Molecular cytogenetic characterization of Philadelphia-negative rearrangements in chronic myeloid leukemia patients

The BCR/ABL gene rearrangement is generated by a reciprocal translocation t(9;22)(q34;q11) in chronic myeloid leukemia (CML) patients. In most cases, it is cytogenetically visualized by the Philadelphia (Ph) chromosome. About 5-10% of CML patients lack cytogenetic evidence of the Ph translocation but show BCR/ABL fusion by fluorescence in situ hybridization (FISH) or reverse transcriptase-polymerase chain reaction (RT-PCR). Deletions around the breakpoints on derivative chromosome 9 including 5'ABL and 3'BCR sequences occur in 10-15% of Ph-positive CML patients and are thought to have prognostic significance.

We explored cryptic rearrangements involving chromosomes 9 and 22 in 3 CML patients with an apparently normal bone marrow karyotypes using multiplex RT-PCR and FISH with commercial and home-brew probes.

The BCR/ABL fusion transcripts were detected by RT-PCR. Using commercial FISH probes, the BCR/ABL fusion gene was found on chromosome 22 in two patients and on chromosome 9 in one patient. Consecutive FISH assays clarified the mechanism of the masked Ph chromosome: in the 3 patients, Ph rearrangement resulted from double mechanism consisting in standard translocation t(9;22)(q34;q11) followed by a second reversed translocation t(9;22)(q34;q11). One patient achieved major cytogenetic response after 6 months of imatinib therapy, and one patient had successful bone marrow transplant.

In this study, we have characterized three Ph-negative CML patients with cryptic BCR/ABL rearrangement generated after an uncommon mechanism involving two sequential translocations and confirm that the BCR/ABL hybrid gene may be located on other sites than 22q11. Ph-negative CML patients with BCR/ABL fusion gene have the same prognosis as patients with classical t(9;22).