Genesis of variant Philadelphia chromosome translocations in chronic myelocytic leukemia

The Philadelphia (Ph) chromosome is found in more than 90% of chronic myelocytic leukemia (CML) patients. In most cases, it results from the reciprocal t(9;22)(q34;q11), with the ABL proto-oncogene from 9q34 fused to the breakpoint cluster region (BCR) locus on 22q11. In 5%-10% of patients with CML, the Ph chromosome originates from variant translocations, involving various breakpoints in addition to 9q34 and 22q11. In our investigation, three CML cases with complex Ph translocations have been analyzed by G-banding and fluorescence in situ hybridization (FISH). FISH with breakpoint-spanning probes for the BCR and ABL genes revealed information about the genesis of complex Ph translocations. The occurrence of one fusion signal indicates a one-step mechanism (case 1). Two fusion signals indicate a two-step mechanism (case 2). Lack of signals indicates deletions of parts of the BCR and ABL genes or of adjacent regions (case 3).