Identification of a novel NPM1 mutation in acute myeloid leukemia

Nucleophosmin (NPM1) is a widely expressed nucleocytoplasmic shuttling protein with prominent nucleolar localization. It is estimated that 25-35% of adult patients with acute myeloid leukemia (AML) carry NPM1 mutations. The classic NPM1 type A mutation occurs in exon 12, which accounts for 75-80% of adult patients with NPM1-mutated AML. It produces an additional leucine and valine-rich nuclear export signal (NES) at the C-terminus, and causes aberrant cytoplasmic dislocation of NPM1 protein. Notably, emerging evidence indicates that besides the classic type A mutation, rare mutants occurring in other exons may also lead to the imbalance of the nucleocytoplasmic shuttle of NPM1. Identification of novel non-type A mutants is crucial for the diagnosis, prognosis, risk stratification and disease monitoring of potential target populations. Here we reported a novel NPM1 mutation in exon 5 identified from a de novo AML patient. Similar to the classic type A mutation, the exon 5 mutation had the NPM1 mutant bound to exportin-1 and directed the mutant into the cytoplasm by generating an additional NES sequence, resulting in aberrant cytoplasmic dislocation of NPM1 protein, which could be reversed by exportin-1 inhibitor leptomycin B. Our findings strongly support that besides the exon 12 mutation, the exon 5 mutant is another NPM1 "born to be exported" mutant critical for leukemogenesis. Therefore, similar to the classic type A mutation, the identification of our novel NPM1 mutation is beneficial for clinical laboratory diagnosis, genetic risk assessment and MRD monitoring.