Silent Philadelphia chromosome: a distinct developmental stage in a philadelphia chromosome-positive chronic myeloproliferation?

Correlation between BCR-ABL expression and tumor burden is restricted to the transition from minor to major cytogenetic response in interferon treated CML patients

The interferon treatment of chronic myeloid leukaemia has been monitored by investigating the tumour burden as revealed by fluorescence in situ hybridization and the expression of BCR-ABL chimera determined by quantitative reverse transcription polymerase chain reaction. These parameters were obtained from the peripheral blood of 51 untreated and 104 follow-up patient samples. Poor correlation (r=0.31) was found between BCR-ABL expression and tumor load in all samples as well as in untreated patients, and this correlation was even less in all follow-up cases (r=0.28). Regarding chimera expression five order of magnitude difference existed in the untreated patients and this value dropped to two in those with complete cytogenetic response. Only the major and the complete cytogenetic response groups differed significantly (p<0.001) in the BCR-ABL expression from that of patients at diagnosis. Among the different cytogenetic response groups the only significant difference (p<0.01) in the BCRABL expression was obtained between the major and the minor responders. In the individual patients not only correlated changes of residual tumour mass and chimera expression, but mainly independent changes of these two parameters were observed. This indicates that the BCR-ABL expression and the tumor burden are largely independent variables.

Problems with interphase fluorescence in situ hybridization in detecting BCR/ABL-positive cells in some patients using a novel technique with extra signals

Interphase fluorescence in situ hybridization (I-FISH) is frequently used to monitor the response to therapy in various hematological malignancies. We performed a comparison of I-FISH, metaphase FISH (C-FISH), conventional cytogenetics, spectral karyotyping (SKY) and PCR for the detection of the t(9;22) and the BCR/ABL rearrangement in 32 patients with chronic myelogenous leukemia (CML). FISH was done using a novel commercial probe set (VYSIS LSI BCR/ABL ES), which is designed to reduce the rate of false-positive results by marking the argininosuccinate synthetase (ASS) gene and thus providing an extra signal on chromosome 9. Our data indicate, that a substantial number of BCR/ABL-positive patients (n=5 patients, 3 with Ph, 2 with masked Ph) present negative results using this probe set in I-FISH analyses, because they did not fulfill the scoring criteria. In fact, the ASS region, which usually remains on 9q in Ph+ CML, appears to be lost or translocated. Due to these results we recommend that the initial diagnosis as well as the follow-up of patients with Ph+ leukemias should not be based on a single technique but should integrate results of cytogenetics and molecular biology.