Published online 11 February 2008
Haematologica, Vol 93, Issue 3, 439-442 doi:10.3324/haematol.12153
Copyright © 2008 by Ferrata Storti Foundation
NPM1 mutations and cytoplasmic nucleophosmin are mutually exclusive of recurrent genetic abnormalities: a comparative analysis of 2562 patients with acute myeloid leukemia
Brunangelo Falini1,,
Cristina Mecucci1,
Giuseppe Saglio2,
Francesco Lo Coco3,
Daniela Diverio4,
Patrick Brown5,
Fabrizio Pane6,
Marco Mancini4,
Maria Paola Martelli1,
Stefano Pileri7,
Torsten Haferlach8,
Claudia Haferlach8,
Susanne Schnittger8
1 Institute of Hematology, University of Perugia, Perugia, Italy
2 Department of Clinical and Biological Sciences, University of Turin, San Luigi Hospital, Orbassano, Turin, Italy
3 Department of Biopathology and Diagnostic Imaging, Policlinico Tor Vergata, Rome, Italy
4 Department of Cellular Biotechnologies and Hematology, Policlinico "La Sapienza", Rome, Italy
5 Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, USA
6 Department of Biochemistry and Medical Biotechnologies, University "Federico II", Neples, Italy
7 Laboratory of Hematopathology, University of Bologna, Policlinico S. Orsola, Bologna, Italy and
8 Munich Leukemia Laboratory GmbH, Munich, Germany
Correspondence: Brunangelo Falini, MD, Institute of Hematology, University of Perugia, Perugia, Italy. E-mail: faliniem{at}unipg.it

ABSTRACT
Acute myeloid leukemia carrying
NPM1 mutations and cytoplasmic
nucleophosmin (NPMc
+ acute myeloid leukemia) represents one-third
of adult
AML (50–60% of all acute myeloid leukemia with
normal karyotype) and shows distinct biological, pathological
and clinical features. We confirm in 2562 patients with acute
myeloid leukemia our previous observation that
NPM1 mutations
and cytoplasmic nucleophosmin are mutually exclusive of recurrent
genetic abnormalities. Taken together, these findings make NPMc+
acute myeloid leukemia a good candidate for inclusion in the
upcoming World Health Organization classification.
Key words: acute myeloid leukemia, nucleophosmin, NPM, mutations, antibodies, immunohistochemistry.

Introduction
One of the most controversial issues in the WHO classification
of myeloid malignancies is
acute myeloid leukemias not otherwise characterized.
1 This category accounts for 60–70% of AML,
including cases with normal cytogenetics (AML-NC) which are
characterized by a great molecular, pathological and clinical
heterogeneity.
AML not otherwise characterized is currently
defined according to FAB criteria which are, however, insufficient
to characterize AML-NC, since they do not identify distinct
disease entities within this heterogeneous group. Recognition
of underlying molecular alterations is crucial for the improvement
of AML-NC classification.
We previously identified nucleophosmin (NPM1) mutations, causing aberrant cytoplasmic expression of NPM,2–4 as the most frequent genetic alteration associated with AML-NC (50–60% of cases). NPM1 mutations and cytoplasmic NPM appear to be mutually exclusive of recurrent genetic abnormalities2 and identify a leukemia subgroup called NPMc+ (cytoplasmic-positive) AML that show distinct biological, pathological and clinical features,5,6 as well as a unique gene expression profile.7 For these reasons, NPMc+ AML (as well as AML with CEBPA mutations that also appear to be mutually exclusive with recurrent genetic abnormalities) is a good candidate for inclusion as a new entity in the forthcoming WHO classification. However, since a few studies8–11 have reported very rare cases of NPM1 mutations coinciding with either inv(16), t(8;21) or t(9;22), we extended our analysis to 2562 patients with acute myeloid leukemia.

