Blastic transformation of essential thrombocythemia: dual expression of myelomonoblastic/megakaryoblastic phenotypes.

Three patients developed blastic transformation of essential thrombocythemia (tET). Morphological studies in all patients showed that the majority of blasts had either myeloblastic or myelomonoblastic differentiation. Immunologic assays of hematopoietic cells were performed in two patients. In patient 1, 86% of peripheral blood mononuclear cells (predominantly blasts) reacted with a monoclonal antibody specific for granulocytes and monocytes (MMA), and 15% of mononuclear cells reacted with Tab, a monoclonal antibody specific for megakaryocyte-platelet glycoproteins (PGP) IIb and IIIa. In patient 2, 41.5% of peripheral blood mononuclear cells (predominantly blasts) were MMA-positive, 22.5% were Tab-positive, and 40% reacted with rabbit anti-human PGP. These results suggest either that two subpopulations of blast cells exist in tET, or that blast cells simultaneously express surface markers of myeloblastic/monoblastic and megakaryoblastic differentiation. In these three and in nine previously reported cases of tET, neither age, sex, nor previous therapy were obvious etiologic factors. tET occurred 24.2 +/- 14.4 mo after diagnosis of essential thrombocythemia, and a majority of patients had hepatomegaly and/or splenomegaly, anemia, leukocytosis, and thrombocytopenia. Leukemic cell morphology was myeloblastic and/or monoblastic in 12/12 patients, 5/12 had marrow fibrosis. Despite various treatments, death occurred in 3.6 +/- 2.7 mo; one patient had a brief complete remission.