Coexistence of Philadelphia chromosome positive cells with and without der(9) deletion in a patient with chronic myelogenous leukemia

Recently, large deletions adjacent to the Philadelphia (Ph) translocation breakpoint on the derivative chromosome 9 have been reported to be found in a substantial number of patients with chronic myelogenous leukemia (CML). The existence of der(9) deletion is reported as a powerful indicator of a poor prognosis. So far, der(9) deletion is considered to be generated when the Ph translocation occurs, because when der(9) deletion is found, it is detected in all the Ph-positive (Ph+) cells of a particular CML patient. On FISH examination of 47 Vietnamese CML patients, we found 11 patients carrying der(9) deletion. Among these, two patients harbored Ph+ metaphase cells with der(9) deletion and also Ph+ cells without it. In CML patients with der(9) deletion, reportedly no ABL/BCR transcript is detected. In these two patients, the proportion of Ph+ cells without der(9) deletion was much smaller than that of the cells with der(9) deletion. Nevertheless, we detected a ABL/BCR (1b–b4) transcript in the two patients. This is further evidence for the existence of Ph+ cells without der(9) deletion. It is possible that in some CML patients, der(9) deletion is generated in the progression of the disease.