Mutations in exon 12 of the JAK2 gene have been identified in a minority of patients with myeloproliferative disorders (MPD) and are associated with a selective increase in erythropoiesis resulting in polycythemia vera (PV) or idiopathic erythrocytosis. We compared the lineage distribution of JAK2 mutations in the peripheral blood of 8 PV patients with mutations in exon 12 and 21 PV patients with the JAK2-V617F mutation. Five different exon 12 mutations were observed in the 8 patients studied. Peripheral blood cells were fractionated to obtain granulocytes, platelets and mononuclear cells, which were further separated by FACS into T cells, B cells, NK cells and monocytes. Using a sensitive and quantitative assay to assess the percentages of chromosomes carrying exon 12 mutations, we detected exon 12 mutations in purified granulocytes, monocytes and platelets of all patients studied. A similar distribution was also found for the JAK2-V617F mutation. Exon 12 mutation was absent in sorted lymphoid cells of 5/8 patients, in 2/8 patients only NK cells were positive and in 1/8 patients also B cells carried the mutation. Exon 12 mutations were absent in T cells of all patients studied and JAK2-V617F was present in T cells of only 1/21 patients. Thus, inter-individual differences are notable in the involvement of lymphoid lineages for both exon 12 and JAK2-V617F mutations. To determine the presence of the JAK2 mutations in the erythroid lineage, we performed colony assays in methylcellulose, picked single erythroid colonies grown in the presence or absence of Epo and determined the allelic ratios for each individual colony. In 4/8 patients an exon 12 mutation (E543-D544del) was present in all EECs examined and similarly, in 8/12 patients the JAK2-V617F mutation was found in all EECs. In the remaining patients, we detected some EECs with only the wild type JAK2, suggesting that additional clonal events may also be present in some patients with exon 12 mutations. Interestingly, one patient carried exon 12 and JAK2-V617F mutations. None of the erythroid colonies in this patient carried both mutations simultaneously, indicating that the exon 12 mutation and JAK2-V617F represent two separate clones. One patient displayed erythroid colonies homozygous for the exon 12 mutation and an allelic ratio greater that 50% in granulocytes, indicating that progression to homozygosity can occur in some patients. The lineage distributions of exon 12 mutations and JAK2-V617F are similar and do not explain why exon 12 mutations are associated solely with PV phenotype, whereas JAK2-V617F can also cause essential thrombocythemia or primary myelofibrosis.

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