Design and Methods
Leukemic samples
The aim of the study for AML patients enrolled in the GIMEMA
LAM99P and the GIMEMA/EORTC AML12 trials was to compare the
subcellular expression of NPM with the results of molecular
studies for recurrent genetic alterations. The rationale for
this approach derives from our previous observations that immunohistochemistry
is fully predictive of
NPM1 mutation status.
12 Bone marrow biopsies
for immunohistochemical studies were available from 1073 unselected,
consecutively observed AML cases, 591 of whom had been previously
reported.
2 Ninety-six acute promyelocytic leukemia cases (not
included in the GIMEMA LAM 99P and GIMEMA/EORCT-AML12) were
also available for comparative immunohistochemical and molecular
analysis. In addition, 1403 unselected AML cases were investigated
at the Munich Leukemia Laboratory (MLL). Cytogenetics failed
in 10 cases and these were excluded from the present study.
Therefore, the Munich cohort consisted of 1393 consecutively
observed, unselected newly diagnosed AML for which complete
cytogenetic/molecular studies were available. In these cases,
we compared the results of
NPM1 mutational analysis
13 and cytogenetic/molecular
studies. Written informed consents for the study was obtained
from each participating center.
Immunohistochemical studies
Immunohistochemistry for NPM was available in 1073 AML cases, 591 of whom had been previously reported.2 NPM subcellular expression was detected in paraffin-sections from B5-fixed/EDTA decalcified bone marrow trephines using a specific anti-NPM monoclonal antibody (clone 376) and the highly sensitive alkaline phosphatase anti-alkaline phosphatase (APAAP) technique, as previously described.2 Monoclonal antibody against nucleolin/C23 was purchased from Santa Cruz Biotechnology.
Cytogenetics and molecular studies for recurrent genetic abnormalities
Cytogenetic investigation was performed after short-term culture. Karyotypes were analyzed after G-banding and described according to the International System for Human Cytogenetic Nomenclature.14 Fluorescence in situ hybridization was carried out according to standard techniques. Reverse-transcriptase-polymerase-chain-reaction (RT-PCR) for AML1-ETO, CBFB-MYH11, DEK-CAN and BCR-ABL, performed as previously reported,2 was available in 1019/1073, 1020/1073, 936/1073 and 1015/1073 AML cases respectively. Southern blotting and FISH for rearrangements of the mixed-lineage leukemia gene (MLL)2 was carried out in 728/1073 patients.
Mutational analysis of the NPM1 gene
Mononucleated cells were isolated by standard Ficoll-Hypaque density gradient centrifugation. Nucleic acid isolation, cDNA synthesis and screening for NPM1 gene mutations were performed using a melting curve based LightCycler assay, as previously described.13 AML samples with an aberrant melting curve underwent nucleotide sequence analysis.

