Essential thrombocythaemia and the Philadelphia chromosome.

Six adult patients presented with clinical features of essential thrombocythaemia. Five of the patients, although Ph-positive, have maintained these features without evidence of leukaemia; in one case for 9 years. A sixth patient developed leukaemic blast crisis following a persistently high platelet count over 4 years. Her cells were Ph-negative, but hybridization of gene probes to chromosomes in situ and to leukaemic DNA showed that the abl oncogene had moved to the breakpoint cluster region (bcr) on the normal chromosome 22. This patient has the same molecular gene change as occurs in some cases of Ph-negative chronic myeloid leukaemia (CML) whose leukaemic cells likewise show no evidence of chromosomal translocation. Molecular studies are essential for the correct diagnosis of these patients. The Ph genomic lesion appears to have a range of leukaemic expression which includes thrombocythaemia as well as chronic myeloid leukaemia and acute lymphatic leukaemia.