Cytoplasmic localization of nucleophosmin in bone marrow blasts of acute myeloid leukemia patients is not completely concordant with NPM1 mutation and is not predictive of prognosis

Abstract

Background: Nucleophosmin (NPM1) gene mutations are reported to predict a favorable prognosis in acute myeloid leukemia (AML) patients. Aberrant cytoplasmic localization of nucleophosmin (NPM) protein is reported be a surrogate for NPM1 gene mutation.

Methods: Using immunohistochemical (IHC) analysis, we assessed for NPM (clone 376) expression in formalin-fixed, formic acid-decalcified bone marrow biopsy specimens. DNA sequencing of exon 12 of NPM1 gene was performed in 104 patients.

Results: The study included 252 AML patients: 192 de novo AML, 33 AML preceded by either myelodysplastic syndrome or chronic myelomonocytic leukemia, and 27 therapy-related AML. The median age was 62 years and 115 patients were <or=60 years old. All patients received intensive chemotherapy. Cytoplasmic NPM was detected in 59 of 252 (23%) patients, including 48 of 192 (25%) de novo AML and 33 of 94 (35%) with a normal karyotype. DNA sequencing identified NPM1 mutations in 30 of 38 cases with cytoplasmic NPM and 10 of 66 cases with nuclear NPM. Cytoplasmic NPM was associated with young patient age (P=.024), FLT3/ITD (P=.005), CD34 negative blasts (P<.001), high peripheral blood blast count (P=.041), and high serum albumin level (P=.028). No statistical differences in overall or event-free survival were found on the basis of NPM localization. Similar results were obtained in patients<or=60 years old with normal karyotype and wild-type FLT3 (P=.768).

Conclusions: IHC assessment for NPM localization did not predict prognosis in this patient cohort. The discordance between immunohistochemistry and DNA sequencing results indicates that DNA sequencing cannot be replaced by IHC assessment.

Figure 1

Examples of acute myeloid leukemia (AML) cases with nuclear (A&B) and cytoplasmic (C&D) NPM expression. Anti-C23 antibody is used as a control and demonstrates nuclear staining in both cases (A and C). In a case of AML with nuclear NPM, anti-NPM antibody is similar to anti-C23 staining pattern (B). In a case of AML with cytoplasmic NPM, anti-NPM antibody demonstrates predominantly cytoplasmic staining (D). (A–D, immunohistochemistry, × 400)

Figure 2

No significant difference in overall survival between patients with cytoplasmic and nuclear NPM in total study group

Figure 3

No significant difference in overall survival between patients with cytoplasmic and nuclear NPM in a subset of patients with diploid cytogenetic and wild type FLT3.