• T H Lim, S L Tien, P Lim, and A S T Lim.
• Department of Pathology, Singapore General Hospital, Singapore. gptlth@sgh.com.sg
• Ann Acad Med Singap. 2005 Oct 1; 34 (9): 533-8.

IntroductionChronic myeloid leukaemia (CML) is characterised by the formation of the BCR/ABL fusion gene, usually as a result of the Philadelphia (Ph) translocation between chromosomes 9 and 22.Materials And MethodsThe incidence of both typical and atypical BCR/ ABL gene rearrangements was determined in 110 patients suspected of CML using dual fusion fluorescence in situ hybridisation (DF-FISH) probes.ResultsEighty-seven per cent of CML patients showed Ph translocation while 13% were negative for the Ph chromosome. About 71.9% of Ph-positive patients displayed the typical DF-FISH signal pattern. Atypical patterns among the Ph-positive patients included the concurrent loss of residual proximal 9q and distal 22q (10.4%), complex translocation with additional partners (9.4%), supernumerary Ph (3.1%), loss of residual 9q sequences proximal to breakpoint (3.1%), and deletion of distal derivative 22q signal (2.1%). Cryptic genetic alterations with loss of proximal 9q sequences were found in 13.5% of CML Ph-positive patients, which is associated with poor prognosis. Fusion signals were detected in 57.1% of CML Ph-negative patients, indicating cryptic BCR/ABL rearrangements (i.e., masked Ph).ConclusionFISH is able to detect BCR/ABL fusion in CML with masked or variant Ph not apparent with conventional karyotyping. Establishment of signal patterns with FISH is important as atypical patterns may have clinical prognostic implications.

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