Nucleophosmin mutations lead to abnormal, but reversible, nucleoli architecture and aggregate formation – Implications for NPM1-targeting therapies in AML

Mutations in the NPM1 gene represent the most common (>30% of patients) genetic alteration in Acute Myeloid Leukaemia (AML) and results in the mis-localisation of the mutated NPM1 protein from a predominantly nucleolar localisation to a predominantly cytoplasmic distribution. Numerous studies of NPM1 mutated AML have focussed on the aberrant cytoplasmic localisation of the mutated protein but efforts to reverse this mis-localisation therapeutically have so far resulted in limited clinical benefit. More recently, attention has shifted towards the nucleus with studies showing that mutant NPM1 binds to specific chromatin regions, where it directly regulates oncogenic gene expression. Here, we use high resolution imaging to demonstrate that Nucleophosmin (NPM1) is critical for maintaining normal nucleoli architecture and specifically the integrity of the nucleoli rim. We report for the first time that NPM1 mutated cell lines and primary samples have aberrant nucleoli architecture and demonstrate that the abnormal nucleoli phenotype is reversible. We also report the novel finding that NPM1 mutated protein forms distinct aggregates in NPM1 mutated cells and characterise these for the first time. This work reveals how nucleolar organisation contributes to the molecular mechanisms underpinning NPM1 driven AML and reveals unexpected novel vulnerabilities to be exploited for therapeutic intervention.