Application of tri-colour, dual fusion fluorescence in situ hybridization (FISH) system for the characterization of BCR-ABL1 fusion in chronic myelogenous leukaemia (CML) and residual disease monitoring

BCR-ABL1 dual colour, dual fusion FISH (D-FISH) was performed on diagnostic samples of 22 CML patients. The tri-colour FISH system was performed on cases that showed aberrant signal patterns other than the classical 1 green (G) 1 orange (O) 2 fusions (F). Using the aqua band-pass filter, random signal overlap in interphase nuclei would be indicated by the presence of an aqua signal (ASS1), while genuine fusion was represented by the absence of the ASS1 signal.Using the D-FISH system, the signal patterns could be categorized into 4 groups: group 1 (n = 17) showed the classical 1G1O2F; group 2 (n = 2) showed 2G1O1F indicating ABL1 deletion; group 3 (n = 1) showed 1G2O1F indicating BCR deletion; group 4 (n = 2) with 1G1O1F indicating reciprocal ABL1-BCR deletion. The tri-colour dual fusion system correlated with the D-FISH system for cases with der(9) deletion. The added aqua-labelled ASS1 probe was useful in differentiating random signal overlap from genuine BCR-ABL1 fusion in the interphase cells (group 4).Although the D-FISH probe was valuable in establishing the different patterns of aberrant signals and monitoring patients with the classic 2-fusion signals in CML, the tri-colour dual fusion probe should be used for patients with der(9) deletion to monitor response to treatment.Chronic myelogenous leukaemia (CML) is a clonal haematopoietic stem cell disorder characterized by the translocation t(9;22)(q34;q11.2). The translocation fuses the 5' sequences of the BCR gene on chromosome 22 with the 3' sequences of the ABL1 gene on the chromosome 9. The dual colour, dual fusion fluorescence in situ hybridization (D-FISH) system can demonstrate the BCR-ABL1 fusion [1]. In classical t(9;22), there is one signal each for the wild type alleles and two fusion signals, one for the fusion gene and the other for the reciprocal product in a positive cell. This dual fusion pattern has provided good analytical sensitivity for the monitoring of disease response to therapy. However,