Essential thrombocythemia with the Philadelphia chromosome and BCR-ABL gene rearrangement. An entity distinct from chronic myeloid leukemia and Philadelphia chromosome-negative essential thrombocythemia.

A 64-year-old woman presented with a platelet count of 3,225 x 10(9)/L. Bone marrow morphology showed massive megakaryocytic hyperplasia; cytogenetic studies showed the presence of the Philadelphia chromosome (Ph). The presence of a rearrangement involving the major breakpoint cluster region (mbcr) on chromosome 22 was confirmed by Southern blotting techniques. A diagnosis of Ph positive essential thrombocythemia (ET) was made. Such cases constitute less than 5% of patients with ET and it has been proposed that they be considered examples of chronic myelogenous leukemia (CML) because of a shared propensity to progress to blast crisis. An argument is presented for retaining Ph positive ET as an entity separate from Ph negative ET and Ph positive CML.