Fluorescence In situ hybridization signal patterns and intrachromosomal breakpoint cluster region-abelson murine leukemia viral oncogene homolog 1 amplification analysis in imatinib-resistant chronic myelogenous leukemia patients using tricolor dual fusion probe

BACKGROUND: Cytogenetic evaluation is required till a complete cytogenetic remission is achieved in chronic myelogenous leukemia (CML) patients on tyrosine kinase inhibitor (TKI) therapy. The routine dual colour fluorescence in situ hybridization (FISH) probes are less sensitive in identifying der(9) abnormalities. BCR/ABL/ASS1 tri-colour dual fusion (TCDF) probe is highly sensitive and specific in identifying der(9) deletions and random signal overlaps. METHODS: Peripheral blood interphase FISH analysis was performed on imatinib-resistant CML patients using TCDF probe. RESULTS: On analyzing 37 adult patients, all had residual Philadelphia (Ph) chromosome. Classic Ph fusion pattern was seen in 33 (89%), derivative chromosome 9 [der(9)] abnormalities in 25 (67.5%) and supernumerary Ph chromosomes in 11 (30%) patients. Coexistence of classical fusion and der(9) abnormalities was seen in 21 patients (57%); and classical fusion, der(9) abnormalities and supernumerary Ph chromosome in 8 patients (22%). None of the patients had BCR-ABL1 gene amplification. There was significant difference in the der(9) abnormal cell percentages between patients with e13a2 and e14a2 transcripts (P = 0.008) and patients with disease transformation (P = 0.007). CONCLUSION: A high frequency of der(9) abnormalities and absence of BCR-ABL1 gene amplification was seen in imatinib-resistant CML patients analyzed. The use of TCDF probe for cytogenetic follow-up in CML patients was found to be useful in identifying BCR-ABL1 related aberrations. The identified patterns in this study, can serve as a reference material for I-FISH signal interpretation using TCDF probe.