Acute myeloid leukemia with translocation t(3;5): new molecular insights.

AML with translocation t(3;5) belongs to the “AML with myelodysplasia-related changes” defined in the 2008 WHO classification. The incidence of this balanced abnormality is less than 0.5% of AML. The identified breakpoints occur at 3q25.1 on chromosome 3 and at 5q34 on chromosome 5, where the nucleolar phosphoprotein nucleophosmin 1 (NPM1) is located. At locus 3q25.1, Yoneda-Kato et al. identified a new gene, myeloid/myelodysplastic leukemia factor 1 (MLF1), and thus highlighted the fusion transcript NPM1-MLF1. The physiological role of MLF1 has not been well characterized. In hematologic diseases, MLF1was found to be overexpressed in more than 25% of myelodysplastic syndrome (MDS) in transformation phase and MDS-associated AML. NPM1, the partner gene of MLF1 in the t(3;5)(q25.1;q34) translocation, is better known for being affected by a 4 bp insertion in exon 12 that occurs in 3035% of all AML cases. To better characterize AML with NPM1-MLF1, we report morphological, immunophenotypic, cytogenetic features, and the first description of gene mutations analysis and gene expression profiling (GEP) in this cytogenetic entity. This study included 7 cases diagnosed between 2002 and 2011 in France; case n. 1 has been reported previously. The main clinical and biological characteristics of the patients studied are shown in Table 1. Molecular analysis was performed on cryo-preserved bone marrow mononuclear cells. The RT-PCR for the detection of NPM1-MLF1 fusion transcript was carried out using the primers described by Yoneda-Kato et al. The screening for mutations in NPM1, FLT3, CEBPA, WT1, IDH1/2, DNMT3A was performed as previously reported. GEP was performed in 4 patients (UPN 2, 3, 6 and 7), according to standard protocol with Genechip Affymetrix HG 133 plus 2.0 array. The analysis of the Acute Leukemia French Association (ALFA) trials database showed a very low incidence (3 of 1333; 0.23%) of AML with NPM1-MLF1 among adult AML patients with available karyotype. This incidence is consistent with that reported by Grimwade et al. (<0.5%). The mean age of patients with NPM1-MLF1 was 24 years. Cytological characteristics showed that a 3lineage dysplasia was present in the great majority of cases, both in peripheral blood and bone marrow smears. Dysmegakaryopoiesis was constantly observed, dysgranulopoiesis was associated with peroxidase deficiency in 3 cases, and dyserythropoiesis occurred in 5 of 7 cases. Flow cytometry analysis showed that blast cells were positive for the myeloid antigens CD117, CD13, and CD33. CD34 was negative in 6 of 7 patients, as frequently observed in NPM1 mutated AML. Cytogenetic analysis showed that the translocation t(3;5)(q25.1;q34) was present as the sole abnormality at diagnosis in all cases. The presence of the chimeric fusion transcript NPM1-MLF1 was confirmed in each case. Furthermore, all 7 cases harbored WT1 overexpression and at least one mutation (WT1 exon 7, FLT3-ITD and/or IDH2R140 mutation) was identified in the 4 adult cases (Table 1). In contrast, no mutation was found in the 3 pediatric cases. Considering age at AML diagnosis, associated gene mutations in NPM1-MLF1 positive AML seem to be similar to those in NPM1 mutated AML. DNMT3A mutations, that occur in 50-60% of NPM1 mutated AML, were not found in our cohort. GEP of AML with NPM1MLF1 was performed searching for a specific signature associated with this translocation. In order to do this, we compared 4 patients with NPM1-MLF1 to a reference