Philadelphia chromosome-positive thrombocythemia and megakaryoblast leukemia.

A case of well-documented and -illustrated megakaryoblastic transformation is described in a patient with thrombocythemia passing through a stage of myelofibrosis without features of chronic granulocytic leukemia. Immunocytologic studies with the use of conventional and monoclonal antibodies against platelet membrane glycoproteins and electron microscopic investigations, demonstrating bull's-eye granules and platelet peroxidase positivity, proved the megakaryocytic differentiation of the blast cells. From the onset of the disease as well as during the megakaryoblastic transformation, the Philadelphia (Ph1) karyotype, 46XX t(9:22) (q34:q11), was found in peripheral blood and bone marrow cells as the only clonal abnormality. Southern blot analysis of DNA extracted from the blast cells revealed a rearrangement within the bcr on chromosome 22 similar to findings in chronic granulocytic leukemia. The presentation with excessive small and abnormal megakaryocytes in the initial and subsequent bone marrow and the rapid progressive myelofibrosis and splenomegaly differentiate the Ph1 chromosome-positive thrombocythemia from the chronic myeloproliferation of thrombocythemia in its primary form or associated with polycythemia vera.