Concomitant Detection of BCR-ABL Translocation and JAK2 V617F Mutation In 6 Myeloproliferative Neoplasm at Diagnosis

Abstract 4832

Background Myeloproliferatives neoplasms (MPN) are clonal stem cell disorders characterized by proliferation of one or more of the myeloid lineages, associated with genetic abnormalities that include translocations or point mutations of genes that encode cytoplasmic or tyrosin quinase (TK) receptor proteins. This produces an abnormal constitutively activation of signal transduction pathways, leading to an unregulated proliferation. According to the WHO criteria, MPN are classified into BCR-ABL⁺/Philadelfia Ph⁺: chronic myeloid leukemia (CML) and MPN BCR-ABL fusion^-/Ph^-. A single acquired point mutation, JAK2^V617F, has been described in 95% of Polycytemia vera (PV), in 50 % of essencial thrombocythemia (ET) and idiopathic myelofivrosis (IMF), and generally absent in MPN Ph+. In the last years, it has been described the co-ocurrence of both BCR-ABL and V617F mutation in few cases of CML patients. We report here the rare and concomitant ocurrence of JAK2^V617F mutation with BCR-ABL translocation at presentation in atypical CML. Methods Blood samples from six patients with clinical suspicion of MPN diagnosis, were referred to our laboratory to cytogenetic studies and molecular analysis of BCR-ABL fusion gene expression by conventional RT-PCR and JAK2^V617F status mutation by ASO-PCR in three of them. All patients showed a slightly elevated white blood cells level (7200-25900), trombocythosis (700 -1036 platelets) and small splenomegaly. Three patients, after CML diagnosis, recieved Imatinib therapy and were monitored by quantitative BCR-ABL real time PCR. Due to persistent thrombocytosis, slightly elevated white blood cells level and small splenomegaly; JAK2 status was analized in these blood specimens, and later retrospectively in the stored initial diagnosis samples. Results BCR-ABL rearrangement (b3a2 isoform) and JAK2^V617F mutation were identified in all 6 patients at diagnosis. Three cases showed lack of Ph chromosome, 1 patient showed 15 % Ph+ metaphases and in the remained two patients no data was available. Quantitative PCR for BCR-ABL expression performed in 3 patients during follow-up (8-12 months) showed BCR-ABL/BCR ratio < = 0.0018 % (scored according to the International Scale) and the presence of JAK2^V617F mutation. Retrospective assessment of stored bood samples showed that JaK mutation was already present at the time of the diagnosis of CML. Conclusions The coexistence of both genetic defects, BCR-ABL fusion gene and JAK2^V617F mutation in NMP patients is a rare and uncommon feature. We found 6 MPN patients hourboring both genetics features in blood dignosis samples. In three cases JAK2^V617F mutation was detected in MPN patients BCR-ABL positive after the remission induction with Imatinib. The rapid remission of BCR-ABL transcript, after a short period of Imatinib treatment, led us to think that BCR-ABL fusion gene expression was present in a low burden at diagnosis. The complete reduction of BCR-ABL rearrangement, after the imatinib therapy, and the persistence of JAK 2 mutation suggests two possible mechanisms for this double genetic alteration: 1) a haematopoyetic cell subclone with a pre-existing JAK2^V617F acquires the BCR-ABL fusion gene, which confers a selective advantage to double mutant progenitor; 2) two clons, one of them having BCR-ABL rearrangement, and the other one the JAK2^V617F mutation (biclonal origin). These cases intend to contribute to the discussion about the onset of the molecular alterations, and their correlation with the differente phenotypes and clinical management.