data suggest that heterozigous mutation of the NPM1 gene is the founder genetic lesion in this kind of leukemia and that NPM1 should be the target of specific treatment. However, given the modular structure of the protein and its multiple functions, it is less clear what to target “within” NPM1 and how to do it. Using a combination of structural and biophysical methods coupled to bioinformatics and site-directed mutagenesis, we have first discovered the interaction of NPM1 C-terminal domain with G-quadruplex DNA and dissected the structural causes for the loss of nucleolar affinity of the NPM1 leukemic variant. These data have suggested a way to deprive blast’s nucleoli of their residual NPM1 pool, which has been tested in model cell lines. In a complementary approach, we have investigated the interaction of NPM1 with several proteins as a possible target to interfere with the cytosolic functions gained by the leukemic variant. We have devised a common surface for protein recognition in the NPM1 N-terminal domain, identified key residues implicated in the binding process and also identified the structural requirements for NPM1 recognition in different proteins. Since studies aimed at the targeted treatment of AML with NPM1 mutations are still in their infancy, our data may help in the definition of the most promising strategies to reach this goal.