Identification of a novel NPM1 mutation in acute myeloid leukemia.

Abstract

e19019 Background: Nucleophosmin (NPM1) is predominant nucleolar localization. NPM1-mutant acute myeloid leukemia (AML) account for 25-35% of adult AML patients, has been defined as a distinct AML entity in the 2022 WHO classification. The majority of NPM1 mutations reported affect exon 12 (classical type A mutation), account for 75-80% of adult NPM1-mutated AML cases, which generate an extra nuclear export signal (NES) in the C-terminal leading to aberrant cytoplasmic localization. Similar aberrant cytoplasmic translocation has also been reported in NPM1 mutants that identified in exons 9 and 11. Identification of novel non-type A mutants is paramount for diagnosis, risk stratification, and prognosis for targeted population. Methods: Somatic mutation analysis is performed on 566 AML patients using next-generation sequencing (NGS). Immunofluorescence and immunohistochemistry were used to identify the cytological localization of mutant NPM1. HEK-293T cells transiently transfected with plasmids for ectopic expression of the GFP-mutated NPM1 fusion proteins were treated with the specific Crm1/XPO1 inhibitor leptomycin B and to evaluate the NES dependence of their subcellular localization. Results: We identified one AML patient with NPM1 mutant located on exon 5. This patient was a 59-year-old female with de novo AML, with an 18-nucletide in-frame insertion at position 405 (c.405_406insGCCCTGGAACTGGGGAAC, named MutSong) in the middle of exon 5 [variant allele fraction (VAF), 11.9%]. It generated a new NPM1 mutant protein (p.135insALELGN), and containing a leucine-rich NES. Notably, different with all NPM1 mutant in other exons reported so far, our exon 5 mutant retained the functional c-terminal but insert one leucine-rich NES, similar finding was recently reported in NPM1 mutants that identified in exon 5 but on different position. This NPM1 mutant in exon 5 resulted in cytoplasmic localization both by confocal and immunohistochemistry in AML patient sample. HEK-293T overexpressing the new GFP-NPM1 exon 5 fusion protein indicated the aberrant localization in the cytoplasm and partially in nucleoli. Moreover, NES-dependent cytoplasmic localization was inhibited by exportin-1 inhibitor leptomycin B, indicated that the novel NES generated by exon 5 mutant is responsible for cytoplasmic localization. This patient carried WT1, IKZF1, JAK2, and NUP98 mutations, and was intravenously treated with daunorubicin plus cytarabine for induction followed by 3 cycles of intermediate-dose cytarabine as consolidation. Then the patient received allogeneic hematopoietic stem cell transplantation (allo-HSCT). Unfortunately, the patient died of severe pulmonary infection and viral encephalitis 14 months after the initial remission, 7 months after allo-HSCT. Conclusions: Our finding of the novel NPM1 mutation in exon 5 supported that beside exon 12, exon 5 mutant is another NPM1 “born to be exported” mutant critical for leukemogenesis.