Results and Discussion
Of the 1073 bone marrow biopsies from patients of the GIMEMA
LAM99P and GIMEMA/EORTC AML12 trials, 368 (34.2%) were characterized
by aberrant cytoplasmic expression of nucleophosmin (NPMc+)
associated with nucleus-restricted positivity for nucleolin
(C23) (
Figure 1). As previously reported,
12 this staining pattern
is fully predictive of a mutated
NPM1 gene. All other biopsies
showed a nucleus-restricted positivity for nucleophosmin (NPMc-)
which is fully predictive of
NPM1 gene in a germline configuration.
12 Comparison of immunohistochemistry with the results of molecular
studies for the recurrent genetic abnormalities is shown in
Table 1. Remarkably, all 300 AMLs carrying various types of
recurrent genetic abnormalities showed a nucleus-restricted
expression of NPM. Representative examples of these staining
patterns in AML carrying the most frequent recurrent genetic
abnormalities, i.e. t(15;17), inv(16) and t(8;21), are shown
in
Figure 2. Completely overlapping results were obtained in
cases from the Munich Leukemia Laboratory, by comparing the
mutation status of the
NPM1 gene with the results on cytogenetic/molecular
studies for recurrent genetic abnormalities. In total, we investigated
1393 AML cases, 835 of which had a normal and 558 an abnormal
karyotype.
NPM1 mutations were detected in 420 out of 1393 patients
(30.1%) while the other cases harboured a
NPM1 gene in a germline
configuration. In 246 of 1393 patients, reciprocal translocations
and the respective fusion genes were detected; RT-PCR and/or
FISH were performed for all respective fusion genes. The results
are shown in
Table 2. Notably, none of the 246 AMLs carrying
different types of genetic abnormalities harbored
NPM1 mutations.
These results were obtained independently from a total of 2562
AMLs from two large series using different approaches, support
our original finding that, cytoplasmic mutated NPM is mutually
exclusive of recurrent genetic abnormalities.
2 Our findings
raise several issues surrounding the significance of the sporadic
cases of
NPM1 mutations coinciding with recurrent genetic abnormalities,
as reported in some studies.
8–10 First, none of these
studies documented whether
NPM1 mutations and concurrent genetic
abnormalities occur in the same or different leukemic cell populations.
Another major concern is specificity control in large multi-center
trials. This is addressed in Falini B.
et al.
2 (Supplementary
Materials). In particular, 4 patients in the GIMEMA EORTC AML12
trial were diagnosed as having cytoplasmic mutated NPM together
with inv(16), t(8;21) or MLL rearrangement. In-depth re-investigation
of these cases revealed errors in sample registration or PCR
contamination. Because of the presence of
NPM1 mutation in association
with a t(16;16)
/MYH11-CBFB transcript, 1 patient from the CCG-2961
AML pediatric trial was also re-investigated and found not to
have a
NPM1 mutation (Patrick Brown, unpublished results). Therefore,
in our experience, re-analysis according to strict criteria
showed that
NPM1 mutations are mutually exclusive of recurrent
genetic abnormalities. Whether the occurrence of two or more
specific genetic markers in exceptional cases is just coincidental
or represents a true association is still not understood. Rare
examples include t(15;17) plus t(8;21),
15 t(15;17) plus t(9;22),
16 and t(9;22) or BCR-ABL plus inv(16).
17,18 Interestingly, 1 case
reported by Thiede
et al.
9 carried both a t(8;21) and an inv(16)
in addition to
NPM1 mutations with a
NPM1 mut/wt ratio of only
0.37 (
Christian Thiede, personal communication on February 6, 2006). More recently, coexistence of
JAK2V617F and
BCR-ABL has
also been reported in chronic myeloproliferative disorders
19,20 and association of
JAK2V617F with t(8;21) in AML therapy-related
21 or secondary to a myeloproliferative syndrome.
22 In conclusion,
our data unequivocally confirm in a large series of patients
with acute myeloid leukemia that
NPM1 mutations are mutually
exclusive of other recurrent genetic abnormalities and that
NPM1 mutations identify a distinct acute myeloid leukemia genetic
entity which should be considered for inclusion in the upcoming
WHO Classification.

Acknowledgments
we would like to thank Roberta Pacini and Manola Carini for
performing the immunohistochemical stainings and Mrs. Claudia
Tibidò for her secretarial assistance

Footnotes
Authorship and Disclosures
BF is responsible for the study concept and wrote the manuscript; GS, FLC, DD, FP, CM, MM were involved ni the molecular and cytogenetic studies of patients from GIMEMA/EORTC AML12; MPM and SP were involved in the immunohistochemical study of NPM expression in AML specimens. TH, CH and SS carried out the molecular analysis of cases from the Munich Leukemia Laboratory. BF and CM applied for a patent on the clinical use of NPM1 mutants.
Funding: This work was supported by the Associazione Italiana per la Ricerca sul Cancro (A.I.R.C.).
Received for publication August 24, 2007.
Accepted for publication December 17, 2007.

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