[1247] Cytoplasmic Expression of Nucleophosmin Detected by Paraffin Immunohistochemistry Predicts Nucleophosmin Gene Mutations in Patients with Acute Myeloid Leukemia and Normal Karyotype

J Lou, C Qi, W Xu, J Brandwein, S Kamel-Reid, H Chang. University Health Network and University of Toronto, Toronto, Canada

Background: Mutations in nucleophosmin (NPM1) exon 12 and the resulting delocalization of NPM into the cytoplasm are the most frequent cellular events occurring in 50-60% of acute myeloid leukemia patients (AML) with normal karyotype. Recent studies have suggested that AML patients with NPM1 mutations make up a subgroup with a better prognosis. However, there have been discrepancies about the ability of immunohistochemical detection of cytoplasmic NPM (NPMc+) to predict NPM1 gene mutations. Here we investigated the correlation between cytoplasmic NPM immunopositivity and NPM1 gene mutations and their prognostic significance.
Design: Paraffin-embedded bone marrow specimens from AML patients with normal karyotype were immunostained with a monoclonal mouse anti-human NPM antibody (clone 376) that detects both mutant and wild-type NPM1. A case was classified as NPMc+ if the majority of the leukemic cells were positive for cytoplasmic NPM. NPM1 mutation and FLT3-ITD (internal tandem duplication) status were evaluated by multiplex RT-PCR. Clinical and laboratory features were obtained by chart review.
Results: Of the 77 patients, NPM1 mutations were detected in 43 (55.8%) and FLT3-ITD in 18 (23%) cases. NPM1 mutation was correlated with FLT3 ITD (p=0.006), CD33 (p=0.01) and lack of CD34 expression (p=0.0001), but not with age, gender, high WBC count, or FAB subtype. Of the 59 specimens immunostained for NPM, all 31 cases (53%) that were shown to be NPMc+ by IHC carried NPM mutations; none of the 28 cases (47%) with nucleus-restricted NPM1 (NPMc-) carried NPM1 mutations (p<0.0001). The complete remission (CR) rate was similar between patients with or without NPM1 mutations (79% vs 70%, p=0.36). However, a NPM1 mutation positive, FLT3-ITD negative subset had a favorable event free survival (EFS) (median not reached vs. 8.1 months, p=0.05) and a trend to a longer overall survival (OS) (median not reached vs 14.8 months, p=0.13) than patients without this genetic profile. FLT3-ITD positive patients had a worse EFS (5 months vs not reached, p<0.0001) and OS (8 months vs not reached, p<0.0001) than those without this genetic lesion regardless of NPM1 mutations status.
Conclusions: In the absence of FLT3-ITD, NPM1 mutation confers a favorable clinical outcome in normal karyotype AML. NPM IHC accurately predicts NPM1 mutations and warrants inclusion in the routine diagnostic and prognostic work-up of AML.
Category: Hematopathology

Wednesday, March 11, 2009 1:00 PM

Poster Session VI # 183, Wednesday Afternoon