Introduction

Polycythaemia vera (PV), essential thrombocythemia (ET), idiopatic myelofibrosis (MF) and BCR-ABL1 chronic myeloid leukaemia (CML) are distinctive haematopoietic diseases from the group of the myeloproliferative neoplasms (MPN). Clinical and biological features set CML apart from PV, ET and MF, and these three entities have some important characteristics in common. They are all caused by the clonal expansion of a multipotent haematopoietic progenitor cell, maintain a relatively preserved haematopoietic differentiation, develop some degree of bone marrow fibrosis, and can evolve into each other or transform into more aggressive haematological cancers.1 In PV, the oncogenic process targets a multipotent precursor committed to the erythroid lineage, causing an expansion of the red cell mass in the peripheral blood and a loss of response to the physiologic regulation of erythropoiesis. In MF, the target is a pluripotent haematopoietic precursor whose transformation leads to an intense stromal proliferation in the bone marrow, and a variety of peripheral blood manifestations.2 In ET, the oncogenic process results in megakaryocytic hyperplasia in the bone marrow and increased platelet counts in the peripheral blood.1 Despite such distinctive phenotypic features, PV, MF and ET can share some important findings at the molecular level, as the identification of a shared clonal marker. Molecular studies have identified a point mutation in the JAK2 gene on PV, MF and ET. The JAK2V617F mutation leads to the constitutive activation of the JAK2 kinase that can be found in 95% of patients with PV, and in 50–60% of patients with MF and ET.3 Due to such high frequencies, the JAK2V617F mutation has become one of WHO's criteria for the diagnosis of PV, MF and ET. Although the JAK2V617F mutation has been also described with low frequencies in different groups of haematological malignancies,4 its identification in cells from the peripheral blood in the appropriate clinical context is virtually diagnostic of a BCR-ABL1-negative MPN. On the other hand, a negative test does not definitely exclude PV, MF and ET in the presence of other clinical and haematologic features of MPN. In this scenario, further molecular investigations are suggested, as the search for mutations in the Exon 12 of the JAK2 gene (Exon12_JAK2), or in the position 515 of the MPL gene (MPLW515K/L). Exon12_JAK2 mutations have been found in approximately 2–3% of PV patients; those mutations are clustered between codons 533 and 547, where they disrupt the pseudo-kinase domain, promoting its constitutive activation.4 MPL gene is predicted to be a haematopoietic cytokine receptor acting through JAK2 intracellular transduction,5 and the MPLW515L mutation results in a constitutive activation of JAK-STAT signalling. Another mutation in this position—MPLW515K—was later identified, but its precise effects on signalling are not yet explained. The 515 tryptophan (W515) is the key amino acid located in a unique amphipathic domain that prevents spontaneous activation of MPL,6 and either MPLW515K/L mutations meet the WHO diagnostic criterion for ET or MF.

Although major diagnostic criteria for MPN are based on clinical features, blood cell counts, hormone levels and bone marrow morphology, these data are not always available or sufficient to conclude a diagnosis. In this regard, novel tools that are fast, convenient and widely available could be helpful in this clinical setting. Thus, the present work was designed to evaluate the frequency of JAK2V617F, Exon12_JAK2 and MPLW515K/L mutations in Brazilian patients clinically suspected to have MPN.