A fluorescence in situ hybridization study of complex t(9;22) in two chronic myelocytic leukemia cases with a masked Philadelphia chromosome.

The t(9;22)(q34;q11) is evident in more than 90% of patients with chronic myelocytic leukemia (CML) and gives rise to the Philadelphia chromosome (Ph). Approximately 5%-10% of CML patients show variant translocations involving other chromosomes in addition to chromosomes 9 and 22. In some variant translocations, additional material is transferred on der(22), resulting in a masked Ph chromosome. In this paper, we report two apparently Ph-negative (Ph-) CML cases showing a t(7;9;22)(q22;q34;q11) and a t(8;9;22)(q12;q34;q11), respectively. A detailed molecular cytogenetic characterization was performed by fluorescence in situ hybridization (FISH), which disclosed the presence of the 5'BCR/3'ABL fusion gene on the der(7) and der(8) chromosomes, respectively. Derivative (22) appeared as a masked Ph chromosome in both cases. FISH analysis with appropriate BAC/PAC clones allowed us to precisely characterize the complex chromosomal rearrangements that were not detected by conventional cytogenetic analysis.