Abnormal nucleoli architecture and aggregate formation in Nucleophosmin mutated Acute Myeloid Leukaemia Open Access

Mutations in the Nucleophosmin (NPM1) gene represent the most common genetic alteration in Acute Myeloid Leukaemia (AML) and result in mis-localisation of the mutated protein from a predominantly nucleolar localisation to a predominantly cytoplasmic distribution. Here, we use high resolution imaging to demonstrate that NPM1 is critical for maintaining normal nucleoli architecture and specifically the integrity of the enigmatic nucleoli rim, the least understood nucleolar compartment. We demonstrate that cell lines and primary AML patient cells with NPM1 mutations have aberrant nucleoli architecture; intriguingly this abnormal nucleolar phenotype is reversible. Using a surrogate for rRNA synthesis, we show that the aberrant phenotype is associated with differences in nucleolar function, specifically activity is increased in NPM1 mutated cells. Perinucleolar chromatin organisation is also markedly different in NPM1 mutant cells. Finally, we report the novel finding that NPM1 mutated protein forms distinct aggregates and characterise these for the first time. This work reveals how nucleolar organisation contributes to the molecular mechanisms underpinning NPM1 driven AML revealing novel therapeutic vulnerabilities.