Does eventually NPM1 mutation in blast phase chronic myeloid leukemia (BP‐CML) exist? That is the question.

Abstract

The difference between chromosome Philadelphia Acute Myeloid Leukaemia (Ph+AML) and de novo blast phase chronic myeloid leukaemia (BP-CML) used to be a bone of contention until clear separation appeared in the WHO 2016 classification, leading to a provisional entity for Ph+ AML. The fact that the NPM1 mutation, a hallmark of AML, could be associated with BP-CML is another matter of controversy. Georgiou first reported a case about the discovery of NPM1+AML in a Phnegative clone during Chronic Phase CML (CP-CML) imatinib treatment. This brought Watkins et al. to check 14 BP-CML and 33 OCT1-Low CP-CML for NPM1 mutations. They found none and concluded that NPM1 mutations play no role in CML disease initiation or progression to blast crisis. Likewise, Konoplev found no NPM1 mutations in five de novo BP-CML and 37 CML with subsequent BP. They rather identified six Ph+AML patients harboring NPM1 mutations and outlined some criteria, in which NPM1 mutation, to discriminate Ph+AML from BPCML. Yet, the following case is more likely to be a de novo BPCML harbouring an NPM1mutation, than a Ph+AML. A 57-year-old man with a history of smoking, hypercholesterolaemia, hypertension and cardiac infarction with a triple coronary bypass was referred into our institution in October 2017 with the following complete blood count (CBC): hyperleucocytosis (222 g/l) with 26% of blasts, 11% basophils, 2% eosinophils and 42% myelocytes and metamyelocytes. Physical examination revealed a spleen enlargement slightly over the costal margin and infiltrative skin papules on the broadsides, suggesting chloromas. Abdominal echography confirmed the splenomegaly (135 9 48 9 120 mm). The other laboratory findings were thrombocytopenia (78 g/l) and anaemia (haemoglobin: 90 g/l), high LDH level (1461 u/l). The bone marrow aspirate showed chronic myeloid leukaemia features with myeloblasts (9%), promyelocytes (8%), myelocytes (21%), metamyelocytes (9%), neutrophils (18%), eosinophils (11%), basophils (15%), erythroblasts (4%), monocytes (2%) and lymphocytes (1%) on a very rich smear. The flow cytometry identified two subsets of blasts out of 310 000 nucleated cells: the first one (4%) positively expressed HLA-DR, CD117, CD13, CD33, CD38, CD7 and MPO, but the CD34 marker was negative. The second subset (6%) expressed the same markers except for HLA-DR, CD7 and CD34. We also observed biological disseminated intravascular coagulation (DIC). As usual, when AML is suspected, entitled mutations are screened on the marrow sample. NPM1 type A (c.956_959dupTCTG) mutation was identified using the RTMLPA methodology. Real time PCR allowed quantification of NPM1 (NPM1-ABL/ABL at 247%) and detected a WT1 overexpression. Granulocytic sarcoma was found in the skin biopsy. The cytogenetic analysis revealed the exclusive presence of the Philadelphia chromosome. RT-PCR for BCR-ABL confirmed the chimeric protein p210(b3-a2). No ABL1 mutations were found by Next-Generation-Sequencing (NGS). Subsequently, using the oncomine myeloid research assay panel of ThermoFisher Scientific (3% sensitivity for point mutations and 5% for indels), the NGS analysis singled out the NPM1 type A mutation with variant allele frequency at 42 6% on the diagnostic bone marrow sample. Array Comparative Genomic Hybridisation (aCGH) (SurePrint G3-Human CGH Microarray 8x60k of Agilent ) showed no unbalanced abnormalities. The patient was treated with dasatinib 100 mg QD. DIC and skin localisations receded in one week. He achieved complete haematologic and minor cytogenetic responses at three months, and complete cytogenetic and major molecular responses (MMR) at six months. Figure 1 shows the BCRABL1 ratio evolution with a deep molecular response, MR4 (4 log reduction in BCR-ABL ratio) by 24 months, then MR5 (undetectable) by 30 months and the total disappearance of NPM1 within 12 months. At first glance the cytological features concurred with CML, although there was no history of leucocytosis (the last available CBC dating from one year before the diagnosis was normal). The peripheral blast count, however – particularly the skin blast localisation – assigned the patient to BP-CML according to ELN and WHO classification. Moreover, in the Neuendorff algorithm (Fig 2), these aligned for BP-CML, not Ph+AML, regardless of the NPM1 mutation. In literature, considering the concomitant association with the NPM1 mutation, only one reported case matched this algorithm for BP-CML. The potential second case lacked initial clinical and biological features to clearly ascertain the diagnosis of BP-CML and the authors finally retained Ph+AML on the grounds of the presence of the NPM1 mutation. The incidence of the NPM1 mutation is 22% in nine Ph+AML patients. Loss of IKZF1 and/or CDKN2A, accompanied with cryptic deletions in immunoglobulin heavy chain (IGH) and T cell receptor alpha locus (TRA), characterised the diagnosis of Ph+AML. Here, no such losses were observed, but our aCGH assay did not cover IGH and TRA loci, and the infiltrative blasts represented less than 20% in the bone marrow (threshold for detection of abnormalities). The NGS techniques could not help to distinguish between the two entities as it showed neither the recurrent genes mutations in BP-CML (ASXL1, RUNX1, DNMT3A, JAK2, TP53, TET2) nor in NPM1+AML (IDH1, 2. . .). Correspondence